Wednesday, 12 August 2026

Petfood Palatability: Why Fat Is the Signal Alternative Proteins Forget

The Friday Conversation · No. 4

A lipid freshness clock: one continuous oxidation axis, from the fresh-kill signal at one end to rancidity at the other.

Oiling the Palatability Wheel

Fat is the only nutrient that carries its own timestamp. The animal at the bowl has been reading that clock all along.

Dr. rer. nat. habil. Dr. Seronei Chelulei Cheison

Open a fresh bottle of fish oil and smell it. Close it, leave it on the shelf, and open it again a week later. The two smells are not the same, and you do not need a laboratory to know it. The nose reports the change instantly, with a certainty that feels almost like alarm. Something has happened.

Here is the strange part. In that week, very little has changed that a nutrient panel would notice. The energy is the same. The fatty acids are, to a first approximation, the same. The degradation that will eventually matter to the body, the slow loss of the delicate omega-3s, has barely begun; the nose sounds its warning long before the nutrition is meaningfully harmed. By every measure a specification sheet cares about, the oil is still the oil. And yet you know, before you have thought about it, that you would not now put a spoon of it in your mouth.

We have a word for what the nose is reporting. We call it rancidity. But what, exactly, are we smelling? Not a loss of nutrition, which has scarcely occurred. Not a change we can see. We are smelling time. The fat has kept a record of how long it has sat, written in molecules too faint to matter to the body and too loud to miss with the nose, and the animal reading that record is doing something a spec sheet cannot do at all. It is telling the hour.

This is the thing about fat that the protein age has almost entirely overlooked. Protein feeds. Carbohydrate feeds. Fat feeds too, and richly. But fat also does something none of the others do: it keeps time. Leave a protein on the shelf and it ages quietly, in ways the nose can barely detect. Leave a fat, and it begins, from the first day, to tell you how long it has been left. Fat is the only nutrient that carries its own timestamp, and every creature that ever hunted learned to read it, because the difference between a fresh kill and a spoiled one was, quite literally, the difference between a meal and a poisoning.

I have spent three of these Friday conversations on what an animal wants to smell and taste in its food: the second on a signal read off a fresh wound, the third on the many voices of meat that are not its protein. This fourth one is about the nutrient that sits underneath all of that, carrying more of the signal than we credit and almost none of the credit it carries. It is about fat, and about a question the alternative-protein transition has been slow to ask out loud. We have learned, impressively, to rebuild the protein. Have we understood the fat well enough to tell the animal it is fresh, and not, a week later on the shelf, exactly the opposite?

Movement I

The Alternative Protein Conversation That Forgot The Fat

Let me be fair to the transition before I press on it, because it has earned the fairness. The work of the last two decades has been, overwhelmingly, protein work, and that was the right place to start. An obligate carnivore needs a great deal of protein, of a particular quality, and building that from plants and fungi and single cells is genuinely hard. The industry has largely solved it. There are complete amino acid profiles, adequate digestibility, the taurine and arginine supplemented where the chemistry falls short. Nobody should wave that away.

But notice what the whole conversation has been organised around. There are protein action plans and protein transitions and protein start-ups. There is no fat transition. Fat has been treated as the easy part, the thing you add back at the end once the hard protein problem is solved, a commodity you buy by the drum and pour on. And this is precisely backward, because a large part of what makes food palatable to a carnivore was never the protein at all. It was the fat.

Consider what the animals themselves choose when you let them. Give a cat the freedom to compose its own diet from separate sources and it settles, with real consistency, on a balance of roughly 52% of its energy from protein, 36% from fat, and 12% from carbohydrate. Give a dog the same freedom and it lands somewhere quite different: around 30% protein and 63% fat, reaching for nearly twice the fat the cat takes. The dog forgives the fat; the cat audits it. These are not idle preferences. The animal defends them, and will override an unpromising flavour to hit its target. There is an old piece of folklore in the trade about some magic ratio of protein to fat that unlocks palatability, and the folklore has it not quite right. What the numbers actually show is two different things braided together: a nutritional target the animal is trying to reach, and a palatability response to what is in front of it. The ratio is real. It is just not the whole of the story, and mistaking the one for the other has cost the field more than it knows.

The dog forgives the fat. The cat audits it.

And here the numbers open onto something genuinely unsettled, which is worth pausing over rather than rushing past. The cat that self-selects 36% of its energy from fat evolved eating prey that delivers closer to 46%. On protein the match is almost exact; on fat the animal we observe in the feeding trial settles well below what its wild prey would have given it, and on carbohydrate it drifts to six times what prey ever supplied. So which is the true target, the profile the cat reaches for on our diets, or the profile its prey actually carried? When whole prey itself ranges from a tenth to well over half fat depending on the season and the animal, is the cat defending a number at all, or a range? How much of what we record as preference is simply what the bowl allowed the animal to reach? I raise these not to answer them, because I cannot, but because a field confident enough to engineer a diet ought to be honest about how loosely we understand the target it is engineering toward.

There is one more thing the protein framing misses, and it is the largest. When we say fat matters to palatability, we tend to mean taste and richness, the mouthfeel of it. That is real, but it undersells the case badly. Fat’s deepest contribution is to aroma, and aroma is the sense that leads. The nose reaches the food before the mouth does; it forms the first judgement, the go or no-go, before a single taste receptor has fired. And aroma lives in fat. The volatile molecules that carry the smell of food are mostly fat-soluble; they dissolve into the fat, are held by it, and are released from it as it warms. Fat is not merely one contributor to flavour among several. It is the reservoir the aromas are kept in, the medium they travel by, and, as we will see, the very material from which many of them are made. Forget the fat and you have not forgotten a garnish. You have forgotten the thing the animal smells first.

Movement II

One Fatty Acid, Two Messages

To see how fat carries a signal, and how the same fat can carry its opposite, we have to follow a single fatty acid on the one journey that matters: oxidation. It is the reaction at the centre of this entire essay, and its most important feature is that it is not an event but a road. A fat does not simply oxidise or fail to. It oxidises progressively, passing through stages, and where it happens to be along that road is precisely what the animal’s nose is reading.

Take linoleic acid, the polyunsaturated fatty acid that sits in most of the fats we care about. Early on the oxidation road, when the reaction has only just begun, it throws off a particular set of light, volatile fragments, and among them is a molecule I have written about before: the compound that signals a fresh kill, the one a carnivore reads off a wound before it has taken a bite. Freshly begun oxidation smells, to the animal built to read it, like something recently and cleanly dead. This is the attractant, and it appears at the very top of the road.

Keep going down the same road, though, and the same linoleic acid tells a different story. The light early fragments give way to heavier ones, the secondary aldehydes with names like hexanal and 2,4-decadienal, and these are the smell we call rancid: painty, cardboard, stale, wrong. Nothing new was added. No different fat arrived. The identical molecule that produced the fresh signal at the start of its oxidation produces the spoiled signal further along, because oxidation simply kept running. The precursor of the attractant is the precursor of the repellent. They are the same fat, one stretch of road apart.

The molecule that says fresh kill and the molecule that says rancid are the same fat, one step apart.

This is worth holding still for a moment, because it resolves something that might otherwise look like a contradiction across these essays. If the fresh-kill molecule is itself a product of oxidation, and rancidity is also a product of oxidation, how can the animal prize the one and refuse the other? The answer is that it is not reading oxidation as a yes or a no. It is reading how far the oxidation has gone. Fresh oxidation, the first tick, says eat now. Advanced oxidation, many ticks later, says too late. The animal is not asking whether the fat has oxidised. It is asking what time it is.

Now bring a real oil into it, because this is where the abstraction earns its keep. Sunflower oil is a useful case, and a slightly awkward one for anyone who assumes plant fat is simply inferior fat. In a controlled trial where the only thing that changed was the fat coating a kibble, dogs preferred sunflower oil over poultry fat and over beef tallow, and the researchers attributed the preference to sunflower’s high content of linoleic acid. Read quickly, that looks like a straightforward win for the plant oil. Read slowly, it is the whole dilemma of this essay in one result. The very thing that made the sunflower oil attractive, its richness in linoleic acid, is the very thing that makes it spoil fastest, because linoleic acid is precisely the polyunsaturated fatty acid that runs down the oxidation road quickest. The oil is preferred because it is rich in the precursor of the fresh signal. It is fragile for exactly the same reason.

The plant breeders, it turns out, have already been fighting this war, and it is worth knowing which side they chose. Conventional sunflower oil is high in linoleic acid, which makes it flavourful and makes it perishable. To improve its shelf life, breeders have spent decades selecting for a high-oleic sunflower instead, trading the polyunsaturated linoleic away for the far more stable monounsaturated oleic. The newer oil keeps for longer on the shelf. But look at what was given up to get there: the linoleic acid that was bred out is the very fatty acid the dogs were responding to, and the precursor of the fresh signal itself. The industry, chasing stability, has been quietly breeding out the molecule of freshness. You may have stability or you may have the signal, and the same acid governs both. There is no variety that gives you both at once, and anyone who tells you a single sunflower oil is both maximally palatable and maximally stable is describing two different oils and hoping you will not notice.

There is a deeper convergence hiding in all of this, and it reaches back to the fire of the last conversation. The fresh-kill molecule is not only born cold, at the wound. It is also born hot, in the pan. When animal fat is heated, its polyunsaturated fatty acids, the linoleic and the arachidonic, break down under the heat and generate the very same molecule, by a different route. The wound writes it at body temperature in seconds; the fire writes it again from the fat at cooking heat. One signal, two origins, and both of them lipid. It matters, too, that this is not the browning reaction people usually credit for meaty smell. The roasted, savoury notes that any protein can be coaxed into giving off under heat are a separate chemistry, and I gave them their due last time. This is not that. This is thermal oxidation of the fat itself, and it is fussier about its raw material: it needs the right polyunsaturated fatty acids, the ones animal fat carries in species-specific proportion, poultry fat notably rich in linoleic, pork fat carrying appreciable arachidonic. The browning any substrate can fake. The fresh-kill note it cannot, because that one is written in the fat, and the fat has to be the right fat.

Movement III

The Lipid Freshness Clock

In the last conversation I described a clock the tongue can read, the slow slide of the meat’s own nucleotides from the savour of freshness toward the bitterness of age. Fat keeps a second clock, and it runs on the nose. The two are worth setting side by side, because between them they explain how an animal with no calendar and no thermometer knows, with such speed and such conviction, how long ago its food was alive.

The lipid clock is the oxidation road we have just walked, read as time. At the first tick, the fresh-kill molecule, cleanly begun oxidation, the smell of something recently dead. Later ticks, the accumulating aldehydes, the smell of something left too long. The fat is not merely spoiling or not spoiling. It is advancing, steadily, along a track the animal has evolved to read as a position in time. This is why I keep returning to the word clock rather than the word freshness, because freshness sounds like a single quality a thing either has or lacks, and that is not what the animal is reading. It is reading a moving hand. Fat does not just feed the animal. It tells the animal the time.

I want to slow down here and be careful, because there is a question folded into this that the science has not actually settled, and it would be easy to skate over it with a confident sentence. When the nose reports that a fat has turned, what is it really reading? Is freshness the same thing as oxidation, simply the chemical fact of it, measurable in a laboratory? Or is freshness something the animal makes of that chemistry, an interpretation laid over the measurement, the difference between what a molecule is and what it means to the creature smelling it? I do not think these are the same question, and I do not think we know the answer to the second one. We can measure the oxidation precisely. What the animal does with that measurement, whether it reads a number or a meaning, is a genuinely open matter, and I would rather leave it open and honest than closed and wrong.

Let me put a human face on it, because I have one, and because it makes a point no diagram can. I grew up in Nandi County, in the Kenyan highlands, and like children across much of the world in those years we were dosed, at the first sign of a cold, with a spoonful of Scott’s Emulsion. It came in a bottle bearing a picture that has never left me: a man striding along with an enormous fish slung across his back, cod liver oil rendered into a thick, sweetened emulsion. The taste was an ordeal. The smell was worse. I am fairly sure some of us recovered from our colds through sheer dread of the next spoonful, willing ourselves well to escape it.

Here is why that memory belongs in an essay about fat. That smell was appalling, and the oil was not spoiled. Cod liver oil smells like that fresh from the factory; the whole elaborate business of the emulsion, the lime and the sugar and the glycerine, was an attempt to bully a wholesome, intensely fishy oil into something a child would swallow. The smell was not the smell of rot. It was the smell of what the oil simply is. And that is the trap at the heart of the freshness question, the thing the nose gets wrong as easily as it gets right: a strong smell does not always mean a spoiled one. Intensity and spoilage are two different axes, and a nose can confuse them. The child gagging on Scott’s Emulsion was reading a strong smell as a bad one, and was, on the evidence of a century of dosed and recovered children, mistaken. Which ought to make us wonder what the cat is really doing when it turns from a fat we have called rancid. Is it reading spoilage, or only reading strength? Is it refusing something dangerous, or something merely loud?

Movement IV

Rotten, Or Merely Not Fresh?

That question, the one the child on the spoon could not answer, turns out to be the question the whole industry cannot answer either, and a great deal rides on it. When a cat turns from a fat we have judged rancid, what is it actually reading?

Begin with a distinction the trade tends to blur. Rancidity and rot are not the same chemistry. Rancidity is oxidation, the fat reacting with air, throwing off the aldehydes we have been tracking. Rot is decay, the work of microbes breaking tissue down into a different family of molecules entirely, the amines I described in the meat of the last conversation. A rancid fat and a rotting carcass smell wrong in different ways, by different routes, because different things are happening in them. So when a cat refuses an oxidised fat, it cannot literally be reading rot, because the molecules of rot are not there. It is reading something else. But what?

There are, as far as I can see, three honest possibilities, and I want to lay them out without pretending to know which is true. The first is that the cat is making a mistake in its favour, reading advanced oxidation as though it were the beginning of decay, treating the rancid fat as a proxy for a rotting one and refusing it out of an abundance of evolved caution. On this reading the refusal is a false alarm, a safety reflex firing at the wrong trigger, and the cat is being fooled by a resemblance. The second possibility is that the cat is reading exactly what is there and nothing more: not danger, but lateness. Not this will harm me, but this is no longer fresh. On this reading the cat is simply far along the lipid clock, registering a position in time and declining a fat that has moved too far down the road, the way you might decline bread that has gone stale without for a moment thinking it would poison you. And the third possibility is that oxidation is its own signal to the carnivore, neither borrowed from the alarm of rot nor merely a reading of freshness, but a distinct thing the animal evolved to weigh in its own right, for reasons we have not yet worked out.

I do not know which of these is correct, and I am not sure anyone does. But notice that the difference between them is not academic in the least, because it decides whether the problem in front of a formulator is a wall or a workbench. If the cat is reading rancidity as danger, as rot, then the refusal is a hard biological veto, wired deep, and no amount of clever chemistry will talk the animal out of it; you would be fighting an alarm evolution spent millions of years making difficult to silence. But if the cat is only reading lateness, only registering a position on the freshness clock, then the problem is not a veto at all. It is a matter of kinetics. It becomes a question of how fast the oxidation runs and how far it has gone by the time the bowl is filled, and those are things a formulator can actually govern, with antioxidants, with packaging, with the freshness of the fat going in, with how long the food sits between the factory and the animal. A wall is a fact you must design around. A workbench is a problem you can work at. And we do not yet know which one rancid fat is, which means we do not yet know whether the single largest sensory obstacle to feeding a carnivore on stable, shelf-friendly fats is insurmountable or merely unsolved. That strikes me as a thing worth finding out before we build much more of the future on a guess.

A wall is a fact you must design around. A workbench is a problem you can work at.

The Machine That Could Not Marble

There is a reason the fat got left until last, and it is not that anyone forgot it. It is that the machine at the centre of the whole enterprise cannot handle much of it. Once you see this, the flat sensory experience of so many meat analogues stops looking like an oversight and starts looking like a consequence.

The plant-based whole cut is built, overwhelmingly, by extrusion. Protein and water are driven through a twin-screw barrel under heat and pressure, and the shear inside tears and realigns the protein into fibres that pull apart on the tongue like muscle. It is a genuinely clever process. It is also, by its own physics, at war with fat. An extruder works by friction: the screws grip the protein mass and drive mechanical energy into it, and that energy is what builds the structure. Fat does the opposite of grip. It lubricates. Add much of it and the screws begin to slip, the energy stops transferring, the fibres fail to form, and the oil weeps out onto the surface. So the machine has a ceiling, and the ceiling is low. Texturised proteins are typically run at something like 0.5 to 6% fat. The meats they are built to imitate carry 20, 40, 60%. The core technology of the industry can hold only a fraction of the fat of the thing it is trying to become.

So the fat is banished to the end of the line. It is sprayed on afterward, cold, onto the finished fibre, as a coating. And this is the quiet defeat beneath so much of the category, because coating fat is raw fat. It has passed through none of the chemistry that turns fat into flavour. It never met the heat that would have generated the fresh-kill note from its precursors; it never oxidised even to the first fresh tick of the clock. It sits on the surface, greasy and mute, doing almost none of the work fat does inside an animal. The problem was never that these foods contain too little fat. It is that the fat is in the wrong place, added too late, in the wrong state, contributing mass and lubrication and nearly nothing to the nose.

Set that against the animal the whole project is chasing. A Wagyu breeder spends the better part of a decade coaxing fat to deposit not around the muscle but woven through it, marbled into the tissue in fine seams, and the world pays extraordinary sums for the result. What is being paid for is not protein; Wagyu’s protein is ordinary beef protein. It is the fat, and specifically the architecture of the fat, its distribution, its low melting point, the way its intramuscular seams liquefy on the tongue at body heat and carry flavour as they go. The animal builds its lean and its fat together, in the same tissue, on the same day, by the same body. That is the standard. And the extruder, for all its ingenuity, cannot approach it, because the extruder cannot marble. It can only paint.

The extruder cannot marble. It can only paint.

Let The Reactors Converge

And yet I do not want to end on a defeat, because the ground is shifting under this problem faster than almost anywhere else in the field, and for the first time the fat is being taken seriously on its own terms. A generation of companies has stopped treating fat as the easy part. Some are growing true fat cells in bioreactors. Some are brewing tailored fats by fermentation, coaxing yeasts and other microbes into producing the specific molecules they want. Some are structuring plant oils to behave, at last, like the marbled fat of an animal rather than a puddle at the bottom of the pan. The framing has finally caught up with the biology. The taste gap, these companies now say plainly, was a fat gap all along.

But I want to press one request on that emerging field, because it is the whole argument of this essay folded into a single ask, and the field is at exactly the moment when the ask can still shape it. It is not enough to solve the muscle in one reactor and the fat in another and marry them at the end. That is how we arrived at the painted extrudate in the first place, only more expensively. An animal does not grow its lean in one place and its fat in another and glue them together before serving. It marbles them, fibre and fat laid down together, in the same tissue, by the same body, on the same day. The hardest and least-solved problem in the whole endeavour is precisely this, the growing of muscle and fat as one marbled structure rather than two ingredients kept apart, and it is the problem most worth solving, because it is the one that separates alternative tissue from alternative meat.

So let the reactors converge. Let the fat and the muscle be grown into one scaffold, marbled from the start, so that what emerges is not lean here and grease there but a single thing an animal would recognise and a mouth would too. That is the difference between building alternative tissue in one place and alternative fat a continent apart, and building, at last, animal-free fatty meat. Alternative meat, in other words, and not merely its parts.

Marbling puts the fat in the right place. It does not make the fat say anything, not even about the species.

But I have to press the point one turn further, because marbling alone is a trap, and it is the very trap this whole essay has been circling. Put the fat in exactly the right place, woven through the muscle in perfect seams, and you may still have built something that says nothing. Structure is not signal. A cut can be flawlessly marbled and aromatically mute, fat in all the right seams and not a word of fresh kill in any of them. If we design only for where the fat sits and not for what the fat carries, we will grow beautiful lipo-bodies: sculptures of fat in the shape of meat, correct in every architecture and silent in the one language the animal is actually listening for, unable, for all their perfect marbling, to say even which animal they are pretending to be.

So the harder design brief, the one that separates a serious attempt from an expensive imitation, is not structural at all. It is chemical. It is to build fat that can still speak: fat that carries the right polyunsaturated precursors, that can run the freshness clock, that will generate the fresh-kill note under heat and read as recently and cleanly dead to the nose evolved to judge it. That is a far deeper problem than positioning a fat correctly, and we have barely begun to pose it, let alone solve it. But it is the problem that matters, because the animal was never grading the marbling. It was reading the smell.

And here is the test, the one I opened a previous conversation with and can now hand back to you sharpened. Stand in front of a barbecue with your eyes closed. You knew, then, what was cooking, because the fat was speaking, each species in its own aromatic dialect, written by the oxidation of its own particular fats under the heat. Now imagine the cultured cut on those same coals. If it marbles like meat and browns like meat and still, with your eyes closed, tells you nothing, then we have not made meat. We have made a lipo-body doing an impression of one. The bowl, and the nose above it, will know the difference long before the spec sheet does.

The transition toward alternative proteins has taught us something genuinely large: that nutrition can be redesigned, taken apart and rebuilt from new materials, and made to nourish an animal as well as the old materials did. That is no small thing, and I do not mean to diminish it. But fat reminds us of something the protein triumph can obscure, which is that biology still keeps the time. Every day a fat sits on a shelf moves the hand of that clock a little further, from the fresh note the animal leans toward to the spoiled one it turns from, and the animal reads that clock whether or not we have thought to consult it. The question is no longer only whether we can understand fat well enough to rebuild it. It is whether we understand what the animal reads when it looks at the time.

Dr. rer. nat. habil. Dr. Seronei Chelulei Cheison
Langwedel, Niedersachsen

Sources drawn on for this essay

Hewson-Hughes, A.K., et al. (2011). Geometric analysis of macronutrient selection in the adult domestic cat, Felis catus. Journal of Experimental Biology 214(6): 1039–1051. doi:10.1242/jeb.049429
Hewson-Hughes, A.K., et al. (2013). Geometric analysis of macronutrient selection in breeds of the domestic dog, Canis lupus familiaris. Behavioral Ecology 24(1): 293–304.
Plantinga, E.A., Bosch, G. & Hendriks, W.H. (2011). Estimation of the dietary nutrient profile of free-roaming feral cats. British Journal of Nutrition 106: S35–S48.
İnal, F., Alataş, M.S., Kahraman, O., İnal, Ş. & Uludağ, M. (2020). Determination of fat preferences of adult dogs. Turkish Journal of Veterinary & Animal Sciences 44(3): 481–486. doi:10.3906/vet-1906-56
Gassenmeier, K. & Schieberle, P. (1994). Formation of the intense flavour compound trans-4,5-epoxy-(E)-2-decenal in thermally treated fats. Journal of the American Oil Chemists’ Society 71(12): 1315–1319. doi:10.1007/BF02541347
Marchioni, G. / Kemin. Impact of ingredients’ freshness, including free fatty acids and the oxidation aldehydes hexanal and 2,4-decadienal, on cat and dog dry pet food palatability.
AFB International. Fat Quality: Impact of Fats on Palatability (technical bulletin), on free fatty acids, peroxide value and hexanal as indicators of rancidity in petfood fats.
On lipid-derived aroma: role of lipids in food flavour generation, and the oxidation of unsaturated fatty acids to odour-active aldehydes (reviews in Food Chemistry and related literature).
On high-oleic sunflower stability: comparative oxidative and thermal stability of high-oleic (~5.5% linoleic) versus conventional (~71.6% linoleic) sunflower oils.

Dr. rer. nat. habil. Dr. Seronei Chelulei Cheison is the founder and Chief Executive Officer of Sinonin Biotech GmbH, a biotechnology company focused on alternative protein and palatability enhancer innovation and application for petfood formulations. He formerly led alternative protein and palatant development projects at the Mars Petcare Global Innovation Centre in Verden, and spent close to two decades in academic research on enzymatic protein hydrolysis, holding a doctorate from Jiangnan University and a habilitation from the Technical University of Munich.

Sinonin Biotech GmbH is a partner in two consortia funded under the Circular Bio-based Europe Joint Undertaking: ZEST, on fungal fermentation of agricultural residues, and PROSCALE, on scalable microbial protein ingredients, which runs from September 2026 to August 2030.

Co-funded by the European Union under Grant Agreement No. 101157382 (ZEST) and Grant Agreement No. 101288362 (PROSCALE). Views and opinions expressed are however those of the author only and do not necessarily reflect those of the European Union or CBE JU. Neither the European Union nor the CBE JU can be held responsible for them.

Thursday, 6 August 2026

Petfood Palatability: Why Replacing Meat Is More Than Replacing Protein

The Friday Conversation · No. 3

A freshness-clock timeline: the biochemical stages meat passes through after death, from the fresh-kill signal to spoilage.

Meat Is More Than Protein

Perhaps meat was never just an ingredient. It is a biological language that every alternative protein must eventually learn to speak.

Dr. rer. nat. habil. Dr. Seronei Chelulei Cheison

Stand near a barbecue with your eyes closed and you can name what is cooking. Chicken smells like chicken. Lamb smells like lamb. Salmon announces itself across a garden, and beef is unmistakable long before it reaches the plate. Nobody finds this remarkable, because everybody can do it. It is one of those competences so ordinary that we never stop to ask how it works.

So ask it. Why does lamb smell like lamb and not like beef?

The comfortable answer is that they are different animals, which is true and explains nothing. Here is the answer that should unsettle anyone whose job is to rebuild meat from something that was never an animal at all: the difference you are smelling is almost entirely not in the protein. Strip a piece of chicken and a piece of beef down to their lean muscle, cook them bare, and they converge. The heat of cooking on lean tissue produces a meaty aroma that is remarkably species-neutral, the generic smell of cooked flesh, and a blindfolded taster would struggle to tell them apart. What makes chicken smell like chicken lives somewhere else.

It lives in the fat. And not the fat you can see and trim, but the fat woven invisibly into the cell membranes: the phospholipids. Remove the phospholipids from beef and its flavour changes markedly; leave the visible marbling and remove nothing else, and far less is lost. The species signature is a membrane phenomenon. The identity of meat resides less in its muscle proteins than in the chemistry of the lipids surrounding them. Lamb carries its identity in branched-chain fatty acids that beef does not use for flavour at all. Pork can carry a note that is, quite literally, a steroid dissolved in fat: boar taint, the reason most male pigs are castrated, is androstenone lodged in the adipose tissue. Chicken loses its chicken-ness the moment you wash the polar compounds out of its adipose tissue. And none of it speaks until fire arrives: raw meat is weakly flavoured, faintly metallic, faintly of blood, a reservoir of precursors waiting for heat to convert them. A substantial proportion of a cooked meat's characteristic aroma arises from lipid oxidation products and their interactions during heating, rather than from the muscle proteins themselves.

The lean is nearly the same across species. The signature was always in the fat.

I open here, at the grill, for the fastest way into the thing this whole conversation turns on. In the second of these Friday conversations I wrote about a single molecule, the one a cat reads off a fresh wound before it has taken a bite. That was the smell of the kill. This is the smell of the fire. Meat speaks in more than one aroma, one from the wound and one from the flame, and in both of them the protein is silent. Which raises the question the alternative-protein transition has mostly declined to ask out loud: if the thing that makes meat taste of anything at all was never the protein, what exactly are we matching when we match the protein?

WHAT WE HAVE REPLACED, AND WHAT WE HAVE NOT

Let me grant the strong version first. It is true, and the argument does not work without it.

What the alternative-protein industry has largely succeeded in replacing is nutrition. We now know how to formulate diets that deliver complete amino acid profiles, adequate digestibility and, with appropriate supplementation, the vitamins and micronutrients an obligate carnivore requires. That is no small achievement. It represents decades of careful nutritional science, and I have spent enough of my own life on protein hydrolysis to refuse to wave it away. The lysine in a pea is the same lysine as the lysine in a muscle; it is a small, defined molecule and it does not carry a passport. On the nutrient certificate, the substitution is honest and complete.

What remains much less certain is everything beyond nutrition.

We are making progress in reproducing the broad savoury character of meat through hydrolysed proteins, fermentation products, yeast extracts and carefully designed flavour systems. Yet recreating the aroma of meat is not the same as recreating the identity of meat. A cooked chicken, a grilled lamb chop and a roasted beef steak all announce themselves long before they are tasted, each carrying a distinctive chemical signature that evolution has written into their lipids and that cooking merely reveals. We understand fragments of that language. We are still far from speaking it fluently.

A legume may provide the vocabulary of nutrition. Meat provides the grammar of recognition.

Harder still is the question of species recognition. We know remarkably little about how a cat weighs one prey species against another, or how many cues must be present, and in what combination, before the brain concludes simply: this is food. In the last of these conversations we met one such cue, a blood-borne signal shared across the whole predatory lineage. But that signal says kill, not chicken. The cues that carry species identity, and the number of them a diet must satisfy, remain largely unmapped. The science is moving. The map is still mostly blank.

Beyond recognition lies an even larger uncertainty. A bowl is emptied not because a formulation is nutritionally complete, but because an animal chooses to return to it, day after day. That decision emerges from a conversation between aroma, taste, texture and the physiological consequences of eating. Preference is learned, reinforced, and sometimes abandoned. We understand parts of that process. We do not yet understand the whole.

Seen in this light, the challenge facing alternative proteins is changing. It is no longer simply to replace the nutrients found in meat. Increasingly, it is to reproduce the biological functions that meat performs. Nutrition may be the first milestone. It is unlikely to be the last. The future belongs not to formulations that merely analyse like meat, but to those that are recognised, accepted and remembered as food by the animal standing at the bowl.

And to see how far that reaches, you have to stop treating meat as a thing with a fixed composition, and start watching what it does when it is left alone.

Movement I

MEAT THAT WRITES ITSELF

Here is the fact that ought to change how a formulator looks at a data sheet, and almost never does. A cut of meat is not finished when the animal dies. It is barely begun.

In the hours and days after death, the tissue sets about digesting itself. No one adds an enzyme; the enzymes were always there, folded away inside the living cell, kept apart from the structures they would otherwise destroy. Death removes the partition. As the muscle runs out of oxygen and turns acidic, drifting down toward a pH near 5.5, the little membrane-bound compartments called lysosomes, the cell's own recycling bins, begin to break open. Out spill the cathepsins, enzymes that are most active in exactly the acid conditions that dying muscle creates. A second family, the calpains, goes to work on the structural proteins and loosens the meat toward tenderness. The cathepsins and the peptidases that follow them do something subtler and, for our purposes, more important: they carve the long proteins into short peptides, and the short peptides into free amino acids.

This is where much of meat's flavour is actually made. Aging is not storage. It is a slow, self-directed hydrolysis, the tissue quietly cleaving its own proteins into the small, taste-active fragments a carnivore is tuned to. Amino acids that are barely present in fresh muscle, tyrosine, phenylalanine, threonine, tryptophan, rise into detectability as the enzymes run. Some of them read as sweet, some as bitter, and the acidic and sulphur-bearing ones, glutamate and aspartate, cysteine and methionine, carry the deep savour we call umami. The meat that a cat would choose is, in a real sense, meat that has been cooking itself without heat.

Aging is not storage. It is the meat quietly cleaving its own proteins into the language of savour.

Now set that beside a specification. When we match the amino acid profile of muscle, we are matching a photograph of a moving thing. The certificate records what the tissue contained at the instant it was sampled, as though composition were the point. But the carnivore did not evolve to eat a composition. It evolved to eat a process, a tissue whose free amino acids and peptides and nucleotides are still being generated on their own clock, arriving in a sequence and a proportion that the animal's chemistry learned to read across millions of years of eating exactly this.

A legume hands you a number, fixed and honest, printed on a page. Muscle hands you an engine that is still running. You can match the number precisely and never build the engine, because the engine was never in the protein content. It was in the enzymes that came free with the animal, the ones that keep working after the animal is gone.

A legume gives you a number on a certificate. Muscle gives you an engine that is still running.

And the engine does not stop at flavour. The same self-digestion that fills fresh meat with savour keeps going, and where it goes next is the second half of this story, because the clock that ripens meat is the same clock that will eventually spoil it. Before we follow it there, though, we should look at what the animal is reading while the meat is still at its best, the signal that sits at the very peak of freshness and begins, quietly, to fade from the first hour onward.

Movement II

THE CLOCK THE TONGUE CAN READ

There is a second clock running alongside the first, and where the enzymes of self-digestion were slow, this one is fast and almost violent in its opening moments. It concerns the nucleotides, and it is the taste-side companion to the smell-side signal I wrote about last time.

At the instant of death, the muscle is still charged with adenosine triphosphate, ATP, the molecule that powered it in life. With the animal gone and the oxygen with it, that ATP begins to fall apart along a fixed staircase: to ADP, to AMP, and then, within the first day or two, to a compound called inosine monophosphate, IMP. And IMP is not a waste product. It is one of the most powerful savoury molecules in all of food, the nucleotide that carries umami, the very compound the tongue of an obligate carnivore is built to detect. In the first hours after death the meat is quietly filling with the taste of savour. This is the peak. This is freshness at its most delicious, and it is why a signal on the taste side and a signal on the smell side both point, in a fresh kill, to the same conclusion.

But the staircase does not stop at IMP. It keeps descending, only more slowly, because the enzyme that breaks IMP down is the rate-limiting step, and so IMP lingers at its peak for a while, a plateau of deliciousness, before it gives way. When it gives way it becomes inosine, and then hypoxanthine, and hypoxanthine is bitter. The savour fades and a faint bitterness rises in its place. Long before any bacterium has done its work, long before the meat has begun to rot in any sense a nose would recognise, the taste has already started to tell the animal that the best moment has passed.

IMP is the taste of freshness at its peak. Hypoxanthine is the taste of that peak already passing.

This is the taste-side clock, and the tools of fish science measure it directly. The freshness of fish is graded by a number, the K-value, that is simply the ratio of the spent compounds, inosine and hypoxanthine, to the whole family of ATP breakdown products. A low K-value is a fresh fish; a high one is a fish whose clock has run down. A carnivore does not carry a laboratory, but it carries the same reading in its mouth. The E2D of the last conversation was freshness announced at the very first instant, ab initio, off the wound. The nucleotide clock is freshness measured across the hours that follow, the savour rising to a peak and then, molecule by molecule, beginning to leave. Between them, smell and taste, the animal has a remarkably precise clock on how long ago something died.

And this is where one ingredient proves the entire argument of this essay, more plainly than any reasoning could.

Consider liver. Ask anyone who has fed cats and dogs which single ingredient they find hardest to resist, and liver will be near the top of every list. Cats can be so taken with it that they will overeat it against their own interest. On a protein specification, liver is unremarkable, roughly comparable to muscle, sometimes lower. If protein were the thing an animal was chasing, liver would be ordinary. It is not ordinary. It is close to irresistible, and the reason is precisely the vocabulary this essay has been assembling.

Liver is not muscle at rest. It is the body's most metabolically active organ, a chemical factory that never idles in life, and it is therefore dense in exactly the compounds a carnivore is tuned to read: nucleotides in abundance, free amino acids and short peptides, the blood that carries the heme and its metallic note, and the fat that carries so much else. Everything the tongue and nose of a carnivore evolved to prize is concentrated in that one organ, not because it holds more protein, but because organ function itself, the ceaseless traffic of a living liver, leaves its signature in the tissue. The palatability of liver is metabolism made edible.

Liver is not more protein than muscle. It is more of everything the protein was never carrying.

There is a quiet lesson folded inside this for anyone tempted to think the answer is simply to add the missing molecules back. Some of what makes the organ precious does not survive the factory. The heat of processing degrades the very taurine and the B vitamins that made the raw organ so complete for a cat, which is why they must be added back as supplements at all. We are already, without quite admitting it, conceding the point: the raw tissue delivered something that our processing removes and then reconstructs in part. Nutrition we can rebuild. Whether we have rebuilt the whole of what the animal was reading is a different and far less settled question.

Movement III

WORDS ONLY FIRE CAN SAY

Everything so far has been about what the raw tissue already holds, or makes for itself: the fat woven into the membranes, the peptides carved out by the cell's own enzymes, the nucleotides rising and falling on their clock. But a large part of meat's language does not exist in the raw material at all. It has to be written, and the pen is heat.

Return to the barbecue we started at. The raw chop on the counter is faint, a little metallic, faintly of blood, and almost nothing like the thing that will fill the garden twenty minutes later. What happens in between is chemistry of real complexity, and it has a name. When the amino acids and the reducing sugars in the meat are brought together at temperature, they enter the Maillard reaction, a cascade of hundreds of steps that generates hundreds of new molecules that were simply not present before. Alongside it runs Strecker degradation, breaking amino acids into their own aromatic fragments, and alongside that the breakdown of the lipids, throwing off volatile pieces of the fat. These are not small effects. The characteristic flavour of cooked meat is very largely a creation of these thermal reactions. The roast, the sear, the brown crust: this is a vocabulary spoken only by fire, and the raw tissue is merely the reservoir of words waiting to be said.

Here, at last, is a part of the language where the alternative-protein project is on strong ground, and I want to say so plainly. The Maillard reaction does not, in principle, care where its amino acids and sugars came from. Give it the free amino acids of a fermented biomass and the reducing sugars of a plant, hold them at the right temperature, and it will brown and roast and generate savour on a meat-free substrate as readily as on a muscle. Much of the industry's real success in savoury flavour lives exactly here, in the deliberate use of thermal reactions and the hydrolysates that feed them. If any part of meat's language can be spoken fluently by a plant or a ferment, it is this one.

The Maillard reaction does not ask where its ingredients were born. This is the part of the language a plant can learn to speak.

But notice the boundary, because it is precise, and it takes us straight back to the grill. The Maillard reaction supplies the meaty. It does not, by itself, supply the chicken, or the lamb, or the beef. The generic roasted savour is shared; the species identity rides on the lipid substrate underneath it, on those branched-chain fatty acids and species-specific phospholipids that the heat is acting upon. Cook two different meats and the Maillard chemistry is broadly the same in both; what differs is the fat it has to work with, and the fat is where the animal's name is written. So heat can give a fungal kibble the word for cooked. Whether it can give it the word for a particular prey is a harder question, and it returns us, every time, to the fat.

There is a second, quieter point in this for the formulator, and it connects the two halves of the essay. Some of the most meaty of all the thermal compounds are built, in part, from the very nucleotides we were just discussing. The IMP that carries umami on the tongue is also a precursor that heat can convert into some of the most potent roasted, meaty aromas known. The freshness clock and the fire are not separate systems. The molecule that signals a fresh kill to the taste is the same molecule that, under heat, helps write the smell of the roast. Meat's vocabulary is not a list of independent words. It is a web, in which the same molecule can mean one thing to the raw tongue and another thing entirely once the fire has spoken it.

Movement IV

CONTROLLED SPOILAGE

Follow the clock past its best moment and it does not simply stop. The same self-digestion that filled the fresh meat with savour keeps running, and now a second cast of characters arrives: the bacteria. What they do next is, on the surface, the story of rot. Read more closely, it is one of the most interesting things in this whole essay, for it is also the story of some of the most prized foods we make.

When microbes settle into meat, they set about the free amino acids that self-digestion so helpfully produced, and they strip the acid group from them. This is decarboxylation, and its products are the biogenic amines, each one traceable to the amino acid it came from: histidine becomes histamine, tyrosine becomes tyramine, lysine becomes cadaverine, ornithine becomes putrescine. The names alone tell you which way this is heading. Cadaverine and putrescine are the smell of decay, and their rise is a reliable index that decomposition has begun.

But the story is not as simple as good versus spoiled, and this is the part worth slowing down for. Some of these amines are genuinely aversive, even dangerous: histamine at high levels is the cause of scombroid poisoning, and tyramine and phenylethylamine can push blood pressure and trigger headaches. Some are close to neutral, present at low levels in perfectly healthy tissue as part of the ordinary chemistry of living cells. And some, in the right food at the right concentration, are not spoilage at all. They are flavour. The depth of an aged cheese, the tang of a fermented sausage, the savour of a fish sauce that has stood for a year: these are, in significant part, the taste of the very same amines, produced by the very same decarboxylation, that in another vessel we would have called rot.

Aged cheese and rotting fish share a chemistry. The only difference is who was invited.

Here is the thing that ought to stop a formulator in their tracks. The enzymes that carry out this decarboxylation are found in the microbes of spoilage and in the microbes of fermentation alike. The reaction is the same. The products are the same family. What separates the cheese from the rot is not the chemistry but the control: which organisms were present, at what temperature, for how long, under what salt and what acid. Choose the culture and hold the conditions, and you get the sausage. Let the wild flora arrive on their own terms, and you get the bin.

Fermentation, in other words, is spoilage you chose the microbes for. Rot is spoilage that chose its own.

Fermentation is spoilage you chose the microbes for. Rot is spoilage that chose its own.

I set this at the end of the essay, and not as a flourish, because of where the alternative-protein industry actually stands. A great deal of the most serious work in the field is fermentation: mycoprotein grown in tanks, precision-fermented ingredients, biomass raised by cultures under tight control. The industry is already, and expertly, working the far end of the very freshness gradient this essay has walked. It has spent its skill on the controlled end, on safety and consistency and the avoidance of the aversive amines, which is exactly right and not to be second-guessed.

The question I want to leave in the room is subtler than safety. Meat is not palatable because it sits at one fixed point on this gradient. It is palatable because a carnivore evolved to read the whole of it: the fresh-kill signal at the top, the umami of the peak, the first bitter hint of the turn, and yes, in aged and cured and fermented forms, some of the controlled notes of the descent. If we are learning to build proteins by fermentation, we are already speaking the last dialect on the gradient. The unanswered question is whether we are shaping it only away from what is dangerous, or also toward what an animal would recognise as the deep, aged, savoury language it has always known food to speak.

THE WHOLE ENSEMBLE

Stand back now from the gradient we have walked, and look at the whole of it at once.

At the top is the fresh kill, and the blood-borne molecule a predator reads off the wound in the first instant. Then the savour rising as the nucleotides climb to their peak, the umami of a thing at its freshest. Then the first faint bitterness as that peak begins to pass, the taste telling the animal the clock has started. Beneath all of it, from the very beginning, the fat and its phospholipids, carrying the name of the species in molecules no scale will weigh. Through the middle, the meat quietly digesting itself, cathepsins carving savour out of structure. And at the far end, the controlled descent, the aged and fermented notes that are spoilage tamed and made delicious. This is not a list of ingredients. It is an ensemble, and every part of it is playing at once.

That is the word I have been circling for three of these conversations. Meat is not a substance. It is an ensemble, a whole orchestra of signals sounding together, and a carnivore did not evolve to eat any one of them. It evolved to hear the chord.

And an ensemble is more than the sum of its players. This is the part the specification can never hold, because it is not written in any one component but in the way they sound together: the proportion, the timing, the interference of one note against another. You could fill every chair on the stand, source each molecule on the list and set it in its place, and still not have meat, because meat was never the collection of its parts. It was their playing together. The chord is not the notes.

And this is the thing I most want to leave with anyone rebuilding meat from something that was never an animal. When we replace meat, we do not replace the orchestra. We replace some of the players. We are very good now at the nutrition, which is the section that keeps the animal alive; we are increasingly good at the roasted, Maillard savour, which any substrate can be taught to sound. But the fat that carries the species, the nucleotide clock, the self-made peptides, the deep controlled notes of the aged descent, many of these chairs are still empty, or filled by a player reading from a different score.

Here is the question that keeps me up, and that I do not think the industry has yet asked itself squarely. An incomplete orchestra is not simply a quieter orchestra. Leave out the strings and you do not get the same symphony played softly; you get a different piece, and sometimes a discordant one. The parts that are present, sounding without the parts that are absent, can be worse than silence, because the animal is not comparing our formulation to nothing. It is comparing it to a chord it has known for forty million years. A bowl that delivers perfect nutrition and half the ensemble may not read to a cat as incomplete meat. It may read as something that is trying to be prey and failing, which is a harder thing to forgive than a food that never pretended at all.

I do not say this to discourage the project. I say it because the project is worth doing well, and doing it well means being honest about what is actually on the stand. Nutrition was never going to be the whole score. It was the first section to learn its part. The work now is the rest of the ensemble: to find which of the empty chairs matter most to the animal, and in what combination, and whether the players we do have are sounding in tune with one another or merely sounding.

Perhaps meat was never just an ingredient. It was an orchestra, and the animal at the bowl has been listening to the whole of it all along. The question for the next diet is not whether it can analyse like meat. It is whether, when the animal leans in to listen, the chord rings true.

Dr. rer. nat. habil. Dr. Seronei Chelulei Cheison
Langwedel, Niedersachsen

Sources drawn on for this essay

Mottram, D.S. (1998). Flavour formation in meat and meat products: a review. Food Chemistry 62(4): 415–424. doi:10.1016/S0308-8146(98)00076-4
Mottram, D.S. & Edwards, R.A. (1983). The role of triglycerides and phospholipids in the aroma of cooked beef. Journal of the Science of Food and Agriculture 34(5): 517–522.
Toldrá, F. & Flores, M. (1998). The role of muscle proteases and lipases in flavor development during the processing of dry-cured ham. Critical Reviews in Food Science and Nutrition 38(4): 331–352. doi:10.1080/10408699891274237
Howgate, P. (2006). A review of the kinetics of degradation of inosine monophosphate in some species of fish during chilled storage. International Journal of Food Science & Technology 41(4): 341–353. The freshness K-value derives from Saito, T., Arai, K. & Matsuyoshi, M. (1959), Bulletin of the Japanese Society of Scientific Fisheries 24: 749–750.
McGrane, S.J., Gibbs, M., Hernangomez de Alvaro, C., Dunlop, N., Winnig, M., Klebansky, B. & Waller, D. (2023). Umami taste perception and preferences of the domestic cat (Felis catus), an obligate carnivore. Chemical Senses 48: bjad026. doi:10.1093/chemse/bjad026
Gassenmeier, K. & Schieberle, P. (1994). Formation of the intense flavour compound trans-4,5-epoxy-(E)-2-decenal in thermally treated fats. Journal of the American Oil Chemists’ Society 71(12): 1315–1319. doi:10.1007/BF02541347
Watson, P.E., Thomas, D.G., Bermingham, E.N., et al. (2023). Drivers of palatability for cats and dogs: what it means for pet food development. Animals 13(7): 1134. doi:10.3390/ani13071134
Prester, L. and the Frontiers review of biogenic amines in seafood (2012), Frontiers in Microbiology 3: 188, doi:10.3389/fmicb.2012.00188; on fermentation as controlled decarboxylation, see the review of biogenic amines in fermented foods (2024), Heliyon, doi:10.1016/j.heliyon.2024.e25553.

Thursday, 30 July 2026

Petfood Palatability and the Fresh-Kill Signal Cats Read as Prey

The Friday Conversation · No. 2

An editorial illustration: aroma rising from a bowl resolving into the skeletal structure of the molecule trans-4,5-epoxy-(E)-2-decenal.

The Biological Signal

Why the cat at the bowl is not smelling meat. It is smelling a fresh kill, and the single molecule that tells it so is one the flavour industry has spent twenty-five years trying to bottle.

Dr. rer. nat. habil. Dr. Seronei Chelulei Cheison

Cut a finger and bring it to your nose, and you will recognise at once a smell you have never been taught: sharp, metallic, faintly like wet iron. You did not learn it. It arrived with you. And it has a name, a single molecule, one of some thirty that gas chromatography can pull out of a drop of blood, and the only one your nose truly needs.

For a human being that smell is a curiosity, occasionally an alarm. For an obligate carnivore it is not a smell at all. It is an instruction.

For decades the petfood industry has spoken of “meat flavour” as though a cat crossed the kitchen because it enjoyed the taste of meat, the way a person enjoys the taste of beef. That is a comfortable anthropomorphism and it is not what evolution built. The cat is not a small person with fur. It is a predator whose sensory apparatus was shaped by a single recurring problem, asked over millions of years and millions of kills: is this prey fresh enough to be worth eating?

That question is answered long before anything reaches the tongue. It is answered in the air.

I raise this now, and not as a curiosity, because the category is in the middle of the largest reformulation in its history. The industry is moving (under pressure from cost, from carbon, from supply, and from a genuine shift in what owners will buy) away from meat and toward alternative proteins: fungal biomass, single-cell protein, insect meal, and at the furthest edge, plant-based and outright vegan diets for animals that evolution designed around a carcass. I am not here to tell that movement it is wrong. Much of it is right, and some of it is overdue. I am here to name, precisely and early, the thing standing between it and the bowl.

Because the transition will not be decided by whether these proteins can nourish a cat. It will be decided by whether the cat recognises them as prey.

A word on how I mean to do it. In the first of these Friday conversations I set five questions on the table and closed none of them. This one moves differently (a single question, followed as far down as I can take it), but the discipline is the same. I do not intend to answer this one either. I intend to show you how deep it goes, name the people who could answer it, and leave it, on purpose, where a conversation can still reach it.

The cat is not a small person with fur. It is a predator asking one question: is this fresh enough to be worth eating?

The nose that threw things away

An omnivore’s senses are a compromise. They must find ripe fruit and reject unripe, detect sweetness and starch, weigh a hundred plant and animal foods against one another. The cat abandoned all of that. Somewhere on the way to obligate carnivory it lost the working sweet-taste receptor entirely: the gene is a pseudogene, a broken cassette, and it is why a cat is indifferent to sugar in a way no dog ever is. What the lineage saved on sugar it spent on meat. The equipment that survived is equipment for one job: to find, assess and commit to prey.

So when we sit down to reformulate that prey (to replace the animal tissue in the bowl with mycoprotein, with fermented biomass, with a protein a European hectare threw away), we are not merely swapping a nutrient. We are addressing a sensory system that was built, at the cost of everything it discarded, to detect the one thing we have taken out.

The molecule

When blood meets air, the fats within it begin to oxidise almost immediately. Lipid peroxidation is not exotic chemistry; it is the same family of reactions that turns fat rancid, running here in seconds rather than months. Among its products is an oxygenated aldehyde with an unwieldy name, trans-4,5-epoxy-(E)-2-decenal, mercifully shortened by the people who study it to E2D.

E2D is the character-impact compound of blood. Of the roughly thirty volatiles a machine finds in a blood sample, this is the one a trained human nose picks out as the metallic, blood-like note, a fact established not by the instrument, which nearly missed it, but by human panellists smelling the gas chromatograph’s output one peak at a time. It is extraordinarily potent. Human detection thresholds sit in the parts-per-trillion range; the mouse, for its own reasons, is more sensitive still.

And here is the detail that ought to reorganise how we think about the whole category: no open wound, no E2D. The molecule is not a property of meat. It is an event, with a clock. It appears when tissue is broken and blood meets oxygen (at the moment of the kill), and it is gone again as the chemistry moves on. It does not signal meat. It signals a fresh kill. What comes later, as flesh actually decays, is a wholly different and largely repellent set of volatiles. E2D is the top of the freshness curve, not the bottom of the rotten one.

It does not signal meat. It signals a fresh kill, and it is gone again before the meat has time to rot.

What the animals actually did

Between 2014 and 2017 a group of Scandinavian researchers did the experiment that turns this from chemistry into biology. They took pure E2D (no blood, just the single synthesised molecule) and painted it onto wooden logs, and gave it to captive predators. Siberian tigers, Asian and African wild dogs, South American bush dogs, later a pack of wolves. The animals sniffed, licked, bit, pawed and dragged the logs exactly as they did for a fresh kill. One molecule reproduced the response of the whole complex mixture. Present the same compound to a mouse and it does the opposite: avoidance, vigilance, retreat. The identical cue, read as dinner by the predator and as danger by the prey, across species that last shared an ancestor a very long time ago. The word the field uses is conserved, and it is the right one.

Now the honest edge of this, which the essay will not step around, because stepping around it is precisely the habit I am arguing against.

The tiger is a felid. The domestic cat is a felid. The lineage, the diet, the obligate carnivory are shared. It is a reasonable and probably correct inference that Felis catus reads E2D as the tiger does. But inference is not evidence, and I want to be exact about what I am and am not claiming. To the best of my search, no publicly published study has put a domestic cat in front of purified E2D, measured its investigation and its intake, and reported the number. I make the distinction on purpose. The great palatability houses run trials they never publish, and it is entirely possible the experiment has already been done behind a confidentiality wall and quietly priced into a product. What I can say is that it is not in the public record, not in the behavioural journals, which stopped at tigers, wolves and wild dogs; and not, I checked, in the flavour patents that first put a commercial value on this molecule, which rest their entire case on human tasters smelling the compound in water. The most commercially consequential species in the palatability business is absent from the one experiment that would matter most to it, at least anywhere the rest of us are allowed to read.

I would be glad to be shown the study I could not find. And if it truly is not there (if the number does not exist, or exists only where none of us can see it), then that absence is not a footnote. It is the conversation.

The most commercially consequential species in the whole business is missing from the one experiment that would matter most to it.

A word to the dog

Before anyone assumes this is a cat’s story alone: it is not. The clearest behavioural evidence for E2D comes from the dog family. The wolf, the domestic dog’s own ancestor, worked the scented logs as avidly as the tiger did, and three wild-dog species did the same.

E2D is not a feline signal. It is a carnivore signal, and the dog reads it plainly.

What differs is not whether the dog detects the kill, but how much rides on the detection. The dog is the better nose of the two, with more than twice the cat’s olfactory neurons, and it is an omnivore with the full five-taste palate, sweet included, and a working flexibility the cat traded away. It reaches acceptance by many roads. The cat reaches it by few: no sweet receptor, fewer taste buds, a narrower diet, a deeper neophobia. So the same molecule that is one appetite cue among several for the dog is, for the cat, closer to the whole case. I have told this through the cat not because the dog is unmoved, but because the cat is where a single wrong note is likeliest to end in a turned back and an untouched bowl. Solve the cat, the finickier customer, and you have cracked the hardest bowl in the business.

Why this is not another palatant story

The systems that carry the category today are genuinely good, and I will not pretend otherwise. Animal digests, hydrolysed proteins, the browning chemistry of cooking, these reproduce, with real skill, much of the flavour a cooked and digested meat delivers. They are the reason a modern coated kibble works at all, and I spent years of my own working life inside that toolkit. Nothing here displaces them.

But E2D is a different kind of message, and the difference is the point. A digest imitates cooked meat: the kitchen, the retort, the long warm chemistry of a meal. E2D imitates fresh prey: the wound, the moment, the kill. These are not two intensities of the same signal. They are two different sentences spoken to two different parts of the animal’s evolutionary memory: one about food that has been cooked, one about food fresh from the kill.

There is a practical consequence that formulators will recognise immediately. E2D is volatile and reactive; it would no more survive the heat of an extrusion barrel than a fragrance would survive a hot iron. You do not scent a shirt and then press it; you press it first and lay the scent on after. A signal like this could not be built into the dough. It would have to be delivered afterward (through the fat coat, the liquid palatant, the topcoat applied once the kibble had cooled), so that its message waited in the headspace the instant the bag was opened and the bowl was filled. Its job would not be nutrition. Its job would be the first sentence the animal reads before it has taken a step.

And is it? Is E2D, or some designed cousin of it, already sitting in a product on a shelf, doing exactly this? I do not know, and I will not pretend to. That is the honest edge of an outsider’s knowledge, and it is precisely the sort of thing I would rather ask in the open than guess at in private. If it is already there, then someone has been speaking the cat’s oldest language for years without saying so. If it is not, then the most potent word in that language is going spare.

The industry knew, and the regulator hesitated

None of this escaped the people whose business is flavour. More than two decades ago the flavour and fragrance house Givaudan patented the isomers of epoxydecenal specifically for their potency, in United States patents resting on the observation that the molecule’s handedness matters, that one optical form of the same compound carries more sensory punch than its mirror image. Even chirality, the difference between a left hand and a right, turns out to be something biology is reading. That is the level of detail the category has been prepared to pursue.

And yet the same molecule sits under a regulatory shadow. In the European Union, the flavouring 4,5-epoxydec-2-enal was assessed by EFSA and could not be cleared: it tested positive for genotoxicity in an in-vitro micronucleus assay, a result the follow-up in-vivo work confirmed in the liver, and the panel concluded it could not be evaluated as safe under the standard procedure. In 2017 it was removed from the EU list of permitted flavourings. That verdict was reached for human food, and petfood is a different regime with different exposures and different rules; one does not automatically carry to the other. But it is a caution written in ink, not pencil, and any honest account of E2D has to carry it too.

It is worth being exact about the two numbers involved, because they sit strikingly far apart. The signal a predator reads operates at the edge of physics: a cat's fresh-kill cue is a matter of parts per trillion, picograms of the molecule in the air above the wound. The genotoxic flag, by contrast, was raised by feeding rats near-maximum-tolerated doses, in the region of three hundred milligrams per kilogram straight into the stomach, enough to leave necrosis in the liver. Between the concentration that carries the message and the concentration that does the damage lies a gap of many orders of magnitude, and the same chemical reactivity that lights up a genotoxicity assay in a dish is precisely what makes E2D a fleeting top-note in the world rather than a compound that lingers or accumulates. None of which clears it; the flag is real and I will not explain it away. But it does sharpen the question. The hazard was shown in a rodent liver, for a human additive, at a dose no palatant use would approach, in a species that is not the one eating the bowl.

Which leaves the honest account not with a verdict but with a gap in the evidence, and it is worth naming plainly. Is there a single published study of what E2D does in a cat, at the vanishing concentrations a cat would actually meet, rather than in a rat gavaged to its limit? If there is, I have not found it. And if there is not, then we are weighing a signal biology has used for millions of years against a hazard measured in the wrong species, by the wrong route, at ten orders of magnitude the wrong dose, and calling the balance settled. Is it?

Applied chemical ecology

Here is where the questions get harder, and I would rather leave them open than pretend I have closed them.

Modern palatant development, at its sharpest, is no longer flavour creation in the culinary sense, the art of making something taste nice. It looks more like applied chemical ecology: the deliberate use of signals evolution spent millions of years teaching an animal to obey. And if that is what the discipline has quietly become, then is the whole alternative-protein transition asking the wrong question of itself? We keep asking whether a fermented biomass can nourish a cat. Should we not be asking, first and far more urgently, whether it can convince one?

We keep asking whether a fermented biomass can nourish a cat. We should be asking whether it can convince it to eat.

Because a diet the animal refuses has an efficacy of zero, and the refusal is decided in the headspace above the bowl, in the first second, before a single requirement on the label has been tested.

Which brings the sharpest question to the table, the one that decides whether the vegan bowl is a biological dead end or merely an unsolved problem. Is the signal the cat is chasing an irreducible property of meat, something that exists only as long as the muscle does? Or is it a molecule, a single defined compound that forms wherever the right fats oxidise, and that a chemist could, in principle, build from feedstocks that never saw an animal? The science leans hard toward the second answer; the whole reason a company could patent E2D’s isomers is that it is a thing you can make. But if that is right, then where does it leave the argument the industry keeps having? Can the kill be written by a factory rather than a carcass? Can the instruction the cat obeys be sprayed onto a fungal kibble that a tiger’s ancestors would not recognise as food, and would the cat obey it anyway?

And there is a further turn to this that is worth holding onto, because it changes what the molecule is for. It would be easy to treat E2D as a beautiful but unreachable thing, a signal to admire and not use, especially with a regulator’s flag against the isolated compound. That would be the wrong lesson. E2D is not only the smell of a raw wound; it is also generated in situ, by heat, from polyunsaturated fatty acid precursors through ordinary oxidation chemistry. The fresh kill writes this word in seconds; heat can write it again from the fats. Which means the question was never “where do we buy the molecule,” but “what chemistry, and which precursors, let a food generate it for itself.” That is a thread for the conversations still to come, on how these signals form, from which precursors and on what timetable, and on the fats themselves, the most underrated players at the bowl.

I do not think those are rhetorical questions. I think they are unanswered, and I think the answers are worth more than another percentage point of protein.

And here is the one I most want to put to the people reading this, because I cannot answer it alone. We have been assuming the difficulty with insect and plant and fermented proteins is that they lack the signal, that there is no E2D in a mealworm, no fresh-kill note in a fungal biomass. But I do not actually know that, and neither, as far as I can find, does the public literature. Insects are lipid-rich and oxidise readily; the raw chemistry that makes E2D is present in them, even if the blood-metallic note has never been reported and their loudest volatiles point the nose somewhere else entirely, toward fish, earth and cheese. So the real question is open, and it is yours as much as mine. Is the signal simply absent from these materials? Or is there a different signal, one native to insects or microbes or plants, that a carnivore could be taught to read as readily as it reads the kill, that we have simply not gone looking for? If you know of such a molecule, or if you know that E2D is in fact hiding in one of these ingredients, I would genuinely like to be told.

Because strip the argument to its frame and this is what we have done. Evolution taught the cat to smell a fresh kill. We have answered by offering it a cooked meal, nutritionally balanced to the last milligram, and set it before an animal that has never cooked, never stored, never in its evolutionary life encountered food that was prepared rather than caught. We solved the gut and may have forgotten the nose. Have we built the perfect dinner and failed to announce it as dinner at all?

So let me put the essay’s own question back on its feet. Is the compound the point, or is the principle? If what an obligate carnivore reads can be decoded, defined and reproduced, then what exactly is standing between the plant-based movement, the alternative-protein developers and the palatant houses and the bowl they all want the animal to empty? Is it really whether the diet is complete, or whether it is credible to a nose calibrated by evolution to detect a fresh kill?

We have spent a generation asking whether these new proteins can nourish a cat.

The cat has been asking a different question the whole time. It has been asking whether something has just died. Can we learn to answer yes, and mean it, without a single animal having to?

Somebody should put a domestic cat in front of the molecule and publish what the bowl says. Until someone does, we are inferring the most important customer in the category from the behaviour of a tiger, and preparing to build the future of the bowl on that inference. Is that good enough?

I am asking.

And while we wait for the answer, here is the shape I think the question is taking.

We have spent twenty years learning how to make proteins without animals.

Perhaps we will spend the next twenty learning how to make them read, to the animal, like one.

Because the future does not belong to alternative proteins or to inclusive diets, as if the two were rivals. It belongs to an inclusive diet built around alternative proteins, and made credible to the creature that has to eat it. E2D is only one word in that language, and perhaps not even the word we will end up using. The point was never the molecule. The point is that the signal can be learned, and that the learning has an examiner.

And the bowl will answer before the laboratory does.

Dr. rer. nat. habil. Dr. Seronei Chelulei Cheison
Langwedel, Niedersachsen

The thread this essay opens, how these signals form, from which precursors and on what timetable, is taken up in the third Friday Conversation: Meat Is More Than Protein.

Sources drawn on for this essay

Nilsson, S., Sjöberg, J., Amundin, M., Hartmann, C., Buettner, A. & Laska, M. (2014). Behavioral responses to mammalian blood odor and a blood odor component in four species of large carnivores. PLOS ONE 9(11): e112694. doi:10.1371/journal.pone.0112694

Arshamian, A., Laska, M., Gordon, A.R., et al. (2017). A mammalian blood odor component serves as an approach–avoidance cue across phylum border, from flies to humans. Scientific Reports 7: 13635. doi:10.1038/s41598-017-13361-9

Buettner, A. & Schieberle, P. (2001). Aroma properties of a homologous series of 2,3-epoxyalkanals and trans-4,5-epoxyalk-2-enals. Journal of Agricultural and Food Chemistry 49(8): 3881–3884. Establishes trans-4,5-epoxy-(E)-2-decenal as the metallic, blood-like character-impact odorant. doi:10.1021/jf0104329

Pettersson, H., Amundin, M. & Laska, M. (2018). Attractant or repellent? Behavioral responses to mammalian blood odor and to a blood odor component in a mesopredator, the meerkat. Frontiers in Behavioral Neuroscience 12: 152. doi:10.3389/fnbeh.2018.00152

Li, X., Li, W., Wang, H., et al. (2005). Pseudogenization of a sweet-receptor gene accounts for cats’ indifference toward sugar. PLOS Genetics 1(1): e3. doi:10.1371/journal.pgen.0010003

Givaudan SA (inventors A. Daniher, S. Furrer, A. Goeke), US Patent 6,335,047 and continuation-in-part US Patent 6,451,366 (both granted 2002), Epoxydecenal isomers: isomeric forms of epoxydecenal with enhanced flavour potency, the enriched (−) optical isomer carrying greater sensory potency than its mirror image.

EFSA CEF Panel (2017). Scientific Opinion on Flavouring Group Evaluation 226, Revision 1 (FGE.226Rev1): genotoxicity data on 4,5-epoxydec-2(trans)-enal [FL-no. 16.071]. EFSA Journal 15(5): 4847. The substance raised a genotoxicity concern and could not be evaluated under the Procedure (and was subsequently removed from the EU flavourings list). doi:10.2903/j.efsa.2017.4847

Gassenmeier, K. & Schieberle, P. (1994). Formation of the intense flavour compound trans-4,5-epoxy-(E)-2-decenal in thermally treated fats. Journal of the American Oil Chemists’ Society 71(12): 1315–1319. Establishes that E2D forms on heating from the hydroperoxides of linoleic acid. doi:10.1007/BF02541347