Production of superfine green tea powder from processing wastes: Characterization of chemical composition and exploration of antimicrobial potential against Ralstonia solanacearum
Introduction
Green tea is one of the most popular beverages with large annual demand in China (McKay & Blumberg, 2002). However, only a small amount of green teas are suitable for commercial sales, while a large number of green tea wastes are abandoned. Green tea wastes are left as industrial waste due to its imperfect shape. Tons of green tea wastes were left inevitably during processing every year. However, this “green waste” is a valuable resource of polyphenol but have been ignored in decades (Balasundram, Sundram, & Samman, 2006; Centore, Hochman, & Zilberman, 2014; Kuppusamy, Thavamani, Megharaj, & Naidu, 2015).
Great efforts have been made to remove undesirable microorganisms from soil and water biologically for recovering their original natural condition. Polyphenols have been considered as the economic, sustainable, and eco-friendly remediation tools without transferring any harmful after-effects to the future (Daglia, 2012; Yanagawa, Yamamoto, Hara, & Shimamura, 2003). It has been reported that plant polyphenols contributed to antimicrobial, antioxidant, anticancer, and anti-inflammatory (Almajano, Carbó, Jiménez, & Gordon, 2008). A powdered form of pure polyphenols extracted from natural plant can successfully inhibit or completely retard the bacterial proliferation (Corbo et al., 2009; Fan, Chi, & Zhang, 2008; Yi, Zhu, Fu, & Li, 2010). Shimamura, Zhao, and Hu (2007) found that (−)-Epigallocatechin-3-gallate (EGCG), a main constituent of tea catechins inhibited the in vitro growth of several bacterial species. EGCG can directly bind to Gram-positive bacterial peptidoglycan layers, thereby breaking biofilm formation and bacterial swarm motility (Yoda et al., 2004; Zhao et al., 2003). In terms of Gram-negative bacteria, EGCG is likely to oxidatively stress bacteria by releasing H2O2, resulting in morphological change of cell wall (Arakawa, Maeda, Okubo, & Shimamura, 2004). Cui (2012) investigated that EGCG caused grooves in Pseudomonas aeruginosa. Besides, some phenolic acids (gallic, caffeic, and ferulic acids) exhibited antibacterial activity against Gram-positive (Staphylococcus aureus and Listeria monocytogenes) and Gram-negative bacteria (Escherichia coli and P. aeruginosa) (Almajanoet al., 2008; Si et al., 2006; Qiang, Lv, & Yao, 2006; Cooper, Morré, & Morré, 2005; Hamilton-Miller, 1995).
Ralstonia solanacearum, which belongs to Pseudomonas, is a Gram-negative soil-borne plant pathogenic bacterium (Agrios, 2008, pp. 647–649; Peeters, Guidot, Vailleau, & Valls, 2013). It is motile with a polar flagellar tuft, causing lethal wilting disease at high populations (108-1010 cfu/g tissue) (Denny, 2006). The bacterium survives in the soil, enters the plant root system, and colonizes the xylem vessels with the production of large amounts of exopolysaccharides (EPS), resulting in blocking water traffic in the plant (Brito, Aldon, Barberis, Boucher, & Genin, 2002; Mole, Baltrus, Dangl, & Grant, 2007; Vasse, Frey, & Trigalet, 1995). Bacterial wilt caused by R. solanacearum is of economic importance due to its lethality, persistence, wide host range, and broad geographic distribution. It invades over 250 plant species, including agronomically important crops, such as tobacco, tomato, pepper, banana, and potato (Elphinstone, 2005; Genin & Denny, 2012). Currently there are no chemical controls available. So far, good rotation system that follows susceptible crops with resistant or nonhost crops can assist in diminishing this pathogen (Denny, 2006).
Bioactive components of tea, including polyphenols, caffeine, gallic acids, and others have antimicrobial potential (Almajano et al., 2008; de Mejia, Ramirez-Mares, & Puangpraphant, 2009). Current technology of tea infusion and extraction leads to more than 40% soluble solids wasted in spent leaf (Xiao, Zhang, Fan, & Han, 2017). Superfine grinding technology improves bioavailability of tea compoents in vivo or in vitro by increasing tea particle surface area and the breakdown ratio of the cell walls (Hu, Chen, & Ni, 2012; Tao et al., 2014). Xiao et al. (2017) significantly raised infusion yield of total polyphenols, caffeine, and water-soluble carbohydrate of black tea by superfine grinding. Therefore, we ground the green tea wastes into superfine powder form to explore their antimicrobial potential again R. solanacearum.
In this study, we analyzed the polyphenols content of superfine green tea powder and investigated the antibacterial potential against R. solanacearum GMI1000, aiming to highlights the use of natural polyphenol-rich green waste. Since green tea wastes are non-toxic, easily biodegradable, eco-friendly, and rich in resource. Utilization of green tea residues against R. solanacearum would extend the scope of green agriculture development and field application strategies in future.
Section snippets
Preparation of superfine green tea powder (SGTP)
Green tea wastes (harvested at summer and autumn, mainly green tea leaves) were prepared through basic tea process (including de-enzyming, rolling and drying) by Shaanxi wo-long tea manufacture Co., Ltd. Green tea wastes were milled into coarse powder first and then mixed with zirconia mill jar balls (3–10 mm in diameter) at a ratio of 1:2. SGTP was obtained by a vertical air cooled planetary ball mill MITR-XH-XQM (Mitrcn Co. Ltd, Changsha, China) after 2 h grinding at 4 °C. The SGTP was stored
Characterization of SGTP (particle size distribution, cell wall breakage ratio, cell morphology and TP content)
Superfine grinding technology is qualified for manufacturing tea powder at micron level and breaking cell wall totally to release maximum bioactive components. Xiao et al. (2017) obtained the superfine black tea powder in the range of 2.4–13.1 μm with a median particle size of 6.9 μm and increased the bioaccessibility of black tea. In present study, the green tea waste, abandoned from industrial manufacture, was ground into superfine powder form. The particle size of the powder varies from
Conclusions
Microbial resistance has currently become an increasing global problem. There is a compulsory need to find out new eco-friendly antimicrobial agents which promote plant growth and be economical. There have been several studies aimed at identifying novel plant-derived active compound against microbial. Our research ground the green tea wastes into superfine powder form with a median particle size of 9.215 μm, which had active compounds as GA, EGCG, CAF, and ECG. We proved that SGTP inhibited
Acknowledgments
Hengkai Zhang and Li Chen contributed equally to this study. This study was financially supported by Shaanxi science and technology research project (2017NY-147), and the promotion of Xianyang Fu Brick Tea under the Belt and Road Initiative (2017k02-103), and the University Key Laboratory of Food Processing Byproducts for Advanced Development and High Value Utilization No.1111090010, and the National Natural Science Foundation of China (31871801), and the national college students' science and
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