Increasing the palatability of pet food

Ingredients
The ingredients of a pet food are the means by which Hill's Pet Nutrition achieves the twin goals of good nutrition and high palatability.

Type of ingredients
Both dogs and cats prefer ingredients of animal origin, and increasing the animal protein and/or fat content will often increase their liking for a food. Carbohydrates do not provoke an equally strong stimulus to eat.

The appetising flavour of meat is due largely to the presence of particular amino acids, nucleotides and inorganic salts, and in particular to nitrogen-sulphur compounds such as cysteine and thiamin and the peptides anserine, carnosine and glutathione.

When meat is cooked, even more intense flavours can be produced by,
• the breakdown by heat (pyrolysis) of amino acids and their interactions with sugars and carbonyls (controlled Maillard reactions) which form a variety of cornplex organic compounds, and
• the release of volatile heterocyclic compounds derived from reactions with lipids.

A number of studies have recorded the food preferences of pets, e.g.
• more cats prefer beef tallow to vegetable oils, and
• the preference of dogs for different meats can be ranked in the order, beef > lamb > chicken.

However, individual animals show considerable variation in their preference for animal-derived ingredients, which limits the value of such studies. For instance, it was found that sardines are highly palatable to some cats (fitting the usual perception that cats prefer fish) but rejected by others.

Preferences can also be affected by processing, with complex interactions between ingredients in the processing of pet foods rendering a previously preferred ingredient unattractive.

Quality and freshness

Palatability is influenced by both the quality and freshness of ingredients, which dogs and cats can readily cletect.

Hill's Pet Nutrition has stringent ingredient specifications, backed up by regular testing and considerate handling during storage and manufacture, which ensures that all products are of high quality. To maximise freshness, many ingredients are incorporated in Hill's foods within hours of their production by suppliers. And taste tests show that this emphasis on freshness is reflected in the products high palatability scores.

Flavorings

Just as the cook at home adds salt, herbs, spices, stock cubes, onion, garlic, lemon, and synthetic essences (e.g. vanilla and caramel) to enhance the palatability of natural ingredients, so does the pet food manufacturer

Flavour enhancers which may he used in pet foods include:
• salt - which is cheap and the most widely used flavouring in pet foods, sometimes used at a level ten times the maximum nutritional requirement (that needed for growth)
• fats and oils
• onion and garlic (although these are used they are not recommended because of their potential toxicity in pets –and they may not be welcomed by pet owners!)
• amino acids -glycine, L-cysteine and Llysine
• 5' -nucleotides -present in large amounts in yeast
• monosodium glutamate -this is naturally present in soy sauce but may be created by the reaction of salt and wheat gluten during extrusion processing
• maltol
• sugar (dogs)
• organic and inorganic acids, e.g. phosphoric acid - dogs like acidic flavourings and they are greatly preferred by cats -one reason is probably that they provoke salivation which, by moistening, helps to release other flavours and softens and lubricates dry food.

Combinations of some of these flavour enhancers are synergistic, for example 5' –nucleotides and monosodium glutamate. Also the amount that is used is critical -too little is ineffective, too much is unpleasant.

These flavourings can either be incorporated into the food or, in the case of dry foods, applied to the surface. Most dry pet foods are coated with 'digests' produced from combinations of meats, offals and yeasts by the action of heat and/or enzymes. This produces highly palatable meat extracts or essences, with concentrated flavours. Digests may be applied as a liquid, sprayed on to the dry food kibble or they can be dried and applied as a powder. As with ingredients, freshness is critical in achieving maximum acceptance, which is why Hill's prepares digests for its dry foods freshly each day using natural ingredients.

However, an unpalatable food cannot be made palatable simply by the addition of flavouring agents, just as changing the shape and size of the kibbles cannot overcome the use of low quality ingredients in attracting pets to eat. Flavourings should be used to enhance a nutritionally balanced pet food and increase the enjoyment of the pet. In the case of animals with a diminished appetite flavourings can help to encourage acceptance and stimulate eating. However, excessive levels of some flavourings, e.g. salt and fat, can result in a nutritionally unbalanced food which may lead to health problems (e.g. hypertension and obesity)

Hill's Pet Nutrition uses only natural flavourings, i.e. flavour preparations made from animal or plant ingredients.

Off-flavours

Off-flavours are those tastes and odours that adversely affect the palatabiIity of food. They may arise from flavour imbalance, chemical reactions within the food, environmental contamination and from packaging.
• Certain flavouring compounds can become bound to ingredients making them unavailable for sensory perception (so-called 'flavour sinks') which allows other flavours to predominate, e.g. sulphur-containing flavourings, added to provide a meaty taste, can be covalently bound to sulphur-containing side chains of proteins and peptides. Fats, by dissolving fat-soluble flavour compounds such as onion and garlic oils, can restrict the release of their volatile flavours, and starches can act as ‘flavour sinks’ for sugars.
• The oxidation of unprotected fat during storage, producing rancidity, is probably the best known example of a chemical reaction in food - a variety of organic compounds, including aldehydes, ketones and esters, are formed giving a characteristic bitter taste. Sometimes this is activated by light; linoleic acid is particularly susceptible to photo-oxidation, leading to the production of high levels of hexanal. Lipolysis, proteolysis and non-enzymatic browning are other chemical reactions that can result in malodours and unpleasant tastes.
• Microbial contamination (bacteria and moulds) can result in unattractive mouldy and musty flavours, and volatile organic compounds leaching from some types of packaging (plastics, printing inks or coatings) can cause equally undesirable off -flavours.

At all stages - formulation, manufacture and storage - Hill's Pet Nutrition takes steps to avoid the possible development of off-flavours.

What determines food and flavour preferences?

Although some reactions to flavours are instinctive, and there may be genetic influences, food and flavour preferences are determined in large part by experience.

Examples of innate behaviour include:
• the rejection of bitter compounds, which is a natural mechanism to protect against the ingestion of toxic substances, and
• eating only a small quantity of a new food initially to assess its safety (neophobia: fear of anything new), often evident in cats; in extreme cases a new food may be rejected persistently.

As animals mature, instinctive behaviour is modified by experience. Animals become not only willing to try novel foods but seek them out, provided that they have good palatability and are not too dissimilar to already familiar foods. New foods are eaten in greater amounts, and may eventually be preferred. However, aversion will develop rapidly to any taste or smell that is associated with an adverse experience, such as vomiting, that occurs up to 24 hours after eating.

Older pets may require stronger flavours to overcome sensory losses and maintain their appetite.

Other factors affecting palatability

The effect of the manufacturing process
The manufacturing process has an influence on various nutritional factors, including palatability.

Dry food manufacture
Most corn mercially-prepa red dry pet foods are manufactured using a batch system to blend the ingredients into a ‘dough’ which is then extruded. Extrusion is a process in which the dough is forced through a die-plate, simultaneously cooking it by friction and shaping it. A rotating knife blade then cuts it into individual kibbles. The high temperature generated by extrusion cooks the food (improving its digestibility), destroys microorganisms and other anti-nutritional factors, and also increases its palatability. Trials have shown that there is an optimal cooking time for greatest palatability, which Hill's Pet Nutrition employs in its manufacturing plants. The kibbles are then dried and enrobed with a flavouring before being packed.

Canned food manufacture
Commercial production again uses a batch system. Meats are ground and mixed with other ingredients before cooking in the optimum time/temperature combination to maximise flavour. The food is then placed into cans, which are sealed and passed through a retort (resembling a giant autoclave) to sterilise the food. After cooling, cans are labelled and packed.

Packaging
Packaging has a vital role to play in preserving the freshness and palatability of the food, and protecting it from contamination.

Performing the packaging process in a controlled environment limits the food's contact with oxygen, to avoid the oxidation of fats which will adversely affect its flavour, and the use of high quality packaging materials avoids the possibility of tainting the food (i.e. contributing off-flavours).

Recent developments in packaging have resulted in alternative formats, such as pouches, although there is no direct evidence that they improve a food's palatability.

The role of antioxidants
Food antioxidants are added to dry foods to prevent fats becoming rancid, thereby maintaining the food's palatability and prolonging its life. These antioxidants are different to the biologically active nutritional antioxidants which provide health benefits. For example, the biologically active form of vitamin E is alpha-tocopherol, whereas gammaand delta-tocopherols are more effective in preserving fat. Pet foods that are advertised as being preserved with vitamin E will not necessarily also provide antioxidant protection for the pet.

In addition to using food antioxidants, nutritional antioxidants are added by Hill's Pet Nutrition to dry Prescription Diet and Science Plan foods in a unique combination which provides health benefits by protecting against oxidative cell damage caused by free radicals.

Antioxidants are not needed in canned foods because they are exposed to air for only a short time.

What can you do to help your pet enjoy its food?

There are several things that an owner can do to increase a pet's enjoyment of its food,
• Store the food carefully to preserve its freshness and flavour, i.e. reseal opened bags and cans to minimise oxidation and avoid contamination with offflavours, such as disinfectants, bleaches and household cleaners
• Refrigerate opened cans and use within 5-7 days
• Ensure that feeding bowls are clean and free from detergent residues
• Avoid leaving wet food in the bowl uneaten; it becomes stale and may attract flies
• Provide a quiet and undisturbed eating environment
• Site a cat's feeding bowl away from its litter tray

For dogs, to release extra flavour from a dry food, moisten it with water at blood heat, and let it soak for ten minutes before feeding. (This isn't recommended for cats because they so dislike sticky food.)

Courtesy of Hill's Pet Nutrition, Inc. © 2003 Hill's Pet Nutrition, Inc.