Applications of High Pressure Processing for Dairy Products  Introduction In the modern world there is a huge demand for high quality, fresh food among consumers. A number of techniques have been developed in order to meet these demands. Many foods such as dairy products, meat products and fruit juices are processed at high temperatures in order to kill bacteria and extend shelf life. Processing food at high temperatures however decreases the nutritional value of the food. High Pressure processing (HPP) is a method of preservation by which products are processed at a very high pressure for a short period of time. High Pressure Processing is also known as Pascalization or High Hydrostatic Pressure (HHP).  According to N. Datta and H.C. Deeth (1999), High Pressure Processing can kill micro-organisms without causing significant changes to the nutritional value and sensory aspects of the food, thus offering a huge advantage over thermal processing methods. Research on High Pressure Processing began several decades ago when Hite (1899) demonstrated that the shelf life of milk and other dairy products could be extended by applying pressure to them, rather than heat. (Maryam Yaldagard et al. 2008). The first products to be processed using High Pressure Processing went on sale in Japan in 1991. In this paper I will discuss the Applications of High Pressure processing for dairy products. The methods and principals involved, the equipment used during processing, the breakdown of micro-organisms using HPP and the applications of HPP on Milk, Cheese and other dairy products will be highlighted throughout the paper.    Methods and Principles  The effect of high hydrostatic pressure is practically uniform and instantaneous in comparison to other preservation methods such as thermal treatments. (Torres and Velazquez 2008)  . According to Hugo Mújica-Paz (2011), HPP treatments are not mass dependent therefore treatment times are short, especially in comparison to thermal heat treatments.  High Pressure Processing is based on two main principals according to Ronit Mandal and Rajni Kant. These are: • Le Chatelier’s principle • Isostatic principle (Pascal’s Law)  According to the isostatic principle, High Pressure Processing is volume independent. This means that pressure is applied uniformly throughout the food and pressure gradients do not exist. (Ramaswamy et al. 1999). The ability to retain nutritional value throughout High Pressure Processing is partly due to the even distribution of pressure throughout the food being processed. This theory is according to N. Datta and H.C. Deeth (1999). Le Chatelier’s principle states that ‘if a change in conditions is applied on a system in equilibrium, then the system will try to counteract that change and restore the equilibrium’. (Ronit Mandal and Rajni Kant). According to this principle, ”process associated with volume decrease are encouraged by pressure, whereas processes involving volume increase are inhibited by pressure” (Butz and Tauscher, 1998). The Department of Primary Industry (Food Authority) states that during High Pressure Processing of foods, food is exposed to pressures between 100 and 800 MPa (14,500 to 116,000 PSI). Process times are relatively short, usually between 2 and 30 minutes and the temperature of production is usually room temperature or a little higher according to N. Datta and H.C. Deeth (1999).  The food that is being processed is placed inside the vessel, which is filled with liquid e.g water and sealed from top to bottom. It is through this liquid that the pressure is transmitted throughout the food. (RUPESH S. CHAVAN et al.)   According to Maryam Yaldagard et al, a typical High-Pressure Processing system usually consists of a vessel that can withstand high pressure, a pressure generation system and a temperature control. See Fig.1 below.   Fig.1 Source: (Yaldagard et al. 2008) The pressure used within the High- Pressure system may be direct or indirect. With direct pressure, pressure is applied to a fluid with a piston using a low-pressure pump. (Rupesh S. Chavan et al.). Yaldagard et al. stated in 2008, that the indirect pressure system uses high pressure to pump the pressure medium into the closed and de-aerated pressure vessel. This takes place until the desired pressure is reached. See Fig.2 below.      Fig.2 Source: (Mahmoud Rashed High Pressure Treatment in Food Preservation 2016).  Common pressure vessels are made from a steel alloy and are called ‘monoblocs’ as they are forged from a single piece of material. These monoblocs are able to withstand high pressures between 400-600 MPa. (Ronit Mandal et al. 2017) Packaging pays a significant role in High-Pressure Processing. Ethylene vinyl alcohol copolymer (EVOH) and polyvinyl alcohol (PVOH) are common packaging material used during High Pressure Processing. These materials are highly flexible which is ideal for high pressure processing as they withstand compression. During processing, the contents of the packaging decreases to approximately 80% of its original volume. However, once the process is complete, the food reverts back to its original volume.        Breakdown of Micro-Organisms One of the most important advantages of High-Pressure Processing is the breakdown of micro-organisms. As mentioned previously, high quality food with a long shelf life is very important to modern consumers. Dairy products such as milk can be heat treated in order to extend their shelf-life. However, this has a negative effect on the nutritional value of milk. An alternative to this heat treatment is High Pressure Processing, which extends the shelf life as well as maintaining the nutritional value of the food. (Rekha Chawla et al 2011) Research carried out by Smelt (1998) showed that the resistance of micro-organisms to pressure is variable depending on the conditions during production e.g. pressure, time, temperature and the constitution of the micro-organisms themselves. It is also known that the rate of destruction of micro-organisms increases with increasing the pressure within the vessel. (Reps et al., 1998).  Pressure effects weaker bonds for example Van Der Waal forces and hydrogen bonds. (Lado and Yousef 2002) In 1992, Hamada et al. noticed changes in colony form after high pressure treatment of Saccharomyces cerevisiae. Microbial cell destruction is caused by a number of changes that occur within the cell when pressure is applied. According to Ronit Mandal and Rajni Kant, these include: Irreversible changes to the structure of membrane proteins and other macromolecules. This leads to the destruction of cell membranes The Breakdown of homogeneity between the cell wall and the cytoplasmic membrane.  The inactivation of the membrane enzyme ATPase. The prevention of protein synthesis by the disruption of nucleic acids and ribosomes.  Gram positive bacteria become inactivated at a higher pressure than gram negative bacteria, according to (Hayakawa et al. 1994). The rigidity of the teichoic acids in the peptidoglycan layer of the gram-negative cell walls is the reason for this (Lado and Yousef 2002). From research carried out by Ronit Mandal and Rajni Kant (2017), it is clear that spores are more resistant to high pressure than vegetative cells due to fact that they contain calcium rich dipicolinic acid. This protects them from excessive ionization and allows them to survive in pressures of over 1000MPa (Smelt 1998). Pressures of between 400-800MPa inactivate the vegetative forms of pathogenic bacteria (San Martin et al. 2002). According to research carried out by the Food Safety Authority of Ireland, High Pressure Processing usually deceases the number of vegetative bacteria by up to 4 log units. However, the inactivation of bacterial spores is not possible by using pressure alone. This is why current HPP products on the market require environments such as refrigeration and lower pH in order to prevent the growth of bacterial spores. (Hugo Mújica-Paz et al. 2011) The inactivation of micro-organisms using High Pressure Processing is evident in the production of cheese from raw goat’s milk. It is reported that cheese treatment using pressures of 450MPa for 10mins or 500MPa for 5mins caused reduction of more than 5.6 logarithmic units of Listeria monocytogene in the cheese. (Gallot-Lavalee, 1998).  Another application of High Pressure Processing used to inactivate harmful micro-organisms in fresh cheese is the reduction of Escherichia Coli. Research shows that high pressure treatments of 400MPa at 2-25°C has led to the reduction of E. Coli by 7 logarithmic units. (Capellas et al., 1996)   Applications for Dairy Products  A number of studies throughout the years have proved that High Pressure Processing can extend the shelf life of dairy products without affecting the products essential nutrients. This is a huge benefit for the production of milk, which is sometimes known as a ‘whole food’ because of the essential nutrients it contains (proteins, fats, vitamins and minerals). Changes that occur in the milk proteins during high pressure processing also effect other aspects of the milk for example the nature of the products produced from it. (cheese)   Milk  Milk treated using high pressure processing can be sold as liquid milk or used in the manufacture of other dairy products such as yogurt, ice cream and cheese. The effect of HPP on milk has been extensively studied by a number of researchers. (6) According to research carried out by R. Gervila et al., proved that ”milk subjected to a pressure of 350 MPa had a shelf life of 25 days at 0°C, 18 days at 5 °C, and 12 days at 10 °C”. A sample of raw milk was processed using a high pressure of 400 MPa for 30 min at 25°C. After being stored at 7°C for 45 days it contained less than 7 log psychrotrophs/ml. This is in comparison to unpressurized milk that contained more than 7 log psychrotrophs/ml after only 15 days (Garcia-Risco et al). This data proves that milk which has been processed using high pressures meets the expectation of consumers that milk is safe and also has a long shelf life. Extend  Ice Cream  Recent studies carried out by T. Huppertz et al. That were published in the dairy journal showed that ice cream mixes that were treated using High Pressure Processing underwent a number of changes. Among these changes was the increase in viscosity increased by approx. 25-fold when treated with pressures exceeding 400MPa. Researchers attributed this viscosity to ”the formation of a proteinaceous network of micellar fragments in the mix by reduction in solubility of calcium phosphate on decompression during HPP cycle”. (Ronit Mandal et al. 2017). The ice cream that resulted also proved to have a greater texture and creaminess, which is a huge appeal to consumers. The results of this research highlights that the changes in proteins during High Pressure Processing has a huge benefit in the manufacture of ice creams. Yogurt  Significant structural changes occurred when yogurt was processed using High Pressure Processing, according to Needs, E. C et al. Extensive research has been carried out on the production of yogurt from milk that has undergone high pressure processing. Yogurts produced from skimmed milk that was processed using high pressures of between 400-500MPa as well as thermal heat treatments showed improved elastic properties, and an increase in yield stress. The results also showed that they had reduced syneresis in comparison to yogurts produced from milk that had not undergone High Pressure Processing. (F. Harte et al 2003)  One useful application of High Pressure Processing for yogurt production is extending the shelf life of the product by preventing “post-acidification”. (Voigt, D. et al 2015). Research carried out on yogurt that was processed using high pressures of between 200-300MPa for 10 minutes at 10-20°C, showed controlling of “post acidification” of the yogurt without having a negative effect on the texture of the yogurt or the number of lactic acid bacteria (LAB) in the yogurt. (Ronit Mandal et al. 2017).   Cream and Butter  High Pressure Processing of cream improves its whipping properties and reduces its serum loss. This is based on research carried out by P. Eberhard et al (1999) where cream was processed using pressure of 600MPa for 2 minutes. It is suggested that this may be due to the better crystallization of milk fat under high pressure. High Pressure processing of cream may have a potential application in the physical ripening of cream for the production of butter, however the use of HPP for the production of butter is not extensively in use.  Cheese 

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