Compression tests between parallel plates are applied to validate pressure as a grape berry maturity indicator and to define precisely the methodology of this measurement. Samples involve Cabernet franc variety of vine from four experimental plots belonging to two geological areas (Saumurois and Anjou). Two compression ratios (70 and 20 p. cent of the berry height) and two berry positions (equatorial and pedunculary) are applied.
Compression ratio of 70 p. cent gives maximum force and maximum pressure arising from various phenomena (skin bursting, juice ejection, pip crushing), which are difficult to separate from each other and to explain. Equatorial compression should be favoured to prevent the peduncle from interfering on the contact area between the tool and the berry, and to avoid berry resistance to the compression. Results should be expressed as pressure in order to compensate for the influence of the size on the developed force and to allow precise comparison between berries with different size. Compression ratio should be lower than that corresponding to the skin breaking. A rate of 1 mm.s-1, a compression ratio of 20 p. cent and an equatorial position give pressure values allowing to differentiate berries according to their terroirs, degree of maturity and heterogeneïty. Physical results are validated by comparison with chemical analysis (anthocyanins, °Brix, malic acid) from similar samples.
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