Yeast Monocellular organisms belonging to the plant kingdom. They are closely related to the fungi. Unlike other plants, fungi and yeasts do not contain chlorophyll and so are unable to utilize sunlight to build up stores of energy (photosynthesis).

Yeast Activity Visible signs of fermentation in a must are the presence of bubbles rising to the surface, together with particles of pulp and clumps of yeast cells.

Yeast and Fermentation See Enzyme ; See Fermentation entries ; See Appendix IX ; See Appendix X

Yeast Autolysis See Autolysis.

Yeast Cells are about .005 mm in diameter. Their shape can be round, oval or apiculate, depending on the species. The membrane surrounding the cell cannot absorb molecules above a certain size so that, in order to survive, much of their metabolism occurs outside the cell by means of enzymes they secrete. To enable them to do this, they survive best in liquid habitats so that their enzymes may diffuse out and foodstuffs in a utilizable form can diffuse in. They reproduce by budding under ideal conditions, although they are capable of meiosis or binary fission. See Osmosis.

Yeast Classification There are many genera of yeasts but only the Saccharomyces and certain of the spoilage organisms are of import to the winemaker. The Saccharomyces (sugar fungus is the translation) are divided into some 30 species of which S. ellipsoideus is the wine yeast; S. cerevisae the beer yeast; S. carlsbergensis the lager yeast. The common spoilage yeasts are the apiculate yeasts of the genus Hanseniaspora; others are Candida and Pichia. The latter is also a constituent of the sherry flor. See Wine Yeasts

Yeast Culture See Culture.

Yeast Deposit See Deposit ; See Lees ; See Sediment.

Yeast for Mead Most pure wine yeasts are suitable for mead production; the variety recommended would seem to depend on the success of the various authorities.

Yeast Growth See Autolysis ; See Budding ; See Enzymes ; See Feeding ; See Fermentation ; See Fermentation Pathways , See Appendix IX; See Appendix X

Yeast Pitching The addition of yeast (preferably as an active started) to a must.

Yeast Preserving It is possible to keep a yeast culture viable and sterile by careful attention. After preparation of a starter, the culture is allowed to multiply. Once active enough to addition to a must, + - + is added to the must and the remaining starter is sealed with aseptic techniques. (This involves flaming the bottle neck and stopper.) After this, the sealed bottle is placed in a refrigerator until required again, when it is reactivated by means of another starter solution. Do not keep the sealed bottle in the warm or it will continue to work, with the possible danger of exploding glass.

Yeast Reproduction See Binary Fission ; See Budding.

Yeast Starter See Starter.

Yeast Tolerance to Acidity Unlike bacteria and spoilage organisms which are killed or inhibited at a pH of 3-5 or less, yeasts continue to flourish. Indeed, the presence of acid (in particular citric) will ensure the establishment of a healthy ferment

Yeast, Agar Slope See Agar.

Yeast, Bottom Fermenting A yeast which lies at the base of the fermenting jar. Small clumps of yeast rise to the surface, brought up by the carbon dioxide gas entrapped in them. Wine yeasts are bottom fermenters. Because of this they form compact lees which makes racking easy.

Yeast, Brewer's is taken to be the yeast used by English brewers. It is a top fermenting yeast and forms a thick "pancake" on top of the fermenting liquor. It gives a rapid ferment and has a lower alcohol tolerance than wine yeasts. If used for a wine, it will confer a beer taste to the wine. Although a bottom fermenting yeast, lager yeast is another species of brewers' yeast.

Yeast, Champagne A species of yeast which is peculiar in that it can continue fermenting under considerable pressures of carbon dioxide which would normally inhibit other wine yeasts.

Yeast, Dried One of the main resting forms in which yeasts can be purchased. The yeast is encouraged to form spores and then dried gently before packing. In this form, yeast can survive extremes of climate and nature. Dried yeast normally has a two-year shelf life at room temperature, usually stamped on the sachet. However, yeast being the hardy creatures they are have often been known to ferment after many years of dry storage without any negligible affects on the wine or beer.

Yeast, Feeding See Exponential Feeding.

Yeast, Film See Film Yeasts ; See Pellicle.

Yeast, Liquid Cultures See Culture.

Yeast, Properties of A good wine yeast will ferment in the presence of a fairly high concentration of alcohol û up to about 16-18%. It will form a firm deposit which is not easily disturbed when racked, after a slow steady fermentation. Should the wine be left on the lees, it should not acquire too great a taint from autolysis.

Yeast, Sherry has a high alcohol tolerance. If maturation is carried out with the wine in contact with air a flor may develop. The bacteriology of this flor would appear to be a complex mixture of sherry yeast, pichia, and acetobacter. The pichia species of yeast are responsible for much of the sherry flavor that develops.

Yeast, Tolerance to Sulphite Yeasts are able to continue fermenting in the presence of sulfur dioxide up to a concentration of about 50 ppm. If greater than this, it is likely that fermentation will be inhibited. To stop fermentation by the addition of sulphite calls for the racking of the wine followed by sulphiting with 50-100 ppm to prevent the establishment of a new colony

Yeast, Wild See Apiculate Yeasts.

Yeast, Wine Over the centuries each major wine producing area has produced its own variety of S. ellipsoideus by generic mutation. Although taxonomically identical, they do confer slightly different flavors on a wine. But the use of a Bordeaux yeast does not mean the automatic production of a Bordeaux style wine. If the must is properly balanced to favor such a sine, the use of such a yeast will enhance these qualities.  See Wine Yeast

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