Alcoholic fermentation metabolic processes

The dismutation of aldehyde to acid and alcohol also plays an important part in cell metabolism, particularly in alcoholic fermentation (p. 403) (Mechanism ) although the chemical process is certainly different in this case. 

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. 

Anaerobic Oxidation of ducose. Historically, the first system of carbohydrate metabolism to be studied was the conversion by yeast of glucose to alcohol (fermentation) according to the equation CnH,Of,2CH)CH,OH + 2CO . The biochemical process is complex, involving the successive catalytic actions of 12 enzymes and known as the Emhden-Meyerhof pathway This series of reactions is summarized in the entry on Glycolysis. 

Alcoholic fermentation occurs when the end product is ethanol, as shown in Figure 4.11. In this process the pyruvate is first converted enzymatically to acetaldehyde. The conversion of acetaldehyde to ethanol produces NAD+ from NADH + H+, and the NAD+ is cycled through the glycolysis process. As with lactic acid fermentation, the glycolysis process produces usable energy contained in two molecules of ATP produced for each molecule of glucose metabolized. 

In Vin Santo, due to the slow sugar metabolism, maturation already starts when the alcoholic fermentation has still not reached completion. Furthermore, depending on the initial sugar content and on the other process parameters, maturation of Vin Santo can generally last 2-i years, and in some case more (Tachis, 2003). 

Alcoholic fermentation usually begins, albeit slowly, by yeast specialized to grow at low temperatures and they metabolize most of the sugars into alcohol. Subsequently, other strains more resistant to the increasing alcohol content prevail and finish the fermentation process. 

The first metabolic pathway that we encounter is glycolysis, an ancient pathway employed by a host of organisms. Glycolysis is the sequence of reactions that metabolizes one molecule of glucose to two molecules ofpyruvate with the concomitant net production of two molecules of ATP. This process is anaerobic (i.e., it does not require O2) inasmuch as it evolved before the accumulation of substantial amounts of oxygen in the atmosphere. Pyruvate can be further processed anaerobically (fermented) to lactate (lactic acidfermentation) or ethanol (alcoholic fermentation). Under aerobic conditions, pyruvate can be completely oxidized to CO2, generating much more ATP, as will be discussed in Chapters 17 and 18. 

With few exceptions, enzymatic processes in carbohydrates cause degradation. Enzymes are used in the form of pure or semipure preparations or together with their producers, i.e., microorganisms. Currently, semisynthetic enzymes are also in use. Alcoholic fermentation is the most common method of utilization of monosaccharides, sucrose, and some polysaccharides, e.g., starch. Lactic acid fermentation is another important enzymatic process. Lactic acid bacteria metabolize mono- and disaccharides into lactic acid. This acid has a chiral center thus either D(-), L(+), or racemic products can be formed. In the human organism, only the L(+) enantiomer is metabolized, whereas the D(-) enantiomer is concentrated in blood and excreted with urine. Among lactic acid bacteria, only Streptococcus shows specificity in the formation of particular enantiomers, and only the L(+) enantiomer is produced. 

The yeast responsible for alcoholic fermentation in winemaking is usually introduced into the must from the surface of the grapes, the surface of winery equipment, or from specifically prepared cultures. The fermentation process can occur either naturally, without inoculation, or by inoculating the must with selected starters. The use of locally selected yeast strains (usually belonging to the species Saccharomyces cereoisiae), with strain-specific metabolic characteristics can positively affect the final quality of the wine (Regodon et al., 1997 Romano et al., 2003). Several studies have clearly shown the effects of indigenous and inoculated yeast populations on the wine volatile composition (Mateo et al., 2001 Nurgel et al, 2003). 

Fermentation itself can be stopped when the desired alcohol content is reached, either by removal of yeast (filtration, centrifugation) or by their deactivation (pasteurization). A further possibility is to create fermentation conditions, which suppress yeast metabolism (Narziss, Miedaner, Kem, Leibhard, 1992). The most practical tool to suppress yeast metabolism is low temperature. This method is called the cold contact process (CCP) , which ensures very slow ethanol prodnction while other metabolic processes, snch as formation of higher alcohols and esters or reduction of carbonyls, may exhibit moderate activities (Perpete Collin, 1999). After interrupting the fermentation at an alcohol content less than 0.5% voL, the AFB is usually matured for at least 10 days at 0-1 °C to enrich flavour and improve the colloidal stability. Then the product is filtered, carbonated, stabilized and sterilized (Branyik, Silva, et al., 2012 Burberg Zamkow, 2009). 

Fermentation a form of metabolism producing incompletely oxidized end products. Per unit of substrate, F. yields far less energy than respiration, e.g. a yeast cell obtains 2 molecules ATP per molecule of glucose when it ferments glucose to ethanol, whereas complete respiration would yield 38 molecules of ATP (see Alcoholic fermentation). Strictly speaking, F. is an anaerobic process (Pasteur defined F. as life without air ) but the term is also widely and loosely applied to certain aerobic processes, such as acetic acid F., and to any industrial production process employing microorganisms in a fermentor (see Fermentation techniques). 

---