Issue link: http://winesandvines.uberflip.com/i/1007939
40 WINES&VINES August 2018 WINEMAKING PRACTICAL WINERY & VINEYARD The analysis of gluconic acid is an essential tool for the enologist not only to classify the health level of the must but also to recognize the grape juice most affected by gray mold and thus be able to establish the most suitable fermentation protocol. When ripening grapes are injured by hail, berry cracking or excessive compaction within tight-clustered cultivars and clones, they are also subject to development of sour rot, espe- cially under warm pre-harvest conditions. This disease, now known to result from the fermen- tation of berry sugars to ethanol by resident yeasts followed by its oxidation by specific bacteria, results in a major increase in acetic and gluconic acid, which can reach levels that make the must completely unusable. Gluconic acid Grapes attacked by gray mold — or worse, af- fected by sour rot — contain variable amounts of gluconic acid, which is a derivative of glucose, oxidized by the enzyme flavin adenine dinucleo- tide (FAD) glucosidase present in Botrytis, in the absence of phosphorylation agents (see Figure 3: "Enzymatic Glucose Oxidation"). The presence of Gluconobacter bacteria re- sults in the formation of gluconic acid, as well as respective 2-oxo gluconic, 5-oxo gluconic and 2.5-dioxo-gluconic acids, which have a com- bined effect in terms of SO 2 that is even more pronounced than gluconic acid (see Figure 4: "Enzymatic D-gluconic Acid Oxidation"). High temperatures and high stress due to water scarcity The best vintages for red wines are those char- acterized by medium-high temperatures and moderate water stress, while the best white wines, which are more delicate aromatically, require cooler vintages and low water stress.2 What are the effects of extreme tempera- tures and water scarcity on composition of the must? From a meteorological point of view, during a "normal" vintage, the ripening process of the grape involves an increase in sugar content and a decrease in acids, especially L- malic acid, the form most used by the plant for respiration. Tartaric acid, however, remains almost constant. Analyses of total acidity, pH, sugars and L-malic acid therefore provide indices useful in evaluating grape maturity. The ac- cumulation of anthocyanins in the skins, and their extractability, are highly vintage-depen- dent, and technological and phenolic maturity often do not occur at the same time. The situation is aggravated by climate change. High temperatures minimize the growth of destructive forms of mold, reducing the negative resulting aspects but contributing to a loss of nitrogen in the grapes. Under these conditions, YAN in the must is heavily de- pleted, and this deficiency will be dangerous for both red- and white-wine musts. Nitrogen, together with sources of carbon in the form of fermentable sugars (glucose and fructose) and growth factors, is a key element for yeast metabolism. Nitrogen is vital to all living organisms, contributing to the formation of peptide bonds, the "support beam" of pro- tein structures. Fundamental to the metabolic activity of yeast, nitrogen in both its inorganic form (am- monium ion) and organic form (represented Your success is our prioritY Understanding YAN enables a winemaker to make important decisions in order to avoid stuck fermentations, and gluconic acid can be used to classify the "health" of the must to choose the most suit- able winemaking protocol.