Spoilage, water activity

Food apphcations of sorbates expanded rapidly after issuance of the original patents in 1945 (92). The first uses were based on their excellent fungistatic properties and thus involved foods with low pH and/or low water activity in which yeasts and molds are the primary spoilage agents. More recent appHcation research has been directed toward utilizing the bacteriostatic properties of sorbates. 

Sorbate combined with mild heat has a synergistic effect with regard to microbial destmction thus, in the presence of 0.025—0.06 wt % sorbate, products such as apple juice, peach and banana sHces, fmit salads, and strawberries can be treated with less severe heat treatments to extend shelf life (119,120). Sorbates increase the heat sensitivity of various spoilage fungi under varying conditions of pH and water activity (121—124). A similar synergistic effect has been reported for the combination of sorbate with irradiation (125). 

Sucrose is often used as a decorative agent to impart a pleasing appearance to baked goods and confections (36). In jams and jeUies, sugar raises osmotic pressure and lowers water activity to prevent spoilage (18). Sucrose is a fermentation substrate for lactic acid in cultured buttermilk (40) and lowers the freezing point of ice cream and other frozen desserts to improve product mouthfeel and texture. 

BCR CRM 402 Trace Elements in White Clover 0, = 0.456 at 2i.r°C w.c. = 7.52 0.04%. Despite its relatively high value, the water activity is still in the range for long-term stability and major differences in the water content as a consequence of sorption processes are not to be expected. This is supported by the fact that no spoilage has been observed, even though the production was in 1987. 

Figure 1-30 Loss of Free Lysine in Milk Powder Kept at 40C C for 10 Days as a Function of Water Activity. Source From M. Loncin, J.J. Bimbenet, and J. Lenges, Influence of the Activity of Water on the Spoilage of Foodstuffs, J. Food Technol., Vol. 3, pp. 131-142,1968.

Moisture. The water content of lecithin products is usually less than 1.0%. As a consequence of lecithin s essentially moisture-free state, lecithin products have very low water activity and do not adversely contribute to the microbiological profile of most food systems. Most lecithin products are preserved well in storage. Higher moisture levels usually indicate a greater potential for spoilage or chemical degradation. Moisture is determined by AOCS Official Method Ja 2b-87 (77). A less accurate moisture level can also be determined by azeotropic toluene distillation (AOCS Official Method Ja 2-46) (77). One cannot determine lecithin moisture by vacuum oven methods. These methods are known to degrade lecithin products and yield false moisture levels. 

The storage condition of raw materials, particularly hygroscopic substances, is important, and as a minimum water activity (Aw) of 0.70 is required for osmophilic yeasts, 0.80 for most spoilage moulds and 0.91 for most spoilage bacteria, precautions should be taken to ensure that dry materials are held below these levels. Some packaging used for raw materials, such as unlined paper sacks, may absorb moisture and may itself be subject to microbial deterioration and so contaminate the contents. For this reason polythene-lined sacks are preferable. Some liquid or semi-solid raw materials contain preservatives, but others such as syrups depend upon osmotic pressure to prevent the growth of os-mophiles, which are often present. With this type of material it is important that they are held at a con-... 

Microorganisms and enzymes need water in order to be active preservation of foods can be sought by reducing their moisture content to a point where the food-spoilage of pathogenic microorganisms are inhibited. Scott [ 1,2] called the equilibrium relative humidity (ERH), which is the availability of water in a food medium. Water Activity (a, ). a, is defined as the ratio of vapour pressure (p) of solution to the vapour pressure (Po) of the solvent, usually water ... 

Care must be taken with any measurement of water activity to ensure that the sample is representative of the food under test. Dried fish, for example, will have moisture and salt contents, and thus water activity, varying widely from thin, exposed flesh to the relatively moist interior. If the worst-case scenario for the growth of potentially toxic or spoilage organisms is of interest, the sample of flesh for water activity determination should be excised from the thickest, most moist region of the fish. 

It is worth mentioning in particular the spoilage of foods and beverages characterized by high sugar contents, low pH, and low water activities (flw) by yeasts as posing a major economic problem (Hazan, Levine,... 

Suhr, K.I. and Nielsen, P.V. 2004. Effect of weak acid preservatives on growth of bakery product spoilage fungi at different water activities and pH values. International Journal of Food Microbiology 95 67-78. 

How is it possible that of two nonsterilized intermediate-moisture foods of about the same type, of the same water activity, and at the same temperature, one shows bacterial spoilage and the other does not ... 

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