Carbon sources for yeast

Glucose is the preferred carbon source for yeast, as it is for bacteria. When glucose is present, most of the GAL genes are repressed—whether galactose is present or not. The GAL regulatory system described above is effectively overridden by a complex catabolite repression system that includes several proteins (not depicted in Fig. 28-29). 

The best carbon source for yeast is glucose, which is usually administered at a 2% concentration. 

The elemental and vitamin compositions of some representative yeasts are Hsted in Table 1. The principal carbon and energy sources for yeasts are carbohydrates (usually sugars), alcohols, and organic acids, as weU as a few other specific hydrocarbons. Nitrogen is usually suppHed as ammonia, urea, amino acids or oligopeptides. The main essential mineral elements are phosphoms (suppHed as phosphoric acid), and potassium, with smaller amounts of magnesium and trace amounts of copper, zinc, and iron. These requirements are characteristic of all yeasts. The vitamin requirements, however, differ among species. Eor laboratory and many industrial cultures, a commercial yeast extract contains all the required nutrients (see also Mineral nutrients). 

R w Until the 1930s, grain worts were used as the principal carbon and energy sources for yeast production. Since then, cane and... 

Cheese whey soHds contain 70—75% lactose, which can serve as the carbon source for lactose fermenting yeasts such as Klujveromjcesfragilis. The total volume produced is considerably smaller than for the other yeasts described. 

Some yeasts and bacteria are able to produce different alcohols like ethanol and butanol as well as polyols like glycerin and 2,3-butandiol. These compounds- are used in drinks such as beer and wines, and also may be used in or as solvents, drugs, chemicals, oils, waxes, lacquers, antifreezing and antifoaming agents, precipitants, dyestuff, pomades, raw materials for chemical syntheses, motor fuels, and carbon sources for SCP production. These products are mainly synthesized from petroleum — derived materials like ethylene and acetaldehyde. However, because of the insufficient availability and high prices of the raw materials, the microbial production of alcohols has become an interesting area for many researchers. 

D-Glucopyranosides are hydrolyzed by many yeasts,530,531 about half the species known being able to utilize cellobiose, for example, as the sole carbon source for aerobic growth (see Table III). 

Bhushan and Joshi (2006) used apple pomace extract as a carbon source in an aerobic-fed batch culture for the production of baker s yeast. The fermentable sugar concentration in the bioreactor was regulated at 1-2%, and a biomass yield of 0.48 g/g of sugar was obtained. Interestingly, the dough-raising capacity of the baker s yeast grown on the apple pomace extract was apparently the same as that of commercial yeast. The use of apple pomace extract as substrate is a useful alternative to molasses, traditionally used as a carbon source for baker s yeast production. 

Glucose and fructose are the preferred carbon sources for S. cerevisiae and many other yeasts. When these sugars are present in the medium, enzymes for... 

Although many facultatively fermentative yeasts utilize xylose as the carbon source for growth, the ability of these yeasts to produce ethanol from xylose is limited. Yeast strains that utilize xylose often produce xylitol from xylose extra-cellularly as a normal metabolic activity. However, only a few can produce significant quantities of ethanol. The prominent strains that produce ethanol from xylose include Pachysolen tannophilus, Candida shihatae and Pichia stipitis. However, the efficient production of ethanol from xylose is limited by the regulation of dissolved oxygen as well as by the imbalance of cofactors in the metabolic pathway during xylose utilization. In recent years, much effort has been put into improving yeast strains in order to produce ethanol from xylose more efficiently. 

It was mentioned earlier that the expression of HIV protease within E. coli gives rise to a phenotype, hi a similar fashion, it has been observed that phosphodiesterases (PDEs) when expressed in yeast affect the cells. These enzymes function to modulate intracellular concentrations of the cyclic mononucleotides cAMP or cGMP Yeast has two endogenous genes encoding PDEs which, when deleted, lead to elevated levels of cAMP within the ceU. The consequence to the yeast of elevated cAMP is increased sensitivity to heat shock and inability to utilize acetate as sole carbon source. These yeast mutants may be complemented by the human PDE gene and the phenotype reversed (Eigure 5.5). The use of such yeast in the search for inhibitors of PDEs with utility in, for example, asthma has been proposed" and certainly works with the known type IV PDEs inhibitor, rolipram. 

A special situation applies to microorganisms such as yeasts and bacteria, which use organic chemicals from the environment as the sole carbon source for growth. 

Many studies on hydrocarbon as the carbon source for cultivating yeasts and bacteria to produce cellular proteins and various substances have been published by a number of investigators since 1960. Especially various findings on hydrocarbon fermentation such as pathway, cellular component, growth rate, respiration, and so forth have been reported (Humphrey, 1967 Sharpley, 1966 Miller and Johnson, 1966). However, many engineering problems remain to be solved before the microbial potential of hydrocarbon fermentation can be exploited on an industrial scale with an understanding of the physical properties of hydrocarbon fermentations. 

In defined mixed culture the substrate is pasteurized or steriUzed and inoculated simultaneously with more than one pure culture. This can be beneficial for complex substrates and where the various strains use different carbon sources. For example, mixed cultures of Trichoderma reesei or Chaetomium cel-lulolyticum with Candida lipolytica resulted in increased protein production from wheat straw because the yeast uses glucose and prevents catabolite repression of the fungal cellulase [66,67]. 

Autotrophic microalgae as well as heterotrophic yeasts, bacteria, and fungi impH-cate limitations for a commerdal SCOs production process. An overview is hsted in Table 5.12. The main reasons are the costly downstream process of the SCOs and the high costs of carbon sources for heterotrophic microorganisms. 

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