Yeast, in fermentation

The role of yeast in fermenting dough maturation is even less clear. The alcohol and carbon dioxide developed during fermentation must influence the elastic properties of the protein matrix. However, experimental procedures that would permit this to be checked in the absence of yeast have not been developed. 

Sulfurous acid inhibits molds and bacteria and to a lesser extent yeasts. For this reason, S02 can be used to control undesirable bacteria and wild yeast in fermentations without affecting the S02-tolerant cultured yeasts. According to Chichester and Tanner (1968), the undissociated acid is 1,000 times more active than HS03 for Escherichia coli, 100 to 500 times for Saccharomyces cerevisiae, and 100 times for Aspergillus niger. 

Marks, V. D., van der Merwe, G. K., van Vuuren, H. J. J. (2003) Transcriptional profiling of wine yeast in fermenting grape juice Regulatory effect of diammonium phosphate. FEMS Yeast Research, 3, 269-287. 

Following Pasteur s work in the 1860s, which elucidated some roles of microbes in acetification (wine turning to vinegar), interest in microbial transformations slowly developed. By the end of the century several processes were in use. Even resolution of some racemates to obtain (at least one) optically pure enantiomer were viable procedures (e.g., lactic and mandelic acids). An observation that yeast (in fermentations) is capable of reducing 17-ketosteroids to 17(3-hydroxysteroids in the late 1930s ultimately led to a search for other microbially mediated chemical transformations. 

Mendonca, R. C. S., Gouvea, D. M., Hungaro, H. M., Sodre, A. D., Querol-Simon, A. (2013). Dynamics of the yeast flora in artisanal country style and industrial dry cured sausage (yeast in fermented sausage). Food Control, 29, 143-148. 

For more information on brewing and the role of yeast in fermentation and beer quality, see Boulton and Quain (2006). Metabolic engineering and the tools available for its application are described comprehensively by Wittmann and Lee (2012). The Journal of Metabolic Engineering (www.joumals.elsevier.com/metablic-engineering) is an excellent resource for up-to-date information regarding all aspects of metabolic engineering. 

Pasteur, Louis (1822-95) French chemist and biologist. Pasteur is famous for his investigations on stereochemistry, fermentation, and vaccines. In his early work he discovered the phenomenon of optical isomers and invented methods for separating such isomers. In 1860 he postulated that optical isomers existed because of the arrangements of atoms in the molecules. This idea was a major stimulus to the development of structural chemistry. His work on fermentation led to the process known as pasteurization, which uses elevated temperatures to kill organisms that spoil milk or wine. He also established the role of micro-organisms such as yeast in fermentation. 

Biochemistry resulted from the early elucidation of the pathway of enzymatic conversion of glucose to ethanol by yeasts and its relation to carbohydrate metaboHsm in animals. The word enzyme means "in yeast," and the earfler word ferment has an obvious connection. Partly because of the importance of wine and related products and partly because yeasts are relatively easily studied, yeasts and fermentation were important in early scientific development and stiU figure widely in studies of biochemical mechanisms, genetic control, cell characteristics, etc. Fermentation yeast was the first eukaryote to have its genome elucidated. 

Fruit and Vegetable Products. Sorbates are appHed at 0.05—0.1 wt % as a fungistat for pmnes, pickles, reHshes, maraschino cherries, oHves, and figs (64,112). The same levels extend shelf life of prepared salads such as potato salad, cole slaw, and tuna salad (99). In fermented vegetables, sorbates protect the finished product by retarding yeasts during fermentation or in the cover brine (r65,r72—r74,r94). 

The fermentative activity of the yeasts is almost the same in aH industries. Differences in fermentation times arise mainly from differences in temperature. For the baking industry, temperature is high, therefore fermentation time is short, with Htde time for ceU multiplication and very high ceU counts at the start. 

Fermentation is carded out in two different, very distinct ways top fermentation and bottom fermentation. The governing principles are the same in both processes the chief differences are in the type of yeast and temperature employed, and consequently the method used for collecting the yeast after fermentation is finished. The alcohol content and, to a higher degree, the taste and stabiUty of the beer, are directly dependent on the normal progress of the fermentation. 

Fermentation Processes. The efficient production of penicillin, yeasts, and single-ceUed protein by fermentation requires defoamers to control gas evolution during the reaction. Animal fats such as lard [61789-99-9] were formerly used as a combined defoamer and nutrient, but now more effective proprietary products are usually employed. Defoamer appHcation technology has also improved. For example, in modem yeast production faciHties, the defoamers are introduced by means of automatic electrode-activated devices. One concern in the use of defoamers in fermentation processes is the potential fouHng of membranes during downstream ultrafiltration (qv). SiHcone antifoams (43,44) seem less troubled by this problem than other materials. 

Brewing Austenitic steels are used in fermentation vessels, storage tanks and attemperators, in yeast processing, and for transport tanks and barrels. 

Microorganisms have been identified and exploited for more than a century. The Babylonians and Sumerians used yeast to prepare alcohol. There is a great history beyond fermentation processes, which explains the applications of microbial processes that resulted in the production of food and beverages. In the mid-nineteenth century, Louis Pasteur understood the role of microorganisms in fermented food, wine, alcohols, beverages, cheese, milk, yoghurt and other dairy products, fuels, and fine chemical industries. He identified many microbial processes and discovered the first principal role of fermentation, which was that microbes required substrate to produce primary and secondary metabolites, and end products. 

The term fermentation was obtained from the Latin verb fervere which describes the action of yeast or malt on sugar or fruit extracts and grain. The boiling is due to the production of carbon dioxide bubbles from the aqueous phase under the anaerobic catabolism of carbohydrates in the fermentation media. The art of fermentation is defined as the chemical transformation of organic compounds with the aid of enzymes. The ability of yeast to make alcohol was known to the Babylonians and Sumerians before 6000 bc. The Egyptians discovered the generation of carbon dioxide by brewer s yeast in the preparation... 

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