Yeasts oxygen requirements

While enantioselectivity during reduction of ethyl 3-oxobutanoate by baker s yeast (Saccharomyces cerevisiae) to ethyl (S)-3-hydroxybutanoate was found to exceed 99%, yields did not exceed 50-70% (Chin-Joe, 2000). Elimination of two of three causes, evaporation of substrate and product esters and absorption or adsorption of the two esters by the yeast cells, increased the yield to 85%. Alleviation of hydrolysis of the two esters by yeast enzymes could increase the yield even more. Low supply rates of glucose as an electron donor provided the most efficient strategy for electron donor provision and yielded a high enantiomeric excess of ethyl (S)-3-hydroxybutanoate, low by-product formation and biomass increase, with a low oxygen requirement(Chin-Joe, 2001). 

Pichia stipitis. P. stipitis is the most effective natural yeast for the conversion of xylose to ethanol. This yeast species shares many characteristics with its close relative, C. shehatae. Toivola et al. [90] performed a systemic screening program with type strains of some 200 yeast species and identified P. stipitis as one of the yeast species that produces ethanol from xylose. There are many studies that have explored the property of this yeast in relation to its oxygen requirement, ethanol tolerance, enzyme cofactor s balance, etc. According to the reported literature [91,92], ethanol production from xylose by P stipitis exhibits the following characteristics ... 

Aeration, Temperature and Duration—Yeast cells require oxygen for carrying on their vital activities, and although fermentation will go on in the absence of air, unless the necessary oxygen is supplied the cells soon lose... 

The membrane is composed of lipid and phospholipid and contains proteins and sterols. The absolute requirement for unsaturated fats and sterols in membranes accounts for much of the oxygen requirement of brewing yeast strains (see Chapter 18). The nature of the unsaturated fats in the cell membrane affects its properties, e.g. in relation to ethanol tolerance [30]. 

Yeast will grow fermentatively in simple media which contain fermentable carbohydrates to supply energy and carbon skeletons for biosynthesis, adequate nitrogen for protein synthesis, mineral salts and one or more growth factors. Yeasts also require molecular oxygen (see Chapters 16, 18 and pp. 604-608, this chapter). 

Kamzolova S, Shishkanova N, Morgunov I, Finogenova T (2003) Oxygen requirements for growth and citric acid production of Yarrowia lipol dica. EEMS Yeast Res 3(2) 217-222. doi 10.1016/ sl567-1356(02)00188-5... 

A suitable means of scale-up for aerobic processes is to measure the dissolved oxygen level that is adequate in small equipment and to adjust conditions in the plant until this level of dissolved oxygen is reached. However, some antibiotic fermentations and the production of fodder yeast from hydrocarbon substrates have very severe requirements, and designers are hard-pressed to supply enough oxygen. 

Oxygen levels. Reactions can be carried out under aerobic conditions in which free oxygen is required or under anaerobic conditions in the absence of free oxygen. Bacteria can operate under aerobic or anaerobic conditions. Yeasts, moulds and algae prefer aerobic conditions but can grow with reduced oxygen levels. 

These findings lead to (he conclusion that the reduction of MHb by its reductase requires a natural cofactor, which is abolished during the purification procedure and can be replaced by methylene blue (G5, H22, H23, K8, K14). Since methylene blue and the other effective dyes are redox intermediates, it is obvious that the postulated cofactor interacts in the electron transport sequence of the MHbR reaction (H23). This is confirmed by the finding that oxygen and cytochrome c serve as well as terminal electron acceptor as does MHb (H22, H23, K14). Nevertheless, it had been possible to separate a cytochrome c reductase from MHbR in yeast extracts (A6). 

When in later years Krebs reviewed the major points which had to be established if the cycle was to be shown to be operative in cells, the obvious needs were to find the presence of the required enzymes and to detect their substrates. As the substrates are present in the cycle in catalytic amounts their accumulation required the use of inhibitors. Krebs also stressed that rates of oxidation of the individual substrates must be at least as fast as the established rates of oxygen uptake in vivo, an argument first used by Slator (1907) with reference to fermentation A postulated intermediate must be fermented at least as rapidly as glucose is. (See Holmes, 1991). This requirement did not always appear to be met. In the early 1950s there were reports that acetate was oxidized by fresh yeast appreciably more slowly than the overall rate of yeast respiration. It was soon observed that if acetone-dried or freeze-dried yeasts were used in place of fresh yeast, rates of acetate oxidation were increased more than enough to meet the criterion. Acetate could not penetrate fresh yeast cell walls sufficiently rapidly to maintain maximum rates of respiration. If the cell walls were disrupted by drying this limitation was overcome, i.e. if rates of reaction are to be... 

The manner in which yeast contributes to the fermentation process was not clearly understood until 1857, when the French microbiologist Louis Pasteur discovered that not only does the fermentation process require any oxygen, but also alcohol yield is actually reduced by its presence. The amount of ethanol generated by this first alcoholic fermentation is about 11%. At this step, "champagne" is still actually a noneffervescent white wine, because the carbon dioxide produced during the first alcoholic fermentation is allowed to escape into the atmosphere. 

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