What is HPP

HPP stands for High Pressure Pasteurization.  Also called hyperbaric pressure pasteurization or high pressure processing, it is used by many pet food manufacturers. 
 
HPP came into prominence in the US in the 1990s because scientists discovered that it was effective with three things:

1. Controlling food borne pathogens like salmonella and e-coli. 
2. Extending shelf life by destroying spoilage microorganisms (bacteria, yeast and molds)
3. Preventing undesirable reactions in food.  Those reactions involve enzymes.  

If you buy deli meat, hummus, guacamole, salsa, sausages, soups, sauces, sausages, salad dressings, baby food, jams, jellies, or juices, you’re likely getting something that’s been HPP'd.  
 
HPP also is used today by many pet food makers on raw dog food and cat food recipes in response to rising regulatory enforcement activities.   It produces substantial changes in the food as you can see from the graphic below.

Enzymes in Raw Food

Most enzymes are protein molecules.  Think of enzymes as catalysts for reactions.
 
One of the benefits raw feeders get really excited about are the enzymes in raw food.  
 
These aren't digestive enzymes.  They are the enzymes that would turn freshly slaughtered muscle into meat; produce off flavors, smells, or textures, or harmful substances as food gets old; or cause the browning in things like apples, avocados, or bananas  
 
In many ways, they are the enzymes that cause rot. But in fresh, raw food, they could help with the breakdown of that food once it is eaten.

HPP Inactivates Enzymes In Food

Research shows that the intense pressure of HPP can cause the complete and irreversible inactivation of enzymes in raw food.  This is how HPP prevents those undesirable reactions associated with rotten food. (5) (10)
 
The pressure at which irreversible inactivation happens depends on the enzyme, the food, pH, temperature, salinity, but it generally begins somewhere above 100 MPa. (5)

So by design, HPP eliminates a major benefit raw feeders love.

HPP Kills Bacteria By Disrupting Protein

This disruption of large protein molecules (like enzymes) is also how HPP kills bacteria, yeast and mold because all cell membranes - including those of pathogenic salmonella strains, commensal bacteria, probiotics, molds, and yeasts  - contain protein molecules.  

This image below is of a cell membrane. and those little hotdog shaped structures are the various types of proteins in cell membranes.  When these protein molecules become damaged from intense pressure, cells can die. This is the aim of HPP.

"Any pressure above 300 MPa, when you apply it, those are going to be irreversible changes to the proteins of those cells," says HPP expert Mary-Grace C. Danao Ph.D. (11)

How lethal HPP is to any cell depends on many factors.  The lower the pressure, the shorter the "dwell" time under pressure, the longer the food is held in refrigeration before or after HPP, and the fattier the food is, the less effective HPP is at killing any microorganism. (11) (12).  

Manufacturers conduct validation studies for each of their recipes to determine the HPP protocols that will effectively achieve their food safety kill steps.  Any change to ingredient supplier sourcing or recipes necessitates another validation study. (12)

Does HPP Kill All Bacteria?

Another thing raw feeders love are the commensal bacteria or probiotics that naturally occur in raw food.  If HPP's objective is to kill pathogenic bacteria and spoilage microorganisms, does it also kill commensal bacteria or probiotics?

Gram negative bacteria like salmonella are easier to destroy in HPP than gram positive bacteria like listeria because the cell wall of gram negative microbes (which contains those targeted protein molecules) is thinner.  Pet food companies utilizing HPP undergo validation studies to make sure that their HPP protocols effectively kill the thick cell-walled gram positive microbes like listeria too.  (12)

This means that most "good" bacteria is destroyed with the "bad."  The "good" and "bad" bacteria that may avoid eradication in HPP are the non-vegetative cells of spore-forming organisms.  Spore-formers are difficult to destroy because the spore forms a protective cover around it shielding it from the high pressure.

Spores go dormant during HPP but can germinate and grow later, potentially producing toxins like "bad" clostridium (C. botulinum and C. perfringens).  An example of a "good" spore-former is Bacillus Coagulans.  (It produces lactic acid, but isn’t the same thing as Lactobacillus, another type of non-spore forming probiotic that is vulnerable to HPP.)

HPP Creates Heat

Pressure does a lot of things.  It creates heat.   This is part of the indisputable 1st Law of Thermodynamics. We may remember it from high school science: “energy cannot be created or destroyed.” 

Nerd Alert:  How the Pressure of HPP Creates Heat As pressure increases, volume decreases. This forces molecules closer together.  Molecules collide more frequently and with greater speed, increasing kinetic energy which manifests as a rise in heat.   The heat dissipates as the pressure decreases and the volume increases back to its original state.  This heating via pressure is called adiabatic heating.

HPP Can Cook High Fat Foods

In HPP, the fattier the food, the more heat it creates.  Research shows, HPP’d fatty foods can gain about 8 – 9 degrees Celsius for every 100 MPa. (3) (7)

To put that in perspective, if we take olive oil out of the pantry and HPP it to 600 MPa it could climb to around 170 degrees Fahrenheit.  

Based on this research, if we were to HPP a ketogenic dog food (which is about 75% fat), it would go into HPP around refrigerator temperatures and could climb to about 130-135 degrees Fahrenheit before it came out of HPP back at the refrigerator temperature.  But 130 degrees F is medium to medium rare.  That's cooked. 

"Pressure-based preservation methods, especially HPP, have the potential to transform solid fat into its liquid form (due to adiabatic heating), which is more prone to oxidation."(4)

HPP Denatures Proteins Affecting Bioavailability and Leads to the Oxidation of Fats and Proteins 

HPP applies so much pressure that it blows apart cell walls and breaks the bonds of fat and protein molecules.  This unfolding of the structure of protein molecules is called denaturation.  

"It's not just temperature that can induce denaturation of proteins, but if you apply a high enough pressure, you can actually get [to a point] where the protein can be totally unfolded and remain unfolded even when you reduce the pressure," Danao (11)
 
When the disruption to the structure of fat and protein molecules happens, chain reactions that create free radicals begin. These free radicals lead to the oxidation of proteins and fats.  This is what causes rancid fats, losses in protein digestibility and bioavailability, and processing-induced toxicants. (1) (2) (3) (4)

A study on raw milk found that HPP at 200 MPa resulted in protein oxidation.  This oxidation increased with higher pressures.  (2)

HPP Contributes to ALEs, COPs, and Protein Carbonylation

Many modern pet parents are learning of the dangers of dietary ALEs, COPs and inflammatory protein oxidation compounds like carbonyls for themselves and their pets.  ALEs (Advanced Lipoxidation End Products), COPs (Cholesterol Oxidation Products), and carbonyls are diverse groups of highly reactive compounds with pathogenic significance in aged chronic disease.  They are tightly tied to chronic low level inflammation, oxidative stress, and diet related pathologies.

Research shows that rancid fat compounds that are precursors to ALEs, ALES, COPs, and protein oxidation compounds (like carbonyl groups) begin to form in HPP'd foods containing, poultry, fish, and meats between 300 - 500 MPa.  How much forms depends on a lot of factors.  (2) (3) (4)

The more fat in the food, the more ALE-precursors, ALEs and COPs are formed during HPP. PUFAs are especially vulnerable to oxidation – making HPP’d poultry, pork and fish recipes especially prone. (2) (3) (4)

"Meat, in particular, is highly prone to oxidation under pressure treatments. This is because certain water soluble components including iron, [copper,] and proteins like myoglobin, hemoglobin, and ferritin can trigger the oxidation process in unsaturated fatty acids," according to published research. (3)

This means that HPP pet foods containing red meats, and copper and iron rich organ meats like liver and giblets, would be more vulnerable to oxidation.

Some efforts to mitigate the aromatic indications of rancid fats (i.e. oxidation) have been attempted with vacuum sealing food during HPP and after in cold storage.  In some cases this has delayed the ability to perceive rancidity, though the reactions could still occur below sensory levels. (2) (9) While these findings may prove interesting for human foods, most pet food is not vacuum sealed.

"Although different meats and products show different effects of high pressure on their oxidative stability... [HPP] treatment between 400 and 600 MPa seems to be critical for oxidative damage in chicken breast, minced chicken meat, turkey thigh and whole beef muscle." (9)

Rancid Fats and Oxidized Proteins Increase in Cold Storage of HPP Foods 

Science shows that the chain reactions leading to oxidized proteins and fats continue in storage of the finished product too.  (3) (4) (2)

Studies have found that the amount of ALEs produced in some HPP'd meats and poultry after just 6 - 10 days in cold storage is equivalent to the levels resulting from cooking the food to internal temperatures of 176 - 212 F degrees for several minutes.  (3) 

That is well above the temperatures we would cook to at home.  A well-done steak has an internal temperature of 160 degrees.

Other studies using pressures between 300 MPa - 500 MPa have found that the amount of oxidized proteins in HPP'd seafood, lamb, mutton, beef and chicken increased during cold storage.  A number of these studies suggest that the amount and irreversibility of protein oxidation correlates to higher pressures.  (2)

The Role of Antioxidants

Without getting too far into the science behind oxidation reactions, when fats go rancid there are primary, secondary and tertiary reactions.  Unfortunately, vitamin E is not helpful in preventing these primary autoxidation reactions according to research.  (6)

Antioxidants like rosemary and sage extracts, some other spices, and carotenoids like beta carotene (pigments found in dark leafy green and orange root vegetables) are good preventers. But most makers of HPP’d pet food are not using those herb extracts, and spices; and pet food in general is seriously lacking in carotenoids. (6) (8)

HPP and Micro-Plastics

Another concern of HPP cat food and dog food is exposure to micro-plastics.  Pet food manufacturers assemble their entire recipe and then portion it into plastic bags.  In order for HPP to be effective, those plastic bags must have a certain degree of flex.  (11)  The entire plastic bag containing the food goes into HPP where it is subjected to tremendous pressure. There are concerns that this pressure could infuse microplastics into the food.

HPP Pet Food Makers Use High Pressures

So what is the pressure used by raw pet food manufacturers?

600 MPa  (87,000 PSI) for 3 - 6 minutes.  (11) (12)  Higher fat foods require longer treatments. (12)

600 MPa is six to seven times the pressure we would find at Challenger Deep - the deepest part of the ocean in the Mariana Trench.

It is the pressure we would find 38 miles below sea level -- well past the point in the Earth's mantle where the pressure and heat are so great that rock begins to melt. 

"There's nothing natural about these pressures."

Food manufacturers want to call HPP a gentle or mild process.  But “there's really nothing natural about these pressures,” according to Dr. Danao, who presented at the American Association of Feed Control Officials (AAFCO) in 2024.  (11)

Pet food makers use HPP because making unprocessed raw food comes with a serious regulatory risk.  The decision to use a mitigation strategy is wise.  But based on the science and common sense, we should be asking if HPP'd pet food is still raw.  Perhaps it belongs in its own category like sous vide, freeze dried (which is commonly HPP'd first), or dehydrated.  What do you think?

Resources:

1. Effect of high pressure treatment on the color of fresh and processed meats: A review.  K. H. Bak, et al 2019  
2. The effects of pressure-based processing technologies on protein oxidation A.M. Ganjeh, 2024 et al
3. Effects of high pressure processing on lipid oxidation: A review. I.G. Medina-Meza, et al
4. Effects of pressure-based technologies on food lipids oxidation. A.M. Ganjeh, et al 2024
5. Effect of high pressure on enzymes related to food quality. M. Hendrickx et al 1998
6. New Insights into the High-Pressure Processing of Meat and Meat Products. H. Simonin et al
7. Compression Heating of Selected Fatty Food Materials during High-pressure Processing Rasanayagam et al
8. Comparing beta-carotene, vitamin E and nitric oxide as membrane antioxidants, FQ Schafer et al
9. Aroma development in high-pressure treated beef and chicken meat compared to raw and heat-treated, Oct 2010
10. Pressure Induced Inactivation of Selected Food Enzymes March 1996
11. High Pressure Processing (HPP): Principles, Equipment, and Best Practices. Mary-Grace C. Danao, Ph.D. AAFCO 2024 Annual Meeting San Antonio, Texas, USA August 7-9, 2024
12. How to use High Pressure Processing (HPP) for Raw Pet Food Diets? |  2023 Webinar Hiberbaric / Instinct  / UNL / Pet Food Industry