Most biscuits will have moisture contents of less than 5%, with some of the crisper varieties, such as saltines, averaging around 2%. Exceptions are the chewy cookies such as brownies and cakelike cookies, which, when fresh, will have moisture contents of more than 10%. Within the range of normal ambient relative humidities experienced in the United States, drier cookies and crackers will tend to absorb water vapor. Chewier cookies will lose moisture, so it is highly desirable to package all biscuits in containers having low rates of moisture vapor transfer, if extended shelf lives are to be achieved. A more detailed discussion of packaging technology has been published elsewhere (5).
Deterioration of cookies and crackers generally takes the form of loss of flavor or acquisition of stale flavors, due to chemical changes (such as oxidation and loss of volatiles by evaporation), and undesirable texture changes. These changes are caused by loss or uptake of moisture, depending on whether a given biscuit is expected to be chewy or crisp, respectively. Sometimes, the appearance also changes because the surface reflects and refracts light differently, due to physical changes such as crystallization of sugars, solidification or liquefaction of fat fractions, or formation of empty vacuoles as moisture evaporates.
Microbiological changes are usually not particularly important causes of storage deterioration of cookies and crackers, because the low "water activity" (actually, rela tive humidity) of the substrate strongly inhibits the growth of yeasts, bacteria, and molds. The water activity (denoted as Aw) should be less than 0.70, which is the point below which most yeast, mold, and bacteria cannot survive. Sometimes, mold inhibitors such as potassium sorbate or calcium propionate are used to control spoilage, but these additives are regulated and are pH dependent. Occasionally, fermentation by osmophilic yeasts will take place in marshmallow and jellies, but formula adjustments to decrease the water activity of these materials, plus improvements in sanitation, can often remedy such problems. An extensive discussion of packaging and preservation methods for biscuits and other baked goods has been published elsewhere (5).
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