Yeast removal

Sweet wines, with potential for yeast refermentation, or wines with potential for malo-lactic fermentation, go through a membrane filtration prior to bottling. The membrane filters come in different porosities 0.65-p, pore size is used most commonly when 100 percent yeast removal is desired, and 0.45-p, pore size is used for malo-lactic bacteria removal. Proper sterilization of bottling equipment downstream of the membrane filter is essential to maintain the yeast- or bacteria-free nature of the wine after filtration. 

Figure 14. Flow diagram of yeast removal—Wineries F, G, and H...

Fig. 4 Role of xylulose-borate complexation in the co-iimnobilized enzyme system. When sodium tetraborate (borax) is added to solution, it dissociates into tetrahydroxyborate (borate, B) ion and boric acid. In the pellet interior, higher pH favors tighter xylulose-borate binding, which effectively reduces the xylulose concentration in the interior and forces the isomerization forward. In the bulk, the lower pH has an uncoupling effect on the Xu-B complex, making the dissociated xylulose readily available to the yeast. Removal of xylulose via fermentation further forces dissociation of the xylulose-borate complex. Dashed lines represent transport of species solid lines represent reactions... 

North American system variant to above Temperature arising rapidly from 10° to 17°C (50 to 63°F) then held Beer at 2°P (SG 1008) chilled rapidly to 0°C (32°F) and yeast removed by centrifuge 3-4 days at 0°C (32°F). No secondary fermentation... 

Yeast removed from the fermenter by suction is usually collected in an enclosed cylindrical vessel called a yeast back. The yeast is usually pumped from the back to a sheet filter press where entrained beer (barm ale) is recovered and after pasteurizing it is blended with racked beer (Fig. 19.16). Sometimes the yeast is held in the filter which is cooled by water or brine circulating in a jacket around it. In other cases, the yeast is discharged from the filter into trays or trucks which are placed in a refrigerator until it is required for pitching. 

The mmen is not functional at birth and milk is shunted to the abomasum. One to two weeks after birth, the neonate consumes soHd food if offered. A calf or lamb that is nursing tends to nibble the mother s feed. An alternative method of raising the neonate is to remove it from its mother at a very young age, <1 week. A common example of an early weaning situation is the dairy calf that is removed from the cow soon after birth so that the cow s milk supply might be devoted entirely to production. In this instance, the neonate requires complete dietary supplementation with milk replacer. Sources of milk replacer protein have traditionally included milk protein but may also include soybean proteins, fish protein concentrates, field bean proteins, pea protein concentrates, and yeast protein (4). Information on the digestibiUty of some of these protein sources is available (4). 

Other bacterial strains identified as biodegrading poly(vinyl alcohol) iaclude Flavobacterium (95) 2in.dFicinetobacter (96) and many others, as well as fungi, molds, and yeasts (97). Industrial evaluations at Du Pont (98) and Air Products (99) iadicate that over 90% of poly(vinyl alcohol) entering wastewater treatment plants is removed, and hence no environmental pollution is likely. 

As vitamin Bg is mainly located in the germ and aleurone layer in cereal grains polishing for the production of flour removes a substantial portion. White bread is therefore a poor source unless fortified. Some nonedible yeasts contain up to 38 mg/100 g dry weight vitamin B, the highest level of the natural sources (4,27). As a rule, these amounts are too low for cost-effective isolation. 

Continuous processes may be used for the production of yeast biomass. Raw Hquid feed is added continuously to the fermentor and an equal volume of fermentor Hquid is removed to harvest the yeast ceUs. These may be a single homogeneous fermentation in a stirred fermentor or two fermentors in series. Growth rates are high a typical dilution rate in the production of C. utilis on sulfite waste Hquor is 0.25, ie, one-fourth of the fermentor volume is harvested hourly. 

The most widely available yeast biomass is a by-product of the brewing industry, where the multiplication of yeast during brewing results in a surplus of ceUs. Eor every barrel (117 L) of beer brewed, 0.2—0.3 kg of yeast soHds may be recovered. In the U.S., a substantial fraction is recovered and made available about 40 x 10 kg of brewers yeast aimually. The yeast is recovered from beer by centrifuging and dried on roUer dmms or spray dryers and sold as animal feed or a pet-food supplement. It can be debittered by alkaline extraction to remove the bitter hop residues, and is then sold mainly by the health-food industry. It is available as tablets, powder, or flakes and is often fortified with additional vitamins. Distillers yeast caimot be readily separated from the fermented mash and the mixture is sold as an animal feed supplement. 

The presence of nucleic acids ia yeast is oae of the maia problems with their use ia human foods. Other animals metabolize uric acid to aHantoia, which is excreted ia the uriae. Purines iagested by humans and some other primates are metabolized to uric acid, which may precipitate out ia tissue to cause gout (37). The daily human diet should contain no more than about 2 g of nucleic acid, which limits yeast iatake to a maximum of 20 g. Thus, the use of higher concentrations of yeast proteia ia human food requires removal of the nucleic acids. Unfortunately, yields of proteia from extracts treated as described are low, and the cost of the proteia may more than double. 

Filtering. Conditioning or 1 agering gives the beer its desired organoleptic properties, but it stiU contains yeast, protein-tannin complexes, etc, ie, it has a hazy appearance. A high quaHty beer must be clear and totaUy sterile, have coUoidal stabiHty, and yeast must be removed to aUow the beer to have biological stabiHty. The protein-tannin complexes must also be removed so as not to upset the coUoidal stabiHty. 

Beet juice contains about 80% of fermentable carbohydrates and nitrogenous compounds. To remove these compounds, a yeast fermentation utilising Candida utillis has been suggested (141). By so doiag, a more concentrated form of the dye becomes available. The red dye from beets is sold as beet juice concentrate, as dehydrated beet root, and as a dried powder. 

Dried Whole Egg and Yolk. Dried plain whole egg and yolk products are either dried as is, or have the glucose removed to improve stabiHty and shelf life of the product. Glucose is removed before drying by use of glucose oxidase or by yeast fermentation (see Yeasts). Bacterial fermentation is not used because of off-flavor and off-odor development. 

Ethanol fermentation is a particularly good example of product accumulation inhibiting the microbial culture. Most strains of yeast have a much slower alcohol production rate when ethanol reaches about ten percent, and the wine or said strains that achieve over 20 percent by volume of ethanol are very, very slow. A system known as the Vacuferm for removal of alcohol by distillation as it is formed is... 

ENZYMATIC ANALYSIS WITH CARBOXYPEPTIDASES. Carboxypeptidases are enzymes that cleave amino acid residues from the C-termini of polypeptides in a successive fashion. Four carboxypeptidases are in general use A, B, C, and Y. Carboxypeptidase A (from bovine pancreas) works well in hydrolyzing the C-terminal peptide bond of all residues except proline, arginine, and lysine. The analogous enzyme from hog pancreas, carboxypeptidase B, is effective only when Arg or Lys are the C-terminal residues. Thus, a mixture of carboxypeptidases A and B liberates any C-terminal amino acid except proline. Carboxypeptidase C from citrus leaves and carboxypeptidase Y from yeast act on any C-terminal residue. Because the nature of the amino acid residue at the end often determines the rate at which it is cleaved and because these enzymes remove residues successively, care must be taken in interpreting results. Carboxypeptidase Y cleavage has been adapted to an automated protocol analogous to that used in Edman sequenators. 

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