High pressure processing (HPP), also called high pressure pasteurization or cold pasteurization, is a food processing technique that has been used in the human food industry for years. Products that are routinely treated with HPP include ready-to-eat meats and meals, fruit juices, packaged dips, and jams and jellies.
Another example – If you enjoy munching on packaged guacamole and salsa with your Friday night margarita, you have consumed at least two HPP-treated foods. Add some sour cream, and you are up to three.
For dogs – if you feed a commercial raw dog food, there is now a reasonable chance that your dog is also consuming an HPP-treated product. Since passing of the Food Safety and Modernization Act (FSMA) in 2011, many raw food producers have started to use HPP to ensure compliance with the act’s zero tolerance stipulation regarding Salmonella contamination. HPP is not required of raw food manufacturers, but rather, it provides a viable means of increasing food safety without the need for heat treatment or irradiation.
What Exactly IS HPP?
High pressure processing is a non-thermal, high pressure food processing treatment. It is used to preserve products that are not destined to be heat treated to temperatures that would normally kill potentially pathogenic microbes. In the pet food industry, HPP may be used with raw (frozen), freeze-dried, and most recently, fresh-frozen/gently cooked foods. In addition to reducing a product’s bacterial load, HPP also functions to increase the shelf life of treated foods.
The actual process involves applying high pressure – typically around 6,000 bar – to a packaged food submerged in a water tank. Pressure is applied for several minutes, which leads to microbial cell wall damage. Organisms that are most sensitive to pressure are molds, yeast and parasites, while bacteria require higher levels of pressure for inactivation. Currently, both the FDA and the USDA identify HPP as the preferred method for ensuring food safety in raw pet foods.
Research Studies with Pet Foods
Although HPP is used by a number of pet food companies, surprisingly little research has been conducted to study its effect on pet foods. Two questions are of interest:
- Does HPP reduce or eliminate microbial load and improve the safety of raw foods?
- Does HPP alter the nutritional value of raw foods?
Although we certainly need more research, there are three published studies that provide us with a few answers (1,2,3).
HPP and Pathogen Reduction
In 2016 researchers at Colorado State University tested the effectiveness of HPP for reducing microbial load in a beef-based raw dog food (1). The food was inoculated with a pre-measured concentration of a mix of non-pathogenic Escherichia coli test species. This approach had been previously validated as a reliable method for estimating the response of pathogenic Salmonella species in foods. Following inoculation, the product was packaged into rolls, which were then treated with a standard HPP protocol. Following treatment, samples were tested for microbial levels (counts) at 24 hours post-HPP and following 5 days of frozen storage post-HPP.
Results: The results provide some good news and a bit of bad news for raw food feeders:
- Good News: HPP processing greatly reduced surviving numbers of microbes in the treated food. Interestingly, freezing and storing the food for 5 days prior to testing resulted in additional microbial deaths and even lower numbers of viable bacteria. This suggests that bacterial cells were damaged during HPP, causing cellular injury that led to continued cell death during freezing and storage. It is of note that more than 90 percent of the samples had microbial levels that were less than the lowest detectable level.
- Bad News: Unlike some forms of food processing that completely eliminate all pathogens, HPP does not appear to achieve total death or sterilization of the product. (This result is of concern given the zero-tolerance standard that food companies are currently required to meet by FSMA).
HPP and Nutritional Integrity
A concern that some dog owners have about HPP is that it will alter the nutritional integrity of the food. Specific worries focus on the effects of HPP on the quality of the food and on the food’s protein.
Two recent studies have examined these issues. The first was a feeding study in which digestibility values were measured in dogs fed either a commercial dry dog food or an HPP-treated raw food. The second was an in vitro (laboratory) study that measured changes in chicken meat digestibility when subjected to thermal (heat) treatment versus HPP treatment.
Feeding Study: In a switch-back design, digestibility values of a dry, extruded food and a raw food treated with HPP were compared when fed to a group of 10 healthy adult dogs (2). A disclaimer – This was not a well-controlled study. The switchback design had a few issues and the two foods differed in multiple ways, not just in the type of preservation method that was used. However, it is a feeding study of a raw food treated with HPP, which we need. Results: The digestibility coefficient (%) of the dry food was very low (less than 60 percent), while the digestibility value of the HPP-treated raw food was very high (greater than 90 percent – a rock star value for digestibility). This difference may have had numerous causes, so we cannot make conclusions in terms of comparing the two foods.. What we can say however, is that in this study, the raw dog food that was treated with HPP had a very high digestibility coefficient when fed to dogs.
Not much, but it is a bit of helpful data.
In Vitro Study: This study was well controlled (3). The researchers used a validated in vitro method to measure the digestibility values of chicken meat that was treated with different preservation methods. These included thermal (heat) processing at three different temperatures, HPP treatment, and two other non-thermal treatments. Results: Thermal processing of chicken meat led to decreased protein digestibility values that were inversely related to cooking temperature and cooking time (i.e. as temperatures and duration increased, protein digestibility decreased). The change was dramatic; ~ 86 % protein digestibility for raw chicken compared with 65 % protein digestibility in chicken treated at the highest temperature for the longest time. These results were interpreted to reflect increased protein damage with higher heat treatment (see “It’s Maillard, Not Mallard” and “How Reactive is Your Lysine“). In contrast, the protein digestibility of HPP-treated chicken meat remained high and did not differ significantly from the digestibility of raw chicken meat (86 % compared with 84 %). These results indicate that the nutritional value of chicken meat was well maintained when HPP was used, but decreased when thermal processing was used.
Take Away for Dog Folks
So, for those of you who choose to feed a raw diet to your dog(s), the results of these three studies suggest that:
- HPP is an effective processing treatment that significantly reduces (though it does not completely eliminate) a raw food’s bacterial load.
- HPP-treated raw foods retain a high level of nutrient (protein and dry matter) availability.
Given the information that we currently have, a practical (and evidence-based) recommendation to those who wish to feed a raw food is to select products from manufacturers who are using HPP (preferably along with other food safety controls, such as test and hold).
After all, if you enjoy packaged guacamole, salsa, and sour cream, and take it for granted that these foods are wholesome and are not going to make you sick, why not give your dog the same assurances?
- Hasty J, Woerner DR, Martin JN, et al. The use of high pressure processing as a pathogen reduction tool in raw pet food. Meat and Muscle Biology 2016; 1(2)119.
- Neshovska H, Shindarska Z. Comparative study of the digestibility of dry and raw food in dogs. International Journal of Veterinary Sciences and Animal Husbandry 2021; 6:(2):01-03.
- Kim H, Jung AH, Park SH, et al. In vitro protein disappearance of raw chicken in dog foods decreased by thermal processing but was unaffected by non-thermal processing. Animals 2021, 11;1256. doi.org/10.3390/ani11051256.