Water activity microorganisms

Some alcohols, eg, propylene glycol, not only lower water activity but also have an additional preservative effect caused by the way they interfere with the ceU membrane transport system of the contaminating microorganisms. Surfactants (qv) may show a similar effect. 

To fulfill the needs for long term stability of a biological material, the optimum range of water activity lies between 0.20 and 0.35, This area represents the best compromise between lipid oxidation and non-enzymatic browning. Enzymatic browning is suppressed under these conditions, and growth of microorganisms is impossible. 

Deep desulfurization method of fossil fuels, comprising a first step of HDS and a BDS step for the removal of HDS sulfur refractory compounds, using an effective amount of a biocatalyst. The fuel is incubated in the presence of one or more BDS-active microorganisms, which converts the organic sulfur compounds into water-soluble inorganic sulfur. Then, in a separation stage, the products of the incubation are separated into a deeply desulfurized liquid fossil fuel, and the water-soluble inorganic sulfur. 

Humectant properties of sucrose and other carbohydrates may be important for certain products, as water activity can be controlled by the level of humectants. Replacement of the carbohydrates by sweeteners may therefore influence the water activity of products and require other technologies, other packaging materials or additional steps to control microorganisms. 

The inclusion of the a routine microbial limit test in a marketed product stability protocol depends on the pharmaceutical dosage form. Typically, the test would be used only for nonsterile products, especially oral liquids, nasal sprays, and topical liquids, lotions, and creams that have sufficient water activity to support the growth of microorganisms. In contrast, tablets, powder- and liquid-filled capsules, topical ointments, vaginal and rectal suppositories, nonaqueous liquids and inhalation aerosols with a water activity too low to allow for the product to support the growth of microorganisms would not be routinely tested. 

Preservatives. Most preservatives do not kill microorganisms present in food. Rather, they prevent further growth and proliferation of anything that is present by either lowering the water activity or increasing the pH of the foods in which they are used. Preservatives include benzoates, sorbates. propionates, organic acids, sulfur dioxide and sulfites, parabens. sodium nitrate and sodium nitrites, and natamycin and ntsin. See also Preservative. 

Figure A2.2.1 Water activity stability map (adapted form Labuza, 1970). A representation of a typical sorption isotherm for food materials and of the effects of water activity on the relative reaction rates of several chemical processes, as well as the growth of microorganisms, in foods are shown.

Water activity has a profound effect on the rate of many chemical reactions in foods and on the rate of microbial growth (Labuza 1980). This information is summarized in Table 1-9. Enzyme activity is virtually nonexistent in the monolayer water (aw between 0 and 0.2). Not surprisingly, growth of microorganisms at this level of aw is also virtually zero. Molds and yeasts start to grow at aw between 0.7 and 0.8, the upper limit of capillary water. Bacterial growth takes place when aw reaches 0.8, the limit of loosely... 

The population (or biomass) of bacteria or fungi acting on many synthetic compounds is typically small, and therefore the surrounding conditions must allow their proliferation for biodegradation to take place. The occurrence and abundance of microorganisms in an environment are determined not only by the available carbon, but also by physical and chemical factors such as dioxygen and nutrient availability, temperature, pH, salinity, and water activity. 

After a final rinse with clean water in tumbler washers, the cork moisture levels arc adjusted to 5.5 to 8% in continuous tunnel dryers to avoid growth of microorganisms on the washed corks. An ideal moisture level achieves a water activity unsuitable for mold growth, yet high enough to maintain the flexibility of the cork necessary for bottling. 

A plausible interpretation of the interaction between residual moisture and microorganism stability can be drawn from the works of Labuza et al. [18]. Figure 7 shows the progression of the lipid oxidation rate as a function of water activity (a ). This parameter reflects the availability of water for chemical reactions within the product. Its value varies from 0 for an anhydrous product to 1 for pure... 

The rate of growth of microorganisms in a food item depends on the characteristics of the food itself such as the chemical structure, pH level, presence of inhibitors and competing microorganisms, and water activity as well as the environmental conditions such as the temperature and relative humidity of the environment and the air motion (Fig. 4—41). 

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