Production of superfine green tea powder from processing wastes: Characterization of chemical composition and exploration of antimicrobial potential against Ralstonia solanacearum

Highlights

• Superfine green tea powder (SGTP) was prepared at 4 °C with D50 9.215 μm.

• SGTP mainly contained GA, EGCG, CAF and ECG.

• SGTP hindered bacterial proliferation by cell damage and decreased swimming motility.

• SGTP exhibited great antimicrobial potential against R. solanacearum.

Abstract

“Green waste” derived from food or agro-industrial has been applied to remove pathogenic bacteria. Green tea waste is polyphenol rich green waste but barely been focused by researchers. We ground the green tea wastes into superfine powder at 4 °C. The median particle size of superfine green tea powder (SGTP) was 9.215 μm. The chemical composition of powder was analyzed by high performance liquid chromatography (HPLC), and was mainly consisting of gallic acid (GA), epigallocatechin gallate (EGCG), caffeine (CAF), and epicatechin gallate (ECG). In vitro experiment indicated that the SGTP significantly inhibited bacterial proliferation of plant pathogen Ralstonia solanacearum GMI1000 in a concentration-dependent manner. However, when the hrpB deletion mutant of GMI1000 was inoculated in medium supplemented with SGTP, the bacterial growth was barely affected. Furthermore, the SGTP significantly decreased the bacterial swimming motility. Scanning electron microscope (SEM) image exhibited that cell damage emerged when bacteria was exposed to medium supplemented with SGTP. Antibacterial activity was compared towards SGTP, tea polyphenol, and tea extract, which suggested that SGTP had potential to be a functional antimicrobial. Our approach of SGTP as antimicrobial would broaden the scope of green waste utilization in a greener, cost-effective, and sustainable approach in future.

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.