Influence of tea polyphenol and bovine serum albumin on tea cream formation by multiple spectroscopy methods and molecular docking
Introduction
Tea is the most highly consumed beverage in the world after water because of consumers’ preference for its attractive and distinct sensory (color, flavor, aroma, and taste) characteristics and healthy functions (Liu, et al., 2020). The health benefits of tea are attributed to its high content of tea polyphenols (TPs) which account for 18–36% of the dry weight. The pH values of TPs aqueous solutions decrease as the concentration of TPs solutions increased (Li, Taylor, Ferruzzi, & Mauer, 2013). Most of the ready-to-drink (RTD) tea beverages suffer from the tea cream that, a noticeable haze and precipitate, forms spontaneously in a hot and strong tea infusion on cooling down. The formation of tea cream affected the sensory profile and limited the shelf life of the tea beverage largely. Tea catechins (account for 70–80% of total TPs) play a key role in the forming of tea cream (Xu et al., 2017), due to their interactions with other molecules like protein, caffeine, and metal ions. Xu et al. (2017) indicated that Ca2+, sugars, caffeine, and polyphenols significantly influence the formation of sediment in green tea infusions. The creaming amount of green tea was highly related to original concentration of tea polyphenols and carbohydrates, while black tea cream amount was determined by proteins, methylxanthines and thearubigins (TRs) concentrations in infusions (Lin et al., 2015). Recent studies suggest that the amount of tea cream can be enhanced by interactions between TPs and proteins, due to the strong creaming affinities between gallated catechins and proteins (Ikeda et al., 2017, Wu et al., 2017). The interaction between polyphenol and proteins can result in changes in the physicochemical and functional properties, including the antioxidant activity of tea compounds, protein secondary structure, and digestion and permeation of polyphenol (Kanakis et al., 2011, Roy et al., 2012, Tenore et al., 2015). Previous studies indicated that protein-polyphenol interaction is one of the main causes of tea cream formation (Lin et al., 2015, Wu and Bird, 2010). However, previous studies mainly focus on the pH, extraction time and temperature, metal ions, tea type, and leaf/water ratios on tea cream formation. Little knowledge is available regarding the effect of different TPs and protein concentrations on tea cream formation.
Proteins have strong binding activity with polyphenols, which are affected by the number of galloyl and hydroxyl groups present in polyphenols. In acidic condition, interactions between proteins and polyphenols are mainly non-covalent (Yildirim-Elikoglu & Erdem, 2017). An understanding of interactions between proteins and TPs is critical for elucidating the formation mechanism of tea cream. The main water-soluble protein in tea is albumin (Ruan & Zhu, 1986), which usually accounts for 1–3% of the dry weight in tea infusion. Bovine serum albumin (BSA) is widely used in a beverage environment for precipitation analytical purposes (Montero, et al., 2019). Electrophoresis results showed that tea protein with high molecular weight (60 KD) is more prone to cream formation, which is much similar to the cream formation ability of BSA (66 KD) (Lu, 2008). Furthermore, BSA has the same amide A band ranging from 3500 to 3100 cm−1 with tea soluble protein (Wu, et al., 2010). This study aims to reveal the interaction mechanism between TPs and BSA, and obtain the optimal TPs and BSA concentration based on the clarity properties and contents of phytochemicals. It is a difficult task to select the optimal concentration in different reaction groups, for the different characteristics.
Therefore, the aims of this study were to research the effects of different concentrations of TPs and BSA on the tea cream formation, and investigate the interaction mechanism of TPs against BSA. The effect of the TPs-BSA interactions on the tea cream formation was investigated by clarity measurements. The optimal concentration was selected by the technique for order preference by similarity to ideal solution (TOPSIS). The binding properties between catechins and BSA were studied using fluorescence, synchronous fluorescence spectroscopy, and UV–visible absorption spectroscopy. Molecular docking was done to recognize the binding site of catechins to BSA, and observe the type of interaction and the amino acid residues involved in the interaction. This study is expected to provide some significant insights into the TPs-BSA interactions, which may facilitate control the tea cream formation and improve performance in tea beverage products.
Section snippets
Materials
TPs (>95%) was purchased from Changsha Huacheng Biotech Inc. (Changsha, China). The catechins content in TPs was 85.9%. BSA was purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA). (−)-epigallocatechin gallate (EGCG), (−)-gallocatechin gallate (GCG), (−)-epicatechin gallate (ECG), (−)-epigallocatechin (EGC), (−)-epicatechin (EC), (+)-catechin (C), glacial acetic acid and methanol were of high-performance liquid chromatography (HPLC) grade and purchased from Chengdu Biopurify
Transmittance, particle size and precipitation of solution
Consumers are longing for high TPs-RTD tea beverage with good sensory quality. However, the high TPs-RTD tea beverages always have a relatively high turbidity, which cannot meet the needs of consumers’ nutritional and sensory needs. The concentration ranges of 234.1 to 533.7 mg/L has been used in commercial tea beverages (Zhang, Zhao, & Zhang, 2016). In order to obtain high-brightness tea beverage enriched high concentration of TPs, the effects of different TPs and BSA concentrations on cream
Conclusion
Our results showed that the concentration of TPs and BSA had substantial effects on the formation of tea cream. The transmittance significantly (p < 0.05) decreased with higher TPs and BSA concentrations while the particle size and precipitation has a reverse trend. Based on the clarity properties and contents of phytochemicals, TOPSIS method was successfully used to ease the samples comparison. The optimal concentration (TPs = 800 mg/L, BSA = 40 mg/L) for tea beverage under this experimental
CRediT authorship contribution statement
Xia Yu: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Software, Writing - original draft, Writing - review & editing. Xinghong Cai: Data curation, Formal analysis, Investigation, Resources, Visualization, Writing - original draft. Liyong Luo: Conceptualization, Methodology, Resources, Supervision, Validation, Writing - review & editing. Jie Wang: Data curation, Formal analysis, Investigation, Resources, Visualization, Writing -
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.
Acknowledgments
This study was funded by the Chongqing Characteristic Profitable Agriculture (Tea) Industrial Technology System Plan 2020(7), Chongqing Science and Technology Bureau (cstc2019jscx-dxwtBX0030), Training of Technical Innovation Talents in Yunnan Province (2019HB089), and the Yunnan Province “Ten Thousands Talent Program” Industrial Technology Leading Talent Project. Xia Yu would like to thank Sheng Li and Yan Liu for their useful suggestions to this research.
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