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foods
Review
Dry Pet Food Flavor Enhancers and Their Impact on
Palatability: A Review
Shilpa S. Samant, Philip Glen Crandall, Sara E. Jarma Arroyo and Han-Seok Seo *
Citation: Samant, S.S.; Crandall, P.G.;
Jarma Arroyo, S.E.; Seo, H.-S. Dry Pet
Food Flavor Enhancers and Their
Impact on Palatability: A Review.
Foods 2021,10, 2599. https://doi.org/
10.3390/foods10112599
Academic Editors: Youngseung Lee
and Yoon Hyuk Chang
Received: 15 September 2021
Accepted: 22 October 2021
Published: 27 October 2021
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Attribution (CC BY) license (https://
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4.0/).
Department of Food Science, University of Arkansas, 2650 North Young Avenue, Fayetteville, AR 72704, USA;
sssamant@uark.edu (S.S.S.); crandal@uark.edu (P.G.C.); sejarmaa@uark.edu (S.E.J.A.)
*Correspondence: hanseok@uark.edu; Tel.: +1-479-575-4778
Abstract:
Pet foods are a vital component of the global food industry. Pet food’s success depends on
its acceptance by both consumers (the pets) and purchasers (the pet owners). Palatability tests using
panels of both trained and untrained pets are often used to measure the preference and acceptability
of pet foods. Human perception of pet foods is usually determined by descriptive sensory analysis.
Since dry pet foods (also known as kibbles), while being the most popular, are the least palatable,
palatants as a flavor enhancer are generally added to dry pet foods to increase their acceptability
to pets. Pet foods can also be prepared to be more appealing to pet owners if the chosen aromas
and flavors are commonly associated with human food. With increasing demand, developing flavor
enhancers to meet the needs of both pets and owners is becoming increasingly important. This review
summarized the current state of flavor enhancers used in the pet food industry and their influence
on food palatability from both animal and human standpoints.
Keywords: pet; pet food; dry dog food; palatants; palatability; flavor enhancer
1. Introduction
The pet food industry is an expanding part of the food industry that is experiencing
significant growth and has great potential for continual growth. According to the American
Pet Products Association (APPA), more than $100 billion were spent on pet-related sales
in the U.S. in 2020, of which $29.6 billion were spent on food alone, and the rest included
supplies/over-the-counter medicines, vet care, live animal purchases, and other services
(e.g., grooming and boarding) [
1
]. Pet food sales are forecast to be level at $33.5 billion by
2025 in the U.S. market [
2
] as pet ownership continues to grow. For example, in 1988, 56%
of households in the U.S. owned a pet compared to 70% of U.S. households in 2021 [
1
].
Among many different types of pets, this review specifically focused on dogs because dogs
are the most popular pets or “companion animals” in the U.S., with 69.0 million households
owning dogs [1].
Pet foods are generally available in three forms: moist, semi-moist, and dry, depending
on their final moisture content [
3
]. The dry pet food category remains the number one
choice for pet owners with $5338.2 million multi-outlet sales in the U.S. in 2020, followed
by the outgrowing semi-moist and moist subsegments with $2027.9 million [2]. Moist pet
foods typically contain proteinaceous materials (e.g., meat, meat by-products, or fish) and
have a final moisture content of 65% or more [
3
]. Moist pet foods have a limited refrigerated
shelf life after opening. Secondly, semi-moist pet foods are prepared using a combination
of proteinaceous and farinaceous ingredients (e.g., wheat, oats, or other cereal grains) with
a final moisture content between 20 and 65%. Finally, dry pet foods with less than 20%
moisture content are generally prepared using primarily farinaceous ingredients along
with a small proportion of proteinaceous materials. Dry pet foods with a moisture content
of 8–9% usually have a dry and crunchy texture, while other formulations with moisture
content between 10–15% have a softer texture [
4
]. Dry pet foods include baked, pelleted,
and extruded foods, with extruded foods the most common. Raw materials used to make
Foods 2021,10, 2599. https://doi.org/10.3390/foods10112599 https://www.mdpi.com/journal/foods
Foods 2021,10, 2599 2 of 19
pet foods generally include grains, meat, poultry, eggs, and vegetable by-products along
with added fats, vitamins, and minerals [
4
]. Dry pet foods have a long shelf-life because of
their low water activity (<0.60 aw) and thus microbial stability. However, dry pet foods
generally are less attractive to pets than moist or semi-moist pet foods, probably because of
their lower flavor appeal [
5
,
6
], while some pets may prefer dry pet foods because of their
textural characteristics.
Since dry pet foods are most popularly used and constitute a major portion of the
pet food market among the three types of commercially available pet foods [
7
], enhancing
their palatability for greater acceptance among pets is extremely important and has been
rigorously investigated. Previous research suggests that odors might be the primary
drivers in a dog’s food choice [
8
–
12
]. For example, in a study where dogs were trained to
discriminate between foods with different meat sources, Houpt et al. [
11
] found that dogs
did not maintain discriminatory ability once they were subjected to reversible peripheral
anosmia that resulted in temporary loss of their sense of smell. Olfactory cues from pet
foods, constituting a major aspect of overall flavor characteristics, are important drivers of
liking for pets [
8
], and such cues also can play an important role in influencing acceptance
of the product from a pet owner’s perspective. For example, Di Donfrancesco et al. [
13
]
found that pet owners do not appreciate pet foods with aromas that are perceived as being
too strong or intense off-odor notes such as aldehydes like canaveron, oxidized oil, or
must/dust, even though these odors may be attractive to the pets. In contrast, pet owners
were found to like pet foods with grainy-type aromas but did not prefer other aroma
attributes [
13
]. Pet owners’ emotions are also influenced by the sensory characteristics
of pet foods. In a study by Delime et al. [
14
], American and French pet owners could
discriminate the dog and cat dry kibbles by emotions evoked by their odors. For example,
the odors “spicy”, “herbs-like”, “yeast-bouillon-like”, and “roasted chicken-like” were
more associated with activation-related emotions, while the odors “fatty-rancid”, “viscera
like”, and “cereal-like” were more associated with de-activation-related emotions [
14
].
Since food-evoked emotions have been found to play an important role in consumer
acceptability [
15
,
16
] and purchasing-related behavior [
17
], the pet food industry should also
consider pet owners’ emotional responses to pet foods. Additionally, the pet food industry
is continually exploring ways to decrease objectional odors capable of being detected by
pet owners while increasing desirable flavor characteristics from a pet’s perspective.
Adding certain chemical compounds to pet foods to enhance flavor characteristics
is one way of increasing palatability. Since some chemical compounds can also work as
masking agents, they can be added with an intention to overcome or mask off-flavors or
odors, increasing human-perceived palatability. However, while many “flavor enhancers”
(also known as “palatants”) have been proposed over the years, they are not always
backed by thorough research to understand their acceptability among pets and pet owners.
Therefore, this review focuses on two points: 1) palatability of dry pet foods from the
consumer (pet) and purchaser (pet owner) standpoints and 2) flavor enhancers used as
palatants in the pet food industry, especially in the category of dry dog foods or kibble, and
their impact on palatability.
2. Palatability of Dry Pet Food
2.1. Pets, Pet Owners, and Palatability
Araujo and Milgram [
18
] characterized the “palatability” of pet foods as “a measure of
subjective food preference and depends on taste, texture, and odor”. In addition, the palatability
of pet foods has also been defined as “pleasantness of taste of feed to animals that is understood
through the sensory characteristics of food, such as taste, flavor and mouthfeel” [
19
]. When
developing new products of pet foods, manufacturers must achieve a balance between
nutrition quality and sensory appeal because pet foods with the high nutritional quality
might not be consumed if they have low sensory appeal to pets, resulting in a low repeat
purchase intention by pet owners. Conversely, if pet foods are “appealing” to pet owners,
they tend to generally be purchased for their pets [
13
,
20
]. Di Donfrancesco et al. [
13
]
Foods 2021,10, 2599 3 of 19
asked pet owners to evaluate eight varieties of dry dog food and rate both how much
they liked each of them and how much they believed their pets would like them, and the
relationship between overall liking of the pet owners with their prediction of dog liking
was found to be very high (Pearson’s correlation coefficient = 0.93). In other words, the
more a product was liked by pet owners, the higher they ultimately perceived it would
be liked by their pet, leading to higher purchase intention. Furthermore, appearance,
color, and aroma likings were the main contributors to the overall liking for pet owners,
while product color intensity (especially darker brown colors) and oily appearance of the
kibble produced a negative reaction by pet owners. Kibble size also influenced product
liking, with products where kibbles were perceived as too small showing the lowest liking
scores. Among the different sensory attributes, both aroma and appearance attributes
have been found to be the primary drivers of pet food acceptability from the pet owner’s
standpoint. From the pet standpoint, even though aromas (i.e., orthonasal odors) may
drive initial preference, consumption of the product is dictated more by its overall flavors
(i.e., retronasal odors) [13,21].
Palatability considerations for dogs tend to be driven by their ancestral history. Since
dogs, the first domesticated animal, are direct descendants of ancient wolves [
22
], they tend
to prefer meat-based diets over cereal-based ones [
23
]. Lohse [
5
] suggested that dogs prefer
beef the most, followed by lamb and chicken, and they also prefer moist or semi-moist
foods over dry ones due to the low flavor appeal of dry dog foods. Another study by Houpt
and Hintz [
12
] found that dogs exhibit similar preferences for beef and pork, which is
higher than those for chicken and lamb. While pork and beef meat ingredients, that tend to
impart distinct and strong (“meat-based” or “meaty”) aromas, could be liked by pets, they
might constitute a turn-off for pet owners since research has shown that pet owners dislike
pet foods with excessively strong aromas [
13
]. In addition to the liking of “meaty” flavors,
dogs have also shown a high preference for sweet-tasting substances [
24
]. For example,
beagles aged two to four months were found to prefer sweet-tasting substances such as
lactose, fructose, and sucrose, while exhibiting indifference to or rejection of maltose-based
substances. Moreover, sweet-tasting substances such as sucrose were found to increase
food intake and induce food selection [
25
]. However, influences of other factors on the
palatability of pet food, such as pet breed, gender, weight, relationship to owners, taste,
and olfactory sensitivity [6], should also be considered.
2.2. Methodologies for Measuring Palatability of Dry Pet Food
2.2.1. Human (Pet Owners) Sensory Analysis
Due to the inherent difficulties of animal-human communication, it is a challenge
for researchers to understand the acceptance and preference of pet foods from the animal
standpoint. Although indirect methods have been developed to deduce this information
(see Section 2.2.2.), pet food manufacturers rely heavily on pet owners’ perceptions with
respect to the palatability of a formulation by their pets. However, this is not a simple
task because of differences in the pet owner’s perception with respect to what their pets
may like or dislike [
19
]. In addition, because the pet owners might have safety concerns
or other factors making them reluctant to consume pet food samples, most research with
pet owners has been focused on aroma and appearance rather than taste and flavor [
13
,
26
].
It is notable that commercially-available pet foods are regulated under the Federal Food,
Drug, and Cosmetic Act (FFDCA) as follows: “requires that all animal foods, like human
foods, be safe to eat, produced under sanitary conditions, contain no harmful substances and be
truthfully labeled” [
27
]. In addition, the Food Safety Modernization Act (FSMA), signed into
law in 2011, requires animal food, including pets, to be processed under the same good
manufacturing practices (CGMPs) and hazard analysis and risk-based preventive controls
that are required for human food. This rule prevents safety hazards that could potentially
impact humans since pet foods are typically stored next to human food.
Foods 2021,10, 2599 4 of 19
Given all these subjective judgments and the potential profits in making a product
formulation appealing to both pets and pet owners, sensory researchers in the field of food
science believe that descriptive sensory analysis could be a better method of accessing the
sensory quality of pet foods. Descriptive sensory analysis techniques involve describing the
product in terms of appearance, odor, flavor, taste, and texture as well as with correspond-
ing numerical quantification. However, such analysis is usually performed by a panel of
trained experts, not by general pet owners, and such a panel would typically undergo
extensive training with a variety of pet foods to detect subtle product differences [
19
].
Using descriptive sensory analysis to understand sensory attributes of pet foods is a rela-
tively new approach, with the results obtained from descriptive sensory analysis giving
an idea as to “pleasant” and “unpleasant” attributes when they are connected to overall
pet-food palatability data. Di Donfrancesco et al. [
28
] used 21 commercially available pet
food samples to develop a lexicon for describing sensory attributes such as appearance,
aroma, flavor, and texture. A total of 72 sensory attributes, reflecting the highly blended
nature of the pet food products, were selected to describe the sensory attributes of dry pet
foods. Some of the common aroma attributes associated with pet foods were “barnyard”,
“brothy”, “brown”, “grain”, “vitamin”, “oxidized oil”, “cardboard”, and “stale”. Some
aroma attributes unique to only certain products were “liver”, “fish”, “burnt”, “spice
brown”, “garlic”, “celery”, “clove”, and “smoky”.
While understanding the sensory attributes of pet foods is extremely helpful, their
relationship to the acceptance of pet foods among pet owners is equally important. Only
a few studies have been done using traditional consumer testing methods to explore the
sensory acceptance of pet foods. Among studies that have been done in this regard, several
ones have involved collective perceptions of the general pet owner population [
13
,
26
].
Di Donfrancesco et al. [
13
] explored the relationships among descriptive sensory attributes
associated with eight dry pet food products and evaluated using a trained panel and
consumer acceptance as evaluated by pet owners. The results suggested that appearance
even more than aroma influenced pet owners’ acceptance of the products. In terms of
aroma, as mentioned in the Introduction Section, pet owners did not like products with
excessively strong aromas or off-flavors such as “oxidized oil” or “musty/dusty”, while
“meaty” aromas were generally acceptable to pet owners. This information provides
direction to pet food product manufacturers to aim at avoiding certain pet-food attributes
while including others. However, when modifying the pet foods to be acceptable to the pet
owners, precautions must be taken because pets’ liking for the food cannot be compromised.
2.2.2. Animal Palatability Testing (Pets)
Methods for measuring palatability of pet foods employ one of two types of panels: an
expert panel (trained dogs at pet centers) or an untrained panel (in-home or family-owned
pets), and both types of panels pose particular advantages and disadvantages [
29
,
30
]. Similar
to the human expert panel described earlier, an expert panel consisting of extensively
trained animals tends to be more accurate and reliable compared to an untrained panel
of pets. In general, since palatability tests with untrained panels are less controlled and
subject to increased variations based on each individual pet’s historical background, testing
with untrained pets usually requires a larger number of animals (~100) compared to tests
conducted with a panel of fewer trained animals (~30) [
29
]. However, an advantage of using
an in-home panel over a trained panel is obtaining results in a “natural setting” perception
of the pets while also including the owner’s reactions and acceptance. Griffin et al. [
30
]
evaluated the extent to which the food preferences of trained dogs are representative of
those reported for in-home dogs. In summarizing the findings in the literature, the authors
found that in-home panels were more stable in their preferences but that expert panels
were better at discriminating small differences. It was also found that the food preferences
were more consistent between the different panels when testing wet products compared to
dry or semi-moist products. Such differences can be explained by the pets’ level of training,
their feeding history, and/or the testing environment condition.
Foods 2021,10, 2599 5 of 19
Testing methods developed for dogs tap into their inherent feeding behaviors as
mentioned in Aldrich and Koppel [
31
]: “dogs are opportunists for meals and will augment their
diet with foraging for anything seemingly edible from other animals’ feces or scat, to insects, berries,
and grass
. . .
In the dogs’ mouth are large canines, small premolars, and they lubricate their food
with a serous saliva, sans amylase, produced from four primary salivary glands located throughout
the orthonasal cavity. Not much time is given to masticating and savoring the food during the
eating bout: Dogs are known to devour the food in a gluttonous manner and then regurgitate and
re-consume at a time and place when away from competing mouths”. Broadly, two classes of
pet food palatability tests have been suggested: consumption and non-consumption tests;
the former being more popular [
31
,
32
]. Classical consumption tests assessing preference
and acceptance of pet food are known as “two-pan” (also called as “two-bowl”, “paired
stimulus”, or “versus”) test and “one-pan” (also called as “one-bowl”, “single stimulus”,
or “monadic”) test, respectively [
29
,
31
]. In the two-pan test for preference, two test foods
(e.g., foods A and B) are simultaneously presented to a dog in identical pre-weighed
pans and test ingredients. Dogs have free access to the pans for the period of feeding
(15–30 min) after which the pans minus test ingredients eaten are again weighed. Some-
times the process is repeated for each animal while switching pan position to control for
position bias [
29
]. Differences in weight correspond to the quantity of a test food consumed
and are interpreted as a preference for that food [
26
]. For example, if more of food A
is consumed than food B, food A is considered to be preferred over food B. This can be
expressed as an intake ratio (IR) (of food A) defined as a proportion of food A consumed
over the sum of food A and food B consumed. The consumption ratio (CR) (for food A) is
defined as a proportion of food A consumed over food B consumed. Another important
parameter assessed using a two-pan test is the first choice, i.e., the food first approached by
the animal, an indication of visual and odor attractiveness of the food. Two-pan tests are
typically performed with a trained animal panel [
29
,
33
]. Although two-pan tests have been
proven reliable and extensively used in palatability studies, they tend not to be sensitive
to long-term satiating effects of food while accounting for nutritional and caloric value.
There is also a possibility, especially for those not trained to self-limit their food intake, that
animals could consume excessive food [
18
,
29
]. Since the preference measured in this test is
only relative between the two foods tested, each combination of pairs must be evaluated
for preference when three food products or more are compared [
31
]. In addition, when the
two products tested showed no difference with respect to intake ratios and first choice, it
is difficult to conclude that the dogs preferred the two products equally because of their
similar odor preference because the dogs might have a lack of olfactory discrimination [
8
].
Therefore, Basque et al. [
8
] suggested a complementary approach, i.e., a combination of
food preference tests and food olfactory discrimination tasks, for providing a better under-
standing of the drivers of dogs’ preference for food products. Another major limitation of
such tests is that, even though the “first choice” parameter reflects olfactory attractiveness,
it does not truly discriminate among visual attractiveness, odor intensity, individual odor
preference, and recognition due to previous experience [
34
]. Recently, Pétel et al. [
34
]
developed false-bottom bowls (FBBs) intended to hold different odor compounds that modify
traditional bowls by adding a drilled, stainless-steel separation plate at the bottom. Using
such bowls, pet preference could be evaluated while varying aromas and measuring the role
of odor acceptance among pets. However, other limitations such as the proximity of the two
stimuli, i.e., test and treatment, during the evaluation have not been addressed yet.
Consumption methods also include acceptance testing using a one-pan test. In this
method, the animal has free access to a single food for a given feeding period. Parameters
measured include quantity consumed, speed of consumption, or enthusiasm with respect
to eating the food [
26
,
29
,
31
]. Since these parameters are similar to those observed by a pet
owner while introducing a new food product to their pet, one-pan tests are most suitable
for family-owned, untrained animal panels [
35
]. Unless pet food samples are extremely
aversive and therefore not consumed by the pets at all, their acceptability can broadly be
Foods 2021,10, 2599 6 of 19
measured with respect to food consumption required to maintain calorie intake without
necessarily evaluating their taste or aroma attributes [31].
Due to limitations of consumption tests, some researchers have used facial expressions
of pets as indicators of food acceptance or rejection [
36
], although this approach has not
gained widespread use. Non-consumption tests, in which pets are trained to associate
their food preferences with objects, tapping their cognitive ability, and thereby validating
discriminability while assessing preference or acceptance, are also being explored. These
methods tend to ensure more robust results and are seemingly not as biased by testing
conditions or pet history [
18
]. Recently, Cheli et al. [
37
] suggested the use of electronic
nose (e-nose) or tongue (e-tongue) as rapid tools for identifying key odorous or tasting
compounds to ensure high nutritional properties of the pet food along with meeting
palatability standards. While such techniques could help discriminate between different
aromas/tastes and hopefully replace traditional animal-preference tests, the use of e-nose
and e-tongue in the pet food industry is still new and requires more validation before being
used commercially.
3. Overview of Palatants Used in Dry Pet Foods
Palatants, or flavor enhancers, applied to pet foods to improve their inherent palata-
bility and increase pet acceptability, were originally referred to as “digests”, essentially
proteins broken down enzymatically to provide a sensory impact of meat flavors [
3
,
38
].
These palatants can be characterized as complex systems that consist of a variety of macros
and micro-molecules improving the sensory experiences of the pets and pet-owners, mask-
ing unpleasant tastes and off-flavors, and enhancing appetite in pets. [
3
,
38
]. Meat-based
(e.g., poultry, pork, beef, or fish) and vegetable-based (e.g., corn, soy, potato, or grains)
palatants include components such as proteins, yeasts, phosphates, antioxidants, antimi-
crobials, and processing agents. Essential oils, aldehydes, and condiments could also be
used as palatants [39].
When designing palatants, it is important to consider their inherent properties and
their interaction with the chemical composition of a pet food matrix. Commercially-
available palatants, used as flavoring agents, are most often classified as either dry powders
(generally added in amounts between 0.5% and 2%) or liquids (generally added in amounts
between 1% and 3%) and are most commonly sprayed onto dry food, although a few could
be added as an ingredient during processing. There are, however, challenges associated
with proposing palatants for commercial and industrial use. First, since food palatability
differs with the food form, palatants used in canned or semi-moist pet foods might not be
useful for dry pet foods. Second, because palatability might differ among different species,
palatants used in cat foods do not generally serve a similar purpose in dog foods. Finally, it
must be ensured that the palatants used do not compromise or impact the digestibility of
the food [7,20].
Extensive research has led to the discovery of some traditional palatants that have
been used in the pet food industry for a long time. A number of patents have been filed
claiming palatability improvement of pet foods using traditional palatants, such as amino
acids [
40
,
41
], fat or fatty acids [
42
], and animal digests from beef, pork, poultry, or fish, etc.
Organic acids such as phosphoric acid, citric acid, tartaric acid, fumaric acid, lactic acid,
acetic acid, and formic acid have also been used in the past to improve the palatability of
pet foods [
43
]. Phosphates, pyrophosphates, and polyphosphates have also been explored
as potential palatants in dry pet foods [
44
], although there are some concerns regarding
their long-term effects on renal functions of pets [
45
–
47
]. Such palatants could be added
either as an ingredient during a mixing process or as a surface coating after product
processing. A combination of different palatants is often used. For example, U.S. Patent
No. 5,186,964 to Gierhart and Hogan [
48
] revealed the palatability-enhancing effects of
phosphate, pyrophosphate, and polyphosphate when optionally combined with organic
acid (e.g., citric, tartaric, fumaric, lactic, acetic, formic, or hexamic acid) and flavorants. In
that case, a two-step coating process was adopted wherein the flavorant and phosphate are
Foods 2021,10, 2599 7 of 19
initially applied to the pet food, followed by spraying of an acidic enhancer. When patents
mention the use of chemical compounds as potential palatants, it is worth noting that other
factors such as the inherent properties of pet food and the method of palatant application
can also impact an increase in palatability.
While it can be recalled that palatants can serve the purposes of improving fla-
vor [
49
,
50
], odor [
50
], texture [
44
], appearance [
26
], or a combination of these attributes to
increase overall preference and acceptance of a product, it is not always straightforward
to determine exactly which sensory attribute is being targeted by a palatant. In a study
using multiple-coating techniques, dry pet foods were coated with a primary coating
comprised of farinaceous and proteinaceous material to impart the desired flavor and
texture, followed by a secondary coating of starch or egg to provide the final product with
a glistening sheen to make it more attractive [
51
]. Pyrophosphates, in particular sodium
tripolyphosphate, have been used as palatants for improving the texture of the product.
They provide high pH, high ionic strength, and higher protein solubility in meat, resulting
in firmer texture [
44
]. Similarly, it is possible that animal digests provide distinct “meaty”
notes to pet foods, thereby increasing palatability for dogs. U.S. Patent No. 9,480,275 B2 to
Brent [
52
] aimed to improve palatability by making products more attractive to consumers.
Specifically, the patent states “the present invention enhances ease of access and manageability by
a pet with improved attractiveness to a consumer and provides improved texture and palatability”.
Even though different palatants may work to improve different attributes of the pet food,
alone or in combination, it appears that palatants that improve flavor quality of the pet
food are most popular, possibly because odor/flavor characteristics are primary drivers of
pet food preference and acceptance. In addition, pet owners’ attitudes toward pet foods are
also modulated by the aromas and flavor attributes of the products. For example, previous
research suggests that pet owners might have a negative attitude toward pet foods if
the foods have strong and objectional aromas such as “musty” or “oxidized oils” [
13
,
53
].
Moreover, although strong “meaty” aromas may not be well-accepted by pet owners, they
might actually be desirable to a pet. Therefore, the most desirable flavor enhancers would
increase the flavorful-ness of dry pet foods for pets while also being acceptable to pet
owners as mentioned in U.S. Patent No. 2015/0056347 A1 [
54
]: “many pet foods proposed
so far have a major disadvantage due to the presence of smells that are not appealing to the pet
owners. Reciprocally, food products that are attractive to pet owners are not systematically palatable
to pets.” Therefore, the off odors must be either masked or overpowered by more flavorful
characteristics, a purpose sometimes served flavor enhancers used as palatants.
4. Flavor Enhancers Used as Palatants and Their Impact on Dry Pet Food Palatability
4.1. Animal Digest and Other By-Products
Animal digest, i.e., partially hydrolyzed animal parts in both dry and liquid forms, is
probably the most commonly used flavor enhancer in the pet industry. In fact, AAFCO
includes animal digest as an ingredient in pet foods and defines it as “a material which
results from chemical and/or enzymatic hydrolysis of clean and undecomposed animal tissue. The
animal tissues shall be exclusive of hair, horns, teeth, hooves, and feathers, except in such trace
amounts as might occur unavoidably in good factory practice and shall be suitable for animal feed.
If it bears a name descriptive of its kind or flavor(s), it must correspond thereto” (p. 360, [
3
]).
For pork and beef, non-rendered clean parts such as lungs, spleen, kidneys, brains, livers,
blood, stomachs, and intestines are used. For poultry parts such as livers, hearts, heads,
feet, and viscera are used. In some ways, animal digest serves as a primary palatant
in pet foods because it provides raw or meaty odor/flavor notes to pet foods, making
them desirable to the pets. The enzymes used in this process are generally proteases and
lipases that break down proteins and fats, respectively. Additionally, the fatty acids and
amino acids produced as a result of reacting with reducing sugars producing flavorful
aromatic substances via the Maillard reaction are characterized to have aromas of “meaty”
and “brothy” notes in the descriptive sensory analysis by a trained human panel [
13
].
U.S. Patent No. 4,211,797 to Cante et al. [
55
] disclosed the palatability-enhancing potential
Foods 2021,10, 2599 8 of 19
of beef digest along with lipolyzed beef tallow added at 4–8% by weight of dry pet food in
the form of a surface coating. Consumption tests showed that the dogs preferred food with
a digest of beef and beef tallow coating over control pet food not containing a palatant.
Researchers have also explored the addition of protein hydrolysates (proteins hydrolyzed
into short peptides and certain amino acids) directly to pet foods as flavor-enhancing
palatants, and both seafood and animal protein hydrolysates have been investigated for
this purpose [
56
,
57
]. U.S. Patent No. 2008/0280274 A1 to Friesen and Yamka [
58
] disclosed
the use of poultry-liver hydrolysate either alone or in combination with poultry fat to
increase the palatability of dry pet foods. Palatability was evaluated using a two-pan test
with 25 dogs revealed a higher preference for food with liver hydrolysate than for pet
food without it. The current research focused on animal digest as a palatant is related to
optimizing the processing conditions of enzymatic treatment or using different sources of
proteins and fats.
4.2. Maillard Reaction Precursors and Products
The Maillard reaction plays a pivotal role in flavoring pet foods. The initial stages of
the Maillard reaction involve condensation of the carbonyl group (reducing sugar) with an
amino compound (amino acids) by the impact of high temperatures, or at a slower rate, by
low temperatures, low pH, and low a
w
levels [
59
]. The condensation products are further
degraded to produce different oxygenated compounds that interact with other reactive
compounds such as amines, ammonia, hydrogen sulfide, and aldehydes, leading to the
production of flavor compounds such as furans, pyrazines, esters, thiopenes, and other
heterocyclic compounds [
60
]. As mentioned above, amino acids produced by animal digest
react with reducing sugars to produce Maillard reaction-related flavors.
Another approach to initiating a Maillard reaction to produce flavorful compounds is
to treat farinaceous ingredients with different enzymes to produce simple sugars that can
react with proteins in the food system. U.S. Patent No. 6,926,917 B2 to Parthasarathy [
61
]
proposed contacting the raw ingredients (including farinaceous together with some pro-
teinaceous compounds) with
α
-amylase from 0.05% to 0.5% by weight of ingredients. The
enzyme breaks down starch and other complex carbohydrates in the farinaceous ingre-
dients into sugars, with reducing carbonyl groups serving as precursors of the Maillard
reaction. The enzyme could either be added on the surface as a coating or added into
the ingredients before extrusion. The inventors did not include palatability testing in
their patent, but they ensured that the inclusion of
α
-amylase prior to extrusion improved
the palatability and texture of dry pet food. Additionally, U.S. Patent No. 3,617,300 to
Borochoff et al. [
62
] indicated that dextrose, a reducing sugar, has the potential to be used as
a flavor-enhancing compound. According to this patent, some of the starch content in the
dry pet foods can be converted to glucose by
α
-amylase and amyloglucosidase. In addition,
U.S. Patent No. 4,393,085 to Spradlin et al. [
63
] revealed that palatability of dog foods
could be improved when the slurry mixture, combined a portion of farinaceous ingredients
(e.g., wheat and corn) treated with amylase and a proteinaceous portion (e.g., meat meal
and soybean meal) treated with protease, was sprayed as a coating on dry dog foods.
Consumption tests with 21 dogs showed that food treated with palatants developed as a
combination of proteinaceous and farinaceous ingredients was preferred by the dogs over
food treated with palatants containing only proteinaceous or only farinaceous ingredients.
It is important to highlight that Spradlin et al. [
63
] did not indicate the type of pet panel
(i.e., trained or untrained) that they employed in their palatability testing. Similar ap-
proaches in modifying processing conditions to treat proteinaceous and farinaceous raw
materials in the production of pet foods were found to contribute to enhancing the flavor
profile of the final product [
64
]. While the approaches discussed in this section have been
focused on initiating the Maillard reaction to produce flavor compounds, some researchers
have explored the direct addition of Maillard reaction-precursors such as amino acids to
the pet foods, as explained below (Section 4.3.).
Foods 2021,10, 2599 9 of 19
The final flavor compounds produced during the Maillard reaction can be directly
used as flavor additives. Recently, Chen et al. [
65
] investigated the optimization of key
aroma compounds, referred to as “dog food attractants” (DFAs), in pet food. In their
study, seven DFAs, based on the Maillard reaction products using protein sources (brewer’s
yeast, chicken meal, and soybean meal) and reducing sugar (xylose), were identified.
Headspace-solid phase microextraction (HS-SPME) and gas chromatography revealed
53 aroma compounds associated with these seven DFAs, including 11 aldehydes, 10 het-
erocycle compounds, seven alcohols, seven esters, four ketones, four organic acids, four
phenols, two terpenes, and two polycyclic aromatic hydrocarbons. Alcohols are generally
characterized by fruity odors, while some aldehydes such as benzaldehyde smell like
almond oil [
65
,
66
]. Benzaldehyde and hexanal (green or rose-like odors) are common
aldehydes associated with pet foods [
67
]. Another reactant compound of the Maillard
reaction, furfural, imparts a caramel-like flavor to pet foods [
65
]. In fact, caramel-like sweet
odors have been found to be liked by dogs [
68
]. For further investigation, Chen et al. [
65
]
performed a preference (two-pan) test and an acceptance (one-pan) test with eight beagle
dogs. For the acceptance test, the intake ratio (IR) was calculated for each food, while for
the preference test, first choice and first-approach data were used to measure preference
for a sample. The results showed that 2,5-dimethyl pyrazine, vanillin, and benzaldehyde,
added at levels of 0.21, 8.90, and 1.82,
µ
g/g of dry food, respectively, were found to make a
significant contribution to enhanced palatability of the DFAs. The advantage of using these
aroma compounds as flavor enhancers is that, along with being attractive to dogs, they
are also deemed pleasant by pet owners. However, it is worth mentioning that while the
Maillard reaction is an important chemical reaction for pet food manufacturers to increase
pet acceptability of the food, there are some concerns about Maillard reaction products
reducing the bioavailability of lysine resulting in reduced nutritive value of the pet foods.
Some Maillard reaction products have also been associated with age-related diseases in
pets [69,70].
4.3. Amino Acids
Amino acid lysine in addition to other amino acids such as tyrosine, arginine, and
tryptophan have been popularly explored as a flavor enhancer in human foods [
71
] as well
as in pet foods. In U.S. Patent No. 4,267,195 to Boudreau and White [
40
], it was disclosed
that a coating of L-lysine on the surface of dry dog foods at a ratio of 0.1 to 500 mM
had the potential to increase the palatability of pet foods while meeting the nutritional
requirements of dogs in terms of adequate proteins, carbohydrates, vitamins, and minerals.
Although palatability testing with dogs is not reported in this study, the authors of this
patent described that “test have shown all of the dog food flavors of this invention to cause strong
taste responses in dogs”. Another patent, U.S. Patent No. 4,282,254 to Franzen et al. [
41
]
revealed the flavor-enhancing potential of L-phenylalanine, L-tyrosine, L-methionine, L-
tryptophan, L-arginine, L-leucine, L-isoleucine, L-serine, and combinations of them in
an amount of between 0.001% and 0.8% by weight. The patent reported improvement in
palatability based on palatability testing using 40 dogs in an incomplete block design. The
patent also claimed that L-methionine at concentrations between 0.005% and 0.5% based
on the weight of dog food had a palatability-enhancing effect in both dry and semi-moist
pet foods. In addition, E.P Patent No. 2731449 B1 to Niceron [
72
] evaluated the palatability
of dry dog foods coated with a mixture of amino acids (alanine, aspartic acid, asparagine,
arginine, cysteine, glutamic acid, glycine, glutamine, histidine, isoleucine, leucine, lysine,
methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine).
Based on a two-pan test using 36 dog panelists, the bowls of a control diet (no amino
acids added) and the experimental diet (amino acid palatability-enhancing composition)
were compared from 15 to 30 min. The patent reported that the palatability of the amino
acid mixture did not differ from the control. However, the same patent demonstrated
higher palatability of the amino acid diet with a cat panel, highlighting the difference in
palatability between the two specifies.
Foods 2021,10, 2599 10 of 19
4.4. Aroma/Flavor Compounds (Direct Addition)
Since the primary goal of flavor enhancers is to improve the flavor profile of pet foods
and thereby their palatability, the addition of aroma or flavor compounds directly into pet
foods seems only logical. In addition to Maillard reaction-related flavors mentioned above,
other aromas have also been used for this purpose. In patent U.S. 2015/0056347 A1 [
54
],
human sensory appeal and pet preference were both evaluated when dry pet foods were
prepared with additions of different combinations of palatability enhancers and aroma
compounds. Dry food aromas could include animal (beef, poultry, pork, or fish), vegetable
(herbs, fruits, or vegetables), and dairy (butter, milk, or cheese) compounds. In addition
to aroma compounds, traditional palatability enhancers such as animal digests, animal
fats, or dairy products were used. In the total composition of aroma and palatability
enhancer, the liquid aroma was varied from 0.25 to 20%, while the remainder (80–99.75%)
was composed of the palatability enhancer. This was one of the few patents that reported
sensory studies validating palatability-enhancing effects using both animal and human
panelists. Pet preference was evaluated using a two-pan test of 36 trained dogs. The first
food consumed, and the amount of each food consumed was calculated by the end of the
test (from 15 to 30 min). Human panelists “trained” with pet foods were used for sensory
assessment of the food samples. Each panelist was asked to rate his/her preference for the
odor on a 9-point scale with respect to liking (1 = I do not like it at all; 9 = like it very much)
and odor intensity (1 = not at all intense; 9 = very intense). It was claimed that adding a
coating of at least one dry food aroma to at least one other palatability enhancer resulted in
a flavor-enhancing effect in dry pet foods, based on both animal and human panel results.
A recent U.S. Patent No. 2016/0309749 A1 to Perez and Dodge [
73
], suggested coating
the dry pet foods with a mixture of liquid palatability enhancer and liquid aroma followed
by a coating of the animal digest to seal the aroma. In the total composition of aroma and
palatability enhancer, the liquid aroma was varied from 1 to 5%, while the remainder of
95–99% was composed of the palatability enhancer. The results revealed that the use of a
traditional liquid palatability enhancer along with liquid aroma used with dry pet foods
increased palatability among pets and pet owners. The aromas were specifically chosen to
impart the “humanization sensory factor without a negative sensory impact for the pets that will
consume the dry pet food” [
73
]. Therefore, “sweet” and “grilled” liquid aromas (referred to as
“human” aroma) were used in this invention. The advantage of using the liquid aroma and
the liquid palatability enhancer over their dry counterparts is that it is more cost-efficient
and negates the requirement for a dry aroma to be dissolved into a carrier for spraying as
a surface coating. U.S. Patent No. 6,379,727 B1 [
74
] revealed the packaging of flavors in
dry powder (in a shaker) or liquid (in a spray bottle) form along with a base dog food. The
flavors explored during this work, including BBQ, beef stew and crackers, pizza, sausages
and eggs, roast beef, peanut butter and jelly, and potato, are popularly found in foods for
human consumption. This method, i.e., directly adding aromas and flavors to pet food,
is gaining popularity although palatability testing to validate the palatability-enhancing
effect is required.
4.5. Lipids (Fats, Oils, and Fatty Acids)
Past research suggests that the macronutrient profile of the food can influence intake
among pets [
75
,
76
]. Specifically, foraging decisions made by dogs could, to some extent, be
based on meeting their macronutrient goals. In a diet selection study, Hewson-Hughes [
75
]
investigated whether dogs regulated their diets to meet certain macronutrient intake. It was
found that irrespective of the breed, dogs regulated their diet to a protein: fat: carbohydrate
ratio of approximately 30%: 63%: 7% by energy. Certain pet food ingredients, such as
lipids, in addition to their essentiality to maintain the macronutrient profile, also function
as palatants. For example, crude fat extracts, generally from sources such as chicken, beef,
lamb, and other land animals, have characteristic aromas that may be desirable to pets [
77
].
However, the application of such crude fats is not preferred due to the presence of saturated
and higher-melting temperature fats that tend to integrate together and trap the aroma
Foods 2021,10, 2599 11 of 19
molecules. Such “fatty” clusters tend to stick to the surface of the pet food, rendering
it less appealing in terms of appearance. In addition, because high levels of dietary fat
(e.g., >10%) may increase the potential for diet rancidity, antioxidants may be added
into pet foods if the nutrient quality and quantity of the foods are to be preserved [
78
].
U.S. Patent No. 3,745,023 [
79
] used a combination of modified lower-melting temperature
land-animal fat (50 parts by weight), vegetable oil (49 parts by weight), and other flavorants
(1 part by weight), prepared by vigorously stirring rendered chicken, beef, and pork fat
along with crude soybean oil at room temperature and cooling to 7.2
◦
C. The precipitated
saturated fats and soybean sludge was filtered off using the filtrate for further processing.
The filtrate was mixed with other flavorants such as garlic and anise oil along with some
antioxidants. This composition, when sprayed over the dry pet food as a coating, was
proposed as a flavor enhancer. Fish oil could also be used. Interestingly, using fats as
palatants could add to the nutritional profile, act as a carrier for other flavors, and impart
specific aromas and flavors. Fournier [
80
] tested the palatability of eleven fat types of three
different origins (pork, poultry, and beef) with a two-pan test by a dog expert panel. The fat
types were used as a coating at a 6% level with 1.5% of premium liquid dog palatant and
tested against a control coated with 6% of poultry fat and 1,5% of the same premium liquid
dog. In general, beef tallow and the mix between beef and pork were the most palatable,
and pork fat was more palatable than poultry fat as shown by higher consumption rates.
Noticed differences in preference among fats of the same origin (poultry or beef) were also
found, suggesting an impact of fat manufacturing process and composition (volatiles and
fatty acids) on palatability. In addition, U.S. Patent No. 2016/0029668A1 [
81
] revealed
a palatant composition derived from cocoa butter or a mixture of different fatty acids.
Specifically, mono-ethaloamine was exposed to heat treatment and an amidation reaction
along with the fats, oils, or fatty acids, thereby enhancing the palatability of pet food when
added at a level of 10–1000 ppm to the pet food. A palatability-enhancing composition
prepared using a mixture of ethanomides with a fatty acid (oleic acid, palmitic acid, stearic
acid, linoleic acid, or linolenic acid) was blended with the other ingredients and extruded
to form dry kibble. This composition could be added as a surface coating on the extruded
kibble in addition to being blended with the ingredients before extrusion. Preference for
the palatant-added food over the control food was validated with a two-pan test using a
dog panel, with higher consumption indicating higher preference. However, the impact
of fatty acid addition on dog food’s palatability seems to be dependent on the chemical
configuration of the molecules (e.g., carbon-chain length). For example, using a two-pan
test, Dahkal and Aldrich [
82
] evaluated the palatability of diets containing medium-chain
fatty acids including caproic acid (C6, 50%), caprylic acid (C8, 50%), and capric acid (C10,
50%) with a 20-dog panel. The results revealed that adding medium-chain fatty acids, to a diet
strongly decreased food acceptance of dry dog foods compared to a control (chicken fat) [
82
].
4.6. Organic Acids
Organic acids have been successfully used in dry pet foods to increase palatability and
provide an antimicrobial effect. The U.S. Patent No. 2014/0154356 A1 [
43
] proposed the
use of fumaric acid, either alone or in combination with sorbic, succinic, and gallic acids as
potential palatants and anti-microbial agents of dog kibbles. The palatability of the organic
acids was tested using the two-pan test with a dog panel. The diets were presented for a
period of four hours, and the amount consumed was recorded for each dog. The results
indicated that gallic acid and fumaric acid displayed acceptable palatability for inclusion
in dry dog palatant formulations compared to the control (off-the-shelf brand name). The
inclusion of sorbic acid on its own or in combination with salt as a successful palatant was
also demonstrated by patent U.S. 2018/0220678 A1 [
83
]. Palatability comparisons between
dry kibbles coated with sorbic acid and a control (no sorbic acid) were carried out using
a two-pan procedure with a panel of 20 dogs. The pair of bowls with weighed amounts
of test products was presented to each dog for 20 min, or when the bowl was empty if
sooner. The test products coated with sorbic acid showed a higher consumption rate over
Foods 2021,10, 2599 12 of 19
the control with no sorbic acid, as well as when compared to commercial dog food. A
recent patent, U.S. No. 2016/0316789 A1 to Aubril and Callejon [
84
], disclosed ascorbic
acid for potential use along with traditional palatability enhancers to improve the flavor
of the pet food. A two-pan test with 36 dogs was conducted, with the overall duration of
study for each dog ranging between 15–30 min. The first food consumed, and the amount
of food consumed were both calculated for each pet food, although the exact role of the
acid, either antimicrobial action of flavor-enhancing effect or both, was not highlighted.
The palatability results showed that the addition of ascorbic acid (between 0.005% and
0.3% by weight) increased the palatability of the dry kibbles compared to the control (no
ascorbic acid in the mix), regardless of the level of ascorbic acid tested.
4.7. Miscellaneous/Others
As mentioned earlier, novel ingredients are being explored as potential flavor-enhancing
agents. One such substance is ammoniated glycyrrhizin, processed from natural sources
and with 50 times more sweetness than sucrose. Glycyrrhizic acid is obtained by grinding
the root Glycyrrhiza glabra (licorice), followed by extraction of the ground material with hot
water, and recovery of the acid-insoluble fraction from the extract containing glycyrrhizic
acid. Glycyrrhizic acid can further be ammoniated to provide ammoniated glycyrrhizin,
known to work synergistically with sucrose, meaning that the sweetness value of gly-
cyrrhizin and sucrose combined is more than the mere additive effect or sum of their
individual sweetness values. This approach both targets taste improvement and improves
overall flavor appeal [
85
]. The flavor-enhancing effect of ammoniated glycyrrhizin was
validated with a consumption test using 20 dogs. Since dogs like the sweet taste and sweet
flavors, glycyrrhizin can be used to impart the desired taste, thereby reducing the sucrose
content of the pet food as well.
Herbs and spices are another class of flavoring compounds used to increase the
palatability of pet foods. W.O. Patent No. 2006/065841A2 [
86
] exclusively calls out the
impact of tarragon essential oil as a palatant. The palatability of the herb was measured
with 25 dogs via a two-pan test. Test foods were dry dog foods prepared with or without
the addition of essential oil of tarragon to the pre-conditioner composition during the
preparation of the foods. The foods were left with the animal for 45 min, and they were
reweighed at the end of testing to determine the intake ratio. The addition of tarragon oil
to the dog food, especially at low concentrations (0.001–0.005 wt%), significantly increased
the palatability of dry dog food compared to the control. Other spices such as garlic and
anise have also been used because of their distinct odors.
Zeolites, essentially microporous, aluminosilicate minerals, have also been explored
as potential palatants because of their adsorbent and catalytic properties. For example,
U.S. Patent No. 2009/0274796A1 [
87
] disclosed the use of clinoptilolite, a zeolite, as a
flavor-enhancing palatant. For dry pet foods, the palatant is sprayed on the surface of the
food to form a coating. The patent reported results from a two-pan preference test using
25 dogs, wherein food with zeolite was found to be consumed more than food without the
zeolite. In addition, food with zeolite was ingested by the dogs more frequently and at a
higher time rate, further indicating higher preference. Barnes [
88
] provided an extensive
market analysis of U.S. pet foods and suggested that many pet food manufacturers, in
particular dog food manufacturers, use cranberries in their products for health reasons.
Since cranberries are used in human food for nutrition and flavoring, there is a possibility of
exploring the use of cranberries, and perhaps other berry flavors, for flavor enhancement in
pet foods. Other palatants such as pyrophosphates have also been explored [
44
], although
pyrophosphates have been shown in the past to improve textural aspects of food more than
flavor [
89
], and the mechanism to improve the palatability of pet food is still unknown. On
similar lines, U.S. Patent No. 6,926,917 B2 [
61
] suggested the use of
α
-amylase to improve
the palatability of dry pet foods by means of increasing product softness. Although new
chemical compounds have been suggested for their role as flavor enhancers, the exact
mechanisms of their functionality are not well-known.
Foods 2021,10, 2599 13 of 19
Table 1shows some patents published for strengthening palatability of pet (dog) food
using flavor enhancers prepared from different types of substances.
Table 1. List of patents published for improving palatability of pet (dog) food using flavor enhancers as palatants.
Category Target Palatants Amount Suggested
Palatability Testing
Patent Number Reference
Pet (Dog) Pet
Owner
Animal digest and other by-products
Beef protein and fat Blend of beef digest and lipolyzed
beef tallow
4–8% based on total weight
of the dry dog food Yes No U.S. 4,211,797 [55]
Vegetable protein in
combination with
animal fat or oil
Bleachable fancy tallow, butter oil,
soy isolate, enzymes
0.1–5% based on total weight
of the dry dogf ood Yes No U.S. 3,857,968 [90]
Protein hydrolysate Poultry liver hydrolysate (alone or
in combination with poultry fat)
0.01–6% based on total
weight of the dry dog food Yes No U.S. 2008/0280274
A1 [58]
Maillard reaction precursors and products
Combination of
reducing sugar,
animal blood, yeast
and fat
Glucose, dried animal blood,
yeast extract, bleachable
fancy tallow
1–5% based on total weight
of the dry dog food Yes No U.S. 4,089,978 [91]
Sulfur compounds,
reducing sugar,
animal digest
Sulfur compound (ammonium
sulfate, ammonium hydrogen
sulfate, diammonium carbonate,
ammonium bicarbonate,
ammonium chloride, ammonium
nitrate, ammonium hydroxide,
di-ammonium phosphate and
mixtures thereof), reducing sugar
(xylose, pentose, glucose, fructose,
starch hydrolysates, molasses,
and mixtures thereof
Varies with composition
of food Yes No U.S. 6,660,319 B1 [92]
Digested
proteinaceous and
farinaceous
ingredients
Soy, whey, blood plasma, egg,
chicken skins, cheese, enzymes
20–80% farinaceous
ingredients and 20–80%
proteinaceous ingredients
Yes No U.S. 4,713,250 [64]
Digested
proteinaceous and
farinaceous
ingredients
Meat and bone meal and soybean
(proteinaceous), wheat corn
(farinaceous), enzymes
20–80% farinaceous
ingredients and 20–80%
proteinaceous ingredients
Yes No U.S. 4,391,829 [93]
Digested
proteinaceous and
farinaceous
ingredients
Meat and bone meal and soybean
(proteinaceous), wheat corn
(farinaceous), enzymes
20–80% farinaceous
ingredients and 20–80%
proteinaceous ingredients
Yes No U.S. 4,393,085 [63]
Digested
proteinaceous and
farinaceous
ingredients and
α-amylase mixture
α-amylase
0.05% to 0.5% of α-amylase
based on total weight of the
dry food ingredients
No No U.S. 6,926,917 B2 [61]
Amino acids
Amino acid(s)
L-phenylalanine, L-tyrosine,
L-tryptophan, L-methionine,
L-arginine, L-isoleucine, L-leucine,
L-serine,
and combinations thereof
0.001 to 0.8% based on total
weight of the dry dog food Yes No U.S. 4,282,254 [41]
Foods 2021,10, 2599 14 of 19
Table 1. Cont.
Category Target Palatants Amount Suggested
Palatability Testing
Patent Number Reference
Pet (Dog) Pet
Owner
Amino acid L-lysine 0.1 to 500 mM No No U.S. 4,267,195 [40]
Amino acid(s)
L-alanine, aspartic acid,
L-asparagine, L-arginine,
L-cysteine, glutamic acid,
L-glycine, L-glutamine,
L-histidine, L-isoleucine,
L-leucine, L-lysine, L-methionine,
L-phenylalanine, L-proline,
L-serine, L-threonine,
L-tryptophan, L-tyrosine,
L-valine, and
combinations thereof
1 to 70% by weight of free
amino acids Yes No E.P. 2731449 B1 [72]
Aroma/flavor compounds (direct addition)
Liquid aroma
Human aroma such as “sweet”
and “grilled” along with
traditional liquid
palatability enhancer
Total palatant composition
made of 1–5% liquid aroma
and 95–99% traditional
palatability enhancer
No No U.S. 2016/0309749
A1 [73]
Dry aroma
Dry food aromas including
animal (beef, poultry, pork, fish),
vegetable (herbs, fruits,
vegetables), dairy (butter, milk,
cheese) along with other
palatability enhancers (PE; animal
digests, animal fats,
dairy products)
Weight ratio of dry aroma:
PE could vary from 0.25:
99.75 to 20: 80. The
palatability enhancer and dry
food aroma is added at
0.25–12% w/w of pet food.
Yes Yes U.S. 2015/0056347
A1 [54]
Dry and liquid flavor
BBQ, pizza, beef stew and
crackers, sausages and eggs,
peanut butter and jelly, roast beef
and potatoes
Depends on type pf
flavor used No No U.S. 6,379,727 B1 [74]
Lipids
Fatty acids or
combination of fats
and oils
Ehanolamides with oleic
acid/palmitic acid/stearic
acid/linoleic acid/linoleic acid
and combinations thereof or
ethanolamides with beef
tallow/cocoa butter/palm
oil/palm stearin/palm
fractions/olive oil/hydrogenated
oils/lard/high oleic safflower and
combinations thereof
10–1000 ppm Yes No U.S. 2016/0029668
A1 [81]
Fats
Fat composition with varying
compositions of C12 (dodecanoic
acid): C10 (decanoic acid) and/or
C14 (tetradecanoic acid): C12
(dodecanoic acid)
Fatty acid weight ratio of 0.85
to 2.5 (C12:0/C10:0) and/or
0.45 to 4.3 (C14:0/C12:0)
Yes No U.S. 2015/0237887
A1 [42]
Fats and oil
Animal fat (chicken, beef, pork),
vegetable (crude soybean oil) and
fish oil
50, 49, 1 parts by weight
animal fat, vegetable oil and
flavors, respectively or 48, 2,
49, 0.1 parts by weight
animal fat, fish oil, vegetable
oil and flavors, respectively
No No U.S. 3,745,023 [79]
Foods 2021,10, 2599 15 of 19
Table 1. Cont.
Category Target Palatants Amount Suggested
Palatability Testing
Patent Number Reference
Pet (Dog) Pet
Owner
Organic acids
Organic acid Ascorbic acid Varies with composition
of food Yes No U.S. 2016/0316789
A1 [84]
Organic acid(s)
Fumaric acid alone or in
combination with sorbic, succinic
and gallic acids
0.1–2.0% based on total
weight of the dry dog food Yes No U.S. 2014/0154356
A1 [43]
Miscellaneous/others
Zeolites Clinoptilolite 0.01–4% based on total
weight of the dry dog food Yes No U.S. 2009/0274796
A1 [87]
Herb and spices Essential oil of tarragon 0.001–0.005% based on total
weight of the dry dog food Yes No W.O.
2006/065841 A2 [86]
Licorice Ammoniated glycyrrhizin Varies with sucrose content
of the pet food Yes No U.S. 4,191,781 [85]
5. Synonymy between Human and Pet Foods: Future Directions
A growing trend in the pet food industry is to be in synchronism with the human
food industry. As mentioned earlier, pet owners’ decision-making in buying food for their
pets tends to be in harmony with their own food-related decisions [
20
]. In other words,
if a pet owner is interested in “organic” and “natural” foods, it is highly likely that they
prefer to see those same claims for the pet food products they purchase for their dogs.
U.S. Patent No. 2015/0079042 A1 to Foley and Harper [
94
] disclosed a method for develop-
ing a nutritious snack in the form of dog treats made of organic and natural ingredients. All
the ingredients, including wheat flour (42.4%), blueberries (15%), cinnamon (0.1%), yogurt
with or without vanilla flavor (21.6%), and almond milk (21.1%), were mixed together into
a dough and baked in the desired shape to form the dry pet food. This patent reflects a shift
in ingredient choice and requirements by pet owners when they make pet food-related
decisions. Another example is U.S. Patent No. 2006/0062892 A1 [
95
] that described a dry
pet food for dogs using a combination of meat and vegetables. In particular, the proposed
pet food is a dry kibble mixed with dried meat jerky, along with dehydrated/freeze-dried
fruits/vegetables and dehydrated natural gravy. A wide selection of meat (beef, pork,
chicken, duck, turkey, buffalo, fish, venison, and other seafood), vegetables (potatoes, sweet
potatoes, carrots, peas, beans, zucchini, squash, green beans, hominy, corns, tomatoes, and
spinach), and fruits (apples, blueberries, peaches, cranberries, cantaloupe, pears, apricots,
blackberries, papaya, strawberries, mangos, and raspberries) could be used to prepare a pet
food that is both nutritious and tasteful. Another invention, U.S. Patent No. 2017/0181449
A1 [
96
], developed a pet-food product in which the food mixture (animal-based) was
positioned on a chew stick comprising of pizzle stick, giving it an appearance of shish
kabob, and providing a long-lasting chewing portion. A vegetarian alternative, U.S. Patent
No. 2017/0181448 A1 [
97
], used a plant mixture of a large variety of fruits and vegetables
for the base food that was then positioned on a rawhide chew. Intriguingly, the use of
probiotics to improve the intestinal microbiota of dogs has also been explored, even though
studies focusing on associated palatability are still scarce [
98
,
99
]. U.S. Patent No. 8,691,303
B2 [
99
] disclosed a method for dusting dry pet foods with a powder containing probiotics,
with the probiotic at least 10
5
CFU/g of the kibble. It is suggested that probiotic dusting
does not negatively hamper acceptance of dry pet food. It will be interesting to examine
how other trends in the human food industry continue to slowly seep into the pet food
industry and further increase the high correlation between them.
Foods 2021,10, 2599 16 of 19
6. Conclusions
While dry pet foods are the most popular pet food category purchased by pet owners
because of their long shelf life, ease of preparation, and low price, they tend to be low
in terms of their palatability to dogs compared to the wet and semi-moist types. Since
olfactory cues (aroma and flavor) are the primary drivers of a dog’s palatability, pet food
manufacturers add certain substances (or flavor enhancers) to pet foods to improve their
flavorful-ness and thereby its overall palatability by dogs. The present work provides a
summary of the current palatants used in the pet food industry and their impact on the
palatability of dog foods. In addition, it reviews the current methodology employed to
evaluate the sensory acceptance of these palatants from both pet and pet owner stand-
points. A promising approach lies in the direct addition of aromas and flavors, along with
traditional palatability-enhancing agents to increase pet-food palatability. Furthermore, the
employment of “humanizing” aromas, i.e., aromas commonly related to human foods (e.g.,
BBQ, grilled, or caramel-like aromas) could be valuable with respect to pet food acceptance
when added in quantities that are both pleasant to the pet owners and do not negatively
impact the pets’ acceptance of food.
Author Contributions:
Conceptualization, S.S.S., P.G.C. and H.-S.S.; Methodology, S.S.S. and H.-S.S.;
Writing – Original Draft Preparation, S.S.S.; Writing – Review & Editing, P.G.C., S.E.J.A. and H.-S.S.;
Supervision, H.-S.S. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Data Availability Statement: Not Applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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