But, since the dynamic nature of research, a need has been to review all the previous works on HPP applications in dairy industry with the latest inclusions in the field. Therefore, the purpose of this literature review is to enlighten the recent technological developments and understandings in the field of HPP. This paper highlights the principles of HPP, equipment developed for food processing, microbial inactivation mechanisms by HPP, and applications of HPP on milk and dairy products based on several published data as well as effect of processing on constituents of milk.
It suggests that there is an increasing interest of HPP adoption and hence paving of way for its commercialization in dairy industry. The Isostatic principle states that the HPP is volume-independent; therefore, pressure is transmitted instantaneously and uniformly throughout a sample, and pressure is gradients do not exist, so that the size and geometry of the product is irrelevant. The HPP is an alternative to thermal processing as it is operated at ambient temperatures, ensuring little or no heat induced sensory changes in its components of food.
It is because of the fact that, the smaller organic molecules responsible for colours, flavours, and nutrients e. Whereas, the large bio-molecules such as proteins, nucleic acids and polysaccharides that depend on non-covalent bonding like hydrophobic interactions, hydrogen bonds etc.
High pressure processing of foods
Denaturation of the critical enzymes is also responsible for cell destruction. Yeasts, moulds and most vegetative bacteria are inactivated by pressures between and MPa. In general, In general, cells in the exponential phase of growth are less resistant to pressure than cells that are in a stationary phase of growth.
Gram-negative bacteria are inactivated at a lower pressure than Gram-positive bacteria, and rod shaped bacteria are more sensitive to pressure than cocci shaped. Spores are more resistant than vegetative cells because of calcium rich dipicolinic acid which protects them from excessive ionization, surviving pressures over MPa . The level of microbial inactivation depends on the pressure applied, duration of treatment, temperature, environment and initial load and types of microorganisms .
Vessels are sealed by a threaded steel closure, a closure having an interrupted thread, which can be removed more quickly, or by a sealed frame that is positioned over the vessel. The product is introduced in the medium. Pressure transmitting medium is generally any food-grade fluid. Commonly used medium are simply purified water mixed with a small percentage of soluble oil for lubrication and anticorrosion , aqueous solution of mono-propylene glycol for high temperature, isopropyl alcohol for low temperature HPP .
It was investigated such treatment as an alternative method for milk pasteurization. Several researches have been done on the applications of HPP on milk and milk products. The effect of HP on milk has been extensively studied . Many researchers have studied inactivation of microorganisms Listeria monocytogenes, Staphylococcus aureus, or Listeria innocua either naturally present or inoculated in milk ; and this offers a promising alternative for the pasteurization of milk .
HP treatment MPa for 15 min or MPa for 3 min of thermally pasteurized milk increased shelf life by 10 days . Further, the combination of high pressure with a bacteriocin lacticin was shown as a promising and natural method for increasing the efficiency and safety of HPP of milk. It resulted in a synergistic effect in controlling microbial flora of milk without significantly influencing its cheese-making properties .
The gram-negative bacteria, in this case, were found to be more sensitive to high pressure, either alone or in combination with nisin, than gram-positive bacteria . Although Casein and lactose present in milk were the major baro-protective agents to Escherichia coli in milk during the treatment.
Milk treated at MPa results in no significant loss of vitamins like B1 and B6 . HPP caused casein micelle disruption, whey protein denaturation, increase in milk pH and cheese yield, and reduction in rennet coagulation time, which indicates its significant potential in the cheese-manufacturing [25, 26]. So, HPP can be good alternative for cheese milk heat treatment.
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HP treated milk cheeses have higher moisture, salt and total free amino acids content than raw or pasteurized milk cheeses. High moisture led to pasty and weak texture defects.
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Such an effect was attributed to the increased water-holding capacity of the milk proteins . High-pressure treatment of cheese curd rather than cheese milk had beneficial effects. Rennet coagulation time was not dependant on the pressure in the lower range less than MPa , whereas at higher pressures — MPa it decreased . HP treatment MPa of pasteurized milk resulted in decreased rennet coagulation time.
At MPa, the rennet coagulation time was found to decrease along with decrease in pH, initial counts of nonstarter lactic acid bacteria, protein and fat content. Using HPP, cheese ripening process can be greatly accelerated. HP induces changes in biochemical processes such as glycolysis, lipolysis, and proteolysis during ripening of cheese leading to reduction in ripening time and quality improvement. Similar results were reported for HP treated Cheddar cheese 50 MPa for 72 h ; however, no textural changes in cheese were observed . Levels of free fatty acids in cheese was decreased by HP treatment at MPa, indicating slower lipolysis .
Ultra High Pressure Treatment of Foods
There are scarce literatures on the effect of HP on the flavour development and volatiles in cheese. Lower levels of butyric acid and acetoin in Gouda cheese after HP treatment between 50 and MPa were reported . Further studies will be necessary to determine the full potential of HP in exploiting cheese ripening. HP treatment has an impact on the functional properties of whey proteins. HP treatment of MPa for 15 min enhanced the foaming properties of whey protein concentrate, which when added to low-fat ice cream to improved body and texture of ice-cream, showed an increased overrun and foam stability and hardness the ice cream than ice cream added with untreated whey protein .
A team of researchers  studied the applications of HP processing in ice-cream manufacture. For this purpose, ice-cream mixes were subjected to pressures of about —MPa for one second to 20 minutes. HPP at or MPa for time as low as one second showed increase in the mix viscosity, which, the researchers attributed 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. Resultant ice-cream showed a strong increase in resistance to meltdown, greatly improved textural characteristics, like mouthfeel and creaminess.
Pressure assisted freezing may be a good area of research for HP treatment of ice-cream. Yogurt made from HP treated milk was less susceptible to undesirable syneresis on storage, probably due to changes in gel structure and water-binding capacity of milk proteins . Institute of High Pressure Physics of PAS, after ten years of applied and scientific research concerning high pressure food processing according to EU legislation, applied to Polish Health Authorities and at received a positive decision from the Chief Sanitary Inspector for production and marketing HPP food products in Poland.
The list of products includes fruit products processed under pressure of MPa and vacuum packed meat products processed under MPa. Research on possibility of elimination of bacteria spores from foods 2. Elimination of histamine producing bacteria, from smoked salmon, with HPP 3. In the nearest future the focus on mortality curves at extremes, i. The Figure shows the mortality curve of E. The black colour is for experimental data. This hypothetical domain is shown as the red — dashed curve. The technique is now used there to preserve fish and meats, salad dressing , rice cakes , and yogurts.
HPP is now being used to preserve fruit and vegetable smoothies and other products such as meat for sale in the UK. It did not change the guacamole's taste, texture, or color, but the shelf life of the product increased to thirty days, from three days without the treatment. Most commercial frozen and freeze-dried raw diets now go through post-packaging HPP treatment to destroy potential bacteria and viruses contaminants, with salmonella being one of the biggest concerns. Experiments into the effects of pressure on microorganisms have been recorded as early as ,  and successful experiments since In , B.
Hite was the first to conclusively demonstrate the inactivation of microorganisms by pressure. After he reported the effects of high pressure on microorganisms, reports on the effects of pressure on foods quickly followed.
High Pressure Food and Soft Matter Processing
Hite tried to prevent milk from spoiling, and his work showed that microorganisms can be deactivated by subjecting it to high pressure. He also mentioned some advantages of pressure-treating foods, such as the lack of antiseptics and no change in taste. Hite said that, since , a chemist at the West Virginia Agricultural Experimental Station had been studying the relationship between pressure and the preservation of meats, juices, and milk. Early experiments involved inserting a large screw into a cylinder and keeping it there for several days, but this did not have any effect in stopping the milk from spoiling.
Later, a more powerful apparatus was able to subject the milk to higher pressures, and the treated milk was reported to stay sweeter for 24—60 hours longer than untreated milk. Unfortunately, the device used to induce pressure was later damaged when researchers tried to test its effects on other products.
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Experiments were also performed with anthrax , typhoid , and tuberculosis , which was a potential health risk for the researchers. Indeed, before the process was improved, one employee of the Experimental Station became ill with typhoid fever. The process that Hite reported on was not feasible for widespread use and did not always completely sterilize the milk. While more extensive investigations followed, the original study into milk was largely discontinued due to concerns over its effectiveness. Hite mentioned "certain slow changes in the milk" related to "enzymes that the pressure could not destroy".
Hite et al.
Experiments were conducted on various other foods, including fruits, fruit juices, and some vegetables. They were met with mixed success, similar to the results obtained from the earlier tests on milk. While some foods were preserved, others were not, possibly due to bacterial spores that had not been killed. Hite's investigation led to other studies into the effect of pressure on microorganisms.