Impact of low temperature storage on active and storage forms of folate in choy sum (Brassica rapa subsp. parachinensis)

Choy sum (Brassica rapa subsp. parachinensis) is a dark green leafy vegetable that contains high folate (vitamin B9) levels comparable to spinach. Folate is essential for the maintenance of human health and is obtained solely through dietary means. Analysis of the edible portion of choy sum by both microbiological assay and LC–MS/MS indicated that total folate activity remained significantly unchanged over 3 weeks storage at 4 °C. Inedible fractions consisted primarily of outer leaves, which showed signs of rotting after 14 d, and a combination of rotting and yellowing after 21 d, contributing to 20% and 40% of product removal, respectively. Following deconjugation of the folate present in choy sum to monoglutamate and diglutamate derivatives, the principal forms (vitamers) of folate detected in choy sum were 5-methyl-tetrahydrofolate and 5-formyl tetrahydrofolate, followed by tetrahydrofolate (THF), 5,10-methenyl-THF, and 10-formyl folic acid. During storage, a significant decline in 5-formyl-THF was observed, with a slight but not significant increase in the combined 5-methyl-THF derivatives. The decline in 5-formyl-THF in relation to the other folate vitamers present may indicate that 5-formyl-THF is being utilised as a folate storage reserve, being interconverted to more metabolically active forms of folate, such as 5-methyl-THF. Although folate vitamer profile changed over the storage period, total folate activity did not significantly change. From a human nutritional perspective this is important, as while particular folate vitamers (e.g. 5-methyl-THF) are necessary for maintaining vital aspects of plant metabolism, it is less important to the human diet, as humans can absorb and interconvert multiple forms of folate. The current trial indicates that it is possible to store choy sum for up to 3 weeks at 4 °C without significantly affecting total folate concentration of the edible portion.

► Total folate in choy sum (Brassica rapa subsp. Parachinenesis) did not significantly decline in the edible fraction during storage at 4 °C for 3 weeks.
► Principal vitamers detected were 5-methyl THF and 5-formyl THF, together with THF, 5,10-methenyl THF, and 10-formyl folic acid.
► The folate vitamer profile changed during storage, with a significant decline in 5-formyl THF, relative to 5-methyl THF.
► 5-Formyl THF appeared to act as a storage folate, maintaining levels of metabolically active folates in the plant, such as 5-methyl THF.