An Overview of High Pressure Processing

Shivani, Anil Kumar Verma, Pritam Chand Sharma,
Manoj Kumar and  Namita Rani

High Pressure Processing (HPP) is a procedure used to achieve microbial inactivation. It inactivates vegetative bacteria, yeast and moulds using pressure up to 600 MPa at ambient temperature and can inactivate spores, when combined with high temperature (High Pressure Thermal Processing) or Thermal Assisted HPP. HPP retains most of the sensory and nutritional quality of liquids or solids or chilled products. Its effect on enzymes is variable. Research into the application of HPP for food technology began when Hite (1899) demonstrated that the shelf life of milk and other products could be extended by pressure treatment. This technology applies high hydrostatic pressure to materials by compressing the surrounding water and transmitting pressure throughout the product uniformly and rapidly (Hyashi, 1989). In general, high pressure inactivates microorganisms, modifies biopolymers including enzyme inactivation, protein denaturation and gelatine, modifies the physico-chemical properties of water, while leaving nutritional values, colour and flavour components largely unaffected.

The first commercial HPP treatment products appeared in the market in 1991 in Japan, where HPP is now being used for products such as fruit juices, jams, sauces, rice, cakes and desserts. Food products provide a good environment for growth of microorganisms, which may cause food-borne diseases. For this reason, the control of microorganisms is an important aspect of food quality and safety. Many methods of food preservation are used for ensuring microbiological safety and extending food shelf life. High Pressure processing also called ultrahigh-pressure or high hydrostatic pressure is a non-thermal food processing technology. In food products, this process causes no damage or distorts as long as the food is treated with pressure. The food product to be treated at high pressure is packed in a flexible container and placed inside a pressure vessel submerged in liquid medium (mostly water), which transmits the pressure.

Introduction

High pressure processing (HPP) is a non-thermal, eco-friendly novel food processing technology whereby food is subjected to high iso-static pressure generally in the range of 100-600 MPa at or around room temperature. HPP is also known by different terms i.e. High-pressure Technology (HPT), high hydrostatic pressure (HHP), Pascalization or ultra-high pressure (UHP) processing. Increasing demand for foods with reduced amount of chemical additives and less physical damage has increased the need for application of hurdle concept, which made good potential for development of new non-thermal processes for food preservation or product modification.

The use of high pressure to destroy spoilage and pathogenic microorganisms in food was first reported by Hite (1899). High hydrostatic pressure treatments are independent of product size and geometry, and their effect is uniform and instantaneous. HPP induces a number of changes in the morphology, biochemical reactions, cell membrane and spore coats. Many intracellular cell organelles well as enzyme activity in the foods. Cell membrane is the primary site of pressure damage to the microbial cells. Pressure up-to 450 MPa at about 20-25 0C has been used to inactivate the vegetative forms of microorganisms. Pressure (450-800 MPa) eliminates sporeformers whereas in case of milk proteins, high pressure showed that no denaturation of β-lactoglobulin occurred up to 150 MPa but at higher pressures significant denaturation occurred during the pressure holding time. Milk treated at pressure of 500 MPa for a few minutes has been shown to have a shelf life at least equivalent to high temperature short time (HTST) pasteurised milk.

High pressure treatment also inactivated heat liable form of pectin esterase (PE) in orange juice and grapefruit juice. The Japanese company ‘meidiya’ introduced the first high pressure processed food, a jam in April, 1990 and followed it in May 1991 with a variety of high pressure processed fruit yoghurts, fruit jellies, salad dressings and fruit sauces. In 1991 also, the Japanese firm Pokka and Wakayama installed a high pressure system for bulk treatment of citrus juices with a capacity of 6000 and 4000 l/h respectively. Now-a-day, HPP is widely adopted because of reported quality improvement and shelf life extension of various foods.

Purpose and application

The main purpose of HPP is to minimize the freshness and flavour qualities of the food stuffs while achieving the required level of food safety. The significant advances of HPP technology is to produce food products which are safe, fresh, nutritious and innovative. In the food industry, the main field of application of HPP is food preservation. Food spoilage is very often caused by microorganisms and biochemical processes catalyzed by enzymes.

With HPP, a great part of microorganisms can be destroyed and most of the enzymes can be inactivated. Using HPP treatment, undesirable changes and thermal degradation of heat-sensitive food components can be avoided, a major advantage of this technique. It also affects biochemical reactions. Pressure reduces the size of the molecules and promotes bond formation between side chains. Protein molecules are denatured under high pressure. This is a complex phenomenon: it depends on the structure of the proteins, the extent of the pressure, the temperature and the pH.

Principle of High Pressure Processing

During HPP, the pressure is applied uniformly and simultaneously in all directions. It is called iso-static pressure and it is the reason why food is not crushed during the treatment. Once loaded and closed, the vessel is filled with a pressuretransmitting medium. Air is removed from the vessel with an automatic de-aeration value by means of a low-pressure fas-fill-and-drain pump, and high hydrostatic pressure is then generated by direct or indirect compression or by heating the pressure medium (Mertens, 1995).

According to Yordanov and Angelova (2010), High-Pressure technology has been cited as one of the best innovations in food processing from the last 50 years. Some physical and chemical changes result from application of pressure. Physical compression during pressure treatment results in a volume reduction and an increase in temperature and energy.

The basic principles that determine the behavior of foods under pressure are: Le Chatelier’s principle: Any reaction, conformational change, phase transition, accompanied by a decrease in volume is enhanced by pressure.

Principle of microscopic ordering: At constant temperature, an increase in pressure increases the degrees of ordering of molecules of a given substance. Therefore pressure and temperature exert antagonistic forces on molecular structure and chemical reactions.

Iso-static principle: The food products are compressed by uniform pressure from every direction and then returned to their original shape when the pressure is released. The products are compressed independently of the product size and geometry because transmission of pressure to the core is not mass/time dependent thus the process is minimized.

Advantages of HPP

• High pressure is not dependent of size and shape of the food,
• Elimination or significant reduction of heating, thus avoiding thermal degradation of food compounds,
• High retention of flavour, colour and nutritional value,
• High pressure is independent of time/mass, that is, it acts instantaneously thus reducing the processing time,
• Reduced requirement for chemical additives,
• Potential for few food product designs due to the creation of new textures, tastes and functional properties,
• It does not break covalent bonds; therefore, the development of flavours alien to the products is prevented, maintaining the natural flavour of the products,
• It can be applied at room temperature thus reducing the amount of thermal energy needed for food products during conventional processing,
• Since high pressure processing is iso-static (uniform throughout the food); the food is preserved evenly throughout without any particles escaping the treatment,
• The process is environment friendly since it requires only electric energy and there are no waste products.

Limitations:

• HPP is not practiced because the capital cost for a commercial scale.
• High Pressure treatment shown substantial economic losses because there is implementation of comprehensive quality assurance programmed to eliminate or reduce micro-organism in processing,
• Food enzymes and bacterial spores are very resistant to pressure and require very high pressure for their inactivation,
• The residual enzyme activity and dissolved oxygen results in enzymatic and oxidative degradation of certain food components,
• Most of the pressure-processed foods need low temperature storage and distribution to retain their sensory and nutritional qualities,
• Foods should be water free for anti-microbial effect.

Applications of HPP The literature pertaining to different applications of HPP in the food industry especially milk and milk products, fruits and vegetables and animal products is summarized in Table I, Table 2 and Table 3 respectively. Effect of HPP on Microorganisms: HPP can be utilized either as a cold pasteurization process or as in combination with thermal energy for pasteurization.

Generally, a moderate level of pressure (10-50 MPa) decreases the rate of reproduction and growth of microorganisms whereas a higher level of pressure leads to microbial inactivation.

1.Bacteria

HPP treatment under proper conditions can result in the inactivation of both pathogenic and spoilage microorganisms in food products. The inactivation of bacteria is ascribed to various types of damage accumulating inside the cell.

The main bacteria that cause food poisoning are Campylobacter spp., Salmonella spp., Listeria monocytogenes, Staphylococcus aureus, Escherichia coli, and Vibrio spp. Among them, L. monocytogenes, S. aureus, and E. coli are probably the three most intensively studied species in terms of use of HPP processing. HPP treatment under proper conditions can result in the inactivation of both pathogenic and spoilage microorganisms in food products. Although significant antibacterial effects can be obtained if the appropriate HP treatment (300-600 MPa) is conducted, the pressure resistance of bacteria is highly variable, being dependent on the species of bacteria and the food system involved. Generally, gram negative bacteria are less pressure resistant than gram positive bacteria, due to structural differences in the cell envelope.

2.Bacterial Spores

Bacterial spores can resist various stresses, including heat, pressure, radiation, chemicals, and desiccation. This high resistance is described to the thickness and structure of the bacterial spore coat (Reddy et al., 2006). Although HPP is effective in inactivating bacterial spores. During HPP processing, it is thought that the spores are first germinated through the activation of nutrient germinant receptors at moderate pressures (50- 300 MPa). However, the resultant germinated spores are sensitive to pressure, and they are subsequently inactivated as higher pressures are reached (Black et al., 2007). In general, bacterial spores are highly resistant to pressure at ambient temperatures and only very high pressures (>800 MPa) can achieve a marked loss of viability in the spores under these conditions.

3.Fungi

Fungi are mainly divided into two groups based on their structures: unicellular fungi (yeasts) and those producing hyphae (molds, mushrooms, etc.). Generally, HPP can produce greater destructive effects in organisms with a greater degree of structural complexity. As a result, yeasts and molds are more susceptible to pressure than bacteria, and can be inactivated using relatively low pressures. In most cases, treatment at pressures from 300 to 400 MPa for a few minutes is sufficient to inactivate most yeast cells. The mycelia of molds are particularly susceptible to HPP, while mold spores appear to be much more pressure resistant. Pressures between 300 and 600 MPa can inactivate most molds (Smelt, 1998). However, the ascospores of heat resistant molds, such as Byssochlamys spp., can even withstand pressures higher than 600 MPa. A combination of pressure greater than 600 MPa and temperature higher than 60 0C has been found effective for inactivating ascospores of heat-resistant molds in practical application. In addition, the pressure resistance of the heat-resistant ascospores of mold increases with their age, and this factor should be considered when using HPP for the inactivation of molds.

Effects of HPP on food quality

HPP treatment at certain conditions can alter the physicochemical, sensory, and functional properties of1 food ingredients, particularly protein, lipid, and starch (Rivalain et al., 2010; Liu et al., 2008). The following sections will discuss the influence of HPP on four aspects of food quality: food color, texture, sensory quality, and yield.

i) Food Color

For fruit and vegetable based products having natural pigments (e.g., anthocyanins, carotenoids, chlorophy moderate HPP treatment has a limited effect on their color characteristics. However, the stability of pigments can be affected by HP at a high temperature and/or a high pressure. For example, raising the temperature to 50 0C during pressurization resulted in the degradation of chlorophyll in broccoli juice (Oey et al., 2008). The color-related compounds in HPP-treated fruits and vegetables, especially anthocyanins, may become unstable during storage, probably because of incomplete enzyme inactivation and the presence of ascorbic acid (Oey et al., 2008). In addition, browning, condensation with phenolic compound, and textural change can also result in the change of color in HPP-processed plant-based products during storage (Cao et al., 2012). HPP can produce dramatic changes in the chromatic parameters of fresh meat and loss of red color (Bajovic et al., 2012; Canto et al., 2012). HP can also produce redness changes in fish and related products. However, the effect of pressure on redness is highly dependent on the fish species and treatment conditions. The effects of HPP on color of some fruit products is given in table 4:

ii) Food Texture

Due to enzymatic and non-enzymatic reactions, texture changes in fruits and vegetables can be related to transformations in cell wall polymers (Oey et al., 2008). To modify the texture of certain food products, HPP has potential as a technique. HPP treatment of Mozzarella cheese significantly accelerated the development of desirable functional properties on melting (O’Reilly et al., 2002). HPP can benefit the meat industry by modifying meat texture and consequently producing novel meat products. For pre rigor meat, pressurization ranging from 100 to 200 MPa is effective for increasing tenderness (Ma and Ledward, 2013). Chan et al. (2011) reported that HPP at 200 MPa increased the springiness in low pH turkey meat and the cohesiveness and resilience in both low and normal pH turkey meat. However, it is necessary to raise the temperature when it comes to tenderizing post rigor meat by means of HPP (Sun and Holley, 2010).

iii)Food Sensory Quality

Sensory analysis is the most straightforward way to evaluate the quality and consumer acceptance of food products. HPP has proven a promising preservation technology that can ensure food safety and retain the sensory characteristics of fresh food products, the sensory quality of foods can still be affected by HPP to a certain extent. Intrinsically, alterations in the sensory characteristics of HPP-treated foods are associated with the physical and chemical changes induced by HPP. On the other hand, the negative influence of HPP on food sensory qualities can be minimized by selecting proper processing parameters.

For example, HPP processing at 600 MPa enhanced in mouth sensations of a red wine, such as sour, bitter, and astringent tastes, whereas no statistically significant difference in overall quality was found among HPP-treated and untreated wines (Tao et al., 2012). As for seafood, the increase in hardness following pressurization is also not detrimental, since post mortem softening occurs rapidly (Murchie et al., 2005). Furthermore, HPP treatment was found to increase the contents of ester compounds in strawberry puree, which are an important group of flavor compounds (Lambert et al., 1999).

In general, although HPP cannot always retain the original sensory properties, the overall quality of many HPP-treated food products have actually proven superior to traditional heat-treated ones by many researchers. Since several food products processed by HPP have already been commercialized, it is necessary to help consumers understand the mechanism of HPP processing and the advantages of this technology, in order to guarantee the success of HPP-treated food products in the market. iv) Food Yield HPP treatment can give a higher yield in food products than thermal treatment and treatment without pressurization. HP treatment can give a higher yield in food products than thermal treatment and treatment without pressurization.

For example, Mor-Mor and Yuste (2003) reported that weight loss was significantly higher in heat treated sausages than in HP-treated samples. In milk, HP treatment for cheese making can augment the yield of Cheddar cheese by increasing moisture and protein retention in cheese.

HPP treatment is effective for oyster shucking. It has been reported that HPP treatment at 241 MPa for 2 min caused detachment of adductor muscle in 88% of oysters, while treatment at 310 MPa, with immediate pressure release, resulted in 100% efficiency of shucking. Fitting oysters with heatshrinkable plastic bands before treatment holds the shells together and reduces the loss of interval fluid.

Oysters treated in this way obviously do not gape and have proved an attractive alternative to traditional live oysters. In addition to reduced labour cost and risks, and increased safety and shelf life of oysters, yield increases of 25-50% using HPP processing.

CONCLUSION

High pressure processing has proven to be an effective technology to reduce the microbial impact in foods for both pathogenic and spoilage microorganisms with less impact on the initial quality of the foods. Application of high hydrostatic pressure in the food industry for various products is growing day-by-day. It has great potential to develop new minimally treated foods with high nutritional and sensory quality, novel texture and with an increased shelf life.

In general, it can be stated that high hydrostatic pressure treatment as a food processing technology in practice, requires optimization in case of every single product type.

This method is microbiologically efficient and quality protective but further researches are necessary to prevent changes in lipid oxidation and regeneration of sub-lethally injured microorganisms in pressure treated foods during storage keeping in view, the current cost and capacity limit of the high hydrostatic pressure technology, it can be said that it is unlikely to replace conventional thermal processing, but it could offer commercially feasible alternatives in the case of novel food products with improved functional properties, that cannot be attained by conventional heating which will reduce the bioavailability of the functional component.

 (Writers’ affiliation: Dr YS Parmar University of Horticulture and Forestry, Department of Food Science and Technology, College of Horticulture and Forestry, Neri, Hamirpur, Himachal Pradesh)