Microbial growth

In (X/X(j) vs. time and calculate the slope of the linear portion of the curve. Another common way to characterize microbial growth rates is by the doubling time (t ), which equals the time required for actively growing cells to double in concentration  

Many valuable fermentation products are produced during the exponential phase. These are known as growth associated products. 

After nutrients and substrate begin to diminish, cell growth decelerates and enters a period of no apparent growth. This phase is known as the stationary phase. In the stationary phase, the rate of growth equals the rate of cell death and no net change in cell mass concentration occurs. Fermentation products produced during the stationary phase are known as non-growth associated products. 

If additional substrate is not added to the culture, the rate of cell death becomes greater than the rate of growth. This is known as the death phase. Additional substrate and inoculum must be added to restart the culture. 

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If there are significant amounts of both volatile and nonvolatile contaminants, remediation may be achieved by a combination of Hquid and vapor extraction of the former, and bioremediation of the latter. This combination has been termed "bioslurping", where the act of pumping out the Hquid contaminant phase draws in air at other wells to stimulate aerobic degradation (20). Such bioremediation requites that there be enough nutrients to allow microbial growth, and fertilizer nutrients are frequendy added at the air injection wells. Bioslurping has had a number of weU-documented successes. 

A recent suggestion has been to use plants to stimulate the microbial degradation of the hydrocarbon (hydrocarbon phytoremediation). This has yet to receive clear experimental verification, but the plants are proposed to help deUver air to the soil microbes, and to stimulate microbial growth in the rhizosphere by the release of nutrients from the roots. The esthetic appeal of an active phytoremediation project can be very great. 

Halogenation of nitromethane is utilized to produce two economically important pesticides, chloropicrin [76-06-2J, a soil fumigant, and bronopol, a biocide useful for control of microbial growth in cosmetics and industrial appHcations. 

For those pesticides which are utilized as microbial growth substrates, sigmoidal rates of biodegradation are frequentiy observed (see Fig. 2). Sigmoidal data are more difficult to summarize than exponential (first-order) data because of their inherent nonlinearity. Sigmoidal rates of pesticide metabohsm can be described using microbial growth kinetics (Monod) however, four kinetics constants are required. Consequentiy, it is more difficult to predict the persistence of these pesticides in the environment. 

The activity of the sorbates at a higher pH is one distinct advantage over the two other most commonly used food preservatives, benzoic and propionic acids, because the upper pH limits for activity of these compounds are approximately pH 4.5 and 5.5, respectively. Although the effect of sorbates can be microbiocidal under certain conditions, activity is most often manifested as a microbial growth retardant. 

Eor fresh poultry, a potassium sorbate dip significantly reduces total viable bacteria and doubles the refrigerated shelf life of ice-packed broilers (133). In cooked, uncured, vacuum-packaged turkey and poultry stored at 4°C, 0.2—0.25 wt % potassium sorbate suppresses microbial growth for up to 10 days (134). Sorbic acid at 0.5% in a marinade mixture for chicken dmmmettes extends refrigerator shelf life (135). Country-cured hams sprayed with a 10 wt % potassium sorbate solution showed no mold growth for up to 30 days (136). A review of sorbate use in meat and fish products has been pubHshed (137). 

The effect of various pHs has been well known for some time. Acidic foods such as fmits tend to retard microbial growth and resist certain types of contamination. For this reason, the standards adopted industry-wide have been based on the processing of foods of high acidity (low pH). In the United States, the FDA has regulatory responsibiUty over the preparation, sterilization, and distribution of foods. 

Microstrainers. Microstrainers are rotating steel screens with extremely fine stainless steel mesh (85—170 perforations per square centimeter (13—26/in. )). The flowing Hquid enters the open end of the dmm and passes through the mesh to the effluent end. The mesh traps soHd impurities and rotates with the dmm. A wash-water spray washes the trapped soHds into a hopper for final disposal. The mesh is washed with filtered effluent discharged from jets fitted into the dmm and then exposed to uv radiation to inhibit microbial growth. The mesh is washed with chlorine water at intervals of 7 to 28 days in order to control slime growth removal efficiencies are 30—55% of the appHed BOD and 40—60% of suspended soHds. 

J. R. Knowles in E. E. Hahn, Antibiotics FT (Modes and Mechanisms of Microbial Growth Inhibitors] Springer-Vedag, New York, 1983, pp. 90-107. 

Preservatives. The carbonation and acid content ia cola and lemon—lime beverages usually act as adequate preservation against microbial growth. Benzoate or sorbate salts are often added to other beverages for protection (Table 2). 

More than 80% of all the sodium chlorite produced is used for the generation of chlorine dioxide. Sodium chlorite or the chlorine dioxide generated from it or from sodium chlorate must be registered with the USEPA for each specific appHcation use as a biocide for microbial growth control or disinfection. These regulations are covered under the Eederal Insecticide, Eungicide, and Rodenticide Act (EIERA). 

Seafood. Citric acid is used in combination with other preservatives/antioxidants to lower the pH to retard microbial growth, which can lead to spoilage, formation of off-flavors, and colors on fish and other seafood products. 

An entirely different type of contamination arises from the presence of microbiota in a product. As in the case of chemical contamination, compendial requirements for microbiological purity exists. Pharmacopoeial standards vary from country to country, and manufacturers must use the specifications and kill times that meet local requirements. As of this writing, the criteria in the British Pharmacopoeia are more stringent than those estabUshed by the CTFA, which are stricter than those in the United States Pharmacopoeia. In order to meet commonly accepted standards of microbial purity, manufacturing faciUties must be periodically cleaned and all products that can support microbial growth must contain an effective preservative (6). 

An antimicrobial preservative serves to protect materials and products from the deleterious consequences of microbial growth and activities. 

Microbiol Stability. Microbial growth is hindered by reducing water activity and adding preservatives. An overview is available (30). Reduction in water activity is typically obtained by including approximately 50% of a polyalcohol such as sorbitol or glycerol. Furthermore, 20% of a salt like NaCl has a pronounced growth inhibiting effect. 

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