Bioactive compounds, antioxidant activity and quality of plum and sweet cherry cultivars as affected by ripening on-tree, cold storage and postharvest treatments

Resumen

Plum and sweet cherry are one of the most important stone fruits grown commercially in Spain and one of the most important commodities consumed worldwide due to their excellent quality attributes and high degree of acceptation by consumers. However, these fruits are highly perishable and the application of cold storage and other postharvest technologies are therefore necessary to maintain fruit quality from harvest to consumption. In this Thesis a comparative study on the evolution of physical, chemical and nutritive parameters and bioactive compounds during development and on-tree ripening and postharvest cold storage was performed in a wide range of plum and sweet cultivars. Eight plum cultivars: 4 purple ('Angeleno', 'Blackamber', 'Black Diamond' and 'Larry Ann') and 4 yellow 'Golden Japan', 'Golden Globe', 'Songold' and 'TC Sun') skin coloured, and 11 sweet cherry cultivars: 'Brooks', 'Cristalina', Newstar', 'Nº 57', 'NY-6479', 'Prime Giant', 'Santina', Somerset', 'Sonata', 'Sunburst' and 'Sweet Heart' were chosen. During on-tree ripening fruit weight evolution demonstrated the double-sigmoid growth pattern for all plum and sweet cherry cultivars and the main changes related to ripening, such as colour, total soluble solids (TSS), total acidity (TA), firmness started at early stages of fruit development, with significant differences among cultivars. The decrease in colour Hue angle was highly correlated with the increase in carotenoids in yellow plums and with total anthocyanins in purple plums and sweet cherries, showing that carotenoids were the main chemical compounds responsible for colour changes from green to orange/yellow occurring during ripening of the yellow plum cultivars, while anthocyanins were the main pigments of sweet cherry and purple plum cultivars. This correlation was found either during on-tree ripening or during cold storage. In sweet cherry the major anthocyanin was cyaniding 3-rutinoside followed by cyaniding 3-glucoside and pelargonidin 3-rutinoside, while the hydroxycinnamic acids derivatives neochlorogenic acid and 3-p-coumaroylquinic acid were the main phenolic compounds. The cold storage of plums and sweet cherries induced a delay in fruit metabolism although the ripening process went on measured by the decrease in acidity, the increase in TSS and softening as well as the ethylene production in those plum cultivars that showed a climacteric-ripening pattern 'Blackamber', 'Larry Ann', 'Golden Globe' and 'Songold'). In addition, the cold storage experiment in sweet cherry was performed with fruits harvested at 3 ripening stages (S1, S2, and S3), showing that at the end of storage time (16 days of cold storage + 2 days at 20 ºC) sweet cherries harvested at S1 reached the ripening stage of S2 at harvest, but the fruits picked at S2 did not get the ripening stage of S3 at harvest. Total antioxidant activity (TAA) due to hydrophilic (H-TAA) and lipophilic (L-TAA) compounds increased during on-tree ripening and during cold storage in all the plum and sweet cherry cultivars assayed, and were correlated to total phenolics and total carotenoids, respectively, demonstrating that phenolics are the main hydrophilic compounds contributing to H-TAA while the lipophilic nature of carotenoids contributed to L-TAA. It was found that picking fruits at the usual commercial harvest dates for these cultivars do not assure the highest content of bioactive compounds and related antioxidant activity, since a delay of 7 or 4 days led to increases in these phytochemicals and TAA, for plums and cherries, respectively. The effect of modified atmosphere packaging (MAP) on maintaining plum quality was assayed on 4 plum cultivars (2 with yellow skin, 'Golden Globe' and 'Songold', and 2 with purple skin, 'Blackamber' and 'Larry Ann') thermo-sealed in baskets with 2 distinct films with medium (film M) and high (film H) gas permeability. Fruit stored with macroperforated film served as a control and lost their quality attributes very quickly, manifested by accelerated colour changes, softening, decrease in acidity and increase in TSS. The use of MAP retarded these changes, the efficacy being higher in the fruit packed with film M compared with film H as a result of the delay in postharvest ripening, which could be attributed to the effect of MAP on reducing ethylene production rates. With the use of these packages, the storage time with fruit having high quality attributes could be increased 3 or 4 weeks more as compared with control plums. The changes in bioactive compounds (total phenolics, total carotenoids, and individual anthocyanins) as well as H-TAA and L-TAA in the peel and the flesh of these plum cultivars during storage under MAP conditions were studied. Results revealed that in all cultivars, total phenolics and H-TAA increased in the peel and flesh during storage, as well as the two identified anthocyanins: cyaniding 3-glucoside and cyaniding 3-rutinoside in the purple cultivars. These changes were significantly delayed in fruit stored under MAP conditions. Total carotenoids and L-TAA increased in the yellow cultivars (in both peel and flesh) while decreases were observed in the purple cultivars, these changes were also delayed by the use of MAP. Positive correlations were found between H-TAA and total phenolics and between L-TAA and total carotenoids. Results suggest that MAP does not impart any negative effects on TAA or phytochemicals and just reflects the delay of the ripening process occurring in the plums stored under MAP conditions, which led to an increase of the storage time with fruit having high quality attributes 3-4 weeks more as compared with control plums. Postharvest treatment with salicylic acid (SA), acetylsalicylic acid (ASA) or oxalic acid (OA) at 1 mM were performed on 'Cristalina' and 'Prime Giant' sweet cherry cultivars harvested at commercial ripening stage and then fruits were stored for 20 days under cold temperature. These treatments delayed the postharvest ripening process, manifested by lower losses in acidity and firmness, lower color changes, and higher quality attributes maintenance in treated cherries as compared with controls. In addition, total phenolics, anthocyanins and antioxidant activity increased in untreated fruit during the first 10 days of storage and then decreased, while in fruits of all treatments, these parameters increased continuously during storage without significant differences among treatments. Thus, postharvest treatments with natural compounds, such as SA, ASA or OA, could be innovative tools to extend the storability of sweet cherry with higher content of bioactive compounds and antioxidant activity as compared with control fruits. Finally, another postharvest treatment was an edible coating based on sodium alginate applied at several concentrations (1, 3 or 5% w/v) in 'Sweetheart' cherry cultivar harvested at commercial maturity stage. A significant delay in the evolution of the parameters related to postharvest ripening, such as colour, softening and loss of acidity, and reducing respiration rate, was obtained in alginate-coated fruits. In addition, the edible coatings showed a positive effect on maintaining higher concentration of total phenolics and TAA, which decreased in control fruits associated with the over-ripening and senescence processes. Overall, the results obtained from quality parameters and antioxidant activity suggested that the maximum storability period for alginate-coated cherries with optimal quality and enhanced antioxidant activity could be extended up to 16 days at 2 °C plus 2 days at 20 °C, while this period for control fruits was only 8 days at 2 °C plus 2 days at 20 °C.