Strains, yeast preservation

The growth of malo-lactic bacteria in wines is favored by moderate temperatures, low acidity, very low levels of S02, and the presence of small amounts of sugar undergoing fermentation by yeast. It is frequently possible to inoculate a wine with a pure culture of a desirable strain of bacteria and obtain the malo-lactic fermentation under controlled conditions. The pure-culture multiplication of the selected strain of bacteria is difficult, however. It is also difficult to control the time of the malo-lactic fermentation—sometimes it occurs when not wanted, and at other times will not go when very much desired. For the home winemaker it is probably most satisfactory to accept the malo-lactic fermentation if it occurs immediately following the alcoholic fermentation. The wines should then be siphoned away from deposits, stored in completely filled containers at cool temperatures, and have added to them about 50 ppm S02. If the malo-lactic fermentation does not take place spontaneously and the wine is reasonably tart, the above described regime of preservation will likely prevent its occurrence. When the malo-lactic transformation takes place in wines in bottles, the results are nearly always bad. The wine becomes slightly carbonated, and the spoiled sauerkraut flavors are emphasized. 

Food can be preserved by fermentation using selected strains nf yeast, lactic acid-producing bacteria, or molds. The production of ethanol, lactic and other organic acids, and anlimicrobial agents in the food, along with the removal of fermentable sugars, can yield a product having an extended shelf life. 

Certain strains of yeast, moulds and bacteria can survive in relatively low pH conditions and some of these can exist and grow in the presence of certain preservatives so it is important that everything is done to prevent their multiplying. Under favourable conditions, a typical rapidly growing yeast strain can double its numbers every 30 min, and at this rate in 12 h one yeast could become 16.7 X 106 cells, provided no inhibitory factor is present. 

Because of mounting consumer resistance to the excessive use of sulfur dioxide and other chemical preservatives in wine, the use of bacteriocins as preservatives has generated interest among researchers. In a study by Schoeman et al (1999), bactericidal yeast strains were developed by... 

The P. aeruginosa PAl strain, previously selected from a petroleum environment as a good biosurfactant producer [12, 13], was preserved in glycerol 10% in an ultrafreezer at -80 C. The preinoculum was cultivated in a plate with YPDA (yeast extract 0.3%, peptone 1.5%, dextrose 0.1%, and agar 1.2%) at 30 °C for 48 h and transferred to 1,000-ml flasks with 300 ml of a culture medium. After 24 h of cultivation, the fermentation medium containing the cells was stocked in cryotubes with 25% of glycerol and preserved at -18 °C to serve as a preinoculum pattern for all the fermentations. 

Breweries are constantly trying to improve the efficiency of the production process while trying to preserve or improve the quality of the product. At the same time, they are developing new products with other flavours or with enhanced sensorial characteristics. A promising direction for the development of new products appears to be the use of different non-traditional yeast strains able to enrich the beers with new flavours with added value. These yeast strains can be combined with traditional brewer s yeast and used at different stages of the fermentation process. Simultaneously with improving the sensorial properties, some non-traditional yeasts (e.g. Saccharo-myces boulardii) would also be able to increase the nutritional value of low-alcohol beers in particular. 

Most breweries have close control of their yeast strains, and, unlike most distillers, they preserve (maintain) and propagate their cultures themselves on a regular basis. 

Jensenin P is a bacteriocin produced by P. jensenii B1264- which inhibits closely related propionibacteria and lactic acid bacteria. It was shown that jensenin P is a new anionic bacteriocin (Ratnam et al., 1998). Jensenin P is stable at lOOT for 45 min, at pH 2 to 10 for 225 min, and in 0.1 to 0.3M NaCl (pH 10) for 225 min (Barefoot et al., 1995). The resistance of jensenins to the extremes of pH and high temperature, their wide inhibitory spectrum allows to consider them as useful natural preservatives in thermal food processing. P. jensenii strain DFl was found (Miesher et al., 1998) to markedly inhibit other P. jensenii strains and to suppress 15 out of 24 yeasts and 3 out of 4 molds tested. From the culture broth of P. jensenii DFl an inhibitory protein substance, named SMI, was isolated and partially purified (Miesher et al., 1998). Crude propionicin SMI was sensitive to proteolytic enzymes and stable to incubation at 30°C (more than 14 days), cold storage (more than 6 months at 4 C) and heat treatment (15 min, 100 C). 

Yeast strain used in this study was K. fragilis PTCC 5193, obtained from the Iranian Research Organization for Science and Technology (IROST). K. fragilis was maintained in agar (65gl ). The culture was sterilized in autoclave at 12 PC for 20 min the yeast inoculum was spread on the surface and incubated at 30 C for 48 hr. At completed growth, the slants were preserved at 4°C. 

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