Determination of optimal harvest dates for three apple cultivars in Weihai, Shandong

Abstract:【Objective】Apple (Malus domestica) is an important cultivated specie worldwide. Fruit ripening and senescence is a programmed process with quality decline and postharvest loss. Maturity at harvest is one of the main factors affecting apple quality and storage. The determination of suitable harvest time is important for fruit storage. At present, the technical standards for the determination of the suitable harvesting period of Golden Delicious, Harlikar and Yanfu 3 apple cultivars in Weihai, Shandong have not yet been established. Fruit ripening process contains the change of many quality indicators such as texture, flavor and aroma, which can be determined by fruit size, weight, soluble solids content (SSC), titratable acid, starch and volatile substances.【Methods】In this study, the fruits of Golden Delicious, Harlikar and Yanfu 3 at different development times were used as the materials. The physiological parameters including transverse and longitudinal diameters of fruit, fruit firmness, solid soluble contents (SSC), ethylene production rate and respiration rate were measured. And the electronic nose analysis was conducted to distinguish the quality of fruits and further determine the maturity. The transverse sections of fruit were stained by iodine to construct the starch-iodine staining pattern of three ap-ple cultivars.【Results】Fruit size is one of the indicators to judge whether a fruit is ripe or not. The transverse and longitudinal diameters of fruit with the three cultivars Golden Delicious, Harlikar and Yanfu 3 exhibited a trend of increase during the developmental period, and tended to be stable until the end of the developmental period, indicating that the sampling time point was enough for the research requirements. Fruit firmness determines the overall palatability and transportability of the fruit. The fruit hardness of the three cultivars showed a decrease trend and remained stable at the end of development. SSC is considered to be the most important intrinsic quality related to fruit flavor and maturity. The SSC of Golden Delicious, Harlikar and Yanfu 3 fruits gradually increased during development, and reached 13.94%, 14.61% and 12.94% at September 30, October 30 and October 20, respectively. Apple, as a climacteric fruit, requires ethylene biosynthesis to initiate its ripening, and the peak of ethylene production rate occurs at the late ripening stage. The rate of ethylene production in the fruits of three cultivars increased slowly in the later developmental stage. The respiration rate of Harlikar and Yanfu 3 decreased slowly during fruit development. An electronic nose is a partially specific electronic chemical sensor and pattern recognition system. Unlike other analytical instruments, electronic noses can identify simple or complex volatile aroma mixtures. In horticultural crops, electronic noses are often used to identify fruit aroma and maturity. The volatile substances of Golden Delicious, Harlikar and Yanfu 3 cultivars during the developmental stage were quite different, but the proportion of hydrogen and alkane volatile substances of fruits in the three cultivars was similar. The methods commonly used in the aroma identification of horticultural crops are LDA and PCA. LDA analysis is a statistical method to study the type of samples. PCA extracts the most expressive features, and LDA extracts more identifying features. Generally, LDA analysis shows better classification results than PCA analysis. Therefore, in this study, LDA was used to distinguish and identify volatile substances in fruits at different developmental stages. The aromas of Golden Delicious, Harlikar and Yanfu 3 cultivars can be clearly distinguished in different developmental stages, indicating that the electronic nose can well distinguish apple volatile substances at different ripening stages, and can be used to determine the fruit maturity. Electronic nose analysis showed that the volatile substance of Harlikar and Yanfu 3 fruits gradually enriched from October 20 and September 30, respectively. The fruit accumulates carbohydrates in the form of starch by using photosynthetic compounds on the tree, and the starch in the fruit is hydrolyzed into soluble sugar after harvesting. Iodine can separate starch from polysaccharides, monosaccharides and disaccharides. Starch interacts with iodine to form a black or yellowish- brown color. The iodine- starch staining is a simple and effective method to determine the maturity of apple fruit. After the fruits of the three varieties were stained with iodine- potassium iodide solution, the staining degree of the fruit changed from deep to light and the staining area changed from large to small with the increase of fruit maturity. During the ripening process, the staining parts of the fruits of the three varieties gradually became lighter from the core area, indicating that the hydrolyzed parts of starch gradually spread outward from the core. For Harlikar and Yanfu 3, the hydrolyzed part of starch had diffused outward to the peel at the period when the SCC of fruit reached 14.61% and 12.94%.【Conclusion】These results showed that the optimal harvest time for fruits of Golden Delicious, Harlikar and Yanfu 3 apple was September 30, October 30, and October 20, respectively. The results of iodine-starch staining at this period can be used as a reference standard for harvest.