Sugar in Grape Berry and Wine---Sugar Transport and Accumulation in Berry

Sugar in Grape Berry and Wine
Sugar transport and accumulation in berry

• Sucrose is a disaccharide sugar composed of one unit of glucose and one unit of fructose, as shown above
• Sucrose is produced through photosynthesis in the mesophyll of mature grape leaves
• The mesophyll of mature leaves are the "source tissue" and sucrose produced in the mesophyll are transported to the grape berries that are the “sink organs” which accumulates the sugar
• Sucrose is the major osmotically active constituent in the phloem and provides the driving force for translocating all other compounds in the phloem sap
• Massive accumulation of glucose and fructose in the vacuoles of mesocarp cells in the grape berry occurs after véraison
• The rapid accumulation of hexose characterizing berry ripening must involve the activity of invertases
• Invertases catalyse hydrolysis of sucrose provided by the phloem conducting complex into glucose and fructose; different invertase isoforms are localized in the cell wall, cytoplasm and vacuole
• Sugar accumulation in the berry is regulated by complex mechanisms and may be affected by various parameters including light, water and ion status, wounding, fungal and bacterial attacks, and hormones
• Once phloem transport has ended, any further increase in berry sugar concentration is exclusively due to water loss
• The fungus B. cinerea can thus act positively on the sugar content in matured berries and is used by grape growers to produce sweet wines
• Sugar content is an indicator often used to assess ripeness and to mark the harvest
• While indicating maturity level, “sugar ripeness” is not, by itself, the best index of optimal maturity
  
• Most of the sugar is fermented to ethanol during the wine making process, the measurement of sugar content, the so-called “must weight”, allows the control of alcohol content in the wine
  

References:
1. Text and illustration from: Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.