|The 2004/05 season in Worcester was characterised by warm and dry conditions. The crop was almost 19% lighter and 10 to 14 days earlier (than the previous year). The result was that some of the vineyards achieved optimum ripeness simultaneously, thereby putting enormous pressure on the cellars to process the grapes.
The white cultivars in particular were affected, to such an extent that pressing of certain vineyards inevitably took place at higher sugars. During vinification it was found that certain tanks experienced sluggish fermentation. Further investigations and analyses found such tanks to have exceptionally high levels of fructose. Chardonnay in particular was affected, and to a lesser extent Chenin blanc. This phenomenon elicited a number of questions. What is the role played by the climate in this phenomenon How can the higher levels of fructose be explained What is the role of the viticulturist in the early identification of such vineyards and in these instances, which measures must be taken during fermentation to limit sluggish fermentation
Growth phases of the berry
For the sake of thoroughness a short overview of the berry’s development is included (Figure 1).
Sources and translocation of sugars
Glucose, fructose and sucrose are the main forms in which sugar occurs in the vine. The first two represent approximately 99% or more of the total carbohydrates in the juice. Other sugars are also present, such as raffinose, maltose, galactose and meliobiose.
Fructose is 1 1/2 times sweeter than glucose. Most yeast strains are better at fermenting glucose than fructose. For the vinification of stable sweet wines with ordinary yeast strains, grapes with higher fructose are preferred.
Sugars are formed mainly through the process of photosynthesis in the leaves. This is transported to the berries via the phloem in the form of sucrose.
During phases I and II of berry development the sugar concentration remains constant and very low and mainly in the form of glucose. (glucose:fructose > 1)
With the onset of phase III (veraison) a drastic change occurs in the metabolism of the berry. The berry now becomes a sugar accumulating organ. Sucrose is now transported from the leaves, trunk, arms and shoots to the berry. It is possible that auxins play a role in the mobilisation of sucrose to the berry. The sucrose inside the berry hydrolyses to glucose and fructose and the ratio is approximately 1:1.
When the grapes become overripe, the ratio shrinks to as little as 1:0,75.
Possible explanations for the variations in the ratio of glucose to fructose
Table 1 gives the results of the relevant research, which was conducted on many cultivars. For the purposes of this discussion, however, five will suffice.
The data/results indicate differences between cultivars with regard to glucose and fructose levels. Varieties are described as high glucose (e.g. Chenin blanc) or fructose forming (e.g. Chardonnay).
In warm and dry seasons a lower glucose:fructose ratio is obtained, thereby indicating increased fructose levels. This phenomenon may be explained by the following theories:
Glucose may be converted to fructose using sorbitol as an intermediary product in the presence of the co-enzyme NADP.
It is suspected that sorbitol and other sugar-alcohols are actively involved in the translocation of carbohydrates. It is not clear whether this reaction is simply aided by higher temperatures and/or whether there is a closer link / correspondence with all the complex biochemical processes.
In the berry glucose is broken down by the pentose phosphate cycle. This means that if sucrose enters the berry on a constant basis and glucose is respirated, the ratio of glucose:fructose will be reduced.
Another (unlikely) reduction of glucose is explained by the phenomenon that most of the micro-organisms in the berry show a high affinity for this sugar (healthy grapes were used throughout in the research.)
Overripe grapes usually hang on the vine for a longer period before being pressed. Through respiration, more glucose than fructose may be broken down and reduced. During warmer seasons the majority of biochemical reactions take place more rapidly, and therefore explain why lower levels of glucose in relation to fructose will be present.
Viticulturally the overripe phase of white cultivars in particular must be avoided. This situation often occurs in warm seasons. Logistical problems at cellars increase the possibility of the grapes becoming even more overripe.
Sampling and analyses may be used to ascertain whether the fructose level becomes higher than that of the glucose. In such instances press earlier rather than later. It is therefore imperative to avoid stress conditions in such vineyards.
Use yeast strains that are able to make good use of fructose if the grapes are suspected to have a low glucose:fructose ratio.
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