Protein Quality of Freeze-Dried Foods

I have written about freeze-dried pet food safety in a previous Science Dog essay. While we have a bit of evidence regarding risks of pathogen contamination in freeze-dried products, there has been little published research regarding the nutritional quality of freeze-dried foods or treats. This is an important question because freeze-dried foods and treats are increasing in popularity and because we often hear claims that freeze-dry processing results in reduced nutrient damage – in particular, less damage to the food’s protein. Until recently, we did not have any research to support or refute these claims.


This month, the Journal of Animal Sciences supplement published an abstract summarizing a recent study conducted by a team of nutritionists at the University of Illinois (1). This group is actively researching and writing about a wide variety of food and treat types – work that is incredibly helpful to dog folks everywhere. While this recent study was presented as an abstract (paper will be published in early 2023), I thought the results were important enough to provide the bare bones now.

The Study

Amino acid digestibility values of three freeze-dried dog foods were measured using a validated feeding assay. All three of the foods were produced by the same company, Primal Pet Food. The foods were: traditional freeze-dried nuggets (FDN), low-temperature freeze-dried nuggets (LTN), and a hybrid product that was a mix of the previous two (HN). Of the three foods, only the first, the freeze-dried nugget, is available for sale. Although the abstract does not state this, it is assumed that the latter two foods, produced using a lower temperature than traditional freeze-drying, were experimental. (Note: One of the many cool things about this study is that it allows us to compare the effects of processing temperature alone on a set of three foods).



Comparisons among the three foods showed no significant differences in amino acid digestibility values, and all of the values were high. Some were extremely good….. (one might even say, rock star values).

  • Essential (aka indispensable) Amino Acids: Always of greatest importance are the 10 essential amino acids. Most of these had digestibility values that were higher than 90%. For example, in all three products, methionine digestibility was between 93 and 95%; tryptophan digestibility was as high as 98 % (!!); and arginine digestibility varied between 94 and 96%.
  • A Bit Lower: A few of the essential amino acids had digestibility values that were less than 90 percent (histidine, lysine and threonine), but these still had very respectable values in the mid to high 80s.
  • Reactive Lysine: Measuring reactive lysine and calculating a ratio of reactive to total lysine (RL:TL) provides a helpful measure of heat damage to a food’s protein (see “How Reactive is Your Lysine for details about this test). The values for all three of the freeze-dried products were high which reflects a low degree of heat-damage to the protein in the foods.


This study found that the amino acids in three forms of a freeze-dried food were highly digestible and had low degrees of heat damage (as reflected by measurement of reactive lysine). The results also suggest that the slight differences that were observed among the three foods suggest reduced heat damage in the low-temperature product.

A Few Considerations

As always, it is important not to overstate the science. In this study, an in-vivo assay (roosters) was used to measure amino acid digestibilities. While this is an accepted and validated estimate of how these foods would do when fed to dogs, the ultimate test will be actual dog feeding studies.

Second, because only a single company’s foods were tested, we do not know how these foods would measure up when compared with other freeze-dried products, and perhaps more importantly, how these digestibility values compare with other food types. (Note: We do have quite a bit of evidence, from other studies, showing that protein/amino acid digestibility and availability values can be quite low in highly processed foods. We also know that heat damage, although not the only factor, is an important consideration when examining protein quality in a food).


Finally, it is commendable that the researchers identify the company that produced the tested foods in this abstract (Primal). If you are a regular reader of The Science Dog, you know that this is not always the case with canine nutrition research. Providing company and brand names in published works promotes transparency in the pet food industry and can help the dog folks who need this information to make more informed choices for their dogs.


Cited Study: Oba PM, Utterback PL, Parsons, CM, Templeman, J, and Swanson KS. Standardized amino acid digestibility of three formats of freeze-dried dog foods using the precision-fed cecectomized rooster assay. Journal of Animal Sciences, 2022; Volume 100, Supplement S3, Pp. 48.

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