An operational model for capturing grape ripening dynamics to support harvest decisions

Introduction

Grape ripening is a critical phenophase during which many metabolites driving wine quality are accumulated in berries. Major changes in berry composition include a rapid increase in sugar and a decrease in malic acid content and concentration. Its duration is highly variable depending on grapevine variety, climatic parameters, soil type and management practices. Together with the timing of mid-veraison, this duration determines when grapes can be harvested. Grapevine sugar accumulation was previously studied from a physiological perspective (among others, see Dai et al., 2009; Suter et al., 2021). Shahood et al. (2020) investigated sugar accumulation and malic acid degradation from a single berry perspective. Bigard et al. (2017) developed an original method to study ripening by sorting berries according to their density. In another study, the dynamics of organic acids and cations was considered together, to assess the effect on titratable acidity (Bigard et al., 2020). Although these studies deepen our understanding of the physiological mechanisms involved in grape ripening, they do not provide an operational tool for growers to support harvest decisions, as the models developed and/or the sampling protocols associated with these studies cannot be deployed in a production setting. Moreover, in physiological studies, grape sugar is expressed in sugar content (mg/berry), while winemakers are interested in concentration (g/L of grape must), as it allows the prediction of wine alcohol percentage.

Viticulturists and winemakers monitor sugar to total acidity ratio (S/TA) during grape ripening and start harvesting grapes when this ratio reaches the optimum value for the desired wine style intended. Duteau (1990) suggested that this ratio, when expressed as a function of thermal summation (i.e. degree days), evolves in a strictly linear way during the first four weeks after mid- veraison. Hence, it can be expressed as:

y = αx + β

where y = S/TA ratio and x = thermal summation.

In this equation, the slope of the curve (α) represents the ripening speed and (-β/α) a proxy for the timing of mid-veraison.

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