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70 WINES&VINES May 2018 GRAPEGROWING PRACTICAL WINERY & VINEYARD thawed over 24 hours the results showed an increase in 3MH and 3MHA content. 14 Wines made from hand-harvested grapes that under- went this cryogenic maceration contained about 300% more 3MH and 3MHA compared to wines that did not undergo cryogenic maceration. The level of thiols obtained were compa- rable with the same grapes that were harvested using the mechanical harvester. The explana- tion for this could be the increased leaching of precursors and enzymes into the grape must due to berry damage from ice crystal forma- tion. Ice formation thus not only increases contact between reactants, but also concen- trates the reactants in the available liquid thereby facilitating the reaction. The expense involved in using this freeze/thaw cycle may not be economical when processing large vol- umes of must, however, this technique could be applied in small batches to obtain larger diversity in wine styles. Pressing and oxidation of juice Studies have shown that wines made from juice obtained from the press (1 bar) contained either the same or a lesser amount of thiols when compared to wines made from free run juice. 17,18 This could be due to the higher po- tential for oxidation due to larger phenolic extraction as the pressing pressure increases. Sufficient SO 2 should be present prior to pressing to achieve a reductive atmosphere (using carbon dioxide or nitrogen gas) during the pressing process. Higher pressing fractions might contain higher concentrations of thiol precursors (including C6 compounds). How- ever you run the risk of increasing the potential for oxidation (due to higher phenolic content). This could be risky seeing that an increase in the conjugated precursors would not necessar- ily lead to an increase in free thiols in the corresponding wine. Juice oxidation (measured by absorbance at 420 nm) can influence the volatile thiol concentration of the corresponding wines. Higher concentrations of 3MH were obtained from juice with lower 420 nm measurements. 17 However, a low 420 nm measurement did not guarantee high thiol concentrations as other important factors could have a greater effect on the formation of precursors and volatile thiols. The oxidation of juice is an important factor to consider, however the addition of sufficient SO 2 at this stage can minimize nega- tive effects occurring due to oxygen addition delivering volatile thiol levels equivalent of juice that was not exposed to oxygen. 18 It is advisable to keep juice fractions that might be in advanced stages of oxidation, separate due to lower potential for volatile thiol formation. The wine can then be bottled or blended if proven to have sufficient volatile thiols present after fermentation. Another op- tion would be to eliminate phenolic com- pounds through the use of a specialized fining agent on the juice. This prevents formation of quinones at a later stage and preserves thiol- containing compounds. 21 Fermentation conditions The yeast strain is extremely important that could determine the amount of precursors converted during fermentation. However, the yeast strain will only have a limited effect. Juice composition (such as the presence of volatile thiol precursors), needs to be of a certain standard and composition for the yeast to be able to convert and form volatile thiols. Other than precursors, the exact com- position needed is unknown. The effect of the same yeast strain was investigated on differ- ent musts and the results showed that juice diversity was the primary thiol determinant, 20 with yeast strain selection having a secondary effect. The strains can, however have an im- portant effect and multiple fold increases have been seen when comparing yeast strain and volatile thiol production. 3MHA is not formed directly from a precur- sor, but rather due to an esterification reaction that occurs during the fermentation process. Not all strains of Saccharomyces cerevisiae have the same capacity to express these compounds. Some strains are good thiol producers in that they release 3MH and 4MMP from correspond- ing precursors or it can create the volatile thiols from other compounds. Other yeast strains can be good converters in that they can efficiently convert 3MH to 3MHA. A mixture of these yeasts can be inoculated to maximize both production and conversion. S. cerevisiae is not the only yeast strain capable of releasing/producing volatile thi- ols from the precursors and other yeast strains, such as Pichia kluyveri, that have proven to be effective in increasing volatile thiol production. 23 Higher fermentation temperatures (irre- spective of yeast strain used) resulted in in- creased volatile thiol concentration when compared to lower temperatures. However, in some cases the extended higher tempera- ture during fermentation led to a decrease towards the end. 24 It is therefore advised to commence fermentation at 62.5° or 64.5° F (17° or 18° C) for about 30 grams of sugar fermented and then, depending on the yeast added, whether it is cold-tolerant or not, gradually lower the fermentation temperature to about 59° to 61° F (15° to 16° C) in order to preserve the released volatiles. Storage temperature It is absolutely vital to keep the wine at a low temperature. 25 Higher temperatures will not only accelerate the oxidation reaction, but will encourage hydrolyses of 3MHA to form 3MH (in some cases even leading to an increase of 3MH concentration). This way you will lose some of the aroma potency due to the higher perception threshold reported for 3MH, and a change in aroma quality. 3MHA is the volatile thiol most affected during storage of wine, while much smaller losses were seen for 3MH. Some studies have shown the effect of temperature to be even more important than oxygen exposure during wine storage. It would thus be advised to keep wine at a temperature as low as possible not only until bottling but for consumption. When hand-harvested Sauvignon Blanc grapes were frozen they reached similar thiol levels as in machine-harvested grapes. CONCANNON VINEYARD