The role of citric acid and enzyme activities on sourness in pineapple fruit after low temperature storage

This study aims to elucidate biochemical mechanisms underlying the sour taste in pineapple fruit after cold storage. The acidity of six pineapple cultivars was evaluated after low temperature storage (LTS) by a trained sensory panel and by assessing their titratable acids (TA). Perceived sourness and TA were most pronounced in ‘Phu Chawa’ and least pronounced in ‘MD2’. Consequently, these cultivars were selected to examine differences in biochemical mechanisms in fruit stored at room temperature (senescence), as well as during and following LTS. In ‘Phu Chawa’ fruit, citric acid levels and citrate synthase (CS) activity increased significantly (p < 0.05) after LTS, whereas levels of glucose, fructose, oxaloacetic acids, and the activities of aconitase (ACO) and isocitrate dehydrogenase (IDH) decreased. In contrast, ‘MD2’ fruit showed no significant change in these attributes after storage compared to freshly harvested fruit. Malate dehydrogenase activity remained low in both cultivars throughout the storage period, suggesting that it is unlikely to be a determinant of LTS-induced sourness. Upon transferring the fruit from LTS to 35 °C for 5 d (POST-LTS), all LTS-induced changes dissipated, leading to similar enzyme activities and metabolite levels during senescence. No decay was observed in this study, regardless of the cultivars and storage conditions. These findings suggest a potential regulatory association between citric acid content and the activities of CS and ACO, contributing to the development of sourness in pineapple fruit after LTS.