Staling is a rather general term applied to a variety of chemical deteriorative changes. Staling is usually manifested as adverse alterations of taste, odor, and texture in prepared foods which are not promptly eaten. The changes may lead to rejection of stale food even though it may not be altered nutritionally.
a. Control Techniques. All of the changes are related to the fact that foods are composed of many substances that are chemically altered by heating. Many of the new compounds formed by heating are unstable and react in time with oxygen from the air or with other chemical compounds present in the food. To prevent such changes, many commercially processed foods are heated and stored in the absence of oxygen, for example, canned foods. Another way of excluding most of the oxygen in frozen or refrigerated foods is to have the food covered with sauce or some other liquid and packaged in a container with little residual air.
b. Bread as an Example. The staling of bread and related products is familiar to everyone. Furthermore, no better example of food can be found to illustrate the changes in both flavor and texture so typical of the staling process.
(1) Desirable flavors in bread and many other foods result from crust formation. For the purpose, the surface of the food, particularly during baking, is subjected to temperature above the boiling point of water. The heat causes intensified breakdown of food constituents.
(2) Upon standing, even in packages which allow no water loss, bread will degenerate in flavor and become harder and crumbly in texture. This latter change is not due to drying out but to rearrangements of the molecules of water, starch, and protein in the bread itself.
(3) Certain substances can retard the rate of the hardening which takes place during the staling of bread. These materials are emulsifying agents which are used as "freshness preservers."
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