The evaluation of grape quality is the first stage of evaluation to ensure that the final consumer’s perception of quality is satisfied. Consumers may differ considerably in their quality perception of the same wine, however, which complicates the initial evaluation of the grapes.

It remains a challenge nevertheless for researchers to accommodate these poles of perception.

Quality may be defined as the intrinsic characteristics of a product that ensure that the consumer’s expectations are satisfied. The “consumer” may range from a winemaker who harbours certain ideals for his product, to a buyer who purchases the wine for a special occasion, to the accountant who wants to maximise the cellar’s profit.

Last year the Australian Wine Research Institute (AWRI) asked consumers to evaluate Chardonnay and Riesling wines with different sensorial profiles. As might have been expected, the consumers differed significantly with regard to their wine preferences. The ideal of a winemaker would obviously be to satisfy the expectations of all the consumers.

The quality of wine grapes may be evaluated over a broad base, notably the following aspects:

  • The balance between vigour and canopy capacity.
  • The evenness of ripening.
  • The vigour of the vineyard.
  • The exposure of the grapes to sunburn.
  • The incidence of diseases.
  • The berry characteristics such as size, colour of the seeds and taste.
  • The sugar content of the grapes.
  • The grape colour composition of red grapes.
  • The quality of compounds such as the methoxypyrazines responsible for herbaceous or vegetal flavours in wines.

Despite the above-mentioned quality parameters of grapes, it remains a problem that the measurement thereof will not necessarily be noticeable in the characteristics of the resultant wine. This is mainly due to the transformation of grape components by yeasts during alcoholic fermentation. The choice of yeast strain will play an important role in this regard. The yeast strain selected by the winemaker can therefore address the requirements of a specific buyer.

Unfortunately there is no such concept as general or absolute quality and the winemaker or cellar will have to decide about the requirement(s) to be addressed. Winemakers would prefer to have recourse to a single parameter that addresses an anticipated perception of quality. Unfortunately this does not exist and hypothetically the grading of grapes might be done in different stages so that the grapes might comply with stricter requirements inasmuch as specific quality perceptions are concerned. A model of such a system may be summarised as follows:

A specific sugar content of grapes may be used as a basic yardstick of quality and grapes that do not comply with this are the lowest grade grapes (Grade 1) that are received.
Grapes that do comply with the above requirement, however, are evaluated in terms of a subsequent quality yardstick, such as colour. Grapes that do not comply with this will be classified as Grade 2, while grapes that do comply, will qualify for a subsequent evaluation.
Berry weight that has an influence on tannin extraction, for example, might well be the next evaluation. Grapes that do not qualify will remain Grade 2, while grapes that do qualify will be Grade 3 or qualify for a subsequent evaluation.
The sensorial evaluation of berries, or determination of methoxypyrazine qualities might well be the next evaluation. Grapes that do not comply with the requirements will remain Grade 3, while grapes that do comply, will be classified as Grade 4.
A grape grading system of this nature may be used to place the eventual wines in the cellar in quality categories.

The methods to determine grape quality will definitely improve, but it must also be borne in mind that it is a dynamic process that will have to be adjusted on an ongoing basis.

Reference:

Francis, L, Hoy, P, Dambergs, R, Gishen, M, Lopes, M de B, Godden, P, Henscke, P, Waters, E, Herderich, M, Pretorius, I, 2005. Objective measures of grape quality – are they achievable- Wine Industry Journal 20 (3): 12 – 18.

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