Micro-organisms choice

Enzymes are frequently used as catalysts to promote specific reactions in free solution. They are typically required in small amounts and are attractive in that they obviate both the need to provide the nutritional support which would be required for micro-organisms to perform the same conversion, and the possible subsequent removal of those microbes. Furthermore, the enzyme need not necessarily be of microbial origin so that a wider choice of operating conditions and characteristics may be available. 

The choice of process depends on the level of microbiological contamination of the raw materials and packaging, and the ability of the product to withstand growth of micro-organisms (e.g. preservative levels and sugar content). Also of great importance is the ability of the product to withstand heat. 

Carbohydrates are the most widely utilised carbon substrate by exopolysacchaiide producing micro-organisms and are used as substrate for commerdal jnoduction. The structure of the exopolysaccharide is generally independent of the carbon substrate. However, choice of carbon substrate can influence both the quantity produced and the extent of acylation of exopolysaccharides. The bacteria that produce dextran are unusually specific in their carbon substrate requirement for exopolysaccharide production they synthesise dextran only when grown on sucrose and are apparently unable to s)mthesise the polymer when grown on other substrates, such as gjucose. 

Bioluminescence has been used for specific detection of separated bioactive compounds on thin layers (BioTLC). After development and drying of the mobile phase by evaporation, the layer is coated with micro-organisms by immersion of the plate. Single bioactive substances in multicomponent samples are located as zones of differing luminescence. The choice of the luminescent cells determines the specificity of the detection. A specific example is the use of the marine bacterium Vibrio fischeri with the BioTLC format. The bioluminescence of the bacterial cells is reduced by toxic substances, which are detected as dark zones on a fluorescent background with picogram level sensitivity. 

Several micro-organisms can survive and propagate on unpreserved cosmetic products. Preservatives are routinely added to all preparations that can support microbial growth. The choice of a preservative for a given product is difficult. Anhydrous preparations and products containing high levels of ethanol or t-propanol may not require the addition of preservatives. 

Choice of Micro-Organisms Involved in HP for Producing Antigens 141... 

Table 7.87 shows the main features of on-line micro LC-GC (see also Table 7.86). The technique allows the high sample capacity and wide flexibility of LC to be coupled with the high separation efficiency and the many selective detection techniques available in GC. Detection by MS somewhat improves the reliability of the analysis, but FID is certainly preferable for routine analysis whenever applicable. Some restrictions concern the type of GC columns and eluent choice, especially using LC columns of conventional dimensions. Most LC-GC methods are normal-phase methods. This is partly because organic solvents used as eluents in NPLC are compatible with GC, making coupling simpler. RPLC-GC coupling is demanding water is not a suitable solvent for GC, because it hydrolyses the siloxane bonds in GC columns. On-line RPLC-GC has not yet become routine. LC-GC technology is only applicable to compounds that can be analysed by GC, i.e. volatile, thermally stable solutes. LC-GC is appropriate for complex samples which are difficult or even impossible to analyse by a single chromatographic technique. Present LC-GC methods almost exclusively apply on-column, loop-type or vaporiser interfaces (PTV).

The aim of this chapter is to draw the attention of experimental organic and catalytic chemists to a new field of catalysis, especially to catalytic methods which use micro-wave irradiation as a new means of activation of chemical reactions, called micro-wave catalysis . It is intended to advise synthetic organic chemists about the choice of catalytic steps which might be more efficient than conventional synthetic methods. 

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