Fruit quality during ripening and cold storage of two Japanese plum varieties cultivated in Tunisia

Abstract: Fruit Quality attributes of two Japanese plum (Prunus salicina L.) varieties 'Black Diamond' and 'Fortune' cultivated in Tunisia were monitored during fruit ripening on the tree and cold storage. A five years old commercial orchard was used for the study. Harvested fruits were stored in cold rooms at temperature less than 2?C and relative humidity around 90%. Beginning at two weeks before harvest, until the end of the cold storage period, samples of 20 fruits were taken at irregular time intervals and used for the determination of soluble solids concentration (SSC), titratable acidity (TA) and fruit firmness. At harvest, values of SSC were similar for both varieties reaching 15 and 14.8%, respectively for 'Black Diamond' and 'Fortune'. During cold storage, the increase in SSC was nil for 'Fortune' and very low (0.2%) for 'Black Diamond'. In contrast, values of TA were different for 'Black Diamond' and 'Fortune', the former was more acidic at harvest and during cold storage. TA values ranged between 0.94 and 1.24 % malic acid at harvest and 0.8 and 1% at the end of storage period. The sugar to acid ratio (SSC/TA) showed a continuous increase during ripening and cold storage for both varieties. Values of SSC/TA, differed largely between the two varieties, at all sampling dates. Fruit Firmness decreased during ripening and cold storage for both cultivars. At harvest 'Fortune' fruits were more firm (28.44 N) than 'Black Diamond' (21.77 N). At the end of cold storage, these values were as low as 10.64 N for 'Black Diamond' and 15.64 N for 'Fortune'. A linear regression analysis showed that the rate of firmness decrease during cold storage was very similar for both cultivar and reached a value of 0.23 N day^-1. Thus, fruit firmness could be used to determine harvest time and to predict cold storage duration