Effect of high pressure processing on food proteins, polysaccharides and triglycerides

1.INTRODUCTION:

High pressure processing is used widely in food industry in combination usually with heat treatment. Some of the most known applications of high pressure processing are homogenization of milk before or after pasteurization or sterilization, in order to produce whole or skimmed fresh milk and other dairy products, the snack production in various shapes through extrusion and filtration of wine tar in industrial scale. The aim of this assignment is to define the effect of high pressure processing on structural and operational characteristics of the major food components, such as proteins, carbohydrates, and lipids.

 

2.EFFECT OF HIGH PRESSURE PROCESSING ON MAJOR FOOD CONSTITUENTS:

 

a. Effect of high pressure processing on food proteins:

High pressure has significant effect on proteins. In this section the effect on proteins in different food and feedstuffs is studied thoroughy, so to have more clear view.

In milk, the 2 major whey proteins β – lactoglobulin and α – lactalbumin undergo severe denaturation of their tertiary structure, which is reversible when pressure ranges between 100 and 300 MPa at ambient temperature, with the latter to be more pressure – resistant. The denaturation of their tertiary structure results from sulphydryl – disulphide interchange reactions between them or even with κ – kazein. Especially, in the case of β – lactoglobulin, high pressure affects the monomer – dimmer equilibrium, causes dissociation of dimmers, and around 1300 MPa a significant decrease to the amount of β – sheet and α – helix, indicating the unfolding of the protein (not completely) which enables aggregation. The change in the structure causes the better hydrolyses of the molecule, which results in in vitro increase of digestibility. The high pressure homogenization of milk causes, also, decrease in alkaline phosphatise and lactoperoxidase activity, which is more apparent as the temperature of milk increases due to the application of high pressure. On the other hand, the activity of lipoprotein lipase – an enzyme which causes lipid degradation – increases in high pressure homogenized milk, causing the appearance of rancidity. In buffalo milk, high pressure between 250 and 600 MPa resulted in slightly reduce of micelle casein, whereas high pressure around 100 MPa had no effect. Furthermore, high pressure treatment on bovine milk increases the size of micelle casein significantly. The rennet coagulation time of buffalo milk increases due to the partial destruction of rennet, whereas in the milk of other species decreases. This can be interpreted due to the destruction of micelle casein in bovine and other species’ milk, whist in buffalo milk micelle casein combines with the whey proteins. Casein micelles at pressures exceeding 300 MPa form large aggregates resulting in precipitation. In Cheddar cheese, chymosin was found to be high – pressure resistant, whereas plasmin was sensitive to pressure more than 400 MPa. Casein micelles undergo, also, disruption and reformation resulted from inter-molecular hydrophobic bonding.

Another food rich in proteins is avocado. Polyphenoloxidase exhibits great stability to high pressure (only after 900 MPa undergoes inactivation). This is characteristic of all polyphenoloxidase in fruits and vegetables. Viccilin, a protein in red kidney beans under the effect of high pressure appears to be more hydrophobic, and has better emulsifying activity and is more soluble due to the transformation of insoluble aggregates to soluble ones with lower molecular weight. On the other hand, the same protein undergoes unfolding. In the case of soybean proteins, the emulsion capacity has increased, due to unfolding. The emulsions derived from these proteins exhibited smaller droplet size, higher percentage of adsorbed proteins and higher susceptibility to depletion flocculation. The solubility decreases when high pressure increases from 200 MPa to 600 MPa, the relative surface hydrophobicity increases gradually and significantly, the free –SH groups increase due to disruption of –SS bonds, and the gelling capacity decreases, something attributed to the sever denaturation. In wheat, high pressure reduces solubility of cysteine containing gliadins due to rearrangement of intrachain disulphide bonds into interchain bonds, the solubility of glutenin, and the strength of gluten. In tomatoes, pectinmethylesterase was found stable at high pressure treatment, in contrast with polygalacturonase.

 

In case of beef strip loins, it was found that high pressure processing decreased solubility, while tenderness in not affected.

Ovalbumin, the most important protein in eggs, appears to form insoluble aggregates and precipitate, only under very high pressure. At the same time, the pressurization accelerates the enzyme hydrolysis, under 300 – 400 MPa, in the presence of trypsin or chymotrypsin. High pressure treatment results in the loss of secondary structure of white egg proteins, escorted by a reduction to –SH groups due to oxidation, which leads to minor (in comparison with thermal treatment) loss of solubility and increase in surface hydrophobicity. This results to smaller aggregates, which make their suspensions less turbid. At the same time, foams created by these proteins which have subdued high pressure were moister and creamier, showing smaller bubble size and lower sensitivity to bubble coalescence.

In case of a tilapia (a fish species) it was found that actomyosin – one of the key proteins – showed a significant decrease of total SH content, which showed that intra – or extra protein disulphide bonds were created by sulphydryl groups. This leads to disruption of actomyosin structure resulting in the formation of aggregates. Denaturation was the result of high pressure on myosin again in tilapia. In other fish species, high pressure treatment dissociates the calpain molecule, which then becomes denaturilized and deactivated. On the other hand, calpastatin is not affected.

 

 b. Effect of high pressure processing on polysaccharides:

 

Carbohydrates and especially, polysaccharides are another major group in both animal and plant food and feedstuffs, which are affected by high pressure processing.

 

One example is the presence and properties of Resistant Starch (RS) in Wheat Starch. RS is determined as the sum of starch and products of starch degradation that are not absorbed by the small intestine of healthy individuals. RS consists of crystallized, linear, unbranched, and short – chain a – glucans. The application of high pressure on wheat starch has no significant effect on RS production; however, the physicochemical properties of the resulting starch suspensions give the opportunity for the production of novel food products with a supplementary enhanced RS content. Furthermore, high pressure treatment of starch suspensions leads to limited retro gradation and limited expansion of molten granules under pressure as well as to differing paste/gel properties with lower viscosity and higher storage moduli, resulting in the production of products rich in starch with the same viscosity. Additionally, high pressure processing enables control of a desired degree of crystallinity by modulating parameters such as pressure, temperature, water content and treatment time. In the case of rice starch, high pressure treatments of 500 MPa induced irreversible modifications of rice starch suspensions. Starch kernels were found to retain their granular form. On the other hand, starch gelatinization under pressure resulted to a reduction of total suspension volume. Moreover, cereal starches are more sensitive to high pressure treatment than tuber starch. In terms of swelling index and amylase release from starch granules, starch suspensions demonstrated peculiar properties. More accurately, pressure – induced gels were characterized by a weaker matrix related to the lower release of amylase from starch granules observed upon low starch content suspensions. specific gravity measurements showed residual effects of pressure on starch suspensions. High pressure treatment at 600 MPa generated increased the density of gels.

 

Another significant polysaccharide – more precisely – disaccharide is lactose or milk sugar. Lactose consists of a glucose molecule combined with a galactose molecule. High pressure processing causes considerable inhibition of isomerization and degradation of lactose. This has a significant role on the final quality of thermally produced products, since the fundamental reactions leading to caramelization and furthermore to milk and dairy products quality deterioration include isomerization of aldose to ketose, fragmentation reactions and browning. On the other hand, high pressure treatment accelerated Maillard reaction between lactose and lysine at alkaline pH, whereas the opposite took place in the case of pH values under 8.0.

 

c. Effect of high pressure processing on lipids:

 

The effect of high pressure processing has been studied mainly in the case of milk and dairy products, fish and their products, and soybean products.

 

Milk homogenization is a high pressure process which precedes or follows milk pasteurization. Its main target is the reduction of fat particles, so as to achieve the better distribution of fat into milk and to block fat accumulation on the liquid phase surface, which is not acceptable, especially in case of chocolate milk production, when it affects negatively the latter’s sensory characteristics. In this case, fat particles break down resulting in the formation of smaller in size particles. At the same time, the number of these particles increases up to 10000 times leading to an up to 10 increase of their total surface. This suggests that a number of these new particles is surrounded not by the original membrane, but by a new one formed mainly by whey proteins.

 

The apply of high pressure on milk fat leads to the exposure of milk triglycerides to the enzymes (lipases), which continue to be active even after the application of high pressure. As it was mentioned above, one of these enzymes, lipoprotein lipase increases and presents even higher activation after high pressure application. Triglycerides, under the effect of lipases, degrade into fatty acids (total hydrolysis) or into a fatty acid, and monoglyceride and diglyceride mixture (partial hydrolysis). The released fatty acids, cause rancidity which deteriorates milk flavor (making it bitter and sour). Hydrolysis is more significant in the case of low molecular weight, volatile fatty acids, especially in the case of butyric acid, because they present bad odor and taste, providing milk and butter with an unpleasant flavor, something which leads mainly to final product discard.

 

In case of fish and fish products, soybean products and other plant and animal tissues, triglycerides consist of polyunsaturated fatty acids (PUFA). These fatty acids are of great importance, because they increase the amount of HDL in blood stream, preventing the expression of cardiovascular and other diseases connecting to the modern way of life. In this case, high pressure processing can lead to a significant fat degradation through a mechanism known as lipoxidase oxidation. Lipoxidase is, in fact, a group of enzymes, which propagate PUFA triglyceride oxidation, which their unsaturated double bonds are separated by a methylene group. Oxidation mechanism includes the intermediate free radical formation. At second phase, the intermediate free radicals are transformed into the second stage products, such as fatty acids, mono – and dihaldeydes and haldeydoacids). At first, the reaction gives to the products a slightly sweet flavor, which with the time becomes more disgusting and appalling due to the production of second phase products.

 

3.CONCLUSIONS:

 

The most significant conclusion is that high pressure processing is a technology for the future in food industry not only because it offers a different processing procedure, which allows the production of safe and hygiene food with all the nutritional and sensory characteristics of the raw materials used to be made of, but also because it will be at the first line in the production of novel food.