Rapid volatiles fingerprinting by dopant-assisted positive photoionization ion mobility spectrometry for discrimination and characterization of Green Tea aromas

Highlights

• A method using ion mobility spectrometry for tea aroma analysis.

• Dopant-assisted positive photoionization is suitable for tea volatiles ionization.

• Rapid and real-time fingerprinting of tea volatiles without prior sample treatment.

• Effective classification of different green tea aromas by using this method.

Abstract

Aroma is one major concern of tea flavor and represents an essential criterion for quality evaluation. Herein, we presented a novel approach for rapid, non-destructive and real-time fingerprinting of green tea aromas using ion mobility spectrometry (IMS) without prior sample treatment. Positive photoionization with acetone as dopant was selected as suitable source for tea volatiles analysis in comparison to other sources. To demonstrate the effectiveness of presented method, chestnut-like aroma, an indicator of high-quality green teas, was distinguished from others using IMS fingerprinting. In particular, three sub-types with subtle difference in sensory evaluation, i.e., tender, pure and roasted chestnut-like aroma, were classified by IMS fingerprinting in combination with chemometrics with an overall predictive accuracy of 95.6%. Furthermore, we demonstrated a strategy of two-dimensional IMS profiling for capturing the “dynamics” of tea volatiles. To our knowledge this is the first report of tea aroma analysis using IMS-based tool, which may serve as a complement to the conventional tea sensory evaluation and is promising for process monitoring during tea manufacture.

Introduction

Tea, manufactured from fresh leaves of Camellia sinensis L., is one of the most widely consumed flavored beverages in the world [1]. Although tea aroma volatiles constitute only trace fraction of the total amount of tea extract, the various compositions of these volatiles, in particular, odor-active compounds, and potentially complicated interaction amongst them, set up important basis for the formation of different tea aroma characteristics, thus have been intensively studied [2], [3], [4], [5], [6]. Tea aroma is one major concern of tea flavor and represents an essential criterion for tea quality evaluation. At present, tea quality is assessed predominantly based on sensory evaluation by human panel. Such method requires special expertise acquired by long-term training and is often inaccurate due to its vulnerability to external and subjective factors [7]. On the other hand, tea aroma formation and rapid transformation, occurring during manufacturing process, tends to be readily influenced by all sorts of operational settings such as temperature. Therefore, rapid characterization and discrimination of different tea aromas would not only serve as a complement to the conventional procedure of tea sensory evaluation but also is significant to process monitoring and oriented manufacturing of tea products with desired aroma characteristics.

Unfermented green tea is the most prevalent tea category in China, Japan, and other East Asian countries [5]. Substantial transformations of aroma take place during green tea processing (withering, heating, rolling and firing), resulting in various aroma characteristics of final tea products including, for example, fresh, sweet, smoky, and chestnut-like aroma [8]. Chestnut-like aroma is one of typical pleasant flavors of green teas, in particular, of those high-grade and well-known green teas, and is regarded as an indicator of green tea quality and grade [9], [10]. Based on subtle difference in tea sensory evaluation, chestnut-like aroma can be further classified into three sub-types, i.e., tender, pure and roasted chestnut-like aroma [11]. Tender and roasted chestnut-like aroma contains relatively small fraction of fresh and roasted flavor, respectively, in addition to the major chestnut-like flavor. It has been observed based on our green tea manufacturing experience, that, the attractive chestnut-like aroma can be transformed into the less-appealing over-roasted or over-baked aroma by inappropriate processing control. Of interest, roasted chestnut-like aroma, as a transitional aroma, represents a signal for potential trend towards such “off flavor”. It is therefore of great significance to distinguish chestnut-like aroma from other aroma types, and in particular, even the subtle differences in between tender, pure and roasted sub-types in a rapid, real-time and valid pattern.

The state-of-the-art gas chromatography coupled to mass spectrometry (GC-MS) or olfactometry (GC-O) technologies have been the most popular tools, with exponentially increased applications in the field of volatile or semi-volatile compounds [2], [12]. By applying GC-MS, a few studies have been reported by far concerning the key volatile compounds responsible for the chestnut-like aroma or the differential volatiles of three sub-types of aroma [9], [10], [11]. However, these approaches are often time-consuming and laborious, and require complicated sample pretreatment prior to GC analysis for volatiles extraction. Besides, due to the high temperature and potential oxidation or thermal reactions involved in volatiles extraction, tea volatiles may be affected, such as those derived from glycoside precursors [2], [13]. Thus, a fast, responsive and valid tool is required as a complement for aroma analysis.

Ion mobility spectrometry (IMS) is an emerging technique for gas-phase separation of ions at ambient pressure under electric field gradient, based on difference of ion mobilities of ionized (semi-)volatile compounds, which is regarded as a novel “sensor” of electronic nose [14], [15], [16]. It has been widely employed in on-site detection of explosives, toxics, and illicit drugs [17], [18], attributed to its rapid response, low cost, operational simplicity and high sensitivity. The scope of IMS applications has been extended in recent years to other fields including, for example, environmental monitoring and food analyses [19], [20]. IMS, either as a stand-alone device or coupled with GC or MS, has been utilized for rapid detection of food freshness [21], [22], adulteration of edible oils [23], [24], assessment of food grade or authenticity [25], [26], food safety control [27], as well as process monitoring of coffee roasting [28]. However, thus far, no study has been reported yet regarding the tea aroma analysis using IMS-based approach.

The aim of this study was to develop an optimal IMS-based approach for rapid fingerprinting of tea volatiles, and moreover, for discrimination and characterization of different green tea aromas. To this end, we optimized IMS method for tea volatiles fingerprinting by comparison of different ionization sources. To demonstrate the analytical potential and effectiveness of presented method in tea aroma analysis, different green tea aromas were distinguished, with specific focus on classification of three sub-types of chestnut-like aroma with subtle difference, i.e., tender, pure and roasted chestnut-like aroma, by applying optimized IMS setup in combination with chemometric analysis.