Food Science and Technology Research
Online ISSN : 1881-3984
Print ISSN : 1344-6606
ISSN-L : 1344-6606
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Dynamics of Water-soluble Pectin during Unrefined Tea Processing: Content and Molecular Weight Distribution
Hisako Hirono
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2016 Volume 22 Issue 2 Pages 297-300

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Abstract

We examined the dynamics of water-soluble pectin (WSP), specifically the content and molecular weight distribution, in tea leaves and tea infusions in relation to steaming time during unrefined tea processing. It was revealed that the fraction of higher-molecular weight (MW) WSP in tea leaves was infused at the beginning of unrefined tea processing at all steaming times, and lower-MW WSP was not infused during any processing steps. Moreover, by increasing steaming time, the increase of higher-MW WSP content in tea leaves was reflected by the increase in WSP content of corresponding size in the tea infusions.

Introduction

Water-soluble pectin (WSP) is contained in tea leaves at less than 1.5% (dry weight) (Takayanagi et al., 1987a; Matsuo et al., 2012; Hirono and Uesugi, 2014). It has been shown that the WSP content in both tea leaves and tea infusions alters significantly with steaming time, in contrast to catechins, caffeine and amino acids, the main components of tea (Takayanagi et al., 1987a, b; Matsuo et al., 2012).

We recently clarified that when tea leaves were steamed, pectin fractions with a molecular weight (MW) of 1,100 × 103 were degraded from water-insoluble pectin and eluted as WSP, resulting in increased WSP content of the steamed tea leaves. Additionally, the longer the steaming time, the greater the degradation of WSP (Hirono and Uesugi, 2014).

Green teas are popular beverages that are composed of the infusions of refined tea leaves. Unrefined teas, which possess the fundamental quality of teas and are the base of refined teas, are typically manufactured using drying and rolling processes after the initial steaming process. Therefore, it was necessary to reveal the transition of WSP during the unrefined tea manufacturing processes and the leachability of WSP in tea infusions in order to clarify the properties of WSP in unrefined teas. In this paper, we investigated the dynamics of WSP in both tea leaves and tea infusions, in terms of content and molecular weight distribution, in relation to the duration of unrefined tea steam processing.

Materials and Methods

Preparation of tea leaves    Tea leaves were cultivated at the National Institute of Vegetable and Tea Science plantation in Kanaya-Shishidoi, Shimada, Shizuoka, Japan. Tea leaves (‘Yabukita’ cultivar) were harvested during the first crop season in May, 2013. Harvested tea leaves were immediately steamed for 30, 60 or 120 seconds using a SD-500 steaming machine (Terada Seisakusho Co., Ltd., Shimada, Japan), and then manufactured into unrefined teas. After the initial steaming process, unrefined tea processing proceeded sequentially from primary drying, tea rolling, secondary drying, to the final rolling; which were all conducted using tea manufacturing machines specified for 2 kg of production (Kawasaki Kiko Co., Ltd., Kakegawa, Japan). For analysis, a small amount of tea leaves from each step in the process was removed and dried to about 5% moisture content using a circulation dryer at 40°C. Manufacturing of each unrefined tea according to steaming time was performed in triplicate.

Determination of WSP content in tea leaves    Dried tea leaves were powdered using a ZM200 cyclone sample mill (Retsch GmbH, Haan, Germany). Preparation of Alcohol-Insoluble Substances (AIS), WSP extraction from AIS, and determination of WSP content by the colorimetric method were conducted according to the 3,5-dimethylphenol method as previously described (Hirono and Uesugi, 2014), except that a UV-1800 UV-VIS spectrophotometer was employed (Shimadzu Corporation, Kyoto, Japan) in the colorimetric method. WSP content was shown as an average analysis value of three independent unrefined teas.

Determination of WSP content in tea infusions    Three grams of dried tea leaves (not powdered) were infused with 200 mL of hot water, immediately after boiling, using a GAV-2/C teapot (Celec Co., Ltd., Tajimi, Japan) and left at room temperature for 5 min. Fifty milliliters of the infusion without tea leaves was freeze-dried, and the lyophilized concentrate was then dissolved in distilled water (DW). A precipitate, formed by the addition of a final concentration of 70% ethanol, was dissolved in DW and subjected to colorimetric determination of WSP content. The WSP content in tea infusions was calculated per 100 g of dried tea leaves. WSP content was shown as an average analysis value of three independent unrefined teas.

Determination of molecular weight    The WSP solutions from each processing step tested (determination of WSP content in tea leaves and tea infusions) in triplicate were respectively combined according to steaming time and concentrated by lyophilization. Size exclusion chromatography using a HiPrep 26/60 Sephacryl S-400 HR column (GE Healthcare, Little Chalfont, England) was performed with this concentrated WSP according to a previously reported procedure (Hirono and Uesugi, 2014).

Results and Discussion

Figure 1 shows the effect of steam processing time on WSP content in tea leaves (a) and tea infusions (b) during unrefined tea processing. The WSP content of each completed step in the process is shown. Table 1 (a) and (b) are ANOVA tables showing the WSP content in tea leaves and tea infusions in relation to steaming time during unrefined tea processing. In tea leaves, the WSP content increased with increasing steaming time. WSP content after the steaming process was higher than the other processing steps for all steaming times. This result is in accordance with the report of Takayanagi and Anan (1986). After the steaming process, adhesive substances, so-called chashibu, are attached to the manufacturing machines during the drying or rolling of tea leaves. It is expected that the chashibu contains not only powdered tea leaves but also pectin, a viscous component, obtained from the surfaces of tea leaves during rolling. Therefore, the reason why the WSP content in tea leaves was higher after steaming might be attributable to the loss of pectin as chashibu. On the other hand, the WSP content of tea infusions also increased with increasing steaming time; however, the content after steaming was the lowest among the five processing steps. It is proposed that the disruption of tea cells by primary drying contributed to the increase in WSP content of tea infusions. Besides primary drying, there were no significant differences in WSP content among the tea processing steps.

Fig. 1.

Changes in WSP content in tea leaves (a) and tea infusions (b) during crude tea processing by steaming time. Steaming times were 30, 60, and 120 s. Data is shown as an average value ± standard deviation.

Table 1. Two-way ANOVA tables for the WSP content in tea leaves (a) and tea infusions (b) during crude tea processing by steaming time. Steaming times were 30, 60, and 120 s.

Figure 2 shows the changes in MW distribution of WSP in tea leaves (a) and tea infusions (b) in relation to steaming time during unrefined tea processing. The WSP content of each completed step in the process is shown. In tea leaves, a wide range of elution volumes, from approx. 100 to 270 mL, was observed for all processing steps. In this volume range, a strong peak was observed for all steaming times at an elution volume of around 112 mL, which corresponded to a MW of 1,100 × 103. Along with increasing steaming time, an increase in the amount of higher-MW fractions eluted (elution volume of approx. 100 to 170 mL), which was eluted as the major peak and following the peak, was observed. This indicated a steaming time-dependent increase in the water solubility of the water-insoluble pectin as well as WSP degradation. On the other hand, in tea infusions, a characteristic elution volume corresponding to higher-MW fractions in tea leaves (approx. 100 to 170 mL), was observed with a broader peak around 121 mL (corresponding to a MW of 960 × 103) throughout all processing steps, which was somewhat lower than that observed in tea leaves, regardless of the steaming time. Moreover, after the steaming process, the MW of the infused fraction was somewhat higher than the other processes. The WSP content in tea infusions, which was present in the higher-MW fractions, was increased with increasing steaming time. This increase of WSP content in tea infusions was reflected in the increased WSP content of corresponding size, adjacent to the peak, observed in tea leaves.

Fig. 2.

Two-way ANOVA tables for the WSP content in tea leaves (a) and tea infusions (b) during crude tea processing by steaming time. Steaming times were 30, 60, and 120 s.

We have demonstrated for the first time the effect of steam processing time on changes in the MW distribution of WSP in tea leaves and tea infusions during unrefined tea processing. The results revealed that the fraction containing higher-MW WSP in tea leaves was infused at the beginning of processing, and subsequent fractions did not contain lower-MW WSP (elution volume of approximately 170 to 270 mL) throughout the processing steps. Moreover, the increase of higher-MW WSP content in tea leaves with increasing steaming time was reflected in the increased WSP content of corresponding size in the tea infusions. Considering that no fractions of lower-MW WSP in tea leaves were infused, this indicates that both WSP content as well as the hydrophilicity of WSP, which varies according to the degree of esterification (Reginald and Timothy, 1990) and hence affects MW, might contribute to the WSP infusion ability. Thus, we propose the hypothesis that higher-MW fractions, containing an abundance of high-methoxyl pectin, have higher hydrophilicity and are easily infused. In contrast, lower-MW fractions, containing an abundance of low-methoxyl pectin, exhibit lower hydrophilicity and are less easily infused.

It is known that WSP is capable of reducing catechin astringency (Hayashi et al., 2005). Additionally, WSP is proposed to contribute to the viscosity of infused tea. Okada and Furuya (1965) reported that the viscosity of tea extracts might be associated with the alcohol precipitation characteristics of tea extracts. Moreover, these characteristics are thought to be correlated with the MW of WSP in tea extracts. Future investigations will focus on the effect of alterations in the MW of WSP on the reduction of catechin astringency and the viscosity of tea infusions.

Acknowledgements    This research was partly supported by a Research Grant from the National Agriculture and Food Research Organization Gender Equality Program. The author thanks Masako Ishikawa for her research assistance.

References
 
© 2016 by Japanese Society for Food Science and Technology
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