The level of sugar accumulates during ripening. The amount of sugar depends on a range of factors including the length of ripening, the number of grapes on the vine as well as crop management practices. Sugar content increases towards maturity, while grape acidity decreases over the same period (Figure 1). Thus it is very important that harvesting coincides with conditions of optimal °Brix /Acid ratios.
Quality standards vary between countries. Many have been established as a result of local preference, taste and tradition.
Consumers preference is usually affected by taste, aroma, skin and pulp texture, and the presence or absence of seeds. A good ‘sufficiently firm’ crunch is desirable. Most consumers seek a large, evenly sized bunch of consistent appearance. Color is important with green, red or black grapes favored over those of more intermediate colors.
As a rule of thumb, when the earliest grapes on a cluster – those nearest the tip – are good to eat, then the bunch is ripe for picking.
Grapes are ‘Graded’ according to a range of quality characteristics. There are also minimum packaging and labeling requirements that need to be met to ensure grapes meet the desired Class.
Various regions or individual importing countries set defined standards for sweetness. The European Community has, for example, adopted United Nations minimum values of 14 °Brix for seedless varieties and 13 °Brix for seeded grapes.
The industry also has a number of defined quality standards for the °Brix/Acid ratio - of table grapes. A good °Brix/Acid ratio is 25:1 or above, though certain varieties are marketable with °Brix/Acid ratios as low as 20:1. In Europe, a °Brix/Acid ratio of 20:1 is accepted as the normal standard for most varieties.