Interaction Effects of Oxalic Acid and Storage Time on Some Qualitative Characteristics of Plum Fruit cv. Santa Rosa

Plum as a climacteric fruit has high ethylene production and respiration rate at the ripening stage which is the reason for its short postharvest life. In order to evaluate the effect of oxalic acid treatment on postharvest storage life and maintain qualitative characteristics of plum fruit cv. Santa Rosa, an experiment was performed based on factorial in completely randomized design with two factors, oxalic acid concentration and storage time, in three replications.
Material and Plum fruits were harvested at the commercial harvest stage from a commercial orchard located in Golestan Province, Iran. On the same day, harvested fruits were transported to the laboratory and selected for the uniformity in size, shape and color. The selected fruit samples were distributed into four groups of 60 (each replicate contained 5 fruits). The selected fruits were immersed in oxalic acid solutions (0, 2, 4 and 6 mM) for 3 minutes and after removing the surface moisture at room temperature stored at 4 ◦C and 80 % relative humidity. The quantitative and qualitative traits of the fruit were evaluated every two week intervals during 42 days of storage. The obtained data were subjected to analysis of variance using SAS (9.1) software, and LSD values were calculated for the mean comparisons.
The results showed that oxalic acid had a significant effect on soluble solids content, titratable acidity, vitamin C, peel anthocyanins and browning index, whilst weight loss, soluble solids content, titratable acidity, vitamin C, fruit flavor index, pH, flesh and peel anthocyanins, total chlorophyll and chlorophyll a, b, total carotenoids, phenols and flavonoids, antioxidant activity, electrolyte leakage and browning index were significantly influenced by storage time. The increment of the oxalic acid concentration led to increase of soluble solids content by the late week 4 of storage. The highest (1.21%) and lowest (0.38%) titratable acidity was recorded for 6 mM oxalic acid in week 2 and 2 mM oxalic acid in week 6 of storage, respectively. At the end of the storage, 6 mM oxalic acid caused to preserve the maximum Vit C content (11.63 mg/100g) compared to control. Oxalic acid had no significance on total phenols and flavonoids; however, the interaction between storage time and oxalic acid had also a significant effect on soluble solids content, titratable acidity, vitamin C, fruit flavor index, peel anthocyanins, phenols, flavonoids and browning index. The weight loss reached 9.96% by late week 6. The electrolyte leakage percent and browning index increased during storage period and the minimum browning index without any significant difference was observed in 2 mM (41.11%) and 4 mM (43.89%) oxalic acid treatments.
In conclusion, oxalic acid significantly increased titratable acidity, vitamin C, plum peel anthocyanins and soluble solids compared to the control. The treated fruits had a lower browning index than the control. As a whole, regarding the results of this research, post-harvest application of 4 mM oxalic acid treatment could be used to maintain plum fruit qualitative properties and increase the time of storage.