Ethanol treatment suppresses the yellowing of fresh-cut yam by repressing the phenylpropanoid pathway and bisdemethoxycurcumin biosynthesis
Graphical abstract
Introduction
In recent years, the consumption of fresh-cut vegetables has grown tremendously (Artés-Hernández et al., 2008), and changes in the lifestyle of the urban population have significantly increased the demand for fresh-cut roots and tubers such as yam and potato (Donegá et al., 2013). Mechanical processes such as washing, peeling, and cutting are known to cause a range of degradative changes that limit the shelf lives of processed products (Lunadei et al., 2011). For fresh-cut yam stored at 25 °C, the above processing methods lead to yellowing (Zhao et al., 2020), which is an undesirable phenomenon that changes the appearance and organoleptic properties of yam.
The increased demand for fresh-cut yam and tighter restrictions on chemical food treatment makes it necessary to improve our understanding of enzymatic yellowing and develop simple and natural treatments to control the yellowing process. However, the control of discoloration has always been a challenge in the food industry (Altunkaya and Göekmen, 2009; Teng et al., 2019). Ethanol is generally recognized as a safe product and exits naturally in plants with low toxicity to fruits and vegetables (Yan et al., 2015). Ethanol has been used to retard tissue discoloration (Bai et al., 2011) and maintain the quality of intact apples, grapes, cherries, peaches, mangos, fresh-cut eggplant (Chervin et al., 2005; Hu et al., 2010), and asparagus spears (Herppich et al., 2014). Based on its anti-discoloration properties, ethanol has the potential to be developed as a simple and fast treatment for repressing yellowing in fresh-cut yam. However, further studies are required to understand the mechanism by which ethanol inhibits yellowing.
Zhao et al. (2020) demonstrated that the primary yellow pigment in fresh-cut yam is bisdemethoxycurcumin, a curcuminoid that is found mainly in the rhizomes of turmeric (Curcuma longa). The formation of curcuminoids has been reported to involve upstream and downstream sections. The upstream section primarily involves the metabolites related to the general phenylpropanoid pathway, while polyketide synthases are the key enzymes involved in the downstream section (Deepa et al., 2017). In this study, we developed a fast and effective postharvest technique based on ethanol treatment to inhibit the yellowing of fresh-cut yam. We also investigated the underlying anti-yellowing mechanisms at the enzymatic and metabolic levels and we speculated that ethanol inhibited the yellowing by regulating the phenylpropanoid and curcuminoid biosynthetic pathways. The findings provide insights into mechanisms of discoloration and suggest an effective method to maintain the color of fresh-cut yam.
Section snippets
Experimental materials and treatment methods
Mature fresh yams (Dioscorea oppositifolia L.) (Tiegun) were purchased from a commercial market (Henan Province, China). The yams were immediately transported to our laboratory. The yams selected for study were uniform in maturity and size and had no obvious signs of defects, mechanical injury, or decay. The yams were scrubbed, washed with deionized water, peeled using a sharp, stainless-steel peeler, and manually chopped into small slices with thicknesses of 5 mm. After soaking the yam slices
Ethanol inhibited the synthesis of yellow pigment
Fresh-cut yam samples showed visible yellowing in the control samples during storage (Fig. 1A), and the yellowing appeared after 9 h of storage. In contrast, the fresh-cut yam samples treated with ethanol did not show yellowing until 36 h, at which point the control samples exhibited serious yellowing. To verify the visual observations of yellowing, the L*, a*, and b* values were evaluated using a colorimeter (Figs. 1B–D). For the control samples, the L* value decreased during the storage
Discussion
Color is an important quality in the food industry as it affects both aesthetic value and quality judgment, which influence consumer choice and preference. Thus, the preservation of the normal tissue color and suppression of discoloration is an important consideration in the processing of fresh-cut products (Homaida et al., 2017).
A previous study demonstrated that fresh-cut yam slices turn yellow during storage at 25 °C, and the primary yellow pigment is bisdemethoxycurcumin (Zhao et al., 2020
Conclusions
The quality of fresh-cut yam is diminished by yellowing, and evidence suggests that the yellowing is primarily caused by the production of bisdemethoxycurcumin. In this study, the treatment of fresh-cut yam with 10 % ethanol inhibited yellowing compared to the control. Ethanol treatment drastically repressed the enzyme activities of PAL, C4H, and 4CL, which corresponded to the downregulated transcription level expressions of PAL, C4H, and 4CL compared to the control. In addition, ethanol
CRediT authorship contribution statement
Shuang Guo: Investigation, Data curation, Writing - original draft. Xiaoyan Zhao: Data curation, Writing - review & editing, Funding acquisition. Yue Ma: Methodology. Yan Zhang: Formal analysis. Dan Wang: Conceptualization, Data curation, Writing - review & editing.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
This work was supported by Beijing Natural Science Foundation (6212008), the Special Training Program for Outstanding Scientists of the Beijing Academy of Agricultural and Forestry Sciences (JKZX201908), the Collaborative Innovation Center of the Beijing Academy of Agricultural and Forestry Sciences (KJCX201915), and China Agriculture Research System of MOF and MARA (CARS-23).
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These authors contributed equally to this work.