Pasteurization effects

Pasteur effect Yeast and other cells can break down sugar in the presence of oxygen (eventually to CO2 and H2O) or in its absence (to CO2 and ethanol). The decomposition of sugar is often greater in the absence of oxygen than in its presence, i.e. the Pasteur effect. With oxygen, less toxic products (alcohol) are produced and the breakdown is more efficient in terms of energy production. 

Since more ATP is produced by respiration of glucose than by fermentation, and since the ATP requirement for biosynthesis of cell mass is the same, it follows that to obtain the same cell yield from glucose, the yeast should consume less sugar under aerobic conditions than under anaerobic conditions, with a resultant decrease in glycolytic flux (Berry, 1982). These phenomena are referred to as the Pasteur effect. Although this effect is observed in some yeasts, in S. cerevisiae it is either absent (Gancedo and Serrano, 1989) or observed only under certain nutrient-limited conditions. The main reason for the absence of the Pasteur effect is that even under aerobic conditions, fermentation is still the main catabolic route for the utilisation of glucose because of the Crabtree effect (Walker, 1994). The Crabtree effect is the repression... 

Glucose and its Role in the Pasteur Effect, Biochim. Biophys. Acta (1967) 135,166. 

Since oxygen plays no role in this process, the system can obviously proceed anaerobically. In fact, the presence of oxygen decreases the net disappearance of glucose (Pasteur effect). 

Traditionally, biologists views of cell-level responses to 02 limitation are formalized in the concept of the Pasteur effect as ATP generation by oxidative phosphorylation begins to fall off due to oxygen lack, the energetic deficit... 

Schmidt H., and G. Kamp (1996). The Pasteur effect in facultative anaerobic metazoa. Experientia 52 440-448. 

Answer The addition of oxygen to an anaerobic suspension allows cells to convert from fermentation to oxidative phosphorylation as a mechanism for reoxidizing NADH and making ATP. Because ATP synthesis is much more efficient under aerobic conditions, the amount of glucose needed will decrease (the Pasteur effect). This decreased utilization of glucose in the presence of oxygen can be demonstrated in any tissue that is capable of aerobic and anaerobic glycolysis. 

A good illustration of how glycolytic flux can rapidly change is seen when yeast are grown under aerobic and anaerobic conditions with glucose as the carbon source. This effect, first observed by Louis Pasteur, is called the Pasteur effect and is depicted in Fig. 11-21. 

Fig. 11-21 The Pasteur effect metabolic activity of yeast grown under aerobic and anaerobic conditions.

Addition of oxygen to cells metabolizing glucose under anaerobic conditions leads to (a) a decrease in the rate of glucose consumption and (b) cessation of lactate accumulation. The latter phenomenon is known as the Pasteur effect. Explain why these changes occur in glucose and lactate metabolism. 

This phenomenon, which is known as the Pasteur effect, has been attributed to several mechanisms (Barnett and Entian 2005). Respiration needs very high amounts of ADP inside the mitochondria as a subtract for oxidative phosphorylation. Therefore, when respiration takes place, the cytoplasm lacks ADP and inorganic phosphate (Lagunas and Gancedo 1983), which in turn decreases the sugar transport inside the cell (Lagunas et al. 1982). These mechanisms explain how aeration inhibits the alcoholic fermentation. 

Lagunas, R., Dominguez, C., Busturia, A., Saez, M.J. (1982) Mechanisms of appearance of the Pasteur effect in Saccharomyces cerevisiae inactivation of sugar transport systems. J. Bacte-riol, 152, 19-25... 

Racket, E. (1974) History of the Pasteur effect and its pathobiology. Mol. Cell. Biochem., 5,17-23. 

Using Brettanomyces claussenii, Custers showed glucose fermentation is inhibited under anaerobic conditions. Glucose was fermented more rapidly under aerobic than anaerobic conditions. He named this inhibitory effect a negative Pasteur effect". Aerobic conditions activated the fermentation of glucose to produce ethyl alcohol, carbon dioxide, and "a considerable amount of acetic acid". Only ethanol and carbon dioxide were produced under anaerobic conditions acetic acid was not. 

In 1961 Wiken (11) showed evidence of a negative Pasteur effect as characteristic of all yeast in the genus Brettanomyces. In 1966 Scheffers (12) described this inhibition of alcoholic fermentation under anaerobic conditions as a consequence of the net reduction of NAD" to NADH in Brettanomyces yeast cells he called this a "Custers effect". 

Burk (13) provided an extensive history of the literature up to 1939 on the mechanism hypotheses of a Pasteur effect in biological systems. For further confirmational and mechanistic work on a Custers effect in Brettanomyces, the articles by Scheffers (14), Carrascosa (15), and Wijsman (16) should be consulted. 

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