Combination Use Of Pressure P And Temperature T

Under ambient pressure, protein denatures at high temperature (heat denaturation) and also at low temperature (cold denaturation). In contrast to temperature-dependent denaturation, pressure-dependent denaturation of proteins exhibits two opposite effects depending upon proteins: an increased pressure-denaturation at high or low temperature (enhancement effect of T and P) and a decreased pressure-denaturation at high or low temperature (competitive effect of T and P). This has been systematically studied by altering T and P factors independently (see ref. 17 for a detailed explanation).

Although bacterial spores are not killed by high pressure treatment at room temperature, they are killed at elevated temperatures (45 - 60 °C) at 600 MPa. Vegetative cells of some bacteria and yeasts can be sterilized at low temperatures (e.g., -20 °C) under pressurization (8, 10).

The enhanced effects of high or low temperature can be applied not only to inactivate bacterial spores but also to inactivate enzymes and improve food texture, especially that of starches. An advantage of using both pressure and temperature together lies in its ability to lead to efficient and economical industrial applications; although, the wrong combination of T and P may lead to inefficient inactivation of bacteria or yeasts.

Pre-treatment by T followed by P treatments or vice versa is an important consideration for developing high quality foods with good taste, flavor, color and texture. Following scheme summarizes how to introduce P in addition to T:

The use of pressure lower than 200 MPa at 0 to -20 °C is of interest because under such conditions the equilibrium temperature between water and ice is shifted to lower temperature; thus, it may be useful in the following applications:

1. Non-freezing food preservation

2. Rapid thawing of frozen food

3. Rapid freezing of food and biological materials (pressure-shift freezing method)

Two factors should be considered for the combinational use of T and P: 1) the effect of pressure on the increased chemical reaction at high temperature, and 2) the temperature rise of the pressure medium accompanying adiabatic compression. Since the rate of some chemical reactions (e.g., decomposition of nutrients) are accelerated by pressure, adverse effects may take place in food. We have started a study to examine the pressure effects on food-related reactions (7, 8). During the compression of water, adiabatic temperature increase at high temperature is larger than that at low temperature (7). Hence, an accurate control of temperature inside the pressure vessel is required.

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