Effect of a combined microwave-assisted drying and air drying on improving active nutraceutical compounds, flavor quality, and antioxidant properties of Camellia sinensis L. (cv. Longjing 43) flowers

Abstract

Drying tea flowers into a high-quality product is important to its commodity value. In the present work, a combination of microwave-assisted drying and air drying (MAD-AD) was applied in the processing of fresh tea flowers and its effects on flavor quality, active nutraceutical compounds, and antioxidant capacities were studied. The results showed that compared to air drying and freeze drying tea flowers, the MAD-AD tea flowers had higher amounts of active compounds such as catechins, flavonol glycosides, and triterpenoid saponins, and possessed high antioxidant activities. Moreover, this drying method improved the tea flowers’ color and preserved a more floral fragrance. This combined method could be of interest as an industrial method for drying tea flowers with the benefit of reduced processing time, more reserved active compounds and high quality of products.

Microwave-assisted drying, tea flowers, active compounds, quality attributes, antioxidant

Highlights

1. The combination of microwave-assisted drying and air drying is firstly applied in the processing of fresh tea flowers to elevate their flavor compounds and antioxidant capacities.

2. Compared to conventional air drying and freeze drying, this combined drying method has the benefit of reducing processing time and reserving more active compounds.

3. The present work provides a feasible method to process high-quality tea flower products.

Introduction

Longjing tea, mainly made from the leaves of Longjing 43 cultivar, is one of the famous green teas in China. Longjing 43 is one of the most widely cultivated varieties in China, with the characteristics of strong drought resistance and high budding rate (Wang et al., 2019a). The production duration of Longjing tea is very short, only from March to May, and the remaining biological resources of Longjing 43 are not used to their full potential. Longjing 43 flowers open every autumn accompanied with high water and nutrient consumption, which has severe effects on the yields and quality of the tea leaves. To avoid this problem, removing flowers from tea bushes is required. At a rough estimation, there are 3000–12 000 kg tea flowers/ha for one year (Chen et al., 2018), and over 2.0 million kg of Longjing 43 tea flower resources are generally available in Zhejiang Province, China. For a better understanding and utilization of tea flower resources, increasing physiology and biochemistry studies have been performed. It has been clearly demonstrated that tea flowers are rich in nutraceutical compounds such as catechins, flavonols, saponins, as well as polysaccharides and amino acids (Chen et al., 2018). Tea flowers contain comparable amounts of catechins as tea leaves and possess the strongest antiproliferative and apoptotic effects on human breast cancer among seven Camellia sp. flowers (Way et al., 2009). Moreover, flavonoids which are separated and isolated from tea flowers proved to exert excellent antioxidant capacities (Yang et al., 2009) and inhibitory activities on lipid accumulation (Morikawa et al., 2013). Triterpenoid saponins, which were especially high in tea flowers, exhibited gastro-protection and inhibited pancreatic lipase activities (Yoshikawa et al., 2009). Large amounts of two characterized aromatic compounds, acetophenone and 1-phenylethanol, were obtained from tea flowers with the benefit of food storage and flavor improvement (Dong et al., 2017). Interestingly, tea flowers contain relatively low caffeine content (0.3–1.1 per cent), which is far below the level of tea leaves. Therefore, it is a potential drink appropriate for the consumers who must limit caffeine intake for health purposes.

The harvested fresh flowers respire and transpire with a loss of their quality. Drying is the best way to keep their qualities and convert these fresh resources into a shelf-stable commodity. Among all drying methods, freeze drying is one of the most advanced drying methods, providing high-quality products with a fine appearance, improved flavor, and more favorable compositions. The food stuff using freeze drying usually has a better quality, but the long drying time, large energy consumption, and high cost of equipment make this method inappropriate for industrial production (Valadez-Carmona et al., 2017). Conventional air drying (AD) is widely used at both small scale and industrial scale because of its low costs and availability of equipment. Drying with hot air can decrease the processing time and improve sample quality. However, heat and mass transferring through the whole drying process resulted in decreases of bioactive compounds (Aral and Beşe, 2016). In our previous work, we also found that tea flowers drying at high temperatures lost amounts of bioactive ingredients (Shi et al., 2019). Recently, microwave drying was introduced to process various kinds of products for its unique characteristics of high heat transmission, high drying rate, and high efficiency (Qu et al., 2019). Microwave drying avoided the internal crack and interior burning from excessive heating, and the product is characterized with uniform and high qualities. More importantly, microwave radiation was shown to rapidly inactivate degradation enzymes present in materials, which was beneficial to the preservation of bioactive compounds. Microwave drying has been used in black tea (Hatibaruah et al., 2013; Qu et al., 2019), green tea (Gulati et al., 2003) and other foodstuffs. In addition, the combination of microwave drying with other drying methods, particularly with AD, was widely used for industrial production, but the application of microwave-assisted drying (MAD) on tea flowers was rare.

Furthermore, the shape and the texture of tea flowers are quite different from conventional tea, therefore a methodological study is required for processing this new material. Optimum drying method and conditions are vital for maintaining tea flowers’ flavor, quality, and bioactive components during processing. To date, there is no available information on the effects of microwave-assisted drying combined with air drying (MAD-AD) on tea flowers’ quality and bioactive compounds. The aim of the present work was to evaluate the effect of MAD-AD on bioactive compounds, flavor, quality, and antioxidant capacity of tea flowers.

Material and Methods

Chemicals reagents and standard compounds

Gradient-grade methanol, acetonitrile, acetic acid, and formic acid for liquid chromatography–mass spectrometry (LC-MS) were purchased from Scharlau (Spain). Standards of catechins, epicatechins, catechins gallate, epicatechins gallate, gallocatechins, epigallocatechins, gallocatechins gallate, epigallocatechins gallate (EGCG), myricetin-3-O-β-d-rhamnoside (Myr-rha), quercetin-3-O-β-d-rutinoside (Que-rut), quercetin-3-O-β-d-glucoside (Que-glu), kaempferol-3-O-β-d-rutinoside (Kae-rut), kaempferol-3-O-β-d-glucoside (Kae-glu) were purchased from Sigma-Aldrich (Shanghai, China). Ginsenoside Rd standard was purchased from Chengdu Must Bio-technology Company (Chengdu, China). 2,2,-Diphenyl-2-picryl-hydrazyl (DPPH), 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH), sodium fluorescein (3′,6′-dihydroxyspiro [isobenzofuran-1[³H], 9′[⁹H]-xanthen]-3-one), and Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) were obtained from Sigma (USA).

Microwave-assisted treatment and sample preparation

Fresh flowers from a Camellia sinensis cv. Longjing 43 bush were harvested 16–20 November 2017 from the tea plantation located in Academy of Agricultural Sciences and Technology (Hangzhou, China). After the withering process the fresh flowers were used for further experimental treatments. The moisture content of the withered tea flowers was 0.60–0.65 g water/g dry matter (abbreviated to d.m.). Three levels of microwave drying powers (4.5 kW, 6.3 kW, 8.1 kW) were used as low, medium, and high levels by different settings with DXCWS microwave drying equipment (Yixing Dingxin Microwave Equipment Co., Ltd, Yixing, China). An optimized 180-s treatment with microwaves could provide intact appearances of tea flowers and were applied in this experiment. Then, a thin layer AD at 60 °C, which was found to keep most bioactive ingredients in tea flowers, were introduced for a further 90 min drying to a final moisture content of 0.08 g water/g d.m. (Shi et al., 2019). The flowers without MAD were prepared at 60 °C AD with a total drying time of 180 min. The fresh flowers were kept in a liquid nitrogen container immediately after harvesting and freeze drying (FD) for 48 hours to reach the same moisture content. These two treatments were used as comparison and named as AD and FD, respectively.

Sample extraction

The dried flowers of different treatments were grinded and passed through a 100 mesh sieve. The tea flower powders were extracted with 70 per cent methanol at a ratio of 1:20, 70 °C for 20 min. The infusions were centrifuged at 10 000g, 4 °C for 20 min, then filtered through a 0.22-μm nylon membrane. The mixtures were further analyzed for the concentration of catechins, flavonol glycosides, and saponins. Each extract was prepared three times. All experiments were repeated several times and typical results are presented.

Determination of catechins, flavonol glycoside, and triterpenoid saponins

The methanol extracts of tea flowers were analyzed for catechins and flavonol glycoside concentrations with a modified high-performance liquid chromatography (HPLC) method and an ultra-pure liquid chromatography–mass spectrometry (UPLC/MS) method, respectively (Shi et al., 2019). The calculation of catechin contents was based on the peak areas of sample injections and catechin standards. The quantification of individual flavonol glycosides was according to external standards of Myr-rha, Que-rut, Que-glu, Kae-rut, and Kae-glu. The HPLC and UPLC chromatogram of catechins and flavonol glycosides in tea flower samples are shown in Figure S1 and S2, respectively.

The UPLC method was employed to analyze the contents of triterpenoid saponins in different tea flower sample extracts (Shen et al., 2017). Seven main saponins were quantified as Rd equivalents concentrations under the range of 0.1–1.0 mg/mL. The standard curve of Rd and the UPLC chromatogram of triterpenoid saponins in tea flower samples in shown in Figure S3 and Figure S4, respectively. All experiments were conducted in triplicate.

Analysis of volatile compounds by head space solid phase micro-extraction gas chromatography–mass spectrometry

Tea flower infusion volatile collection, identification, and quantification were conducted according to Shi et al. (2019) using a 20-mL glass vessel and a head space solid phase micro-extraction (HS-SPME) fiber, coupled with gas Shimadzu QP2010SE head space solid phase micro-extraction gas chromatography–mass spectrometry (GC/MS, Shimadzu Corporation, Japan).

Color analysis

Color measurement of tea flower samples was carried out using a colorimeter (Minolta Chroma, CR-400, Japan) and calibrated with the white and black standard tiles. Hunter color values of L*, a* and b*, where L* is used to express lightness, a* redness (+) and greenness (–), and b* yellowness (+) and blueness (–). Total color differences (ΔE), chroma (C) and browning index (BI) were calculated according to Aral and Beşe (2016). Hunter values of the samples were measured in triplicate.

Determination of antioxidant activities

The antioxidant capacity was measured by Trolox Equivalent Antioxidant Capacity tests against ABTS⁺ and DPPH· radicals adjusted to spectrophotometer (Spectronic 20 Genesys^TM, IL, USA). The protocol of DPPH radical scavenging activity test was adapted from Yang et al. (2007) with some modifications. The protocol of the ABTS test was adapted from Podsędek et al. (2014) with some modifications.

The Oxygen Radical Absorbance Capacity Assay (ORAC) was performed as described by Uribe et al. (2014) and Xu et al. (2006). Briefly, 200 μL of fluorescein solution (100 nM) was added to each micro well and 40 μL of a diluted sample solution, standards (solutions of 25–100 nM Trolox) and blank (phosphate buffer pH 7.4) were added to the respective micro wells. The micro plate was incubated 20 min at 37 °C under shaking. After this time reactions were initiated by the addition of 35 μL of AAPH diluted in 75 mM phosphate buffer (pH 7.4) to 360 mM to all the wells. A Multimode Plate Reader, Synergy H1 (BioTek Instruments, VT, USA), was used at 485 nm excitation and 535 nm emission. ORAC values were calculated according to Uribe et al. (2014). The DPPH, ABTS and ORAC assays were calculated with standard solutions of Trolox (0.1–0.5 mM) to express results as μmol Trolox equivalents (TE) per milligram of tested dried sample.

Data analysis

All analyses were carried out in triplicate, and data are presented as mean ± standard deviation. The data were analyzed using the SAS 9.0 software (SAS Institute Inc., USA) using one-way analysis of variance (ANOVA) and Duncan’s multiple range tests. The statistical significance level was set up at P ≤ 0.05.

Results and Discussion

Effect of MAD-AD on phenolic contents of tea flowers

The catechins and flavonol glycosides represent the basic phenolic profiles in tea flowers. Catechins are the characteristic ingredients. They are essential to sensory properties and health benefits. They are indispensable for the brisk and pungent taste in tea beverages (Qu et al., 2019). Catechins with multi-OH groups have been proved to possess excellent anti-inflammatory, hypoglycemic, antidiabetic, anticancer, antihypertensive, antioxidant, antiviral, anti-Alzheimer and anti-Parkinsonism activities (Afzal et al., 2015). Therefore, maintaining more catechins during tea flower processing is important to sensory qualities and their bioactive functions.

In this work, eight catechin monomers from tea flower samples were analyzed, and the catechin contents of MAD-AD are shown in Table 1. The catechin contents of hot air drying at 60 °C and FD were also tested and used as comparisons (Table S1). The total catechin contents of MAD-AD samples ranged from 47.69 mg/g d.m. to 52.86 mg/g d.m. These values were significantly higher (P < 0.05) than the level of AD (33.19 mg/g d.m.). Except for the low level power-treated microwave-assisted drying combined with air drying (LMAD-AD) sample, the medium and high level power-treated (MMAD-AD and HMAD-AD) samples had higher catechin levels than that of FD (49.95 mg/g d.m.). EGCG was the most abundant catechin in tea flowers. The highest level of EGCG (19.71 mg/g d.m.) was found in the HMAD-AD samples. There is no difference in EGCG content between HMAD-AD, MMAD-AD, and FD samples, and they were all higher than that of AD (12.09 mg/g d.m.). The results showed that MAD maintained the most catechins in tea flowers and is as efficient as FD. The epicatechin and total phenolic contents were reported to increase by MAD in jujubes (Gao et al., 2012), Vitex negundo Linn (Mohd Salleh, 2013). Microwave radiation was characterized to rapidly inactivate the enzymes in fresh material, especially polyphenol oxidases (PPO) (Chen et al., 2017; Wang et al., 2019b). The highest inhibition of PPO activity in cacao pod husks was observed after microwave drying (Valadez-Carmona et al., 2017). The MAD in the present work could quickly inactivate PPO activity in fresh tea flowers, which is beneficial for preserving more catechins from oxidation. Additionally, the drying times of MAD-AD, AD, and FD are different, at 1.5 h, 3 h, and 48 h, respectively. The MAD-AD method took the shortest drying time, and could avoid thermal degradation of catechins from a long drying time. Therefore, MAD-AD, especially at high and medium microwave power, could efficiently conserve more catechins in tea flowers, which are important to the briskness and coordination of taste.

Table 1.

Effect of MAD-AD on catechins contents of tea flowers (mg/g d.m.)

Catechins content	MAD-AD
	Low	Medium	High
EC	5.47 ± 0.20b	6.36 ± 0.17a	6.27 ± 0.17a
C	1.15 ± 0.03c	1.40 ± 0.02a	1.30 ± 0.03b
EGC	7.63 ± 0.32b	8.18 ± 0.22a	7.56 ± 0.20b
GC	0.94 ± 0.03b	1.07 ± 0.02a	1.05 ± 0.03a
ECG	14.99 ± 0.53b	16.18 ± 0.38a	15.55 ± 0.43ab
CG	0.12 ± 0.01a	0.12 ± 0.01a	0.08 ± 0.01b
EGCG	16.69 ± 0.63b	18.76 ± 0.53a	19.71 ± 0.62a
GCG	0.71 ± 0.03b	0.79 ± 0.03a	0.85 ± 0.04a
Total catechins	47.69 ± 1.78b	52.86 ± 1.38a	52.37 ± 1.53a

MAD-AD, microwave-assisted drying combined with air drying; EC, epicatechins; C, catechins; EGC, epigallocatechins; GC, gallocatechins; ECG, epicatechins gallate; CG, catechins gallate; EGCG, epigallocatechins gallate; GCG, gallocatechins gallate. Different letters (a, b, c) for the same individual compound indicates significant difference (p < 0.05).

Flavonol glycosides are an essential subclass of flavonoids present in tea flowers, which have powerful antioxidant properties and protection from free radicals. The consumption of flavonoids can prevent cardiovascular diseases including hypertension and atherosclerosis (Tapas et al., 2008). In tea beverages, flavonol glycosides are very important to the organoleptic characteristics of a silky, mouth-coating sensation, and color (Scharbert et al., 2004). Flavonol glycosides in tea flowers are mainly composed of quercetin, kaempferol and myricetin aglycones combined with O-glycoside moieties at C-3. In the present work, including two myricetin glycosides, five quercetin glycosides, and five kaempferol glycosides were separated by the UPLC method and are analyzed in Table 2. The total contents of flavonol glycosides (FG) in all tested samples ranged from 12.24 mg/g d.m. to 13.26 mg/g d.m. The highest contents of FG were noted in the tea flower sample treated with the medium-power microwave, and there are no differences between the three levels. Compared to the total FG contents of FD and AD samples (Table S2), the total FG contents of the MAD-AD samples increased by 19.30 per cent to 42.43 per cent. Large increases of quercetin-3-O-β-d-galactoside, quercetin-3-O-β-d-galactosylrutinoside, kaempferol-3-O-β-d-galactoside, kaempferol-3-O-β-d-glucopyranosyl (1→3)-α-l-rhamnopyranosyl (1→6)-β-d-galactopyranoside, kaempferol-3-O-β-d-glucopyranosyl (1→3)-α-l-rhamnopyranosyl (1→6)-β-d-glucopyranoside, and myricetin-3-O-β-d-galactoside contents were observed in MAD-AD samples. A similar result was also reported in cacao pod husks where quercetin glycoside content increased twofolds through microwave drying (Valadez-Carmona et al., 2017). Here, compared to the FD and AD samples, phenolic compounds including catechins and flavonol glycosides were highly preserved in tea flowers by MAD, owing to the superiority of microwave radiation in rapid inactivation on hydrolysis oxidases and shorter exposure to oxygen during the dehydration process. These results suggested that MAD-AD might be an optimal method for keeping higher contents of phenolic compounds in finished tea flower products which are benefit to the sensory quality and human health.

Table 2.

Effect of MAD-AD on flavonol glycosides contents of tea flowers (mg/g d.m.)

Flavonol glycosides content	MAD-AD
	Low	Medium	High
Myr-gala	0.74 ± 0.03b	0.81 ± 0.03a	0.77 ± 0.02b
Myr-glu	0.21 ± 0.01b	0.25 ± 0.01a	0.21 ± 0.01b
Que-rut	0.06 ± 0.01a	0.06 ± 0.01a	0.06 ± 0.01a
Que-gala	2.26 ± 0.04a	2.49 ± 0.18a	2.29 ± 0.11a
Que-glu	0.57 ± 0.03b	0.62 ± 0.02a	0.57 ± 0.01b
Que-gala-rut	1.01 ± 0.05b	1.11 ± 0.04a	1.01 ± 0.01b
Que-glu-rut	1.21 ± 0.07b	1.33 ± 0.05a	1.12 ± 0.02b
Kae-gala	1.80 ± 0.08a	1.86 ± 0.08a	1.82 ± 0.03a
Kae-rut	0.05 ± 0.01b	0.05 ± 0.01b	0.05 ± 0.01b
Kae-glu	0.41 ± 0.02a	0.43 ± 0.02a	0.42 ± 0.01a
Kae-glu-rha-gala	1.73 ± 0.07b	1.90 ± 0.05a	1.76 ± 0.02b
Kae-glu-rha-glu	2.19 ± 0.12a	2.35 ± 0.09a	2.19 ± 0.04a
Total flavonol glycosides	12.24 ± 0.30a	13.26 ± 0.61a	12.25 ± 0.31a

MAD-AD, microwave-assisted drying combined with air drying; Myr-gala, myricetin-3-O-β-d-galactoside; Myr-glu, myricetin-3-O-β-d-glucoside; Que-gala, quercetin-3-O-β-d-galactoside; Que-gala-rut, quercetin-3-O-β-d-galactosylrutinoside; Que-glu-rut, quercetin-3-O-β-d-glucosylrutinoside; Kae-gala, kaempferol-3-O-β-d-galactoside; Kae-glu-rha-glu, kaempferol-3-O-β-d-glucopyranosyl (1→3)-α-l-rhamnopyranosyl (1→6)-β-d-glucopyranoside; Kae-glu-rha-gala, kaempferol-3-O-β-d-glucopyranosyl (1→3)-α-l-rhamnopyranosyl (1→6)-β-d-galactopyranoside. Different letters (a, b) for the same individual compound indicates significant difference (p < 0.05).

Effect of MAD-AD on triterpenoid saponin contents of tea flowers

To date, 25 saponins have been found in the tea flowers which are cultivated in China, Japan, and India (Chen et al., 2018). Triterpenoid saponins are a group of acylated oleanane-type saponin glycosides, which are widely distributed in Chinese and Japanese tea flowers. The major identified saponin groups were floratheasaponins and chakasaponins (Shen et al., 2017). The beneficial health properties of floratheasaponins and chakasaponins have been extensively studied. In an olive oil-treated mice model, floratheasaponins showed higher inhibitory effects on serum triglyceride than tea seeds theasaponins E1 and E2 (Yoshikawa et al., 2005). The chakasaponins I–II and floratheasaponin A possessed potent antiproliferative activities in HSC-2, HSC-4, MKN-45, and Caco-2 cells and induced more apoptotic cell death (Kitagawa et al., 2016).

Different from tea leaves, there were considerably high levels of saponins in tea flowers (Shen et al., 2017; Chen et al., 2018). The main saponins, chakasaponins (Chaka I, Chaka II, and Chaka III) and floratheasaponins (Flora A, Flora B, Flora D, and Flora J) content in the tested tea flower samples are presented in Table 3 and Table 3S. The MAD-AD samples showed high values of total saponin levels, which varied from 62.52 mg/g d.m. to 66.80 mg/g d.m. There are no differences between the three microwave treatments, but significant differences were observed between the MAD-AD, AD, and FD samples. Compared to the AD and FD flower samples, 46.9 per cent and 29.2 per cent increases of total saponins were obtained by MMAD-AD, respectively. Flora A and Flora B are the two most abundant floratheasaponins in tea flowers, accounting for over 70 per cent of total saponins. The highest levels of 28.63 mg/g d.m. Flora A and 25.93 mg/g d.m. Flora B were recorded in the MMAD-AD sample, while the lowest levels of 16.23 mg/g d.m. Flora A and 13.76 mg/g d.m. Flora B were observed in the AD sample (Table 3S). Our results showed that MAD could keep high retention of saponins in tea flowers. Popovich et al., (2005) reported that greater amounts of two ginsenosides Rb1 and Rd in ginseng root could be obtained by microwave vacuum drying. Here, the microwave pretreatment showed the same effect as the reported vacuum microwave drying. In this work, the microwave drying shortened the drying time, which avoided the continuous reaction of hydrolysis and degradation of saponins during tea flower drying.

Table 3.

Effect of MAD-AD on triterpenoid saponins contents of tea flowers (mg/g d.m.)

Triterpenoid saponins content	MAD-AD
	Low	Medium	High
Chaka I	3.80 ± 1.07a	3.82 ± 0.86a	3.25 ± 0.34a
Chaka II	0.24 ± 0.03a	0.23 ± 0.01a	0.23 ± 0.02a
Chaka III	0.22 ± 0.03a	0.24 ± 0.02a	0.22 ± 0.01a
Flora A	28.05 ± 0.32ab	28.63 ± 1.19a	26.54 ± 0.83b
Flora B	25.54 ± 1.91a	25.93 ± 0.73a	24.84 ± 0.89a
Flora D	4.41 ± 1.34a	4.94 ± 0.88a	4.71 ± 1.25a
Flora J	2.17 ± 0.52ab	3.02 ± 1.35a	2.73 ± 0.34a
Total triterpenoid saponins	64.44 ± 5.22a	66.80 ± 5.04a	62.52 ± 3.68a

MAD-AD, microwave-assisted drying combined with air drying; Chaka I, chakasaponin I; Chaka II, chakasaponin II; Chaka III, chakasaponin III; Flora A, floratheasaponin A; Flora B, floratheasaponin B; Flora D, floratheasaponin D; Flora J, floratheasaponin J. Different letters (a, b) for the same individual compound indicates significant difference (p < 0.05).

Effect of MAD-AD on color evaluation of tea flowers

Color is one of the most important stimuli recorded by human receptors; therefore, it plays an important role in the evaluation of food quality. The chromatic parameters of tea flower infusions from different drying treatments were measured by the CIE L* a* b* system and are listed in Table 4 and Table 4S. Among the different drying samples, the infusions of AD sample (Table 4S) had the lowest L* value and the highest a* and b* values, whereas the MAD-AD samples (Table 4) had higher L* values and lower a* and b* values. The increased L* value indicated an increase in bright color, and the reduction in a* and b* values indicated less red and yellow hues. The ΔE, chroma (C) and browning index (BI) values are also very important for the dried products, as these indexes express the color difference between different samples which could be discriminated by eyes. The ΔE, C and BI values of the MAD-AD samples varied from 9.57 to 13.92 units, 23.24 to 28.17 units and 32.97 to 43.16 units, respectively, depending on the applied microwave power. These values were greater than those of AD (ΔE = 18.87, C = 32.05, BI = 54.54), and indicated that the AD sample was darker than all MAD-AD samples. Above all, air-dried tea flowers showed a little reddish-brown color while MAD-AD tea flowers had a greener and brighter color. Here, MAD was found to improve the color of tea flowers; the same effect was also reported in jujube fruits and mint leaves (Therdthai and Zhou, 2009; Wojdyło et al., 2016).

Table 4.

Effect of MAD-AD on color evaluation of tea flowers

Color evaluation	MAD-AD
	Low	Medium	High
L*	78.99 ± 0.18c	79.31 ± 0.16bc	79.68 ± 0.38b
a*	0.31 ± 0.06b	-0.07 ± 0.03c	-0.65 ± 0.13d
b*	28.16 ± 0.36b	25.26 ± 0.41c	23.23 ± 0.70d
ΔE	13.92 ± 0.4b	11.37 ± 0.38c	9.57 ± 0.76d
C	28.17 ± 0.36b	25.26 ± 0.41c	23.24 ± 0.69d
BI	43.16 ± 0.85b	37.27 ± 0.81c	32.97 ± 1.50d

MAD-AD, microwave-assisted drying combined with air drying; a*, redness (+) and greenness (–); b*, yellowness (+) and blueness (–); BI, browning index; C, chroma; L*, lightness; ΔE, total color differences. Different letters (a, b, c) for the same individual compound indicates significant difference (p < 0.05).

Effect of MAD-AD on volatile compound contents of tea flowers

Two aromatic compounds, acetophenone and 1-phenylethanol, are highly abundant in fresh and dried tea flowers, but relatively low in tea leaves (Dong et al., 2017). Acetophenone is a precursor of aromatic compounds, with aromatic properties as the flavor of almond, cherry, jasmine, and strawberry. 1-Phenylethanol is widely used as a fragrance in the cosmetic industry because of its mild floral odor (Chen et al., 2018). The relative proportions of acetophenone and 1-phenylethanol of different tea flower samples are shown in Figure 1 and Figure S5. The ratio of acetophenone to 1-phenylethanol in FD sample was 8:1, while the ratio of acetophenone to 1-phenylethanol in MAD-AD samples was nearly 4:5. Compared to FD samples, the 1-phenylethanol values of all MAD-AD samples were higher, while the acetophenone values were relatively low. Apparently, more acetophenone converted to 1-phenylethanol during microwave radiation and thermal dehydration, which accounted for the balanced ratio of acetophenone to 1-phenylethanol in the MAD-AD samples. Some other volatile compounds, such as 2-phenylethanol, benzene–methanol, nonanal, α-farnesene, linalool and α-ionone, also contribute to the unique floral scent of tea flowers. Compared to the AD sample, the relative proportions of sweet flavor compounds such as benzene–methanol and 2-phenylethanol slightly decreased in MAD-AD samples, while terpenes such as α-farnesene and α-ionone with a fragrant floral flavor increased. Microwave treatment of tea flowers reduced the sweet floral flavor and introduced an additional fragrant floral flavor through the autoxidation of unsaturated hydrocarbons, which changed the aroma type of the tea flowers. Qu et al. (2019) also reported that microwave drying could increase the content of terpene alcohols and terpene aldehydes to improve the aroma of black tea, possibly due to the change of molecular structure of volatiles caused by the special microwave frequency.

Figure 1.

Effect of MAD-AD on volatile compound contents of tea flowers. MAD-AD, microwave-assisted drying combined with air drying; LMAD-AD, low level power-treated microwave-assisted drying combined with air drying; MMAD-AD, medium level power-treated microwave-assisted drying combined with air drying; HMAD-AD, high level power-treated microwave-assisted drying combined with air drying.

Effect of MAD-AD on antioxidant activities of tea flowers

Antioxidant abilities of tea flowers extracts have been evaluated by in vitro systems using DPPH radicals (Yang et al., 2007) and in vivo cell systems (Lin et al., 2003). To estimate the effect of different drying methods on the antioxidant activities of tea flowers, the DPPH, ABTS, and ORAC testing models were used, and the results are shown in Figure 2 and Figure S6. Of all applied drying methods, the lowest antioxidant capacity was found in the AD sample, showing values of 358.72, 512.28, and 863.31 μmol TE/g d.m. in DPPH, ABTS and ORAC, respectively. Compared to AD, great increases in the antioxidant capacity were observed in all MAD-AD samples. The DPPH values of MAD-AD treatments ranged from 366.65 to 374.72 μmol TE/g d.m.; ABTS from 538.83 to 600.18 μmol TE/g d.m.; ORAC from 1429.47 to 1929.11 μmol TE/g d.m. The antioxidant activities of MAD-AD samples were significantly higher than those of the AD sample, and were comparable to the FD sample. Research on jujubes showed a clear relationship between phenolic compounds and the antioxidant activity (Du et al., 2013). Here, the antioxidant activities were also in accordance with the content of active compounds, catechins and flavonol glycosides. MAD shortened the drying time and preserved more phenolic compounds that contribute to inhibit DPPH, ABTS and ORAC radicals.

Figure 2.

Effect of MAD-AD on antioxidant activities of tea flowers (μmol TE/g d.m.). MAD-AD, microwave-assisted drying combined with air drying; LMAD-AD, low level power-treated microwave-assisted drying combined with air drying; MMAD-AD, medium level power-treated microwave-assisted drying combined with air drying; HMAD-AD, high level power-treated microwave-assisted drying combined with air drying. DPPH, 2,2,-Diphenyl-2-picryl-hydrazyl; ABTS, 2,2′-azinobis- (3-ethylbenzthiazoline-6-sulfonic acid); ORAC, Oxygen Radical Absorbance Capacity Assay.

Conclusion

In this work, MAD-AD was firstly applied in the processing of fresh tea flowers to improve their flavor, quality, and antioxidant capacities. The MAD could effectively avoid a long exposure to thermal degradation of active nutraceutical compounds and rapidly inactivate some enzyme activities in tea flowers, which was beneficial to the preservation of catechins, flavonol glycosides, triterpenoid saponins, and antioxidant capacities. MAD-AD could also improve the color appearance, introduce additional floral fragrance, and shorten the drying time. Tea flower was approved as a new resource food by National Health Commission of the People’s Republic of China in 2013. The application of MAD and AD will improve the processing efficiency and product quality of tea flowers, which provides the basic reference for the further development of this new resource food.

Author Contributions

Y.W. and L.S. conceived the experiments. L.S. conducted the experiments. L.S., E.K., L.Y. and Y.H. analyzed the results. L.S., E.K., and Y.W. wrote the manuscript. All authors reviewed the manuscript.

Acknowledgments

We are grateful to Haitao Huang and Mingmin Guo of Hangzhou Academy of Agricultural Sciences and Technology for sample preparation.

Funding

This work was supported by the Natural Science Foundation of Zhejiang Province [LY16C200004], the Key Research and Development Projects in Zhejiang Province ‘Industrialization Model Projects on Exploring Functional Components and Related Products from Tea Flowers and Fruits’ [2018C02012], China.

Conflict of Interest

The authors declare no conflict of interest.

Author notes

Linzuo Shi and Eunhye Kim are co-first authors.