Determination of the optimal harvesting period for ‘Marselan’ grape in east foot of Helan Moutain Region

Abstract：【Objective】The maturity of wine grapes is one of the important factors affecting the quality of wine. A fully ripe grape berry is characterized by high sugars and low acids, and also by high levels of pigments, antioxidants and flavor compounds. Therefore, multiple components should be used to evaluate the optimal harvest time of grape berries. High sugars and lower acids are crucial for high quality of wine. Farmers tend to delay the harvest time to obtain the desired component contents in grape berries. However, with the decrease in photosynthesis, the accumulation of sugars may be reduced relative to the accumulation of acids, leading to decrease in berry quality. The aim of this work is to determine the optimal harvest time for the wine grape‘Marselan’in the east foot of Helan Mountain by comparing the components in grape berries in different harvest times.【Methods】Samples were collected from August 21 to September 20 at an interval of 5 days. Approaching the commercial harvest time estimated by previous experience, the sampling interval was reduced to 3 days. Samples were collected from 10 marked grape vines, and berries were randomly harvested from the top, middle and bottom parts of clusters obtained from both the sunny and the shady sides of the canopy. Some of fresh samples were stored at -4 ℃ for a short period, and the remaining samples were frozen in liquid nitrogen andstored at -80 ℃ in an ultra-low temperature freezer. Soluble sugars (S) were determined by thermal titration with Fehling reagent (GB/T 15038—2006). Titratable acids (A) were determined by acid-base titration (GB /T 15038—2006). The sugar-acid ratio (S/A) was then calculated. The pH value was determined using a pen electronic pH meter (kedidGT-6023). Total soluble solids were determined with an ATAGOPAL-1 handheld saccharometer. Total phenols were determined with Folin-Ciocalteu reagent, and tannins with Folin-Denis reagent. Total anthocyanins in fruit was determined using the pH differential method. Cluster analysis and principal component analysis (PCA) were used to comprehensively evaluate the grape berry quality.【Results】The content of soluble sugars in the berries showed a constant increasing trend during ripening. In the early stage of ripening, the content of soluble sugars increased rapidly, while in the late stage, the growth rate slowed down, and the content of soluble sugars stabilized gradually. The content of titratable acids decreased continuously, while the content of soluble solids, sugar-acid ratio and solid-acid ratio increased slowly but decreased slightly after September 13. The pH value in the berries was relatively stable. Total phenols fluctuated in the early stage, then gradually increased towards mature stage; tannin content decreased in a fluctuating manner. As for PCA, the first two principal components were selected as the effective principal components. Principal component 1 had a good correlation with pH value, sugar-acid ratio and solid-acid ratio. Principal component 2 had a good correlation with soluble sugars, soluble solids, total phenols and tannins. In addition, the principal component 1 reflected the quality index of grape berry, and the principal component 2 was related to the quality index of wine.【Conclusion】The combination of cluster analysis and principal component analysis can effectively determine the optimal harvest time of the wine grape. The main factors affecting the harvest time of wine grape are sugar-acid ratio and solid-acid ratio. In addition, the suitable harvest time of wine grape‘Marselan’in the east foot of Helan Mountain was suggested to be September 10 to 20, and the optimal harvest date was September 13 in 2018.