There is continued interest among consumers in purchasing quality products that support or guarantee the health and well-being of pets. Descriptive words are used to communicate product attributes like flavor, recipe, palatability, aroma, nutritious, complete and balanced. The terms hydrolysis or hydrolyzed are not typically among these descriptors. The definition of hydrolysis is likely unknown or misunderstood by consumers, yet hydrolyzed proteins are widely used in the pet food industry.
Definitions of hydrolysis and its place in pet food
One consumer definition of hydrolysis is what happens when a substance is split into smaller compounds. The Association of American Feed Control Officials’ (AAFCO) feed term defines “hydrolyzed” or “hydrolyzing” as a process where complex molecules are split into simpler units by chemical reaction with water, usually by catalysis. There are accepted AAFCO ingredient definitions for hydrolyzed casein, hydrolyzed whey, hydrolyzed poultry byproducts, hydrolyzed soy protein, hydrolyzed whole chicken, hydrolyzed whole pork, hydrolyzed feathers and hydrolyzed yeast. These definitions do not describe the extent of hydrolysis nor the method.
Proteins are largely major themes among pet foods (type, quantity, percentages, limitation). Approximately 20 amino acids (essential and non-essential) contribute to the building blocks of proteins and provide different levels, balance and bioavailability. Proteins are of different lengths and shapes and are measured in Daltons (not inches and centimeters). Proteins can be several hundred thousand units long before hydrolysis. The tertiary structure or three-dimensional structure varies depending upon the peptide content and side chains.
From a food science point of view, proteins provide nutrition, palatability and functionality in products. Proteins are used in canned, baked and extruded applications. Proteins can be used for product structure, or they may be used to develop emulsions, foams and gels, and to add firmness in products. Modified proteins or hydrolyzed proteins will influence the functionality range of the protein. In canned applications, plasma is used to provide structure by its gelling capability, nutritional content and physiological effects. Meat tissue is an arrangement of protein containing connective tissues and myofibrils in muscle designed for contraction. These same protein components provide unique textures. Milk and whey proteins are colloidal, creating semi-solids or soft materials like yogurt or cheese. Some proteins are considered plastic-like and are useful in molding and baking, while others are more amorphous for gelling and texture. Still other proteins provide structural development in products to support dental cleaning and chewing. Proteins are also connected to aroma and taste development (palatability), color, allergenicity and digestibility.
In human foods, hydrolyzed proteins are seen with growing interest as “bioactive peptides.” Some of these peptides have been shown to have beneficial properties in hypertension, inflammation, immunity, and as antimicrobials and antioxidants. These health impacts are linked to the specific amino acid composition and sequence. There are suggestions that protein hydrolysates may delay and/or prevent immune-related diseases in humans. Mildly hydrolyzed proteins are used in sports nutrition to improve bioavailability. Extensively hydrolyzed proteins have been used in infant formulas as hypo-allergenic alternatives to cow’s milk proteins.
Hydrolysis of proteins
Hydrolysis of proteins will vary depending upon the types of enzymes or acids used but also on the time, pH, temperature and pressure of the process conditions. Some (not all) hydrolyzed proteins can become bitter. Further hydrolysis may remove this bitterness. Knowing the desired endpoint is critical for the targeted purpose and the consistency of the output.
The extent of hydrolysis is determined by the number of cleaved peptide bonds and is called the degree of hydrolysis (DH). DH does not indicate the presence or absence of specific bioactive peptides, nor does it describe the length or shape of a protein. DH is controlled by process specification to certain desirable endpoints. Hydrolyzed proteins are then made up of individual amino acids, dipeptides, tripeptides and longer polypeptides of various lengths.
Several veterinary pet food products use hydrolyzed proteins as primary protein sources. One food uses hydrolyzed chicken for dogs with food sensitivities, claiming it helps support the immune system and normalizes the stool. Another product uses hydrolyzed soy protein isolate for dogs with food allergies, dermatitis, food intolerance, irritable bowel disease (IBD) or pancreatitis. Two companies use hydrolyzed salmon as low molecular weight proteins for dogs with food sensitivity.
Dermatological issues remain a major issue in dogs, with 15% to 30% of the world’s population impacted. Among the pruritic cases, nearly 20% of the dogs have true immune-mediated reactions to antigens present in food with cutaneous and gastrointestinal signs. In human nutrition, it is suggested that food sensitivity or allergy of specific proteins can be reduced or eliminated if the overall size of the protein is 3% to 5% of the original. One set of authors states that pet foods with protein of “shorter length” are less likely to elicit an immune response as the hydrolyzed protein is made up of free amino acids and small peptides. Several authors have observed clinical improvements with hydrolyzed protein in dogs with known allergies. Hydrolysate-containing foods have helped in dermatologic cases, alopecia and IBD. Even extensively hydrolyzed feathers have been shown to have antigenic properties in dogs.
Protein hydrolysis has also been used in the pet food market for decades to create new taste and aroma cues to stimulate intake in pets. Hydrolysis of specific sources of chicken, pork and fish create unique palatants that are used widely in the industry across most foods.
Hydrolysis of proteins also improves the digestibility of proteins by cleaving the overall protein length and shape into smaller sizes. With more complete hydrolysis, digestibility (bioavailability) can rise dramatically.
The picture of hydrolyzed proteins and nutrition
A few years ago, protein nutrition and food science misunderstanding and over-simplification by a few hurt the pet food industry when legume proteins were targeted as direct causative factors in DCM (dilated cardiomyopathy) in dogs. One wonders what happened to the cross-pollination of these sciences to further examine these observations more thoroughly. Clearly understanding health is connected to the content, balance, bioavailability and fortification of nutritional content, but it is also connected to the structure of proteins, fats, starches and fibers. To see nutrition simply as content is to see only toothpicks in a forest. To blame a classification of protein ingredients as the causal agent is like blaming ash for urinary tract disease (as was done in the 1980s with “low ash” cat foods).
As with most things, we must step back and see the bigger picture. Food structure is three-dimensional. Proteins are three-dimensional.
- The primary structure is the amino acid sequence in the protein chain. Amino acid content and bioavailability are a normal focus in animal nutrition.
- The secondary structure describes the “folding patterns” of a protein (alpha helix, beta sheets, hydrogen bonding).
- The tertiary structure is the overall three-dimensional shape of the protein peptide chains.
Starches are like this. Fibers are like this. They are linear, branched, cross-linked and bound by tight hydrogen bonding. Hydrolysis changes the basic tertiary shape of a fiber to allow more water absorption. Hydrolysis can change a starch from being sticky to free-flowing.
Enzymatic, acid hydrolysis or other processes can impact protein structures, which can lead to multiple nutrition and health advantages or disadvantages. Over-drying protein can severely reduce the bioavailability of the protein through direct reshaping of the protein. Shape can limit potential enzyme penetration and ultimate availability in digestion. The tertiary structure can be “denatured” with the protein unfolding and losing its function and physiological impact. Proteins can react with sugars to create Maillard (browning) reaction products, which can provide positive taste cues. Alternatively, as mentioned before, some hydrolysis may induce bitterness which can be overcome through further hydrolysis or masking.
Going forward, hydrolyzed proteins will offer opportunities to create many new products with flavor and eye appeal but will support nutrition and health innovation for pets into the future.