The hydrolysis process and what it does to protein
Protein hydrolysis is the breakdown of intact proteins into smaller peptides and free amino acids using either acid hydrolysis or enzymatic digestion. In commercial pet food production, enzymatic hydrolysis is the dominant method — specific proteases (trypsin, chymotrypsin, alcalase, papain, or proprietary enzyme blends) are applied to the source protein under controlled pH and temperature conditions. The reaction is allowed to proceed until the average peptide molecular weight reaches the target range, then the enzymes are heat-inactivated and the hydrolysate is dried for incorporation into the kibble.
The functional consequence: an intact chicken protein with molecular weight ~50,000 Da can be hydrolyzed to peptides averaging 1,000-3,000 Da (extensively hydrolyzed) or 5,000-10,000 Da (partially hydrolyzed). Per Cave 2006 (Journal of Nutrition) review of veterinary hypoallergenic diets, the practical threshold for IgE-binding loss is around 10 kDa — below that size, most dogs with cutaneous adverse food reactions tolerate the protein without allergic response.
Why molecular weight matters for allergy management
Per Olivry 2015 (BMC Veterinary Research) on the ICADA-defined cutaneous adverse food reactions criteria, food allergy in dogs is most often an IgE-mediated hypersensitivity to specific protein epitopes — recognized regions of the intact protein structure. The dog’s immune system has produced IgE antibodies that bind these epitopes, triggering mast cell degranulation and the cascade that produces pruritus, otitis externa, and gastrointestinal signs.
When the protein is enzymatically cleaved into small peptides, the epitopes are physically destroyed — the linear sequence is broken, and any conformational (folded-structure) epitopes are unfolded. Per the Olivry 2010 (BMC Veterinary Research) systematic review of canine food allergy management, this physical destruction is sufficient to prevent IgE recognition in the majority of allergic dogs. Roughly 80-90% of dogs with confirmed cutaneous adverse food reactions tolerate extensively hydrolyzed diets without allergic flares per the Cave 2006 review.
Commercial hydrolyzed prescription diets
The major commercial hydrolyzed therapeutic dog foods are veterinary prescription products. Hill’s Prescription Diet z/d uses extensively hydrolyzed chicken liver as the primary protein source, with target peptide molecular weights below 3 kDa per the manufacturer’s technical documentation. Royal Canin Hydrolyzed Protein uses hydrolyzed soy. Royal Canin Ultamino uses ultra-low molecular weight hydrolyzed feather protein for severe allergy cases not responding to standard hydrolysis. Purina Pro Plan Veterinary Diets HA uses hydrolyzed soy isolate. Blue Buffalo Natural Veterinary Diet HF uses hydrolyzed salmon (the only commercial fish-based hydrolysate at typical pet-food scale).
Per the AAVCN 2024 Position Statement on Veterinary Therapeutic Diets, hydrolyzed prescription products require veterinary authorization for retail purchase. The prescription requirement reflects the AAFCO “intended for therapeutic purposes” classification rather than a regulatory bar — the products are not pharmacological drugs, but their use is intended to be supervised by a veterinarian for diagnostic and therapeutic reasons. See our dog food for allergies guide and puppy food for allergies guide for KibbleIQ’s pick context.
Hydrolyzed vs novel-protein elimination diets
Per Olivry 2010 systematic review, the two diagnostic approaches to canine cutaneous adverse food reactions are hydrolyzed elimination diets and novel-protein elimination diets. Both target the same outcome (IgE non-recognition of the dietary protein) via different mechanisms.
Novel protein diets use a protein the dog has never been exposed to — rabbit, venison, kangaroo, alligator, or duck (in the US market, where chicken and beef are baseline exposures). Per the Olivry 2010 review, the diagnostic accuracy of novel-protein diets is compromised by widespread cross-contamination in commercial pet food manufacturing — many “single protein” foods test positive for multiple unlabeled proteins. Per Raditic 2011 (Journal of Animal Physiology and Animal Nutrition), 4 of 12 commercial novel-protein dog foods tested contained protein not declared on the ingredient label.
Hydrolyzed diets do not have this cross-contamination concern because hydrolysis destroys IgE recognition of any contaminating protein at the same molecular weight threshold. For ongoing therapeutic management (as opposed to one-time diagnosis), hydrolyzed prescription diets are generally preferred per the AAVCN 2024 statement.
Limitations and special cases
Per Olivry 2010 systematic review, roughly 10-20% of dogs with confirmed cutaneous adverse food reactions still react to extensively hydrolyzed protein at the standard 10 kDa threshold. Per the Olivry 2017 update (Veterinary Dermatology), residual reactivity in those dogs is typically managed by switching to ultra-low molecular weight formulations (Royal Canin Ultamino at less than 1 kDa average) or to amino acid-based diets (where free amino acids replace any peptide content). For dogs with severe IBD or food-responsive enteropathy, hydrolyzed diets are also used in non-allergy contexts as part of multimodal management per the ACVIM 2022 GI consensus.
How KibbleIQ scores hydrolyzed prescription diets
The KibbleIQ Dry Kibble Rubric v15 evaluates hydrolyzed prescription diets on the same rubric as standard formulations — ingredient quality, processing, AAFCO substantiation, preservation. The therapeutic intent of the hydrolysis (diagnostic elimination, allergy management) is not explicitly weighted; the rubric scores ingredient quality, not pharmacological intent. As a result, hydrolyzed prescription diets often grade in the C-B range under the standard rubric since the hydrolyzed protein source typically scores lower on whole-protein quality metrics — the trade-off is exactly what makes them therapeutic. Per the KibbleIQ Methodology page, the rubric grade and the clinical appropriateness for an allergic dog are two different evaluations — a C-grade hydrolyzed diet may be exactly the right food for a dog with confirmed cutaneous adverse food reactions, while an A-grade non-hydrolyzed food might trigger flares. See our Hill’s z/d review for a worked example. To check the ingredient quality of a hydrolyzed product, paste the ingredient list into the KibbleIQ analyzer.