# Characterization of the Aroma of an Instant White Tea Dried by Freeze Drying
## 1 Introduction  
Various teas are popularly consumed around the world [1], due to its unique pleasant flavors and outstanding health benefits [2; 3]. In addition, teas could be processed to instant teas via water extraction, concentration and spray/freeze drying. In decades, the market of instant teas keeps on increasing rapidly, because it is convenient to carry and easy to use in food industry.  
The aroma is an essential characteristic that influences the perceived quality, price and consumer acceptance of tea and instant tea products [4]. In addition, the aromatic volatiles play important roles in regulation of human health [5; 6]. Aromatic volatiles could be extracted by direct solvent extraction, Soxlet extraction, simultaneous distillation-extraction (SDE), solvent-assisted flavor evaporation, headspace solid-phase microextraction, etc. [7]. Among them, SDE combines the advantages of liquid/liquid and steam distillation-extraction, and is convenient, simple for extracting high boiling-point volatiles in thermal processed products such as fried foods and instant teas [7; 8]. Aroma characteristic could be evaluated by sensory evaluation [4; 9; 10; 11] and instrument analysis such as GC-MS determination [8; 9; 12; 13]. In addition, the aroma-active compounds could be determined witholfactometric techniques [14; 15; 16] and odor active value (OAV) analysis [10; 11], as well as confirmation via aroma reconstruction and omission test [11; 14; 17]. Volatile compounds with concentrations over thresholds could contribute to tea aroma with their respective odors [18], as well as their antagonism and synergism interactions [12]. Moreover, manufacturing processes of tea and instant tea have noticeable effects on volatile compounds and overall aroma profiles of tea products [19]. In current, over 600 volatile compounds have been reported in tea and tea products [1], however, the understanding is still limited about the effects of antagonism and synergism interactions among volatiles on overall aroma.  
White tea is a kind of slightly fermented tea. It is made from tea leaves only by a prolonged gathering process and a drying process [9; 20]. White tea has protective effects against cardiovascular diseases, cancer, diabetes mellitus, obesity, central nervous system and microorganism-based diseases [21]. In addition, white tea has unique sweet and umami tastes as well as smoothly fresh and green odors [20]. The market and production of white tea is increasing in recent years due to the desirable flavor and excellent health benefits. Its production was 5140 tons in China in 2009, while rapidly reached 15,700 tons in 2014 [21]. The main volatile compounds in white tea were hexanal, linalool, 2-methyl-butanal, phenylethyl alcohol, benzaldehyde, benzene acetaldehyde, (\(Z\))-3-hexen-1-ol, 1-hexanol, (\(E\))-2-hexenal, geraniol, etc. [20]. In recent years, more and more white tea are made into instant white tea (IWT) through hot water extraction, vacuum/reverse osmosis concentration, spray/freeze/vacuum drying [22]. Among IWTs, the IWT dried with freeze drying possess a more comfortable aroma in comparison to the IWT dried by spray drying. More than 50 kinds of flavor substances are determined in white teas from Fujian Province of China [23]. Although researchers have performed studies on the aroma characteristic of white tea, the aroma characteristics of IWT products made from white tea have been seldom elucidated.  
On this context, this study aimed to: (1) investigate the aroma profiles and aromatic contributors of a IWT dried by freeze drying via SDE extraction, GC-MS and GC-O analysis, aroma reconstruction, omission test and synergistic interaction analysis; (2) investigate the effects of antagonism and synergism interactions among volatiles on overall aroma. This study elucidates contributions of volatiles and their interactions on the aroma of IWT, and thus helps in-depth understanding of the aroma of tea and tea products.