Harvesting white grape

Mechanically harvested white grapes must be protected against oxidation. Sulfiting, however, must be avoided since it promotes the extraction of phenolic compounds. The addition of dry ice to the crop is a preferable alternative. Some countries use ascorbic acid, but this antioxidant is only authorized for treating wines in the EC. 

White musts and wines made without maceration contain very low amounts of flavonoids. However, when making white wine from white grapes, skin contact at low temperature is sometimes performed before pressing and fermentation to increase extraction of volatile compounds and aroma precursors. After 4h of skin contact, the concentration of flavanol monomers and dimers in must was increased threefold. Delays between harvest and pressing, especially if sulfur dioxide is added to prevent oxidation, as well as thorough pressing, similarly result in increased concentrations of flavonoids in white musts and wines. " " ... 

The difference in wine making is also the reason for the differences in ochratoxin A concentration between white, rose, and red wine. White grapes are immediately pressed after harvest, whereas red wine grapes are mashed and macerated to extract anthocyanins from the berry skins. Maceration lasts either for several days at elevated temperature or for several hours using pectolytic enzymes after heating of the must to 80°C. 

In general, grape maturation results from several biochemical transformations that are not necessarily related to each other. To simplify matters, the increase in sugar concentration and the decrease in acidity are monitored. The accumulation and refinement of white grape aromas and phenolic compounds in red grapes should also be taken into account. The essential property of a quality wine-producing area is to permit a favorable maturation. This corresponds with a harmonious evolution of the various transformations to reach the optimum point simultaneously at the time of the harvest. 

Red grapes are certainly less sensitive to maceration and oxidation phenomena than white grapes, but microbial contamination is likely to occur in a partially crushed harvest, left in the vineyard, especially in the presence of sunlight. These risks must be avoided. 

These terroirs have slow and complete maturation conditions. Conversely, excessively hot climates, early harvests and excessive water stress in the summer are unfavorable to the aromatic evolution of white grapes. 

The choice of harvesting method depends on grape maturity and disease status on one hand and on economic constraints on the other. White grapes... 

The native grapes and their hybrids were generally easy to harvest and crush—at first by feet and later with roller crushers. Open wood tanks were used for red wine fermentations and closed tanks for whites. Eastern grapes are difficult to press because the skins slip from the pulp when pressed. Therefore, with the traditional hydraulic presses, it was difficult to secure a high juice yield. Various expedients were used rack-and-cloth presses, heating the skins and pulp before pressing, etc. See Figure 2. 

The presence of indole or an indole derivative in a food is not always desirable. Some white wines develop an off-flavor described as ...floor polish like... within a few months of storage [33]. The chemical responsible for this unpleasant flavor was identified as 2-aminoacetophenone 16 in 1993 [34]. Reported in a series of papers over the next 10 years, researchers traced the ultimate source of this agent to indole-3-acetic acid 15 that was present in the grapes before harvesting Indole-3-acetic acid itself is derived from another indole L-tryptophan. The proposed mechanism for this transformation is shown in Scheme 14 [35]. 

Wine production is a fundamentally simple process, with the potential to be highly nuanced (Figure 19.1). Following harvest, grapes are crushed and the resulting must (juice H-seeds-t-skins) is used either directly for fermentation (red wines) or pressed to release the juice from the skins and seeds (white and blush wines). Grapes are not... 

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