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Enzymatic browning is the main quality issue of fresh-cut foods. This study investigated the effects of different concentrations (0.05, 0.1, and 0.2 mM) of melatonin (MT) treatment on the enzymatic browning of four fresh-cut foods, including apples (Malus domestica), pears (Pyrus spp.), potatoes (Solanum tuberosum), and taros (Colocasia esculenta), and found that only 0.05 mM MT was most significant at enhancing the L* values and reducing the browning index (BI) in all four foods. This suggests that 0.05 mM MT might be a universal concentration for the browning alleviation of fresh-cut foods. MT treatment increased the total phenolic contents and PAL activities but reduced the activities of POD, PPO, and LOX in fresh-cut taros. Furthermore, MT significantly affected the expression of sixteen browning-related genes in fresh-cut taros during storage. These results suggest that MT reduced fresh-cut food browning by regulating the activities of browning-related enzymes. However, MT did not significantly influence the browning degree or PPO activity of the taro mash in vitro, suggesting that MT might regulate enzyme activity in an indirect manner. Overall, these results indicate that MT might be a promising anti-browning agent to alleviate the browning of fresh-cut foods. PRACTICAL APPLICATIONS: Melatonin (MT) is an endogenously produced indoleamine. Previous studies have reported that MT displays protective activities in many agricultural products. However, the reported protective MT concentrations vary between different products. To extend this agent to the fresh-cut industry, it is necessary to determine the universal dosage of MT needed for application efficiency. In this study, the effects of three concentrations (0.05, 0.1, and 0.2 mM) of MT on four fresh-cut foods, including apples, pears, potatoes, and taros, were investigated. The results showed that MT treatment alleviated browning development in the slices of these four foods and that 0.05 mM MT was the most effective treatment. Moreover, 0.05 mM MT significantly affected the activities of browning-related enzymes. These results suggest that 0.05 mM MT is a universal dosage for reducing surface browning in fresh-cut foods. This study provides a foundation for the application of MT in the processing of fresh-cut foods, especially fresh-cut taros.

Additional file 2: Table S1. The corresponding information of 107 CabHLHs. Table S2. Functionally characterized partial bHLH proteins from tomato and Arabidopsis thaliana. Table S3. Features of the CabHLH proteins motifs. Table S4. List of the bHLH genes in rice, tomato and Arabidopsis thaliana. Table S5. List of the primer for real-time quantitative PCR. Table S6. List of the primer for yeast two-hybrid assays. Table S7. Putative cis-elements of capsorubin and capsaicin biosynthetic genes promoters.

Additional file 1: Figure S1. Carotenoid (A) and capsaicinoid (B) biosynthetic pathways. Figure S2. Chromosomal localizations of CabHLHs. Figure S3. Multiple alignment analysis of the bHLH domain in pepper bHLH proteins. Figure S4. Phylogenetic tree of the bHLH family in Capsicum annuum, Arabidopsis thaliana, tomato and rice. Figure S5. The expression profiles of CabHLH genes in different tissues. Figure S6. Diagram of yeast two-hybrid assays experiments

Hydrosols are byproducts produced by steam distillation of aromatic plant materials. Previous studies reported hydrosols owned effectively inhibitory effects on microbials, but few focused on the effects in reducing enzymatic browning of fresh-cut products. Here, application of citronella hydrosol (CH) and rose hydrosol (RH) obviously reduced values of L*, a* and b* in surfaces of fresh-cut taros, suggesting these hydrosols could reduce browning severity. About 500 ml/L of RH and CH reduced total phenol contents, and activities of phenylalanine ammonia lyase (PAL), peroxidase (POD) and polyphenol oxidase (PPO), suggesting hydrosols suppressed browning development by reducing PAL, POD and PPO activities. Studies reported terpenoids had potential inhibition on PPO activity. Chemical composition analysis showed that RH and CH extracts contained abundant terpenoids. These imply that terpenoids contained in two hydrosols might have contributed to the reduction of enzyme activity. This study indicates that hydrosols might be promising natural anti-browning agents for fresh-cut foods. PRACTICAL APPLICATIONS: Hydrosols are the byproducts of essential oil from steam distillation. The work described here shows that application of citronella and rose hydrosols significantly reduced the browning of fresh-cut taros. Moreover, aromatic hydrosols also present inhibitory activity against microorganisms and are generally considered safe for human health. These suggested that aromatic hydrosols are the natural and useful anti-browning agents to preserve the quality of fresh cut foods, and can be well integrated into industrial procedures.

The efficacy of cinnamic acid (CA) as an antibrowning agent was evaluated in fresh-cut taro. Different concentrations (0.1 g L−1, 0.2 g L−1 and 0.4 g L−1) of CA treatments reduced the browning index and a* and b* values but maintained L* values, suggesting that CA treatments prevented the browning development of fresh-cut taro during storage. Among the concentrations used, 0.1 g L−1 CA was sufficient to reduce browning. CA treatment (0.1 g L−1) reduced the total flavonoid content and the activity and gene expression of four main enzymes (PAL, C4H, 4CL and CHS) in the flavonoid biosynthesis pathway, suggesting that CA treatment reduced taro browning by suppressing the de novo biosynthesis of flavonoid compounds. Furthermore, CA treatment significantly reduced the lignin content, activity and gene expression of cinnamyl-alcohol dehydrogenase and slightly reduced the firmness of taro slices. Moreover, CA treatment showed strong inhibition of PPO activity in vivo and in vitro, suggesting that CA reduced phenolic oxidation by inhibiting PPO activity. CA-treated taro showed significantly reduced hydrogen peroxide content but enhanced activity and gene expression of catalase and peroxidase. In addition, CA had strong radical scavenging activity, suggesting that the application of CA to fresh-cut taro enhanced antioxidant activity. Overall, CA treatment reduced the surface browning of fresh-cut taro by suppressing the activity of browning-related enzymes and by improving antioxidant activity. The results presented in this study indicate that CA is an effective natural antibrowning agent for fresh-cut taro.