Film sherry

Growth of film-forming yeasts is inhibited by ethyl alcohol concentrations higher than about 16 vol %. Fortunately, most of the bacteria and undesirable yeasts which will spoil wines when oxygen is available and which could be a great problem in the production of film wines are inhibited or killed by alcohol levels above 14%. The flor sherry producer thus has a range of about l%-2% ethyl alcohol content in his wines within which it is reasonable to expect film sherry rather than vinegar to result. 

Recently Webb (16) explained the aromas of film sherries as by-products of an incomplete metabolic sequence where ethanol is being metabolized by film yeasts to carbon dioxide and water. Webb also refers to Pasteur s Etudes sur le Vin, published in 1866, as the basis of the specialization of zymology, which recognizes different types of film formers that yield the wines of the Jerez district of Spain, Vinsjaunes of the Jura region of France, and the flor wines of Australia, South Africa, the Soviet Union, and California. 

The Spanish sherry type of film yeasts tolerate high alcohol concentrations (up to 16 percent) and cause compositional and taste changes which are considered to be serious defects in nearly all wines except Spanish-type flor sherries. The most obvious compositional changes in... 

Acetaldehyde. In routine winery operation acetaldehyde is seldom measured. However, in the production of sherry, either by the film yeast or submerged culture processes, regular acetaldehyde determination is necessary. 

The wine yeast, Saccharomyces fermentati, is able to form a film or veil on the surface of dry white wines of about 15-16% alcohol. This yeast produces agreeable smelling and tasting substances which dissolve in the wine and give it the aroma and flavor characteristic of Spanish fino sherries. To provide itself with energy for growth while in the film form on the surface of the wine, the yeast utilizes some of the oxygen from the atmosphere above the wine in the partially filled butt or barrel to oxidize some of the ethyl alcohol from the wine. The ethyl alcohol of the wine is not completely metabolized to carbon dioxide and water, however, but is oxidized to acetaldehyde—probably the principal compound in the complex mixture responsible for the aroma of this type of appetizer wine. 

Fatichenti, F., Farris, G. A., and Deiana, P. (1983a). Improved production of a Spanish-type sherry by using selected indigenous film-forming yeasts as starters. Am. ]. Enol. Vitic. 43, 216-220. 

Sherry wines are obtained from young wines, carefully selected soon after completing fermentation. These are typically fortified by adding vinous alcohol until they reach an alcohol content of 15-15.5°. They are subsequently transferred to oak barrels before being aged. In most sherries, wine aging occurs in the so-called solera and criaderas system under the flor film of yeast. Once alcoholic fermentation is finished, races of Saccharomyces cerevisiae that can grow on the surface of the wine switch from fermentative to oxidative (respiratory) metabolism. They spontaneously form a biofilm called flor on the wine surface. 

Several botrytized wine specialties, other than Aszu, are also matured in the Tokaj cellars. Traditionally, these were exposed to the air for a short period. Today, this is a characteristic only of dry Szamorodni. During this phase a Saccharomyces film develops spontaneously on the wine s surface. This donates a flor sherry-like character to dry Szamorodni wines (Alkonyi, 2000 Magyar, 2010). The similarity involves only the flor character, since Tokaji Szamorodni never contains added alcohol. Moreover, its acidity and sugar-free extract content is very high and botrytis notes are present in the taste. 

The development of an oxidized odor in white table wines was common. However, those who burnt sulfur freely in their casks (to produce SO2) had no trouble. Film yeast growth on dry white wines was also a problem, indicating poor cellar practice. The baking of wine to produce sherry was practiced already in California in 1884—in sherry houses. ... 

Guijo, S., Mauricio, J.C., Salmon, J.M., Ortega, J.M. (1997). Determination of the relative ploidy in different Saccharomyces cerevisiae strains used for fermentation and flor film ageing of dry sherry-type wines. Yeast, 13,101-117. 

Wurz, R. E. M., Kepner, R. E., Webb, A. D. (1988) The biosynthesis of certain gamma-lactones from glutamic acid by film yeast activity on the surface of Flor Sherry. American Journal of... 

Aldehydes also arise as normal by-products of yeast fermentation. Acetaldehyde is the ultimate electron acceptor in the conversion of glucose to ethanol. In this pathway, aldehyde dehydrogenase (ADH) reduces acetaldehyde to ethanol with the corresponding oxidation of NADH. Acetaldehyde levels are therefore dependent on the fermentation conditions, e.g., temperature, O2 levels, pH, SOj levels, and yeast nutrient availability (13, 14). Yeast strain can also affect aldehyde formation and excretion (15-17). For example, film yeasts used in sherry production are selected for their ability to produce very high acetaldehyde levels (18). 

The spoilage bacteria of dessert wines isolated by Fomachon (1943) were very sensitive to a pH of below 3.5 this is probably the primary reason why so many European wines are relatively resistant to bacterial spoilage. A decrease of pH during growth of the film yeast in sherry production was reported by Marcilla et al. (1936). The changes in pH during the malo-lactic fermentation are summarized by Schanderl (1950). 

At least 2-5 years are needed to make dessert wines. In the production of sherry the wine is stored in partially filled butts, i.e. in the presence of excess air. Ror yeasts develop on the wine surface in the form of a continuous film or wine cover (sherry yeast). The typical sherry flavor is derived from the aerobic conditions of maturation. During this time the concentrations of the following compounds increase at the expense of alcohol and volatile acids ethanal, acetals, esters, sotolon (cf. 5.3.1.3) and 2,3-butylene glycol. In port wine production the wine is drawn off to casks before the end of fermentation and is fortified with wine distillates. The fortifying procedure is repeated several times ( multiple addition ) until the desired alcohol content is reached. Sotolon is the key aroma substance of Port wine. Its odor threshold in this wine is 19 pg/1. Its concentration increases linearly during storage. Port stored for one year and for 60 years contained 5 and 958 pg/1 sotolon respectively. 

It may not be conunonly known that the tank wall above flie liquid level in a LNG tank is wetted by surface tension driven film flows up flie tank wall. This effect, an example of the so-called Marangoni effect, was first studied at Southampton in 1974 [2]. Have a look to see for yourself Then compare what you see with the tear-drops round the edge of a glass of sherry or port (see Fig. 6.1). 

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