Palatability is defined as the subjective pleasure that an individual experiences in association with eating a particular food. ² Therefore palatability should not be considered to be an intrinsic property of the food, but rather a property of an animal’s perception of the food and the tendency to select one particular food over another. Historically, palatability in dogs and cats has been measured using food acceptability and preference tests. A food acceptance test (also called a “one-pan test”) examines initial response to a new food and may also record the time that it takes an animal to taste and consume the new food. Because no choice is involved in this initial screening, this test only provides information about whether or not the dog or cat finds the new food to be appealing and is willing to eat it. Factors that can affect initial acceptability include the animal’s degree of hunger and an individual’s response to novelty. Food preference tests (also called “two-pan tests”) are based upon the assumption that a greater intake of one food over another is an indication of higher palatability. ³ Using this approach, animals are offered the simultaneous choice of two foods for a predetermined number of days and intake is recorded. One food will be ranked higher in palatability than a second food if a greater volume of the first food is eaten. However, limitations of this approach include the confounding effects of satiety when foods of differing caloric density are fed and the inability to discern short- or long-term effects of novelty (see below).

In recent years, the assessment of food preferences and palatability in dogs and cats has become significantly more sophisticated as methods have been developed to measure animals’ responses to the smell, taste, and texture of foods. ⁴ Because of the highly developed olfactory acuity of dogs and cats, it is not a surprise that the odor of a food significantly influences food preferences in both species. ^{5. and 6.} When presented with more than one food choice, cats first smell the foods and will preferentially consume the food with the most attractive odor, usually without tasting the less attractive food(s). ⁷ Olfaction is intrinsically linked to taste, which appears to be second in importance to dogs and cats during food selection. ⁸ For example, when cats cannot discriminate among foods using olfaction, they will then use taste to make a choice. ⁷ The sense of touch is involved in food selection when dogs and cats react to a food’s shape and texture. Together with the size of kibble pieces, these sensations make up the “mouth feel” of a food. For example, increasing the size of kibble pieces generally decreases rate of eating in dogs, because they spend more time chewing and masticating the food pieces than they do when fed foods with small kibble sizes. Cats are especially sensitive to the size and shape of kibble pieces because of their tendency to eat slowly and because they chew foods less thoroughly when compared with dogs. For example, cats tend to reject kibble pieces with sharp edges, presumably because these pieces have an uncomfortable “mouth feel.”⁹ Other animal-related factors that may influence a pet’s acceptance of a particular food include past experiences, age, and breed. The feeding environment and an owner’s behavior can also affect a pet’s response to a new food. Most reputable pet food manufacturers recognize this and conduct controlled palatability tests both with kenneled animals and within a wide variety of home environments.

Two conflicting effects that can significantly influence an individual dog or cat’s food selection are the primacy effect and novelty effect. ¹⁰ The primacy effect occurs when an animal is fed a specific type or flavor of food for a long period of time (most commonly beginning at weaning) and the individual shows an enhanced preference for that food to the exclusion of novel foods. For example, an early series of studies found that limiting the flavor experiences of puppies and kittens at an early age led to fixed food preferences in which all novel foods are rejected later in life. ¹¹ When the same experimenters fed foods of varying flavors and textures early in life, the dogs and cats showed increased acceptance of novel foods as adults. This form of neophobia is most commonly reported as a problem when a cat has been fed one type or flavor of food for many years and a change in nutrient needs or health status suddenly requires a change in food. Although neophobia can also occur in dogs, it is less common, possibly because the dog’s scavenging nature and more omnivorous diet make dogs more resistant to the development of fixed food preferences. Although it has been speculated that there are breed-specific differences in dogs in the tendency to accept or reject novel foods, there is currently little evidence to support or refute this. ⁶

Paradoxically, the novelty effect can also occur with animals that are fed a single food item for a long period. In these cases, instead of rejecting novel foods, the individual shows an enhanced (though sometimes transient) preference for a new food or treat when it is offered. ^{12. and 13.} In another early study, puppies that had been fed a single type of canned food starting at weaning and continuing until they were 16 weeks of age showed a strong preference for novel foods when they were offered. ¹⁴ Dog trainers regularly capitalize on this tendency by using treats that differ from a dog’s daily food or by varying the type of treats that are used as food reinforcers during consecutive training sessions. It is possible that the attraction to a new food (novelty) in some animals is actually a reflection of a mild aversion to or boredom with a food that has been fed for a long period of time. This has been called the “monotony effect” and has been used to explain attraction to novel foods in cats more often than dogs. ^{15. and 16.} It is theorized that cats may have an evolutionary bias toward selecting more than a single food choice or prey species to avoid nutrient deficiencies from developing.

Although the primacy and the novelty effects have both been observed in dogs and cats, the incompatibility of these two responses and factors that determine which behavior an individual animal will show are not completely understood. Recently, a series of studies with cats led investigators to speculate that these opposing responses may be explained by cats possessing certain innate flavor preferences. When a varied diet is fed to kittens, being exposed to multiple food flavors would tend to “mask” inborn flavor or food preferences; conversely, feeding a limited diet early in life would be more likely to allow expression of inborn flavor preferences. ¹⁷ Cats in this study appeared to possess an innate preference for the flavor of tuna when compared with beef. This preference was only expressed when kittens that were raised on a beef-flavored food were presented with tuna as the novel flavor. Conversely, other investigators argue against the existence of a select set of innate flavor preferences in cats and suggest that observed differences in preference are a result of early learning from the mother and possibly litter-specific predispositions. ¹⁵ More studies are needed to explore innate and learned flavor preferences and to further elucidate the importance of primacy and novelty in food selection in both dogs and cats.

There are several properties of food components that can directly affect palatability and food selection in dogs and cats. ¹⁸ The quality of ingredients, and the way that they are cooked, processed, and stored, significantly affects acceptability and palatability. For example, the extrusion of grain starches imparts a desirable texture and flavor to dry pet food kibbles. However, if mold growth has occurred or if the product has not been properly extruded, grains will be perceived as highly unacceptable. Poorly extruded starches cause food particles to have high bulk densities, which negatively affect the texture and chewiness of the product. Poorly processed or poorly stored foods may also contain high levels of oxidized oils and fats. High concentrations of aldehydes produced by fat oxidation are unpalatable to animals. Food palatability for both dogs and cats is positively correlated with protein level. Animal-source proteins are generally considered to be more palatable than plant-source proteins, although the extrusion process significantly increases the palatability of plant-based protein sources such as soybean. ^{19. and 20.} However, just as with starch, the overprocessing of protein or the inclusion of poor-quality protein sources in a food can lead to compounds that are perceived as unpalatable.

A pet food that is low in digestibility contains a high proportion of ingredients that cannot be digested by the enzymes of the gastrointestinal tract. These components pass through to the large intestine, where they are partially or largely fermented by colonic bacteria. Rapid or excessive bacterial fermentation leads to the production of gas (flatulence), loose stools, and occasionally diarrhea. In addition to these side effects, a greater quantity of a poorly digested food must be fed to the animal because the pet is absorbing a smaller proportion of nutrients from the food. As the quantity of food that is consumed increases, rate of passage through the gastrointestinal tract also increases. A more rapid passage of food through the intestines further contributes to poor digestibility, high stool volume, and gas production. A food’s digestibility is decreased by the presence of high levels of nonfermentable fiber, ash, phytate, and poor-quality protein. Improper processing or excessive heat treatment can also adversely affect digestibility. In contrast, pet food digestibility is increased by the inclusion of high-quality ingredients and increased levels of fat, as well as the use of proper processing techniques.

In general, dogs and cats digest foods of animal origin better than those of plant origin. This difference is primarily the result of the presence of lignin, cellulose, and other nonfermentable fibers in plant ingredients. However, it is important for owners and professionals to recognize that low-quality animal products containing high amounts of skin, hair, feathers, and connective tissue are also not well digested by dogs and cats. Although a pet food that contains high-quality animal products has a higher digestibility than a plant-based food, pet foods that contain poor-quality animal ingredients may have lower digestibilities than plant-based products with similar nutrient profiles. ²²

There is some evidence that apparent digestibilities of the major organic nutrients in commercial pet foods are higher for dogs than for cats. 23. ^{and 24.} Both the dog and the cat belong to the order Carnivora and are classified as simple-stomached carnivores. The cat is a strict carnivore, but the dog is more omnivorous in nature. One way in which this difference is reflected is in the abilities of the two species to digest certain types of dietary components. A study comparing the digestive capabilities of dogs and cats found that when fed the same dog or cat food, dogs had higher apparent digestibility coefficients and obtained more digestible nutrients per unit of food eaten than cats for almost all nutrients and types of foods. ²⁴ It was suggested that some of these differences could be explained by a greater ability of the dog to digest (ferment) certain types of fiber in the large intestine. Data from studies of the fermentative capacities of canine and feline colonic microflora support this. While both dogs and cats are capable of fermenting certain fiber types, the cat is less tolerant of wide ranges of fiber fermentability than the dog. ^{25.26. and 27.}