Mung bean sprouts turn brown during cold storage. Here we showed that heat-shock treatment inhibited this browning by repressing the induction of phenylalanine ammonia-lyase (PAL) activity and the biosynthesis of polyphenols during cold storage. Mung bean sprouts were soaked in warm water at 50℃ for 60 s as heat-shock treatment and then stored at 8℃ for 6 days. Heat-shocked mung bean sprout turned less brown than that of the control sprout. The PAL activity in the control significantly increased during cold storage, while that in heat-shocked mung bean sprouts did not. The polyphenol content in heat-shocked mung bean sprouts was lower than that in the control at 3 and 6 days. There was no significant difference in polyphenol oxidase activity between the control and heat-shocked mung bean sprouts during cold storage. These results show that mild heat treatment inhibited the enzymatic browning of mung bean sprouts by repressing the induction of PAL activity during cold storage and the accumulation of polyphenols.

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Mung bean sprouts turn brown during cold storage. Here, we cloned polyphenol oxidase (PPO) mRNA, and then examined its expression and active form to clarify the mechanism of this browning. A PPO cDNA encoding 592 amino acids showed high homology to PPO genes of Fabaceae plants and had highly conserved motifs, including the active site and transit peptide to the plastid thylakoid. Expression of PPO mRNA was almost constant. The translated PPO protein was considered to migrate into the thylakoid, but the active PPO was mainly present in the cytosol or soluble fraction, and its molecular mass was 31 kDa, smaller than the translated protein. The membrane structure of mung bean sprouts is heavily disrupted during cold storage. These results showed that PPO transported into the thylakoid is solubilized and decomposed to its active form, which oxidizes phenolic compounds in the cytosol to turn sprouts brown during cold storage.

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