Hydrolyzed protein is a solution derived from thehydrolysis of a protein into its componentamino acids and/orpeptides. Hydrolyzing down to the amino acid level is most commonly achieved using prolonged heating withhydrochloric acid.^[1] Hydrolyzing down to the peptide level can be achieved with anenzyme such aspancreaticprotease to simulate the naturally occurring hydrolytic process.

Protein hydrolysis is a useful route to the isolation of individual amino acids.^[1][2] Examples includecystine from hydrolysis of hair,^[3]tryptophan fromcasein,^[4]histidine from red blood cells,^[5] andarginine fromgelatin.^[6]

Protein hydrolysis release savory free amino acids (especiallyglutamic acid) and peptides.Hydrolyzed vegetable protein andyeast extract are commonly used asflavor enhancers (sources ofumami) as a result. The non-protein components in these products also contribute to the flavor.^[7]

Protein hydrolysis also increases their digestibility and rate of digestion.^[8][9] Some hydrolyzed beef protein powders are used for specialized diets for athletes.^[10]

Protein hydrolysis can be used to destroyepitopes involved in recognition byantibodies involved inallergy. "An allergen must have at least 2 IgE-binding epitopes, and each epitope must be at least 15 amino acid residues long, to trigger atype 1 hypersensitivity reaction."^[11] As a result, it has been used to reduce the allergenicity ofinfant formula: Reducing the size of cow milk proteins in the formula makes it more suitable for consumption by babies suffering frommilk protein intolerance. The US FDA has approved a label for this usage of partially-hydrolyzed proteins in 2017,^[12] but ameta-analysis published the same year shows insufficient evidence for this use.^[13]

Food allergies alsooccur in cats andin dogs. When the allergy is due to a protein in the food, the issue can be avoided by hydrolyzing the proteins into smaller components. Hydrolyzed protein is therefore used in specially formulated hypoallergenic pet foods. Partially hydrolyzed protein may also increase the development ofimmune tolerance. The increased protein digestibility is also potentially useful for pets with digestive issues such as inflammatory bowel disease, being in recovery from acute enteritis, and exocrine pancreatic insufficiency.^[14]

Hydrolysing a usually indigestible protein can provide an additional source of digestible protein. An example of this is hydrolyzedfeather meal, made fromkeratin-rich waste features from the poultry industry. With partial hydrolysis, it becomes a cheap, low-carbon source of protein that is digestible by ordinary livestock and pets. Although it retains the poor amino acid profile from feather, the good digestibility and high protein content makes it a good companion to other protein sources.^[15]

Hydrolyzing a protein improves the digestibility not only for animals, but also for soil microbes. As a result, it becomes useful as a fertilizer providingnitrogen and potentially alsosulfur. Hydrolyzed keratin from chicken feather is the most common type,^[16] though other kinds of waste proteins (e.g. waste wool, waste fish) are also re-used this way.^[17][18]

Hydrolyzed protein fertilizer is considered anorganic fertilizer.^[19] It contains a high amount of available nitrogen, and unlike unhydrolyzed feather meal, tends to release the nitrogen quickly.^[20] It helps the growth of soil microbes and improves soil quality.^[21]

Partially hydrolyzed protein can contain peptides that taste bitter, as more and more hydrophobic side chains are exposed. With soy protein, maximum bitterness occur at a mean size of 2 to 4kDa; further hydrolysis leads to reduced bitterness.^[14][22]

If a hydrolyzed protein for use in hypoallergenic food is not sufficiently hydrolyzed, it can still contain epitopes and trigger an allergic response.^[14]

Hydrolyzed protein has higherosmolarity than the original protein. As a result, it could make the food act more like ahyperosmotic laxative and lead to diarrhea. Infant formulas with hydrolyzed protein has been blamed for causing diarrhea. Dogs and adult humans do not appear to be affected, and dogs with gastrointestinal signs of allergy do see their stool improve on hypoallergenic diets.^[14]

• Acceptable daily intake
• Acid-hydrolyzed vegetable protein
• E number
• Food allergy
• Food intolerance
• Food labeling regulations
• Glutamic acid
• Monosodium glutamate
• Protein allergy

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