Principle And Method

The basis for applying high pressure to food is to compress the water surrounding the food (Fig. 1). A decrease in volume of water with increasing pressure is very minimal compared to gases. The volume decrease for water is approximately 4% at 100 MPa, 7% at 200 MPa, 11.5% at 400 MPa and 15% at 600 MPa at 22 °C (4). At above 1,000 MPa and room temperature, however, water changes to a solid (type VI ice) whose compressibility is very small. Usually, irreversible effects on biological materials are observed at pressure of >100 MPa. Therefore, the pressure of 100 to 1,000 MPa could be useful in food treatment. For reversible effects, the pressure up to 200 MPa may be used. Microbial death at higher pressures is considered to be due to changes in permeability of cell membranes.

When a protein solution is compressed as described above, protein is denatured reversibly or irreversibly depending upon the nature of the protein and the applied pressure. This is because non-covalent bondings (hydrogen bonds, ionic bonds, and hydrophobic bonds) are destroyed or formed, corresponding to the decrease in volume of the system (see ref. 17 for a review). Covalent bonds do not undergo a change during the pressurization used here. Protein, nucleic acid and starches, whose tertiary structures are composed of non-covalent bonds, change their structures at high pressure and lead to denaturation, coagulation or gelatinization.

Pressure effects are, thus, similar to heat effects on biological materials and foods. In other words, a high pressure is as useful as a high temperature (Table 1). The unique advantage of the high pressure-treatment is that the covalent bonds are kept intact as the liquid water was compressed. Effects such as the Maillard reaction and formation of cooked flavors do not occur during the pressure-treatment. Thus, it is possible to retain natural flavor and taste by application of high pressure treatment to foods.

In a typical experiment, compression of an egg in water to several thousand atmospheric pressure induces coagulation of egg proteins without causing chemical changes, although high temperature sometimes destroys covalent bonds. The egg shell was not crushed under the high pressure and the egg white and yolk were coagulated completely at 620 and 400 MPa, respectively. The coagulated egg proteins increased the proteolytic susceptibility, retained natural taste, color and flavor without lowering

Pressure

Water

Fig. 1. A schematic view of the application of hydrostatic pressure to an egg or the food enveloped in a sealed plastic bag.

Water

Fig. 1. A schematic view of the application of hydrostatic pressure to an egg or the food enveloped in a sealed plastic bag.

vitamin content. High pressure treatment generally denatures proteins; thus, the high pressure is useful to inactivate enzymes, to gelatinize starches, to sterilize microorganisms and to kill insects and parasites without accompanying destruction of nutrients and without changing flavor and taste.

Table 1

Possible uses of high pressure in cooking, processing, and preservation of foods as compared to high temperature processing.

Table 1

Possible uses of high pressure in cooking, processing, and preservation of foods as compared to high temperature processing.

Phenomenon

Temperature

Pressure

Denaturation of protein

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