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Optimizing the methodology to measure firmness of grape berries (Vitis vinifera L.) during ripening

Fuentes, S., Manso, A., Fenoll, J., Cava, J., Garrido, I., Molina, M. V., Flores, P., Hellin, P.
Acta horticulturae 2018 no.1194 pp. 1103-1110
Vitis vinifera, cultivars, deformation, firmness, freshness, fruits, markets, maturity stage, ripening, sensory properties, small fruits, stainless steel, table grapes, texture
Firmness in grape is considered an important attribute of organoleptic quality and is a desirable feature for good storage. A firm crunchy berry is considered a sign of freshness and health, while allowing longer storage times and permitting the fruit to reach markets in optimal condition. The first aim of this study was to develop a methodology to objectively measure the firmness of table grape (Vitis vinifera L.). For that, two cultivars classified as soft and crunch ('Muscat Hamburg' and 'Sugraone Seedless', respectively) were used. Afterwards, the methodology was applied for classifying nine cultivars of table grape ('Muscat Alexandrie', 'Muscat Hamburg', 'Moscatuel', 'Autumn Seedless', 'Dominga', 'Ruby Seedless', 'Dona Maria', 'Italia' and 'Napoleon'). For the methodology development, berries were sampled at nine maturity stages for 'Muscat Hamburg' and eight for 'Sugraone Seedless'. To determine firmness, each berry was subjected to a deformability test with a texture analyzer, equipped with a stainless steel plate and a 100-mm stainless steel probe. Settings included a pre-test speed of 10 mm s-1, a test speed of 2 mm s-1 and post-test speed of 10 mm s-1. Firmness was estimated as the force (Newton) needed to deform the berry a given percentage of its equatorial diameter (2, 5, 10, 15, 20 and 30% in this case). To estimate the influence of the grape skin, the test was carried out using whole and peeled grape berries. In whole berries, the most significant differences between the first four stages were obtained in the case of 15 and 20% deformation. Deformability with the skin on was always much higher than without skin, but no difference in loss of firmness was measured between grapes with and without skin. Among the evaluated cultivars, one was classified as very soft ('Moscatuel'), two as soft ('Muscat Hamburg' and 'Muscat Alexandria'), four as medium ('Autumn Seedless', 'Ohanes Cid', 'Dominga' and 'Ruby Seedless'), two as crisp ('Dona Maria' and 'Italia') and two as very crispy ('Napoleon' and 'Superior Seedless').