Grapes and Musts

It is generally accepted that in the case of sound, undamaged grapes, the viable population of yeasts ranges from 10 to 10 CFU/mL (Parish and Carroll, 1985 Fleet and Heard, 1993). The most frequently isolated native species is Kloeckera apiculata, which may account for more than 50% of the total yeast flora recovered from fruit. Lesser numbers of other yeasts, such as species of Candida, Cryptococcus, Debaryomyces, Hansenula, Issatchenkia, Kluyveromyces, Metschnikowia, Pichia, and Rhodotorula, have also been reported (Heard and Fleet, 1986 Holloway et al., 1990 Longo et al., 1991 Fleet and Heard, 1993 Sabate et al., 2002). If grape juice or concentrate 

Saccharomyces can be found in grape musts, but the populations are often less than 50CFU/mL (Fleet and Heard, 1993). This is true even in vineyards where fermented pomace is returned as a soil amendment. Failure to routinely isolate Saccharomyces from the vineyard could reflect the preference of this yeast for the high-sugar environments of grape juice and fermentation. 

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A review of the vitamin composition of grapes and musts has been reported in recent texts (2, 3). Also, vitamins in grapes were reported by Amerine (21) in an extensive review of organic components of wines. 

Parish, M. E. and Carroll, D. E. (1985). Indigenous yeasts associated with Muscadine (Vitis rotundifolia) grapes and musts. Am. J. Enol. Vitic. 36,165-169. 

Sage, L., Krivobok, S., Delbos, E., Seigle-Murandi, F., and Creppy, E.E. 2002. Fungal flora and ochratoxin A production in grapes and musts from France. J. Agric. Food. Chem. 50, 1306-1311. 

Rodopulo, A. K. 1950a. Oxidizing enzymes of champagne varieties of grapes and musts (transl.). Biokhimiia vinodeliia 8, 43-52. 

The nitrogenous components of grapes and must that are metabolically available to yeast are present as ammonium salts (NH4 ) and amino acids primary, collectively known as yeast assimilable nitrogen (YAN). Therefore, a complete evaluation of the nutritional status of juice or must requires measurement of both fractions. The total nitrogen content of juice not only contains YAN but peptides and proteins as well. However, the latter two fractions are not thought to play a significant role in the nutritional needs of Saccharomyces during fermentation. 

It is estimated that wines contain 400-600 flavour-active compounds in a total amount of 0.8-1.2 g/1. In wine terminology, aroma is the term used for the smell of young wines. The transformation of aroma during agingby chemical reactions leads to a wine s bouquet. Wine flavour is classified into primary - the bearer of which are substances present in grapes and musts - and secondary, the bearer of which are substances produced during fermentation. 

An initial inoculum of 10 cells/ml is generally considered necessary to obtain good fermentation kinetics. In view of the current constraints on white winemaking, this initial level is rarely achieved so the use of yeast starter has become practically compulsory. In red winemaking, there may be insufficient inoculum in the first few vats filled, but grape and must handling operations in the winery... 

The difference in fermentability of various grapes and musts is finked with many poorly controlled factors. In the same way that specific... 

Considering the rapidity of oxidative phenomena, grape and must sulfiting is only effective if the sulfur dioxide is intimately and rapidly incorporated into the total volume before the start of fermentation. If a fraction of the grape must ferments before being sulfited, it is definitively shielded from the action of the SO2, because it immediately combines with the ethanal produced by the fermenting yeasts. 

After a botrytis attack, grapes and must contain polyosides with phytotoxic and fungistatic activities. B. cinerea also produces divers antibiotic substances botrytidial, norbotryal acetate and botrylactone. Some of these substances can be the source of fermentation difficulties. 

Depending on winemaking techniques, the duration of grape solid maceration in must varies. During this period, hydrolase-type enzymes act on grapes and must. These enzymes are responsible for the hydrolysis of diverse macromolecules such as proteins, polyosides, heterosidic derivatives and various esters. Their action often improves the grape/must mixture. This maceration phase should therefore sometimes be prolonged. 

In red winemaking, these preparations are used in particular for press wines and heat-treated grapes and must. In the latter case, the must is very rich in pectic compounds and devoid of endogenous grape enzymes. These are destroyed by heat (Martiniere and Ribereau-Gayon, 1973). Pectolytic enzymes can also be used at the time of running off after a traditional maceration. 

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