WINETECH Technical Yearbook 2020

influential on wine quality, which can consequently lead to the higher sugar concentration of grapes. The development of grape berries consists of three growth phases. During phase one the berries are formed, its respiration rate is fast and the berries are green. Photosynthesis is consequently sufficient to fulfil the nutrient requirements of the berries. The acid concentration of the berries is high and the sugar concentration low and remains constant. During this phase the glucose-fructose fermentation ratio is more than one. During phase two the growth of the berries decreases, the acid concentration maximises and glucose begins to accumulate. This phase is terminated when the berry colouring commences. Phase three is characterised by a mass and size increase of the berries and the glucose-fructose ratio is balanced. During further ripening or raisin formation the fructose concentration increases. The

which is dependent on the species and strain. Wine yeast strains vary regarding their fermentation kinetics, nitrogen requirements, ethanol resistance, tempe­ rature tolerance and glucose or fructose degrading. These differences are more obvious under stress conditions, which are influenced by the adaptability of strains thereto. The degrading of fructose is enhanced by nitrogen additions during fermentation, whi le higher alcohol concentrations inhibit fructose degrading more than glucose degrading. Although the use of fructophilic yeast strains cannot prevent fermentation problems, it can definitely contribute to limiting the possibility of such problems. If fermentation problems can be attributed to the glucose-fructose ratio of grape juice, it must be differentiated amongst different cultivars. Chardonnay for example, is more likely to cause problems than Chenin blanc. Different vintages can also play a role,

time that grapes remain on the vine from this point has far-reaching results. It is not only the wine quality that is influenced, but it can also lead to sluggish or stuck fermentations. Residual fructose results in a lower alcohol concentration and can also lead to microbiological problems. These problems can be prevented to a certain extent by using fructophilic yeasts, which have a preference for the degrading of fructose. It has already been known since 1932 that the sugar preference of yeast strains differs. The preference is however not constant and is influenced by the culture circumstances during yeast growth. Further research however indicated that residual fructose is not necessarily the reason for fermentation problems, but must rather be seen as a symptom thereof. The performance of yeasts is partially determined by its genetic composition,

because drier years usually result in a lower ratio. The choice of yeast strain is consequently influenced by the ratio. If the fructose concentration is more than the glucose concentration, more attention must also be given to stress factors like the yeast assimilable nitrogen (YAN) and the expected final alcohol concentration. Fermentation problems are usually not caused by a single factor, but rather a combination of factors. If the nitrogen concentration of grape juice is for example replenished, 6-9% more glucose and 13-17% more fructose are degraded by yeasts. A di-ammoniumphosphate addition at the end of alcoholic fermentationwill also favour the degrading of fructose (Mabbett, 2019). REFERENCE Mabbett, J., 2019. Fructophilic yeasts. Consequences of yeast strain selections. Wine Business Monthly , June 2019: 44-48.

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