Extraction of tea aroma

The analysis and determination of tea aroma is always an important topic in the field of tea research, which has been studied by many tea researchers at home and abroad Up to now, more than 700 aroma compounds, including alcohol, aldehyde, ketone, vinegar, acid and nitrogen, have been isolated from various teas The first step of aroma research is to extract and separate it, which is directly related to the qualitative and quantitative analysis results of aroma The aroma components in tea are low, complex, volatile and unstable During the extraction process, due to the influence of external conditions, they are prone to complex chemical reactions such as oxidation, condensation, polymerization, group transfer and so on The extracted essential oils can not reflect the aroma characteristics of tea leaves very well, so that the quality of tea leaves can not be correctly judged Therefore, the previous scholars have carried out a lot of research on the extraction and separation methods of aroma, mainly including atmospheric water vapor distillation and simultaneous extraction (SDE), vacuum distillation extraction (VDE), headspace adsorption (has), supercritical carbon dioxide extraction, vacuum water vapor distillation (SDRP), column adsorption (TLA), etc 1 SDE is a method designed by likens and Nickerson in 1964 and widely used in the analysis of aroma components In this method, distillation and extraction are combined, the operation is simple, and the extraction rate and recovery rate of aroma substances are high (R) In the process of extracting essential oils by SDE, under the heating conditions, volatile aroma compounds are mixed with water vapor and extractant (ether) vapor at the top of the closed device, and then extracted, then condensed and refluxing, so that the volatile matter of 10-9 concentration level can be concentrated thousands of times from lipid or water medium repeatedly, and it can also be found in most of the 10 concentration range The organic matter is extracted quantitatively, so a small amount of sample and extractant can be used for chromatographic analysis However, the whole process of SDE is carried out under the high temperature and airtight environment The secondary reaction is violent, and there are many artificial products For example, some non-volatile sugar II compounds in tea are oxidized and degraded by heat to produce some aromatic substances, such as linalool and geraniol, which are different from the raw materials Some thermosensitive aroma components will be decomposed by heat, and unsaturated fatty acids will also change in structure Some fatty aldehydes and alcohols will be formed due to thermal degradation Therefore, tea flavor oil extracted by SDE method can not completely reflect the aroma characteristics of raw materials to a certain extent Li Yongjun and other "3] studies" showed that the flavoring oil extracted by 15SDE method had a scorching flavor, which was quite different from that of the original sample Chen Yuejiao et al [2] got the same result According to Mitsuya Shimoda [41, the aroma extracted by SDE method has water tightness, wood astringency and irritation Zhang Zhengzhu and other [s] used SDE method to extract and analyze the efficiency of tea essential oil The results showed that SDE method was not only inefficient, but also could not extract aroma substances proportionately, and could not be used for quantitative analysis However, SDE can play a positive role in the analysis of some volatile aroma compounds with thermal stability Chen MEIXIA et al 161 extracted the aroma components of apricot by SDE method, and 74 kinds of aroma components were obtained (only 32 kinds in the control), which can better reflect the aroma components of apricot The SDE method has high extraction efficiency and good reproducibility Therefore, in the extraction and separation of aroma substances, different extraction methods should be selected according to different extraction objects Vacuum distillation extraction (VDE) is a common method of aroma extraction The sample and distilled water are placed in the flask connected with the rotary evaporation instrument, the electric heating sleeve is used to heat it to a slight boiling point, then the electric heating sleeve is removed, and then the flask is kept in a 50 ℃ water bath for vacuum distillation to collect the condensate, and then the re evaporated ether is used for extraction The whole process of the method is carried out at lower temperature, thus avoiding the influence of high temperature on aroma components of tea The extracted essential oil can better react with the aroma characteristics of raw materials, and is a better aroma analysis method Zhu Qi et al (B]) extracted the aroma of green tea by different extraction methods (SDE, HAS and VDE) The results showed that VDE method had lower extraction and recovery rate, but its extracted essential oil could better reflect the aroma characteristics of tea The same results were obtained for the extraction and analysis of aroma components of instant green tea (E3 The sensory and physicochemical tests of tea soup extracted by different methods showed that VDE method had the least effect on tea and could better reflect the brewing process of tea VDE method has a high extraction ratio of aldehydes and acids, but the capture of cool aroma is very low, and the recovery rate of the standard sample of glyoxylate added in the analysis process is only 3.77% "'' '' '' '' ''" A large number of samples and reagents are needed in the extraction of aroma substances by VDE method, and its long sample processing cycle is gradually replaced by other methods 3 Head space analysis (has) in 1972, tennina et al First reported the head space gas capture analysis method (SIS) It is a technology of direct sampling and parallel gas chromatography analysis of volatile components above liquid or solid substances, which is divided into static headspace analysis and dynamic headspace analysis Static headspace analysis is a method of directly absorbing the gas above the sample and injecting it into the gas chromatograph for analysis It can well reflect the aroma characteristics of raw materials Zhu Guobin et al [to] have proved that the aroma components analyzed by static headspace adsorption method are the closest to the smell felt by human body It is easy to operate, does not need tedious and lengthy pretreatment, and can reduce the overload or pollution caused by water, high boiling point or non-volatile substances However, the absolute amount of tea aroma captured by this method is small, which limits the detection of trace components in tea Dynamic headspace analysis can make up for its shortcomings Firstly, porous polymer adsorption resin is used to adsorb and concentrate aroma substances, then heated or eluted with solution, and then analyzed by gas chromatography It is very important to choose the adsorption resin for dynamic headspace analysis At present, gdx-104 GDX-502, porapakq resin, XAD-2 resin are the most widely used However, when Zhu Qi et al [it] studied the effect of different adsorbents on the aroma extraction of green tea, they concluded that gdx-104, GDX-502 and porapakq resin could produce some substances different from tea aroma in the process of aroma extraction due to material problems After GC analysis, some impurity peaks appeared, which greatly interfered with the analysis of aroma, and the analysis of XAD-2 resin was ideal In order to make the aroma molecules as much as possible to adsorb resin, the dynamic headspace analysis method needs to continuously air the whole process of extraction, namely, the adsorption of aroma on the top space of tea leaves in open environment This has little effect on the components and properties of aroma, and makes the analysis closer to the aroma of tea itself Zhu Qi and other 121 methods of HAS and SDE extract the essence of green tea The oil was compared It was considered that the essential oil extracted by HA method had green tea chestnut fragrance, which could better reflect the aroma characteristics of green tea However, it also brings other negative effects when it inhales air, such as the air entering accelerates the oxidation of some aroma molecules Other data show that the method of [u '' '' '' '' '' 1has has great influence on the soluble sugar and free amino acid of the tea soup after the aroma extraction due to the air entry, which proves that it also has certain influence on the aroma substances It can be seen that although the has method can better reflect the tea aroma to some extent, the effect on tea aroma is still obvious by 4 supercritical carbon dioxide extraction (SFE) Supercritical carbon dioxide extraction is to use CO: in the solid, liquid, gas three-phase equilibrium supercritical state, with a strong ability to extract natural products, this ability depends on the pressure and temperature of compressed C02 When C02 is in supercritical state, it contacts with the substance to be separated, and selectively extracts the components of polarity, boiling point and molecular weight successively In the process of extraction, different components in the substance to be separated can be obtained by controlling the pressure, and then the supercritical fluid can be changed into ordinary gas by means of reducing pressure and increasing temperature, and the substance to be extracted can be completely or basically separated out, and then other analysis can be carried out As early as 1975, supercritical CO2 method was applied to the extraction of tea aroma components Because the critical temperature of C02 is 31 0C, supercritical C02 extraction can extract aroma components from tea leaves near room temperature, and the whole process is carried out under the cover of CO: It can effectively prevent oxidation and degradation of heat sensitive substances, and the essential oil extracted from it is almost the same as that in raw materials However, it requires high equipment Supercritical CO: method has been widely used in medicine, food, perfume and other industries In addition, the extraction methods of tea aroma include SDRP and TLA There are advantages and disadvantages in each method of tea aroma extraction The analysis results of each method can not be completely overlapped, but are complementary to each other If we want to analyze the whole components of tea aroma, sometimes we need several capture methods at the same time.