Beer fermentation metabolism

The main limitation to the clinical use of the MAOIs is due to their interaction with amine-containing foods such as cheeses, red wine, beers (including non-alcoholic beers), fermented and processed meat products, yeast products, soya and some vegetables. Some proprietary medicines such as cold cures contain phenylpropanolamine, ephedrine, etc. and will also interact with MAOIs. Such an interaction (termed the "cheese effect"), is attributed to the dramatic rise in blood pressure due to the sudden release of noradrenaline from peripheral sympathetic terminals, an event due to the displacement of noradrenaline from its mtraneuronal vesicles by the primary amine (usually tyramine). Under normal circumstances, any dietary amines would be metabolized by MAO in the wall of the gastrointestinal tract, in the liver, platelets, etc. The occurrence of hypertensive crises, and occasionally strokes, therefore limited the use of the MAOIs, despite their proven clinical efficacy, to the treatment of atypical depression and occasionally panic disorder. 

Esters. Esters are extremely important aroma compounds and there are many reports that esters are biosynthetic products of bacterial action. Thus, the fruity flavor defect sometimes found in cheddar cheese is due to the presence of esters, principally ethyl butyrate and ethyl caproate (25). Similar esters can be found in beer in which both fusel alcohols and the short chain fatty acids, acetic and butyric, are also present. These materials can undergo esterification, which in this case is mediated by the enzyme alcohol acetyltransferase present in the yeast used for beer fermentation (26). There are a number of esters present in wine which are metabolically produced by the yeast. Of these,... 

Kinetics of Yeast Growth and Metabolism in Beer Fermentation... 

The objectives of wort (unfermented beer) fermentation are to consistently metabolize wort constituents into ethanol, carbon dioxide and other fermentation products to produce beer with satisfactory quality and stability. It is also important to produce yeast crops that can be confidently repitched into subsequent brews. This is unlike distiller s yeast strains where the yeast culture is used only once it is not repitched. 

Tyramine is an amino acid which is present in large quantities in protein rich, fermented and stored products like some cheeses, sausages, red wines, beers etcetera. Tyramine is metabolized into nor-adrenaline by the enzyme mono-amino-oxidase (MAO). If MAO is inhibited by drags nor-adrenaline is accumulated and can give hypertensive crises. 

Two bacterial strains, one from soil and the other from infected local beer, which utilised calarene as the sole source of carbon and energy have been isolated by enrichment culture techniques [149]. Both these bacteria were adapted to grow on valencene as the sole carbon source. Fermentations of valencene (5) by these bacteria of the genus Enterobacter in a mineral salts medium yielded several neutral metabolic products dihydro alpha-agarofuran (200) (7.5%), nootkatone (6) (12%), another ketone (201) (18%) and a-cyperone (202) (8%), Fig. (40). 

In fact, fermentation is both the oldest and newest type of catalytic processing. In the very old days— back to several thousand years BC— beer, bread and winemaking were t) ical fermentation processes. In the present, modern recombinant DNA technology allows us to reconstruct the cell factories, i.e. the metabolic pathways, and so to develop novel multi-step conversions by fermentation that are not available in nature. 

Beer, on the other hand, is produced by more complex biochemical and technological processes, which all affect its flavor. Yeast amino acid metabolism, a key to the development of beer flavor as described earlier, is affected by process temperature and use of cell immobilization techniqnes. Therefore, technologies based on these features as well as other process conditions and strain selection have been developed to control beer flavor. The combination of immobilized yeast and low-temperature primary fermentation was found to produce beers with low diacetyl amounts, therefore indicating potential of low-cost industrial application since maturation is a high-energy-consuming process. Finally, Perpete and Collin showed that during alcohol-free beer production, the enzymatic reduction of worty flavor (caused by Strecker aldehydes) by brewer s yeast was improved by cold contact fermentation. 

Modern fermentation technology emerged from beer and wine production, and it is based on batchwise processing of highly concentrated media where most of the sugar taken up by the cells is efficiently channeled into the metabolic... 

Under the reducing conditions of a brewery fermentation, sulphur dioxide is formed (up to 10 mg/1). Other volatile sulphur compounds have been reported in beer, e.g. thiols and mercaptans. Many of these may be derived from hops or other materials rather than from yeast metabolism. 

PRODUCTS OF NUCLEIC ACID METABOLISM Nucleotides are excreted by yeast early in fermentation [91] and also under certain conditions of storage [91, 92]. Yeast is not the sole source of these compounds, malt being a particularly rich source. Free bases are reported to modify beer flavour, thus xanthine, guanine and cytidine enhance bitterness whereas adenine slightly depresses flavour [93]. 

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