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Access Practical Winery & Vineyard article archive online: practicalwinerylibrary.com TECHNICAL RESOURCE FOR GROWERS & WINERIES 63 Mineral taste in wine, minerals in the vineyard … Are they connected? By Alex Maltman 71 Cluster elongation to control bunch rot in winegrapes By Stan Grant grapegrowing Predicting harvest date using berry sugar accumulation BY Alain Deloire, Department of Viticulture & Oenology, Stellenbosch University, South Africa (present address: NWGIC, Charles Sturt University, Wagga Wagga, Australia) S ugar loading can be defined as the accumulation of the quantity of sugar per berry, expressed as milligrams (mg) per berry, from véraison onward. Véraison corresponds with the onset of fruit maturation. In the grapevine, fruit maturation starts with an abrupt softening of the berry (within 24 hours). This softening goes hand in hand with sugars being actively introduced into the berry (sucrose rapidly hydrolyzed into hexoses: glucose and fructose). In red and black cultivars, véraison is characterized, after softening, by skin coloring as a result of the biosynthesis of anthocyanins. Accumulation of sugar loading in grape berries gives an indication of the ripening process from a new perspective and is a novel approach to identify practical indicators to obtain particular styles of grapes and wine. Sugar loading may also provide information about ripening kinetics and enable the principal phases of ripening to be distinguished.8 This information provides a greater understanding of how grape quality develops in the vineyard. Sugar loading calculation Phloem sugar transport, principally to 58 p r acti c al w i ne ry & v i ne yard MAY 20 13 the flesh cells, has been characterized in studies of plant-to-berry sugar loading and phloem sugar unloading, notably by the peripheral vascular system of the berry.11 Phloem sugar unloading into cell vacuoles occurs mainly via an apoplastic mechanism, which requires the intervention of hexose transporters.1,9 From the above-mentioned studies, it can be concluded that sugar loading into the berry, coupled with the dynamics of sugar concentration changes, may be considered a useful indicator of grape quality. It takes into account the accumulation of sugar per berry (mg per berry) and therefore enables the kinetics of sugar concentration changes to be monitored. Kinetic monitoring of the quantity of sugar per berry may be considered as a method of measuring the vine's physiological functioning 4,5,7,9 and in particular photosynthesis, which is a reliable indirect indicator of temperatures to which the vine is subjected under given conditions for a specific time period and grapevine water status. Active sugar loading is calculated on the basis of berry volume (or berry fresh mass) and sugar concentration.3,9,10 Example of sugar loading calculation For a berry with a ripeness level of 25° Brix: 1. Convert 25° Brix to probable alcohol: 25 x 0.59 = 14.75° in probable alcohol, where 0.59 is the coefficient used for yeast activity to convert Brix to probable alcohol (this coefficient to be adapted to the probable alcohol level). 2. Determine mg of sugar per ml probable alcohol: To obtain 1° probable alcohol, 17 grams per liter (g/L) of sugar is required. 14.75° x 17 g/L = 250.75 g/L, which is equivalent to 250 mg of sugar per ml (in this example). 3. Calculate quantity of sugar per berry: In addition to measuring °Brix, the volume of a berry (or berry fresh mass thereof) should also be measured so that the quantity of sugar per berry can be calculated. Approximately 50 berries should be used to determine berry fresh mass so that the single berry fresh mass is an average of a berry population. To determine quantity of sugar per berry, multiply 250 mg x the volume of a berry (or the berry fresh mass thereof, because for many varieties there is a linear correlation between berry volume and fresh mass).