S. cerevisiae

S. cerevisiae is produced by fed-batch processes in which molasses supplemented with sources of nitrogen and phosphoms, such as ammonia, ammonium sulfate, ammonium phosphate, and phosphoric acid, are fed incrementally to meet nutritional requirements of the yeast during growth. Large (150 to 300 m ) total volume aerated fermentors provided with internal coils for cooling water are employed in these processes (5). Substrates and nutrients ate sterilized in a heat exchanger and then fed to a cleaned—sanitized fermentor to minimize contamination problems. 

C. uti/is yeast is produced by either fed-batch or continuous processes. Aerated-agitated fermentors range up to 300 m total capacity and ate operated in the same manner as described for S. cerevisiae (2,5). C. utilis is capable of metabolizing both hexose and pentose sugars. Consequendy, papermiU wastes such as sulfite waste Hquot that contain these sugars often ate used as substrates. 

Significant protection of mice by several polysaccharides other than glucan isolated from S. cerevisiae has been described (209). A 2.16-fold protection in the LD q q is observed for one modifier, MNZ, when given 15 min prior to irradiation. Glucan protects 2.25-fold in this same protocol. Many of these polysaccharides may act through activation of the complement system, rather than directiy on ceHs. 

Saccharomyces cerevisiae S. cerevisiae var. ellipsoideus S. carlsbergensis S.fragilis S. rouxii S. delbrueckii... 

Manufacturing procedures of riboflavin have also appeared using Saccharomjces bacteria, eg, fermentation with a purine-independent S. reverse mutant (61) and with S. cerevisiae NH-268 (62) produced 2.79 g/L and 4.9 g/L, respectively. 

The majority of industrial research describes classical approaches to yield improvement (49). However, there has been some work using genetically modified organisms. In the case of these recombinant organisms, the carotenoid biosynthetic gene cluster has been expressed in noncarotegenic species such as E. coli (50) and S. cerevisiae (51). 

Fig. 1. Scanning electron micrograph showiag ceUs of S. cerevisiae. Bud scars are visible at the ends of ceUs. Scale bar is 5 p.m.

Fig. 2. Karyotype of 10 S. cerevisiae strains. Intact chromosomes have been separated electrophoreticaUy by size in a TAPE gel. (a), gel mn to separate most chromosomes. Large chromosomes are at the top, smaller at the bottom. Since most strains are polyploid, more than 16 bands may be observed, (b). Chromosomes from the same strains have been separated in a gel mn to enhance resolution of the smaller chromosomes, corresponding to the 4—5...

Mutation. For industrial appHcations, mutations are induced by x-rays, uv irradiation or chemicals (iiitrosoguanidine, EMS, MMS, etc). Mutant selections based on amino acid or nucleotide base analogue resistance or treatment with Nystatin or 2-deoxyglucose to select auxotrophs or temperature-sensitive mutations are easily carried out. Examples of useful mutants are strains of Candida membranefaciens, which produce L-threonine Hansenu/a anomala, which produces tryptophan or strains of Candida lipolytica that produce citric acid. An auxotrophic mutant of S. cerevisiae that requires leucine for growth has been produced for use in wine fermentations (see also Wine). This yeast produces only minimal quantities of isoamyl alcohol, a fusel oil fraction derived from leucine by the Ehrlich reaction (10,11). A mutant strain of bakers yeast with cold-sensitive metaboHsm shows increased stabiUty and has been marketed in Japan for use in doughs stored in the refrigerator (12). 

Beer taste can be spoiled by contaminating bacteria or yeasts. The most common bacteria are lactic and acetic acid producers and T ymomonas. Wild yeasts can be anything other than the intended strain S. uvarum is considered a contaminant of ale fermentations and S. cerevisiae a contaminant of lager fermentations. The common wild yeast contaminants are S. diastaticus and species of Picbia, Candida and Brettanomjces. It may be noted that the flavor of beer may be improved by the ability of yeast to adsorb bitter substances extracted from hops, such as humulones and isohumulones. 

Addition of up to 200 ppm sulfur dioxide to grape musts is customary. Strains of S. cerevisiae and S. bayanus grown in the presence of sulfite, become tolerant of fairly high concentrations of SO2. Cultures propagated in the winery are added in Hquid suspension, usually at 1—2% of the must volume. Many strains are available in pure culture. Factors such as flocculence, lack of foaming, fast fermentation, lack of H2S and SO2 formation, resistance to sulfur dioxide and other inhibitors, and flavor production will affect strain choice. No strain possesses all the desired properties. 

The dry yeasts have excellent storage stabiUty, up to a year or more if packaged under an inert atmosphere (N2, CO2, or vacuum). First introduced into the United States and then AustraUa, they are now being introduced into European winemaking as well. A number of strains of S. cerevisiae S. bayanus and S. fermentati are available. 

Saccharomyces yeasts are rapid fermentors. S. cerevisiae and S. bayanus produce up to 18—20% ethanol. The cells are ovoid to spherical, eUiptical, or elongated (especially under conditions of nitrogen starvation). Vegetative propagation is by multilateral budding. S. uvarum and S. rosei occur earher in the fermentation, when S. rosei may produce up to 6—8% ethanol before being overgrown by the other Saccharomyces yeasts. S. cerevisiae may produce up to 18-20% ethanol (28). 

Enzymes. Invertase (P-fmctofuranosidase) is commercially produced from S. cerevisiae or S. uvarum. The enzyme, a glycoproteia, is not excreted but transported to the cell wall. It is, therefore, isolated by subjecting the cells to autolysis followed by filtration and precipitation with either ethanol or isopropanol. The commercial product is available dry or ia the form of a solutioa containing 50% glycerol as a stabilizer. The maia uses are ia sucrose hydrolysis ia high-test molasses and ia the productioa of cream-ceatered candies. 

FIGURE 12.40 Phylogenetic comparison of secondary structures of 16S-Uke rRNAs from (a) a eubacterium (E. coli), (b) an archaebacterium (H. volcanii), (c) a eukaryote S. cerevisiae, a yeast). 

A seed culture of S. cerevisiae ATCC 24860 (American Type Culture Collection, Manassas, VA, USA) was grown in a media of 5g glucose, and 0.5 g yeast extract, respectively, 1.5 g KH2P04 and 2.25 g Na2P04 phosphate buffer up to a total volume of distilled water, 500 ml. The media was sterilised at 121 °C for 15 min. The stock culture of the microorganisms was transferred to the broth media for preparation of seed culture. 

Fig. 8.4. Electron micrographs of the outer surface of immobilised S. cerevisiae beads, (a) Outer surface of the fresh beads with magnification of 300 p,m. (b) Outer surface of the fresh beads with magnification of 2000 p,m. (c) Outer surface of the used beads after 72 hours with magnification of 300 p,m. (d) Outer surface of the used beads after 72 hours with magnification of 2000 p,m. Reprinted from Najafpour et al. (2004). Copyright with permission from Elsevier.

Fig. 8.8. Relative activities of S. cerevisiae in batch fermentation. Reprinted from Najafpour et al. 2004.18 Copyright with permission from Elsevier.

The volume of reactor without beads was 1.4 1. The column was loaded with the solidified uniform beads of S. cerevisiae. The void volume of the reactor was 660 ml when it was packed with immobilised beads. The growth of beads with different proportions of column packing is shown in Figure 8.9. A fresh feed of 10 g l 1 glucose solution was pumped from the bottom of the reactor. The optimum amount of packing obtained was 65-70% of the reactor volume. The trend of the collected data resembles the growth curve of yeast in... 

A high glucose concentration of 150 g l 1 was used in continuous fermentation with immobilised S. cerevisiae the obtained data for sugar consumption and ethanol production with retention time are shown in Figure 8.13. As the retention time gradually increased the glucose concentration chopped, while the ethanol concentration profile showed an increase. The maximum ethanol concentration of 47 g l 1 was obtained with a retention time of 7 hours. The yield of ethanol production was approximately 38% compared with batch data, where only an 8% improvement was achieved. 

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