WINETECH Technical Yearbook 2021

solids or percentage solids. Practically spoken, NTU measurement is the most suitable measurement. Different opinions exi st however regarding the most suitable NTU reading of white juice prior to alcoholic fermentation. In order to ensure that sufficient lipids are available to the yeast. NTU readings of 50 to 150 are recommended. In the case of clearer juice, oxygen additions can be considered at the end of the yeast growth phase to compensate for the lipid shortage, seeing that yeast can use oxygen to synthesise lipids. Italian research investigated the juice of six different cultivars at NTU levels,

which varied from 15 to 350, to make certain conclusions. More turbidity led to faster fermentations and lower residual sugar concentrations, irrespective of assimilable nitrogen concentrations. A NTU reading of 100 resulted in the best balance between the appearance of fruity flavours and the absence of sluggish fermentations or reductive flavours. The concentration of solids in juice influ - ences at least 40 non-volatile and volatile compounds. More turbidity causes lower volatile acid and acetaldehyde concentra- tions and higher glycerol concentrations.

There was also a trend that more solids cause lower esters concentration, but result in higher concentrations volatile sulphur compounds and higher alcohols. This caus- es a lower appearance of cultivar character. With certain cultivars, the juice with more turbidity results in more polysaccharides, irrespective of the extraction method which was applied. This has an influence on the taste of the wine. Sometimes there is concern about the potential negative results that too much solids in white grape juice will have on the resulting wine. The most polyphenol

oxidase enzymes that cause browning and sulphur spray residues which can cause reductive flavours, occur in the grape skin. The latter is an important source of the solids in juice, which can lead to the mentioned potential negative results in wine. If winemakers are however aware of this and manage it correctly, it should not be a problem (Godden, 2019). REFERENCE Godden, P., 2019. Grape solids in white winemaking. Australian and New Zealand Grapegrower and Winemaker , December 2019: 73-74.

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