Fermentation, botrytized musts

Using direct isolation, without enrichment, Saccharomyces species were not found on Tokaj aszu berries (Csoma, 2008 Magyar, 2006 Magyar and Bene, 2006), although Naumov et al. (2002) reported the presence of S. uvarum and S. cerevisiae on Tokaj grapes (method of isolation unknown). During spontaneous fermentations, however, diverse Saccharomyces populations can be detected in botrytized musts (see Section IV.C). 

Fermentation of botrytized musts is a slow process. It may take 1-6 months, 1 year not being exceptional. These musts possess particular initial yeast biota and provide extremely difficult nutritional and environmental conditions for yeasts. 

The unique chemical composition of botrytized must greatly impacts the products and by-products of alcoholic fermentation, as well as subsequent reactions. The changes have been extensively studied by German and French authors and have been reviewed by Dittrich (1977, 1989), Jackson (2008), Ribereau-Gayon et al. (2000), and Dittrich and Grossmann (2011). The chemical composition of some traditional (German and Hungarian) botrytized wine styles are illustrated in Table 6.5. 

Antunovics, Z., Irinyi, L., and Sipiczki, M. (2005). Combined application of methods to taxonomic identification of Saccharomyces strains in fermenting botrytized grape must. ]. Appl. Microbiol. 98, 971-979. 

The yeast strain used for fermentation had no impact on the enantiomer distribution of these volatile thiols. 3SHA is generally considered to be formed by esterification of 3SH by yeast during alcoholic fermentation. The esterase or lipase involved probably acetylates 3SH with a certain enantioselectivity. In contrast, the enantiomer distribution of 3SH in wine made from botrytized grapes (Botrytis cinerea) is 25 75 in favor of the S form, which has also been found in botrytized must (Thibon et al. 2007,2008a). 

FIGURE 6.4 Course of alcoholic fermentation and evolution of the yeast populations during spontaneous fermentation of Tokaji Aszu. Botrytized berries were macerated with fermenting must (A) or dry wine (B) (Magyar, 2010). 

Eder et al. (2002a,b) surveyed 117 Austrian wines including 55 potentially botrytized Pradikat wines (Auslese, BA, Ausbruch, TBA) for OTA. None of the samples contained the toxin at a detectable levels. In 121 different wines studied by Valero et al. (2008), the wines with the highest OTA contents were those produced from must fortified before fermentation (4.48 gg/1) and those made from sun-dried grapes (2.77 gg/1). Wines affected by noble rot contained no detectable OTA. Icewines and late-harvest wines were also not contaminated. Nonetheless, an elevated OTA concentration has been reported in some South African botrytized wines (Stander and Steyn, 2002). 

It is certainly necessary to protect must during fermentation, especially in the case of botrytized grapes. However, new techniques for total stabilization involve carefully controlled, high-level oxidation (hyperoxygenation). 

Fig. 2.15. Effect of an alcohol-induced precipitate of a botrytized grape must on glycerol and acetic acid formation during the alcoholic fermentation of healthy grape must (Dubourdieu, 1982). (1) Evolution of acid acetic concentration in the control must (2) evolution of acid acetic concentration in the must supplemented with the freeze-dried precipitate (1 ) evolution of glycerol concentration in the control must (2 ) evolution of glycerol concentration in the must supplemented with the freeze-dried precipitate...

Experts from the OIV estimate that the concentrations recommended by the EC can be decreased by 10 mg/1, at least for the most conventional wines. In this perfectly justified quest for lowering SO2 concentrations, specialty wines such as botrytized wines must be taken into account. Due to their particular chemical composition, they possess a significant combining power with sulfur dioxide. Consequently, their stabilization supposes extensive snlfiting. The EU legislation authorizing 400 mg/1 is perfectly reasonable, but this concentration is not always sufficient. In particular, it does not gnarantee the stability of some batches of botrytized wines and will not prevent them from secondary fermentation. 

Considering these phenomena, the addition of SO2 to a fermenting must should be avoided. It would immediately be combined without being effective. When the grapes are botrytized, the variation in the ethanal content of different wines when 50 mg/1 of SO2 is added to the must accounts for a combining power approximately 40 mg/1 higher than that of non-sulfited control wines. When stopping the fermentation of a sweet wine, a sufficient concentration should be added which... 

A sufficient and controlled addition of oxygen as soon as cellular structures are degraded provokes the denaturation of tyrosinase during the oxidation reactions that it catalyzes. The disappearance of the enzyme and the depletion of oxidizable phenolic substrates thus make the must stable with respect to oxidation. The condensation products responsible for browning should be eliminated before fermentation. Dne to its possible impact on aromatic elements, this technique seems better adapted to certain cultivars. It is not applicable to botrytized grapes, due to the resistance of laccase. 

As with dry white winemaking, overclarification can lead to large fermentation problems and increased acetic acid production. Must turbidity should not be as low as in dry white winemaking (100-200 NTU) 500-600 NTU or even a slightly higher turbidity is perfectly acceptable. Moreover, botrytized sweet wines are not subject to the same problems related to insufficient clarification as dry white wines—the development of reduction odors and vegetal tastes, oxidability, etc. 

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