Flow Diagram of HPP

Process flow below shows how HPP is used; 

What is High Pressure Processing?

High Pressure Processing also commonly called as HPP is a cold pasteurization technique by which products that was already sealed in its final package will undergo high pressure by introduced into a vessel and subjected to a high level of isostatic pressure (300–600MPa or 43,500-87,000 psi) transmitted by water. Figure below shows the range of pressure assigned to different conditions.

Pressures above 400 MPa / 58,000 psi at cold (+ 4ºC to 10ºC) or ambient temperature inactivate the vegetative flora (bacteria, virus, yeasts, moulds and parasites) present in food, extending the products shelf life importantly and guaranteeing food safety.

Limitation : Product That Is Not Suitable Using HPP

Although HPP can pose lot of benefits, however it can't be used on certain types of food product such as ; 

Spices, powder and extracts

Dry products such that are not suitable for HPP because of isostatic pressure needs water inside the food product for its homogeneous, uniform, efficient transmission and consequently to cause an inactivation of the microorganisms present in the product. Powders and spices have;

• too low water/humidity content
• too low Aw values

HPP will create a more compact matrix, but the pressure will not be transmitted throughout the product and will not tackle the microbes.

Dried nut, fruit or cereals

Similar to the above mentioned case. Such products have low water content and low values of water activity. HPP is not effective in extending the shelf life or generally improving the safety of such products.

Whole fruits

HPP makes all the sense if you clean a fruit; slice it, chop it or dice it; mix it perhaps with other ingredients; pack it in adequate flexible, sealed formats… and then high pressure process it for shelf life and safety while maintaining its attribute.

But it does not really make sense for whole, raw fresh fruit. Fresh pieces of produce do not constitute such a high value product, and more importantly, the isostatic pressure causes certain changes in the texture and in the cell walls. Raw, fresh fruit submitted to high pressure will certainly have a great quality from the microbial point of view; but with the noticeable changes in the fruit matrix and the evacuation of all the air that was trapped in the fruit flesh, it is not a marketable proposition, in general.

Vegetable leaves and leafy salads

Similarly, HPP does not pose a good solution for leafy vegetables. Isostatic pressure provokes certain changes in the structure of the vegetable cell walls. After HPP, leaves of i.e. spinach, rocket, lettuce etc. appear more mushy, more flaccid and more exudative. High Pressure Processing is, on the other hand, a fantastic value proposition for wet salads and vegetable based sandwich fillings

Bread & pastries

The comments below apply to any product with a lot of trapped air, fluffy or with a mousse-like structure:

Basically if you put a fan of bread, or a croissant, through an HPP system, you will obtain a flat, collapsed piece of pastry. Why? Breads and pastries have a huge amount of “empty spaces” in their matrix, a lot of trapped air. Under isostatic compression in high pressure conditions, all that air is compressed until it occupies no space. When we decompress and unload the product from the machine, we can realize that such product could not possibly have enough “plasticity” or “compressibility” as to go back to its previous volume and shape. All that air that was originally inside of the product, is now outside of it, occupying the volume around it, inside of the package.

Raw fresh meat

This topic is of considerable importance and though high isostatic pressure induces changes in the texture and color of raw meat, there are important potential and current applications in this space. We will focus on this question in a next entry of our blog. To open mouth, a few examples of current commercial applications with raw, never cooked meat:

•       Cargill Fressure ground beef
•      Zwanenberg Filet Americain and Leverwurst
•      Research on Innovative HPP meats and Marinated Meats  

Disadvantages of HPP

Some of disadvantages of using HPP is listed below;

• Bacterial spores are resistant to the high pressure
• Some food enzyme also resistant to high pressure such as polyphenoloxidase and pectic esterase which these two are contribute to food spoilage
• Required low temperature storage to retain their sensory qualities
• Residual of oxygen and enzyme activity can lead to degradation of food
• Required aseptic packaging  

Techniques and Equipment used in HPP

High-pressure technology of foods in similar to conventional heat processing, using two main types of industrial equipment. One is a batch system for processing packed foods.  A typical high pressure system consists of a pressure vessel and pressure generating device. Food packages are loaded into the vessel and to top closed. The pressure medium, usually water containing a small amount of soluble oil, is pumped into the vessel from the bottom. Once the desired pressure can be maintained without further need for energy input. The process is isostatic, so pressure is transmitted rapidly and uniformly throughout both the pressure medium and the food with little or no heating. It is equal from all sides so there is no ‘squashing’ effect and product is not affected. Pressure in the range of 300, 700 MPa are used for foods (500 MPa is equivalent to 20 family cars bearing down on an area the size of postage stamp). These pressures, maintained for approximately 15 minutes, can cause more than a 10,000 fold reduction in numbers of food poisoning bacteria such as Salmonella, Compylobacter and Listeria in milk and poultry meat.

Principle of HPP

Any phenomenon in equilibrium (chemical reaction, Phase transition, change in molecular configuration) accompanied by a decrease in volume, can be enhanced by pressure. Thus HPT affects any phenomenon in food systems where volume change is involved and favors phenomena, which results in a volume decrease. The HPT affects non-covalent bonds (Hydrogen, Comic, Hydrophobic bonds) substantially as some non-covalent bonds are very sensitive to pressure, which means that low molecular weight food components (responsible for nutritional and sensory characteristics) are not affected, whereas high molecular weight components (whose tertiary structure is important for functionality determination) are sensitive. Some specific covalent bonds are also modified by pressure. High pressure technology acts instantaneously and uniformly throughout a mass of food independent of size, shape and food composition. Compression will uniformly increase the temperature of foods approximately 30 C per 100 MPa. The temperature of homogenous food will increase uniformly due to compression. An increase in food temperature above room temperature and to a lesser extent a decrease below room temperature increases the inactivation rate of microorganisms during HPT treatment. Temperatures in the range of 450 C to 500 C appear to increase the rate of inactivation of food. 

HPP effects toward the food products quality and microbial food safety

From the previous posts, we already know that High-pressure processing (HPP) is a non-thermal process capable of inactivating and eliminating pathogenic and food spoilage microorganisms.

In this post we will look at effect of the method towards the food product;

Microbial Food Safety

Different species of microorganism has different degrees of resistance as well as various sensitivity towards High-pressure-processing treatment (Alpas H, Kalchayanand N, Bozoglu F, Sikes A, 1999).

• It is well established that bacterial endospores are the most-pressure resistant life forms known (Farkas & Hoover, n.d.)

• Pathogen found which is the most heat –resistant and most lethal to human being is C.botulinum primarilytypes A,B,E and F. C.botulinum is the most pressure-resistant and dangerous organism faced by HPP (Farkas & Hoover, n.d.).

• Gram-positive bacteria are more resistant to heat and pressure than gram-negative bacteria (Smelt, 1998)

• Cocci-shaped bacteria are more resistant than rod-shaped bacteria (Smelt, 1998)

• Non-sporeforming gram-positive bacteria , Staphyloccocus aureus have a high resistance to pressure (Smelt, 1998) .

• Organisms with simpler structure such as bacteria was found to be more resistant to HPP inactivation (Farkas & Hoover, n.d.).

• HPP is said to be effective in eliminating foodborne parasites, such as Toxoplasma gondii, Cryptosporidium parvum, Anisakis simplex, Trichinella spiralis and Ascaris, in low pressure ranges (Farkas & Hoover, n.d.).

•  Moulds and yeast have intermediate resistance with mould mycelia being particularly susceptible but mould spores that are quite resistant to HPP have been reported (Farkas & Hoover, n.d.).

   FOOD QUALITY

•      Produce high-quality foods that display characteristics of fresh products, are microbiologically safe and have an extended shelf life (Hogan et al., 2005; Patterson, 2005) while retaining the sensory characteristics of fresh food products (Patterson, 2005)
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•      Changes in physical functionality and/or changes in raw product colour are significantly less than those experienced using conventional thermal processing techniques (Hogan et al., 2005).
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•      The physical structure of most highmoisture food products remains unchanged after HPP exposure as the pressure exerted does not generate shear forces (Hogan et al., 2005); however, colour and texture may change in gas-containing products post-HPP treatment due to gas displacement and liquid infiltration into the collapsed
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•      Product yield is of immense economic importance to food manufacturers and HPP treatment in general gives a higher food product yield compared with heat treatment, with effects depending on product type and treatment intensity (Hugas et al., 2002).