Inhibitory effect of chlorogenic acid on polyphenol oxidase and browning of fresh-cut potatoes
Graphical abstract
Introduction
Fresh-cut potatoes are a major option for consumers duo to their convenience and freshness (Zhou et al., 2019). However, tissue damage during processing mainly resulted to surface browning, such as peeling and cutting. (Liu et al., 2019). As a consequence, the surface browning of fresh-cut potatoes resulted in the poor appearance, quality deteriorations and losses of water and nutrients (Cabezas-Serrano et al., 2009). Surface browning is usually caused by enzymatic browning and polyphenoloxidase (PPO) is an essential enzyme in the melanogenesis pathway of enzymatic browning (Feng et al., 2020).
PPO is a type-3 copper protein that contains two copper atoms (CuA and CuB) at the active site (Fronk et al., 2015). In the presence of oxygen, PPO could catalyze the hydroxylation of monophenols to o-diphenols and the oxidation of o-diphenols to o-quinones, which resulted in enzymatic browning (Lim and Wong, 2018). A series of adverse changes could lead to the decrease of bioavailability of nutrients and limit shelf-life of fresh-cut potatoes (Kim and Uyama, 2005; Lin et al., 2011). Therefore, effective inhibition of PPO activity is desirable for delaying enzymatic browning and preservation of foods (Ma et al., 2017). A few of the methods have been reported to inhibit enzymatic browning by using antioxidants such as cinnamic acid (Shi et al., 2005), ascorbic acid (Landi et al., 2013), oxalic acid (Yoruk and Marshall, 2006), apigenin (Xiong et al., 2016) and sodium sulphite (Palma-Orozco et al., 2011). Therefore, finding highly effective inhibitor from natural sources is on demand in fresh-cut potatoes industry.
In recent years, it is noteworthy that natural polyphenols are widely applied for the preservation of fruits and vegetables. The phenolic compounds in the extracts of citron (Cedrus deodara) pine needles could be used as a potential anti-browning agent in protecting against fresh-cut apple slices browning (Liang et al., 2014). Polyphenol extracts from rice bran inhibited PPO activity and showed anti-browning effect on potato and apple puree (Sukhonthara et al., 2016). The phenolic acids in the dog rose and pomegranate extracts could control enzymatic browning of artichoke stems (Zocca et al., 2011). Ferulic acid treatment could increase activities of related enzymes in the phenylpropanoid pathway and prevent the gray mold of apple fruit (He et al., 2019). Chlorogenic acid (CGA) which is an ester between trans-cinnamic acid and quinic acid, is an important phenolic acid. CGA treatment significantly reduced ethylene production and retard senescence of the apple pulp (Xi et al., 2016). Furthermore, the treatment of CGA could reduce the content of malondialdehyde and effectively maintain the quality of nectarine fruit (Xi et al., 2017). However, the lack of sufficient understanding about the regulating action induced by CGA and inhibitory mechanisms of CGA on PPO has seriously hampered the application of this potential inhibitor on fresh-cut potatoes.
In this study, the reversibility and inhibition types of CGA on PPO were evaluated by kinetic analysis. The conformational changes of PPO induced by CGA was studied through circular dichroism and fluorescence emission spectra analysis. The potential interactions between CGA and PPO were simulated by molecular docking program. Based on the inhibition mechanism of CGA on PPO, we further investigated the anti-browning effect of CGA on fresh-cut potato slices during storage by color analysis, weight loss analysis and PPO activity assay. Hence, our study has provided efficient information for application of CGA as a PPO inhibitor in fresh-cut potatoes.
Section snippets
Materials
Chlorogenic acid (CGA), catechol and mushroom (Agaricus bisporus) polyphenoloxidase (PPO, EC 1.14.18.1, 128 kDa, 1380 u/mg dw) was purchased from Solarbio (Beijing, China). All commercial chemicals were of analytical grade and solutions were prepared with pure water.
Potatoes (S. tuberosum, cv Netherlands 7) were purchased from Agricultural Market in Tianjin, China. The time interval was less than 12 h between potato harvest and selection from the market according to the retailer’s statement.
Effect of CGA on activity of PPO
The inhibitory effect of chlorogenic acid (CGA) on the activity of polyphenoloxidase (PPO) was shown in Fig. 1A. Our results showed that the relative activity of PPO remarkably reduced with increasing concentration of CGA. Meanwhile, the half-maximal inhibitory concentration (IC50) of CGA was used to assess the effect of CGA on activity of PPO. The IC50 of CGA was calculated as 1.55 × 10−4 mol L-1. Moreover, we found that the PPO activity could be completely inhibited when the concentration of
Conclusions
In our study, we investigated the CGA had been shown a suppressing effect on PPO in a mixed-type inhibition during the oxidation of catechol. In addition, CGA could interact with PPO and quench the fluorescence intensity. The secondary structure of PPO was rearranged after treating with CGA by CD analysis. Docking results revealed that CGA was embedded into the hydrophobic cavity of PPO structure and prevented the formation of oxidation product. Furthermore, the present study demonstrated that
CRediT authorship contribution statement
Dai Cheng: Conceptualization, Methodology, Writing - review & editing. Guangliang Wang: Data curation, Writing - original draft. Jinlei Tang: Visualization, Investigation. Congying Yao: Software. Pengfei Li: Investigation. Qi Song: Visualization. Chunling Wang: Supervision, Validation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank all participants for their participation and kind assistance. This work was supported by the National “Thirteenth Five-Year” Plan for Science & Technology Support (Grant Number 2016YFNC010104), National Natural Science Foundation of China (31801453), the Open Project Program of State Key Laboratory of Food Nutrition and Safety, Tianjin University of Science & Technology (SKLFNS-KF-201906), the key program of the Foundation of Tianjin Educational Committee (2017ZD07) and China
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