BIOLOGY OF YEAST
Yeasts were the first microorganisms which people began to use to meet their needs. The main property of yeast, which has always been attractive to human - Is the ability to form relatively large amounts of alcohol from sugar. Another group of processes that have long been used yeast, is also linked with their ability to alcoholic fermentation: the formation of carbon dioxide under the action of yeast - a key step in the preparation of bread, resulting in fermentation test. This process is also very old. Already by 1200 BC. er. Egypt was well known to the difference between bread made of unleavened dough and sour, as well as the benefits of of yesterday's test for fermentation of fresh.
Features of yeast in nature have become interested in microbiology from from the earliest studies of the processes of traditional winemaking. Initially, the study of yeast limited to those species and strains that caused брожение при приготовлении пива и вина. Однако уже в конце XIX в. М.Бейеринк высказывал мысль о том, что эти культурные виды represent selektsionirovannye form «wild" yeast , widespread in nature.
term "wild yeast" usually refer to species and strains of yeast contained in the wine stum or wort to the fermentation process. In the traditional process of winemaking and brewing it "Wild yeast" fermentation process started, but then were replaced by a more rapidly growing strains of Saccharomyces also initially present in the fermentation raw material or contained in sourdough added. With significant development of wild yeast can cause undesirable changes in quality drink, that is to become weeds fermentation . In modern fermentation industries in the wort is commonly added yeast culture race, and "wild yeast "usually kill sulfation. In yeast taxonomy the term "wild yeast" is also used to refer to natural populations yeast, which closely related cultural races and, perhaps, are their ancestral forms.
Naturally, the question arose about the sources of their falling in fermenting substrates. The first study, performed the founders zymology E. Hansen and A. Klekerom, were devoted to this theme: the search of natural sources of wine yeast Saccharomyces cerevisiae . saccharomyces were found on grapes, but as it turned out, here is dominated by very different types of yeast, not involved in the subsequent fermentation of grape juice.Bakery
All yeast used in bread baking are to the form Saccharomyces cerevisiae and historically come from strains of brewer's yeast. Flour usually contains almost no free sugars, which can ferment with yeast. In low-grade flour may contain enzymes that cleave starch, but high-purity grades of flour, these enzymes are destroyed, and for fermentation dough into the flour must be added sugar. When fermentation is intense release of CO 2 , which lingers in the dough, causing it to rise. The resulting alcohol is removed by baking.
used yeast for bread prepared with breweries. In the late nineteenth century. An entire industry has evolved to produce pressed or dry baker's yeast. Modern production of baker's yeast has a number of significant features in comparison with the fermentation industry. The main goal of this production - getting the yeast to produce a high rate in the test of carbon dioxide by fermentation under anaerobic conditions. However, they need to perform with good aeration to achieve greater yield of yeast biomass ( effect Pasteur ). Obtained yeast should not only have high fermentative activity in the test, but a well kept without losing its properties in a frozen or dried state. Baker's yeast are grown in large vessels under vigorous stirring and aeration. In this culture medium, which usually serves as a basis for molasses, applied gradually, or portions. If you add just a lot of sugar, then switch to the metabolism of yeast fermentation ( effect Crabtree ) and the yield of biomass reduced. Upon completion of the growth of yeast was concentrated by centrifugation and filtered. Formed on the filter cake can be converted into briquettes compacted yeast. Dry yeast obtained by drying the mass in special spray dryers.
As the baker's yeast added to dough in a concentration about 1% by weight of flour, they constitute an important source of microbial biomass in food. Despite the fact that the strains Saccharomyces cerevisiae do not have any toxic effect, they contain relatively large amounts of nucleic acids that can cause high levels of uric acid in the body, resulting in to gout. Reduction of nucleic acids - an important direction in the selection of nutritional yeast.
Pasteur effect and Crabtree effect
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Demonstration joint Pasteur effect and Crabtree effect in continuous hemostatnoy Yeast culture: changes in cell concentration, glucose and ethanol depending on the rate of dilution. The rate of dilution reflects the rate of income Wednesday glucose and approximately equal to the rate of growth of yeast. At a low dilution rate of all glucose utilized through respiration and ethanol is not formed. At a higher rate of dilution in the culture begins to accumulate ethanol, and the concentration of cells decreases. At still higher rate of dilution in the fermentor glucose concentration begins to rise, as the speed of its revenues exceed the rate of recycling. |
Saharomitsetovye yeast can grow under conditions of fermentation with a very high rate (doubling time about 1.6 hours), but the final yield of cells at This turns out to be insignificant. Under the conditions conducive to aerobic metabolism, the growth of yeast occurs at the same rate, but reached much greater biomass growth. The fact that a given quantity of sugar in aerobic conditions, yeast cells is greater than the anaerobic, first noticed Pasteur , and therefore it is called the Pasteur effect. The reason for this effect became clear after deciphering biochemical pathways of fermentation and respiration: anaerobic conversion of one glucose molecule during glycolysis formed four ATP molecules, and at its full decay in the Krebs cycle - 36.
Alcohol fermentation in yeast occurs not only under anaerobic conditions. Yeast growing in the presence of oxygen, but at a high level of glucose in the medium and fermentation glucose into ethanol. Suppression of aerobic respiration high glucose (more precisely, high-speed glucose uptake) is called Crabtree effect, or catabolite repression. This effect was not observed in growing Saccharomyces cerevisiae on the medium with a less easily digestible sugars such as galactose. Crabtree effect found in yeast, as well as in cancer and embryonic cells. Detail the mechanism of this phenomenon remains unclear.
metabolism
Although yeast is not as varied in their metabolism , such as bacteria, different types of yeast can katabolizirovat different connections carbon and nitrogen and form various end products.
Alcoholic fermentation
most well-known property of many yeast - the ability to fermentation . Many types of yeast can switch from fermentation to respiratory metabolism and vice versa, depending on conditions: if oxygen inhibited fermentation and yeast begin to breathe in the absence of oxygen, the mechanism of alcoholic fermentation. Since oxygen breathing - energetically more favorable process than fermentation, the yield of yeast biomass per unit of substrate used higher in growing them under aerobic conditions than anaerobically. This phenomenon is called effect Pasteur . Alcoholic fermentation can take place not only in anaerobic conditions. If the yeast is grown in the presence of oxygen, but with a high content of glucose in the medium, then the yeast and ferment glucose. Thus, glucose inhibits the processes of aerobic respiration. This phenomenon is called effect Crabtree , or catabolite repression.
Breath
With the growth in aerobic conditions at low glucose concentrations in the environment of the yeast ATP obtained by the processes of respiration, as it does the majority of aerobic organisms. Complete oxidation of the carbon substrate to carbon dioxide and water can occur in yeast in cycle tricarboxylic acids , and pentose phosphate cycle . In the operation of each of these cycles in the cell leads to the formation of reduced pyridine nucleotides. They can be used either for renewal processes in the biosynthesis, or to obtain ATP through oxidative phosphorylation .
If under anaerobic conditions, only a few types of yeast can ferment pentatomic sugar, in the presence of oxygen pentoses (arabinose, xylose, etc.) and metilpentozy (Rhamnose) can serve as substrates for many yeasts. This results in the conversion of pentoses in the corresponding intermediates of the pentose phosphate ways .
Many yeasts are also capable of aerobically assimilate a variety of sugar derivatives, such as saharospirty (sorbitol, ribit, erythritol, mannitol, inositol) или гликозиды (арбутин, салицин).
Полисахариды также могут усваиваться дрожжами, способными к синтезу соответствующих гидролаз. Довольно часто встречаются yeast assimilating starch. Many yeast species are able to digest polysaccharides such as polifruktozany (inulin), poligalaktouronidy (pectins), hemicellulose (Xylans). Significant changes in the structural organization of the cells occur by changing the growth conditions: the transition from anaerobic to aerobic cultivation, replacing the medium composition and concentration of limiting substrate, with a lack of growth factors, as well as the physiological effects of stress. Particularly noticeable ultrastructural reorganization of the membrane apparatus in aerobic-anaerobic shifts. Under anaerobic fermentation mitochondria unrecognizable change their appearance: they are united, Christy straighten. Conversely, when the aeration mitochondria actively share their number increases, they are clearly visible cristae. Under anaerobic conditions in the cells accumulate more glycogen and he presented large granules, and sometimes forms a conglomerate. Significant structural changes occur in the cell when it is dehydrated. Viability it does not lose and dried cells easily reactivated. Exiting anabiotic state begins with the rehabilitation and reactivation of mitochondria. Cytological features of yeast under different conditions of growth
Secondary fermentation products
during alcoholic fermentation in addition to the two main products - ethanol and carbon dioxide - is formed many side, or secondary products. Many of them are sensory connections, and play an important role in the production of alcoholic wine and beer, since the relations depend on their taste and aroma of the drink. Organoleptic compounds can be divided into at least five chemical groups: higher alcohols, acids, esters, aldehydes, sulfur-containing substances. From alcoholic beverages allocated hundreds of organoleptic compounds. Many of them are present in very small quantities and are difficult to identify and quantify. Even more difficult to determine the contribution of these compounds in the final taste of the product as for various substances characteristic smell different thresholds, ie the concentration at which individual compound can be caught by the olfactory receptors. Moreover, this threshold is also dependent on the presence in the beverage other sensory compounds.
temperature
yeast species vary considerably in temperature range of growth . In most species of yeast minimum temperature growth is within 0-5 ° C and a maximum - 30-40 ° C. Almost all of the yeast can grow at room temperature 20-25 ° C. Bazidiomitsetovye yeast are generally characterized lower maximum temperatures of growth than askomitsetovye. However, these rules have some exceptions.
Water activity
Water activity (a w ) in a solution is defined as the ratio of vapor pressure above the solution to the vapor pressure of pure water. For pure water a w = 1. Without exception, the yeast can grow at a w , approaching 1. Most species of yeast ceases to grow at a w = 0.9.
Such activity in the water has, for example, 50% glucose solution or 14% solution of NaCl * salt. However, a number of yeasts able to grow at a concentration of glucose in the medium to 60% or at a concentration NaCl to 20%.
Such yeasts are called kserotolerantnymi .
These include askomitsetovye yeast Zygosaccharomyces bailii , Zygosaccharomyces rouxii , Schizosaccharomyces pombe , Debaryomyces hansenii
, etc. These types of often encountered in various preserves, jams, syrups, dried fruit and can cause damage to these products . Yeast Debaryomyces hansenii particularly resistant to high NaCl concentration in the medium and are often found in a variety of pickles, for canned meat products in seawater.
acidity
optimum pH for growth of most yeasts are found in the medium acidity (pH 4-6). However, some species can grow in a more acidic environment. For example, some strains Saccharomyces cerevisiae grow well at pH 2.5-3. By acid-yeast is also inhabits the intestinal tract Animal Cyniclomyces guttulatus , growing at pH 2. Yeast, which can grow at alkaline pH (8 or more) is not izvestny.Prakticheski all kinds of yeasts can grow in the pH range 4-4.5.
At the same time, in a weakly acidic medium is not growing, most banal of bacteria that are most common in a variety of natural habitats (Eg, Pseudomonas, Bacillus, coryneform bacteria, actinomycetes). based on this simple method of selective isolation of yeasts: Nutrition environment (eg, wort-agar) acidified with HCl or lactic acid to pH 4-4,5. In most cases such an environment, grow the yeast and the fast-growing filamentous mushrooms.
Killer effect in yeast
Killer effect was first discovered in Saccharomyces cerevisiae Makourom and Bevagna in 1963 Killer yeast secrete a toxin to which they are stable, but that is lethal for others - a sensitive strains.
Killer yeasts have been studied intensively in recent in connection with the prospects of their practical use. In the fermentation industry, for cooking wine, beer, often the infection of fermenting wort so-called « weeds » fermentation, which alter the course of fermentation in an undesirable direction. Using killer strain Saccharomyces cerevisiae fermentation eliminates the development of wild yeast - killer toxin will simply inhibit their growth.