Pure yeast strain fermentation

Rankine (6, 38) believes that the differences in the secondary products formed in wines during fermentation by various yeasts are quantitative rather than qualitative, and careful selection of pure yeast strains can eliminate wine disorders caused by large amounts of undesirable by-products such as hydrogen sulfide, mercaptans, acetaldehyde, acetic acid, ethyl acetate, higher alcohols, etc. 

Yeast Disorders. Many genera and species of yeasts are found on grapes. Undesirable yeasts must be inhibited as soon as the grapes are crushed, and the primary fermentation must be controlled by inoculating the must with pure strains of yeast. This ensures a clean fermentation in a reasonable time with efficient conversion of the grape sugars to alcohol. Selecting pure yeasts is a matter of preference for individual wineries, but the preference in the United States has been for yeasts which do not produce undesirable by-products. 

In the alcoholic fermentation of juice to wine, SO2 is added at the crushers at the rate of about one pound per one thousand gallons. Pectic enzymes may be added to increase juice yields and clarity. The fermentation is conducted with the native yeast of the grape or various pure culture strains of Saccaromyces cerevisiae. Whether or not yeast cultures are added depends on the quality of the grapes and the winemaker s preferences. Dehydrated wine yeast is in general use at the beginning of the crushing season. 

Ballesteros et al. [72] studied SSF of pure cellulose with K. maxianus and K. fragilis at 45 °C with cellulase loading of 15 FPU/g substrate. Both yeast strains produced close to 38 g/1 ethanol in 78 h. The results also confirmed the importance of using thermotolerant yeast in SSF processes in order to improve hydrolysis rates and achieve higher ethanol production. Possible benefits of using a thermotolerant yeast to carry out ethanol fermentation at a supraoptimal temperature are ... 

This yeast strain is excellently suited for the production of bottom-fermented beers of all types. The resulting beer possesses an extremely pure flavor, a fine, subtle aroma, and a mild overall Impression. Fermentation is rapid with optimal yeast flocculation. The length of time that the yeast remains in suspension is dependent on conditions present in the brewery. The degree to which the color lightens during fermentation is sufficient. 

Techniques for growing/expanding LAB starters vary somewhat but can generally be divided into either pure culture or coculture (LAB and wine yeast) methods. In the latter case, most utilize the same yeast strain that is used to carry out alcoholic fermentation. In either case, a properly prepared and expanded LAB starter should yield a stationary-phase inoculum of >10 CFU/mL. 

Yeasts and Bacteria. One of the purposes of adding S02 is to inactivate bacteria and wild yeast so that the fermentation may be conducted with a chosen desirable strain of yeasts. Fortunately the wild yeast and the bacteria on grape berries (frequently confused in the older literature with the wax-like bloom which is naturally present on some berries) are susceptible to inactivation by relatively low doses of S02. A clear field is thus available to the large inoculum of S02-tolerant pure culture yeast added by the enologist. 

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. 

Fermentation. The juice should be inoculated with an actively fermenting culture of one of the selected strains of S. fermentati. Such yeasts may be obtained from winery supply houses as pure cultures which must be multiplied to obtain enough inoculum for the large fermentation or as dried yeast which may be added directly to the juice. Alternatively, the juice may be fermented dry with one of the standard, non-film-forming yeasts, and then the dry white wine may be inoculated... 

---