Predicting wine grape weight loss

Joel Characu Perez

Extending the time before harvest in wine grapes can lead to weight loss in grape berries due to dehydration (shrinkage). Grape berry shrinkage occurs following a brief period of maximum berry weight, concentrating total soluble solids (TSS; measured in °Brix), though it is unknown how environmental conditions (temperature, humidity, soil moisture) affect the time of maximum berry weight. Berry shrinkage is known to occur in situ and post-harvest, negatively affecting crop yield. The lack of quantifiable data regarding wine grape weight loss has made it difficult for industry personnel to estimate economic losses as a result of extending the hang-time before harvest. A series of observational experiments were conducted during the 2014 and 2015 growing seasons at the Washington State University (WSU) Irrigated Agriculture Research and Extension Center (46°17’N; 119°44’W; 365 m a.s.l.), using an 3.2 hectare experimental vineyard to measure seasonal weight loss of 22 cultivars in 2014, and 24 cultivars in 2015. The main difference between the 2014/2015 growing seasons was the crop yield. The 2014 average crop yield (measured in metric tons/hectare) was 17.7 t/h, while the average crop yield for 2015 was 7.6 t/h. Findings indicate a relationship between grape berry transpiration (water vapor loss) and sugar accumulation late in the season, with increases in °Brix levels not necessarily resulting from continued sugar production, but from solute concentration. Evaluation of individual berry transpiration from nine cultivars, measured by weighing detached berries over time under constant conditions, revealed a general decrease in transpiration as cultivars advanced in maturity from the blue stage (< 20°Brix) to the overripe stage (> 24°Brix). A cross evaluation of berry transpiration results using laboratory environmental conditions (vapor pressure deficit of 1.53 kPa) and transpiration regression lines obtained under field conditions using a custom cluster transpiration chamber revealed non-consistent results. The use detached berries to estimate field transpiration of whole clusters has been proven to be valid in previous studies. However the application of this technique on various cultivars, each with different cluster morphology, was not validated and requires further investigation. The 2014 growing season saw only 2 cultivars undergo significant weight loss, with none showing physical symptoms of shrinkage. Weight loss percentages ranged from 20% to 26%. However in 2015, 15 of the 24 cultivars showed significant weight loss, with physical symptoms varying greatly between cultivars. Weight loss percentages ranged from 15% to 45%. Grape berry cuticular conductance was also found to decrease during the advanced stages of maturity, despite the maintenance or in several cases the increase in berry size. Organic acid content of grape berries was found to be stable during the advanced stages of maturity, indicating cessation of organic acid degradation.

Predicting wine grape weight loss

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