High-pressure treatments have been reported to cause inactivation of microorganisms and spoilage enzymes (Ludikhuyze et al., 2001). A pressure of 350 MPa has been cited by Knorr (1995) as the threshold in plant systems for an effect on structure and texture. Severe texture loss occurs through rupture of cellular membranes and consequent loss of turgor pressure. Pressure affects individual components such as starch, proteins and poly-saccharides. Fruit and vegetable enzymes such as polyphenoloxidase and pectin methylesterase have been widely studied and just as their heat resistance varies considerably so does their pressure sensitivity. Inactivation or enhanced activity have been observed depending on source and conditions. Studies on eating have shown beneficial textural effects on meat that is subsequently cooked, although in some cases a combination of pressure and temperatures is effective (Cheftel and Culioli, 1997).
For a number of foods, dehydration is an unsatisfactory way to preserve them. Many dried foods rehydrate slowly in boiling water, remaining in part tough and unappetising. Kozempel et al. (1989) described modified and improved dehydration that included a step they called 'explosion puffing'. A partially dried piece of apple, for example, is subjected briefly to high temperature and pressure, then released into the atmosphere, where it expands instantly, or explodes. The result is a lightweight, porous piece of apple that can undergo further drying more quickly than an unex-ploded one. Researchers found that apples, celery, carrots, and potatoes so processed will reconstitute in water quickly, fully and evenly. The technique was claimed to have many applications. Explosion-puffed blueberries (Sullivan et al., 1982) are suitable for inclusion in cereals and muffin mixes. Sliced mushrooms also can be explosion-puffed (Sullivan and Egoville, 1986), retaining their nutrients and delicate flavour. The mushrooms can be stored for more than a year, then rehydrated in only 5 minutes. The puff-dried mushrooms can be used in dehydrated soup mixes and similar products, and can also be eaten in salads in place of conventional croutons.
In many food processes onions are heated to modify their texture and flavour. As an example of a prototype process and using the white, outer fleshy layers of onion waste as an illustration, pressure cooking is one step in pursuit of useful cell wall material (Le Cain et al., 1999). At the tissue level there was cell separation and at the cellular scale there was swelling of the cell wall. Sequential extraction of the cell wall material showed an increase in the water-soluble pectic polysaccharides accompanied by a small reduction in their peak molecular weight.
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