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啤酒酵母.ewer's Yeast

What Is Yeast?

Yeast are a group of unicellular fungi a few species of which are commonly used to leaven bread and ferment alcoholic beverages. Most yeasts belong to the division Ascomycota. A few yeasts, such as Candida albicans can cause infection in humans. More than one-thousand species of yeasts have been described. The most commonly used yeast is Saccharomyces cerevisiae, which was domesticated for wine, bread and beer production thousands of years ago. See Yeast.

Yeast physiology can be either obligately aerobic or facultatively fermentative. There is no known obligately anaerobic yeast. In the absence of oxygen, fermentative yeasts produce their energy by converting sugars into carbon dioxide and ethanol (alcohol). In brewing, the ethanol is used, while in baking the carbon dioxide raises the bread and the ethanol evaporates.

An example with glucose as the substrate is:  

C6H12O6 (glucose) →2C2H5OH + 2CO2 Yeasts can reproduce asexually through budding or sexually through the formation of ascospores. During asexual reproduction a new bud grows out of the parent yeast when the condition is right, then after the bud reaches an adult size, it separates from the parent yeast. Under low nutrient conditions, yeasts that are capapable of sexual reproduction will form ascospores. Yeasts that are not capable of going through the full sexual cycle are classified in the genus Candida.

Top-fermenting yeasts (so-called because they float to the top of the beer) can produce higher alcohol concentrations and prefer higher temperatures. An example is Saccharomyces cerevisiae, known to brewers as ale yeast. They produce fruitier, sweeter, real ale type beers. Bottom-fermenting yeasts ferment more sugars leaving a crisper taste and work well at low temperatures. An example is Saccharomyces uvarum, formerly known as Saccharomyces carlsbergensis. They are used in producing lager-type beers. Brewers of wheat beers often use varieties of Torulaspora delbrueckii.

Fig. 1.   Schematic representation of proposed intermolecular hydrogen bond interactions between isomaltose and -glucosidases from baker's and brewer's yeast and from oligo-1,6-glucosidase from baker's yeast. ^a , only for oligo-1,6-glucosidase. Invariant glycoside hydrolase family 13 side chain candidates of interaction with the four nonreducing substrate ring OH-groups are described in detail in a recent review.

Fig. 2.  Hydrolysis of isomaltose by baker's yeast -glucosidase followed by ¹ H NMR. (A) before addition of enzyme; (B) 4 min; and (C) 16 h after addition of enzyme.

Fig. 3.  Catalytic mechanism for retaining glycoside hydrolases including steps of protonation, formation of a covalent intermediate, and product release, respectively, but not the intermediate two transition-states

Recognition of diastereoisomeric isomaltoside derivatives:
   

Isomaltose is flexible due to rotation around the C5–C6 bond. It is possible to block this conformational flexibility by alkylation of C6 (Fig. 4). Previously, methyl 6-R - and methyl 6-S -methyl--isomaltoside were used to determine the preferred rotational conformer for glycoamylase. Hydrolysis catalyzed by baker's yeast -glucosidase (this enzyme was chosen as it has the highest activity of the two -glucosidases; see Table 1) was similarly examined using methyl 6-R -ethyl- and methyl 6-S -ethyl--isomaltoside as the pair of conformationally biased substrate analogs (Table 3). While methyl 6-S -ethyl--isomaltoside was hydrolyzed with twofold lower V _max , but the same K _m as isomaltose (Table 3), the 6-R enantiomer was a poor substrate V _max being 150-fold lower and K _m twofold higher than for isomaltose (Table 3). Baker's yeast -glucosidase thus preferred the 6-S isomer. In contrast, glucoamylase from A. niger hydrolyzed the 6-R enantiomer with 230-fold higher k _cat /K _m compared to the parent isomaltoside, the difference being essentially in the K _m and not in the rate of hydrolysis as for the -glucosidase. This distinct preference for one of the two diastereoisomers of the C-6 alkyl isomaltose derivatives reflects the fact that one of the rotamers adopts a conformation with more favorable spatial distribution of the groups that play an important rolein the enzyme recognition. This finding stresses the fundamentally different active site architecture that exists for the inverting glucoamylase and the retaining -glucosidases. Glucoamylase, in contrast to -glucosidase, applies a single displacement mechanism and belongs to a different fold family, glycoside hydrolase 15. The specific activities and substrate affinities are similar for these retaining and inverting enzymes, all of which have reasonable capacity in the glucose release from the nonreducing end of disaccharides and small substrates. However, the -glucosidase showed large variation in rate of hydrolysis between the methyl 6-S - and 6-R -ethyl -isomaltosides, with small differences in affinity for the two distereoisomers, whereas the discrimination by glucoamylase was associated with the K _m  and not with the rate of hydrolysis (Table 3).

Fig. 4.   Structure of the conformationally biased diastereosisomer substrates methyl 6-R -ethyl--isomaltoside (A) and methyl 6-S -ethyl--isomaltoside (B) .

 Table 3.  Kinetic parameters for the hydrolysis of conformationally biased isomaltosides.

 EXAMPLE 1:

    Brewer's yeast was subjected to autolysis, for breaking up of the cell membranes. The lysed product was then separated by centrifugation to produce yeast debris the fraction containing the cell bodies having a viscosity of 10 cP (5% aqueous solution).

    The yeast debris was then treated with sodium bicarbonate to as to produce an extracted mix having a pH of 8.5. The mix was then stirred for about one hour at room temperature, and centrifuged for further separation.

    The centrifuged material resulted in two streams namely cell wall rich material and undegraded cells. The cell wall rich material was resuspended in 2.5% sodium hydroxide and then 40% sodium hydroxide added so as to adjust the pH of the material to 12.5. The cell wall rich material was then indirectly heated on a water bath to about 65° C. for about one hour and then bleached by treatment with hydrogen peroxide, for about one hour with mixing. Concentrated hydrochloric acid was then added to the bleached material so as to achieve a pH of 7.0; the material was further centrifuged and the pH further lowered to 5.0. The resulting product was substantially free of any whole yeast cells and predominantly comprised yeast ghosts or shells having substantially uncollapsed walls. The yeast ghosts confined a lower quantity of yeast cell contents relative to the whole cells of the debris.

    Water  was acidified to pH 2 with dilute hydrochloric acid and turmeric added to the resulting solution. The yeast cell ghosts were added to the solution and the mixture stirred for one minute, centrifuged, washed twice with water and freeze-dried to give a stable canary yellow lake.

EXAMPLE 2:

    Water was acidified to pH 6 with dilute hydrochloric acid and annatto added to the resulting solution. Yeast cell ghosts as prepared in Example 1 were added to the solution and the mixture stirred for 1 minute, centrifuged, washed twice with water and freeze-dried to give a stable orange lake.

Properties:

Brewers yeast contains many different vitamins, minerals and amino acids.  The major vitamins are:-

Vitamin B1 (thiamin) –releases energy from carbohydrate in the diet, and is important for the heart and nervous system.

Vitamin B2 (riboflavin) – releases energy from carbohydrate in the diet, and maintains healthy skin, eyes and digestive tract.

Niacin (Vitamin B3) – releases energy from carbohydrate, and helps to maintain healthy skin, digestive tract and nervous system.

Contra-indications/Precautions

Best avoided by anyone taking monoamine oxidase inhibitors and people suffering from gout.