The hidden reason processed pet foods are so addictive – BBC.com

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The cue might be a hand in a pocket, the opening of a cupboard door, or even a word said carelessly aloud – “dinner”. Before you know it, you’re tripping over a pet excitedly awaiting a portion of… dull-brown dried pellets. What’s in these mysterious morsels, that makes them as delectable as roasted chicken, wild salmon, or bundles of fresh herbs? 
Take my flatmate, a small black rabbit. For a large part of every day, he can be found sitting attentively with his paws on his empty food bowl, awaiting his next portion of kibble – even though it looks like his droppings and smells equally unappetising. He used to have an automatic dispenser with a timer, but he learnt to throw it across the room to access its contents prematurely. No matter what delicacies I place before him – home-grown parsley, soft-cut hay, fresh carrot-tops, organic kale – he would always rather eat processed pet food.
It seems that this is not unusual. Anecdotes abound about pets whose thoughts are largely preoccupied with kibble, such as the cat that has a daily panic attack when it realises it has eaten all its pellets and the pragmatic German shepherd found carrying a bag of dog food around the streets of Houston after Hurricane Harvey.  
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As it happens, this addictive quality is carefully engineered. Big Pet Food is a multi-billion-dollar industry which invests heavily in research into “palatants” – ingredients that make our pets want to eat their products. And from potently smelly chemicals usually found in rotting meat to an additive commonly added to potatoes to stop them discolouring, the quest to make the most scrumptious pet food has led to some surprising insights.
“Big [pet food] companies have huge departments that make palatants,” says Darren Logan, head of research at the Waltham Petcare Science Institute, part of the company Mars Petcare. “Just like we make them for humans, we make them for pets as well.” 
Upper-class dogs
The first pet food was invented in 1860 by James Spratt, an enterprising lightning-rod salesman from the US state of Ohio. Legend has it that he had travelled to England for his business, and was looking out over the docks of Liverpool one day when he noticed stray dogs knocking back leftover hardtack biscuits.
This was a revelation for two reasons.
Rabbits eat fresh vegetables in the wild, but many kept as pets feast instead on processed pellets (Credit: Getty Images)
Firstly, hardtack were famously unappealing – loathed by generations of the soldiers and sailors who ate them, these simple slabs of baked flour and water were tougher than wood and sometimes hard enough to break your teeth. Their nicknames included “sheet iron” and “worm castles“, the latter because of the high proportion that were infested with maggots and weevils. The oldest piece of surviving hardtack was baked just nine years before Spratt’s dock visit, and still looks suspiciously well-preserved 170 years later.
Secondly, until that moment it hadn’t occurred to anyone to check what their pets would like to eat – or that this could be monetised. For as long as we had kept domesticated animals, they had been fed more or less the same food as humans, or expected to fend for themselves.
One striking example is the husky. In their native territory of Arctic Greenland, Canada and Alaska, Inuit hunter-gatherers have traditionally fed these dogs on seal meat, which comprises the majority of their own diet. Sled-dogs are so well-adapted to this that when the British Antarctic Survey brought them to Antarctica as a form of transport in 1945, they found that they struggled to digest commercial dog food. In the end, they had to kill a number of local seals each year, just to feed the dogs, before they were largely replaced with skidoos in the 1960s and 70s.
Meanwhile in Victorian London, dogs that were lucky enough to be looked after were either given table scraps or gruel. Even specialist exotic animals were fed everyday human food – the 20,000 or tortoises imported from Morocco each year were mostly expected to survive on ordinary garden vegetables or bread soaked in water. Cats were considered street animals and rarely fed.
But Spratt had hit upon something entirely new. Over the coming months he developed the “Meat Fibrine Dog Cake”, a biscuit-like concoction of beetroot, vegetables, grains and beef of dubious origins that claimed to meet all the nutritional needs of his customers’ hounds. (While its packaging implied that it was the finest prairie beef, what it was actually made from was a secret he took to his grave.)
Read recipes and more in BBC Good Food’s Beyond Natural section
 
Spratt’s innovation coincided with a cultural revolution in the way people saw their pets – dogs and cats went from being viewed as mere utility animals or borderline-vermin to beloved family members to be coddled. Consequently, the Meat Fibrine Dog Cake was marketed as a luxury food for aristocratic pets.
The husky dogs taken on early Antarctic voyages had to be fed with freshly killed seals (Credit: Ozge Elif Kizil/Anadolu Agency/Getty Images)
The adverts labelled them “Dog’s Delight” and included gushing testimonials from wealthy customers. Ironically, Spratt’s also promoted the fact that they were chosen to feed the sled dogs on Captain Scott’s 1901 trip to the Antarctic, though we now know they would much rather have eaten seal.
Eventually the company branched out into cat food – “Spratt’s puts pussy into fine form!”, they said – and the rest is history. However, the science of pet food palatants still had some way to go.
Disgusting smells
Today it’s possible to buy specialised kibble for almost any kind of pet, from frogs to sugar gliders (a small marsupial). Most follow roughly the same formula – they usually contain some kind of base carbohydrate, assorted proteins and fats, sugars, a source of fibre, antioxidants or other preservatives, emulsifiers (which keep the fat in the food and prevent it from separating), vitamins and minerals, and colouring agents.
More sophisticated versions may also contain probiotics or digestibility enhancers – such as chicory, which is often added to dog food – as well as enzymes, anti-parasitic compounds and minerals to prevent the build-up of tartar on teeth.
To turn these ingredients into a dry pet food, it’s formed into a paste and “extruded” via a process that involves heating it up and forcing it through a plate with holes in it, to form an aerated product that matches the shape of the holes. It’s the same process that’s used to make puffed snack foods, with flavourings added in the final step – in the case of pet food, they’re either sprayed on or added as a powder.
Oddly, there is very little relationship between how healthy a pet food is and its inherent deliciousness. That’s because in the US, the EU and many other parts of the world, in order to describe one as “complete” – containing everything the body needs to be healthy – it must meet certain nutritional standards. These set out acceptable ranges for most ingredients, so manufacturers can’t just load up on sugar and fat to make it compelling.
“From my standpoint as a nutritionist, all pet foods are the same,” says Marion Nestle, professor emerita of nutrition, food studies, and public health at New York University.
The first processed dog foods were luxury products which ushered in the idea of the "pampered pet" (Credit: Kirn Vintage Stock/Corbis via Getty Images)
Instead, companies turn to chemistry.
Many animals rely heavily on smell to navigate the world around them, and this is often the main sense that’s targeted. While human noses contain around 50 million olfactory receptors, cats have 67 million, rabbits have 100 million and dogs have around 220 million. On the other hand, their sense of taste is generally less discriminating than ours – our relatively high density of taste receptors is thought to have evolved to help us cope with our diverse omnivorous diets. 
The catch is that appealing to animals that find the smell of roadkill, sweaty socks, and vomit utterly enchanting – as carnivorous pets often do – while not making their human companions feel violently ill, is extremely tricky. “There is a slight paradox there, because the smells that cats particularly but also dogs seem to like are often the opposite of what humans like,” says Logan.
Nestle puts it more bluntly – “animals eat faeces”, she says. “They like strong animal odours and pet food manufacturers have a really difficult time, because they have to make it disgusting enough so that the animal will eat it, but not so disgusting that the owners won’t buy it.”
Examples include putrescine and cadaverine, colourless chemicals produced naturally by the breakdown of proteins. They’re largely responsible for the revolting smell of rotting flesh – and cats love them. While in human food, their levels are sometimes closely monitored as a way of ensuring the freshness and safety of meat, they’re often actively added to cat and dog food, either as offal extracts or lab-made additives.
In the case of naturally vegan animals, such as rabbits and guinea pigs, irresistible smells such as mint and oregano are sometimes added in the form of concentrates.
Japanese-inspired cuisine
Other insights are arguably more surprising. A recent study identified nine volatile compounds in common pet food flavourings that are linked to how delicious they are to dogs, including heptanal, nonanal, and octanal, which all have strong, fruity odours.
However, taste is also important – and here the preferences of carnivorous pets are not so different from ours.
One of the most popular additives in human food is the enigmatic “hydrolysed protein”, which is formed by breaking down the long strands of proteins into their constituent amino acids, usually using enzymes or hydrochloric acid. It imparts a flavour similar to that achieved by meat or vegetable stock, and often comes with MSG, which is produced as a by-product of the same reaction and is responsible for the savoury taste of tomatoes, cheese and Iberico ham.
Though hydrolysed proteins are produced artificially, the process is similar to what happens when you cook food for a long period of time – it’s a kind of pre-digestion, and is thought to contribute to the enticing smell of many brands of kibble.  
Early processed dog food resembled the tooth-challenging hard-tack biscuits often taken on long sea voyages (Credit: Getty Images)
“The understanding of cat palatability is very similar to Japanese or Asian cuisine, where there’s a lot of focus on umami and another taste modality called kokumi,” says Logan.
Kokumi was discovered in Japan in 1989, and has been proposed as the sixth taste in humans, after sweetness, saltiness, bitterness, sourness and umami. It’s described as a kind of mouth-feel rather than a flavour per se – a texture that imparts richness and “thickness” to foods. Unlike the others, kokumi hasn’t yet been linked to a specific set of compounds, but foods that conjure this sensory experience include scallops, soy sauce, shrimp paste, yeast and beer.
The sixth taste is thought to be particularly popular with carnivorous animals, which may discern it via receptors in their mouths that evolved to detect calcium. And as you would expect, pet food companies have already begun targeting it with cocktails of flavour-enhancing chemicals.   
But there are some flavours that you will never find in certain pet foods.  
For example, most wild carnivorous animals lack the receptors for tasting sugar or carbohydrates. And unlike dogs, which have been living around humans and feasted off our scraps for up to 40,000 years, domestic cats have only been around for about 4,300. For the majority of that time, they were considered a kind of free pest-control that could fend for themselves.
So, while cats are particularly drawn to Japanese food, which is rich in meat and seafood, you’re unlikely to find them stealing ice-creams or doughnuts – unlike dogs, they simply haven’t been around humans for long enough to have evolved the ability to taste sugar.
Cats are irresistibly drawn to the chemical compounds found in rotting fish (Credit: Getty Images)
On the other hand, because vegan animals eat exclusively vegetable matter, which is often rich in fibre and carbohydrates, they tend to prefer sweeter pet food.
Finally, no list of palatants would be complete without pyrophosphate, described in Popular Science as “cat crack”. This common additive performs a number of roles in human food, such as preventing potato products from going dark after they’re cooked – none of which involve improving its taste. Nevertheless, cats go nuts for it, possibly because it intensifies the flavour of amino acids.
Pet food companies are now so successful at making food delicious that they’re increasingly encountering a dilemma – it’s almost too good. “The danger for cats and dogs today is the same as for people, it’s overconsumption,” says Andrew Knight, a professor of animal welfare and ethics at the University of Winchester.
Pet obesity is a growing problem in the developed world, with one survey of veterinary professionals at a vet show in London suggesting that around 51% of dogs, 44% of cats and 29% of small mammals are now overweight or obese.
According to Logan, this is not down to the way pet food is formulated, but humans succumbing to their beloved pets’ pleading gazes. “The reason we make pet food palatable is that if they don’t eat all the food that we give them, it won’t meet the nutritional needs that they require,” he says. “The real problem is owners feeding them too much – pets can’t open the packets themselves.”
However, there is an upside. There are mounting concerns about the environmental impact of pet food, too – in 2009, two New Zealand scientists estimated the planetary cost of keeping a dog as roughly twice that of having a medium-sized SUV.
Pets given too much calorific processed food may, just like humans, put on extra weight (Credit: Getty Images)
This is where palatants come in. Because most pet foods comprise a fairly tasteless base that is spruced up with delicious flavourings and smells, pet foods made from more sustainable ingredients such as insects or soya are generally just as acceptable to carnivorous pets as the real deal. (Though cats cannot be fed a diet that is meat free.)
“According to this really large-scale study that we’ve just finished, the animals on vegan pet foods seem to be just as happy as animals on meat ones,” says Knight, who is hopeful about their future potential.
“There is a broad recognition that the need to be more sustainable will have a big impact on the pet food business,” says Logan, who explains that the pet food company he works for has just released its own brand of insect-based pet food.
So, why do our pets find pet food so addictive? Well, because it’s been made that way. Just like us, our pets find it hard to say no to the food we have designed to be tasty.
* Zaria Gorvett is a senior journalist for BBC Future and tweets at @ZariaGorvett
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