FOOD ENGINEERING

Lesson 47. HIGH PRESSURE PROCESSING

Module 6. Recent trends in food processing

Lesson 47

HIGH PRESSURE PROCESSING

47.1 Introduction

High hydrostatic pressure processing (HPP), a relatively new technology to the food industry inactivates microorganisms without causing significant flavour and nutritional changes to foods. On the other hand, the effectiveness of thermal processing technologies explains why it remains as the prevailing method to achieve microbial safety and the inactivation of enzymes and microorganisms responsible for food spoilage. However, the high temperatures used in these processes cause significant chemical changes in foods. Particularly important are thermal degradation reactions leading to off-flavours, destruction of nutrients and other product quality losses.

For example, high-temperature short-time (HTST) pasteurization treatments (72°C for 15 s) impart a slight cooked, sulphurous note that has become acceptable to milk consumers but its refrigerated shelf life is only approximately 20 days. Ultra pasteurization (UP), a process similar to HTST pasteurization using more severe treatments (e.g. 1 s at 89 ⁰ C, 0.1 s at 96 ⁰ C or 0.01 s at 100 ⁰ C) lowers flavour quality and causes more nutrient damage but yields milk with a refrigerated shelf life of approximately 30 days. Pressure treatments of 400 MPa for 15 min or 500 MPa for 3 min at room temperature achieves microbiological reductions similar to thermal pasteurization but it is not used commercially because long pressure processing times are not financially viable. HPP treatments (586 MPa for 3 and 5 min) at moderate temperature (55°C) extend the refrigerated shelf life of milk to over 45 days while retaining milk volatile profiles similar to those observed after conventional HTST treatments. Finally, ultra high temperature (UHT) processing (135–150°C for 3–5 s) yields milk that is stable at room temperature for 6 months; however, this process induces strong ‘cooked’ off-flavour notes thus limiting its consumer acceptance in important markets.

Future advances are expected from the synergistic effects of using high pressure and high temperature combinations in the rapidly evolving pressure-assisted thermal processing technology (PATP). PATP is not yet a commercial application and will require more complex safety validation procedures than HPP, particularly for the case of low-acid foods (pH under 4.5). PATP conditions are sufficiently severe to achieve the inactivation of bacterial spores and recent studies suggest that pressure can lower the degradation rate of product quality caused by high temperature treatments.

High pressure processing at refrigeration, ambient or moderate heating temperature allows inactivation of pathogenic and spoilage microorganisms in foods with fewer changes in food quality as compared to conventional technologies. Pressure acts by disrupting mainly hydrogen bonds without affecting covalent bonds. Therefore, high pressure processing (HPP) treatments at low (approximately 0–30°C) and moderate (approximately 30–50°C) temperature cause minimum losses in quality factors associated with small molecules such as vitamins, pigments and volatile flavours. Research has confirmed that the sensory characteristics of HPP products make them often indistinguishable from untreated controls. Five decimal reductions in pathogens including Salmonella typhimurium, S. enteritidis, Listeria monocytogenes, Staphylococcus aureus and Vibrio parahemolyticus can be achieved by HPP.

47.2 Principle

The high pressure technique is essencially additive-free, mostly non-thermal or involves reduced heat treatments. Based on the pascal or isostatic principle, the hydrostatic pressure at a given point is the same in all directions and pressure is transmitted uniformly and immediately through the pressure transferring medium. Thus, the effects of pressure are independent on product size and geometry. It is often stated that HP processing is a uniform way of processing foods.

The effectiveness of a high pressure treatment is influenced by various intrinsic and extrinsic factors. Treatment time, pressurization or decompression rate, temperature and the number of pulses are critical to the effectiveness of the process. Moreover, the factors which include the effect of pressure on water, adiabatic heating and heat dissipation, food composition and the physiological states of microorganisms to be inactivated must be taken into account when optimising pressure treatments for the production of safe, quality foods.

47.2.1 Effects of HPP

· Microorganism inactivation

·Modification of biopolymers including enzyme activation and inactivation, protein denaturation and gel formation.

· Quality retention (colour, flavour, nutritional value)

· Modification of physiochemical properties of water

General Description of HP Equipment For Food Industry

The main components of an HP system are a pressure vessel, a pressure generation system, a temperature control device and a material handling system. Most pressure vessels are made from a high tensile steel alloy ‘monoblocs’ (forged from a single piece of material), which can withstand pressure of 400-600 MPa. For high pressures, pre-stressed multilayer or wire-wound vessels are used. In operation, after all air has been removed, a pressure-transmitting medium (either water or oil) is pumped from a reservoir into the pressure vessel using a pressure intensifier until the desired pressure is reached. Temperature control in commercial operations can be achieved by pumping a heating/cooling medium through a jacket that surrounds the pressure vessel. This is satisfactory in most applications as a constant temperature is required but if it is necessary to change the temperature regularly, an internal heat exchanger is fitted.

There are two methods of processing foods in high pressure vessels: in-container processing and bulk processing. Since foods reduce in volume at the very high pressure used during processing, there is considerable stress and distortion to the package selection is an important issue in using this method. Materials handling for in-container processing is achieved using equipment similar to that used to load/unload batch retorts. Bulk handling of liquids is simpler, requiring only pumps, pipes and valves.

There are two main types of High pressure equipments:

1. Batch type: A batch press can be used for any kinds of food in flexible packages, such as pouches, cups, or bulk bags. With the food already packed in the final consumer package at the processing stage, the risk of contamination is eliminated. The food packaged, are placed in the pressure vessel where they are isostatically compressed.

2. Continuous type: Continuous systems can be used for pumpable food. The system is installed with other equipment, and in the end the liquid food reaches an asceptic or clean filler. Thus any kind of consumer package can be used. Top of-the-line, high quality juice may be perceived as more valuabe if sold in glass bottles, rather than PET or other plastic that would require for batch cycling.

The volume in a pressure vessel for continuous use is better utilized that in a batch press, where there is dead space between the food packages. Thus the output volume is large despite the fairly small dimensions of the vessels used.

The principle of the continuous system is comparable to a four stroke engine. A valve is opened at the top of the press cylinder to let the product in and then closed. A floating piston inside the cylinder acts as a movable divider, and separates the water from the product. The pressure is the same on both the sides of the floating piston. When full pressure has been reached, it is held for a short period of time. After the hold time is over, the water is released through the bottom of the vessel and thus the vessel is decompressed. Another valve is then opened at the top, and the product leaves the cylinder when water is pumped in at the vessel filling, one for holding, and one for emptying the product. As the vessels take turns delivering the high pressure cycled product. There is an almost continuous output of product. With a balance tank in line with the system, the output will be continuous.

47.3 Advantages of HPP

· Retention of flavour and texture of the product

· Increase in Microbiological safety and shelf-life

· Low energy consumption

· Minimal heat input

· Minimal effluent and losses

· Uniform isostatic pressure & adiabatic temperature distribution

· Combination with heat gives better effects

47.4 Applications of HPP

• Milk treated at pressures of upto 500 MPa for few minutes has been shown to have a shelf-life at least equivalent to HTST pasteurized milk. Most vegetative cells, including non-sporeforming thermodurics, can be eleminated.
• HHP treatment (200 MPa, 10 min) after acidification (rise of acidity after acidification) in yogurt, increases the water binding capacity of whey proteins.
• The cheese yield is not influenced when milk treated at pressure ≤ 250 MPa, but at 600-800 MPa, it gets increased by up to 25% with increase in moisture content in curd and decrease in protein content in whey.
• Cheese Ripening can be accelerated by using the High Pressure treatment, which avoids the usage of elevated temperatures, addition of cheese slurries or exogenous enzymes or by the use of adjunct starters.

Cheddar cheese, when exposed to HP from 5 to 300 MPa for 3 days at 25°C, shows free amino acid levels of 26.5 mg/g at 50 MPa compared to 21.3 mg/g in the 6-month old cheese (which had not been HP treated). The taste of both the cheese were described as ‘‘excellent’’. This shows a considerable reduction in the ripening times of the cheese, attained through the application of HPP.

High pressure sterilization is possible by starting at elevated temp. e.g. 60–90°C, & using the adiabatic compression for rapid heating to higher temperatures. High pressure sterilisation is a combined process where both pressure and temperature contribute to sterilisation by the inactivation of spores and enzymes. The result is a shelf stable product, and in many cases a higher general quality than those products obtained using conventional processing.