Yeasts nutritional requirements

S. cerevisiae is produced by fed-batch processes in which molasses supplemented with sources of nitrogen and phosphoms, such as ammonia, ammonium sulfate, ammonium phosphate, and phosphoric acid, are fed incrementally to meet nutritional requirements of the yeast during growth. Large (150 to 300 m ) total volume aerated fermentors provided with internal coils for cooling water are employed in these processes (5). Substrates and nutrients ate sterilized in a heat exchanger and then fed to a cleaned—sanitized fermentor to minimize contamination problems. 

The easiest cells to grow are microbes that live independently in their natural environment. These include bacteria, yeasts, and molds. The hardest are the cells extracted from higher order plants and animals since they normally rely on complex interactions with other cells in the parent organism. Bacteria and yeasts are single-celled. Molds are multicelled but have relatively simple structures and nutritional requirements. 

Mead is a traditional alcoholic beverage obtained by fermenting mead wort that contains 8-18% (v/v) ethanol. Its production has been known since ancient times. However, mead fermentation and maturation requires an extended period, often lasting several months to years. Mead still remains a relatively empirical and manual exercise, requiring large capacity vessels and the investment of considerable capital in terms of the raw material. In addition, the fermentation rate depends on several factors, such as honey variety, yeast strain, yeast nutrition, and pH. Due to the lack of scientific investigation, mead production has suffered in comparison with other alcoholic beverages and so more research is needed to optimize the production process. 

However, for mead production to become profitable, it is necessary to decrease production time. A major concern in mead fermentation is the notoriously long period required to reach completion. Although fermentation rate depends on the honey variety and its characteristics, through proper selection of yeast strain and fermentation conditions, such as, mixing during fermentation, yeast nutrition, and pH s control, it may be possible to dramatically increase fermentation rate. 

Two major pathways are known for the reduction of sulfate. One is the assimilatory pathway, which reduces sulfate to the extent necessary for satisfying the nutritional requirements of the organism. In this pathway, which has been extensively studied in yeast by Robbins and Lip-mann (S68) and Bandurski and his colleagues 369, 370), sulfate is first activated in the presence of ATP by the enzyme ATP-sulfurylase to form adenosine 5 -phosphosulfate (APS). Then in a second reaction, APS is phosphorylated in the 3 position by ATP to form 3 -phosphoadenosine 5 -phosphosulfate (PAPS)... 

Various protein ingredients are utilized to satisfy nutritional requirements for protein quantity and quality in pet foods. Typical sources of protein ingredients are soybean oil meal, soybean flour, soy protein concentrate, soy protein isolate, meat meal, meat and bone meal, meat by-products, fish meal, blood meal, dried blood plasma, yeast, and milk protein (Balaz et al., 1977). In addition, vitamins, minerals, colorants, and other supplements such as choline chloride MgO vitamins A, Bj2,... 

The yeast cell requires a slight amount of nitrogen in its nutrition and to obtain this it produces an enzyme, endo-tryptase, which has some effect on the gluten of bread. However, the amount of carbohydrate which the yeast cell splits is enormous as compared with the weight of the yeast cell itself. 

Yeast cells grovhng aerobically synthesize sterols and incorporate them into membranes. However, under anaerobic conditions yeast cells do not survive unless they are provided "with an exogenous source of sterok. Explain the metabolic basis for this nutritional requirement. 

The second method available to us for a desaturase functional assay involves the expression of the desaturase cDNA in a desaturase-deficient yeast strain olel (12J3). This mutant has an absolute nutritional requirement for unsaturated fatty acids, which can be met by supplementing the growth medium with the natural Z9-16 and Z9-18 acids (palmitoleic and oleic acids, respectively) as well as with the unusual Zll-16 and Zll-18 acids 12,13), The olel strain s unsaturated fatty acid auxotrophy can also be relieved by transformation with a rat stearoyl-CoA A9 desaturase cDNA (ii). On the basis of these findings, we reasoned that an acyl-CoA A11 desaturase cDNA would also complement the olel defect. 

Lactic acid bacteria have very limited synthetic capabilities and, reflecting this, the group is frequendy described as being fastidious. Simply stated, this means that LAB have complex nutritional requirements that they are metabolically incapable of meeting. This is seen in their habitat specificity which is restricted to decomposing plant tissue and in association with other microbes (i.e., yeasts) where they are able to glean decomposition products for their own needs. 

Reported isolations of either yeast outside of fermentation facilities are rare. Van der Walt (1970) reports isolation of Brettanomyces from honey and tree exudates. It appears likely that subsequent transmission to fermenting juice and wine is the result of insect vectors. Aside from this report, the generally accepted habitat of both Brettanomyces and Dekkera is from fermenting products and their associated environs. It is likely that this ecological restriction is the result of their rather fastidious nutritional requirements. 

Thanks to this last characteristic, rotifers can be cultured on many feed sources (e.g. microalgae or yeast cells) and, even more important, their nutritional composition can be adjusted in a relatively short time (called enrichment) to better suit the nutritional requirements of the predator (e.g. fish or shrimp larvae). In the culture of fresh water rotifers, B. calyciflorus, pH is important due to the ammonia-ammonium equilibrium. 

Like yeasts, lactic acid bacteria are also present in vineyards. However, given their nutritional requirements, species diversity and population density is limited. Sound, undamaged fruit contains <10 CFU/g and so populations in grape musts during the early stages of processing tend to be low (Lafon-Lafourcade et al., 1983b). Species that have been isolated... 

Section I, Grape and Wine Microorganisms, describes those microorganisms found in grape must, juice, and wines namely yeasts, lactic and acetic acid bacteria, and molds. Here, taxonomy, metabolism, nutritional requirements, and potential impacts on wine quality are areas of focus. 

---