| Longan (Dimocarpus longan Lour.) is one of subtropical or tropical fruits with special flavor, high nutritional and commercial value. With the enlargement of Longan's field, its deep processing is more and more concerned. The important way of its sustainable development is to produce Longan juice, and it is the key point of processing high quality juice that Longan juice's aroma components and its change in the processing are determined and controlled. Therefore, in the current study, on the base of establishing the extraction method for Longan juice's volatile components, the active aroma of Longan juice was captured and evaluated by GC-Olfactometry (GC-O) through the analysis for different varieties of Longan juice. Meanwhile, change of its major aroma components duringβ-D-glucosidase hydrolysis processing and storage period were concerned as well as main aroma components'transforming relationship. The major results are as follows:1. Establishment for extracting method of Longan juice's volatile components. Method of different extracting method including SDE, SE, SHS and SPME were compared to study their extracting effects on volatile components, SPME was determined as the most effective one.2. Analysis of volatile components and physicochemical properties in different varieties of Longan juice. Aroma components in eight Longan cultivars were determined and a total of 59 compounds were detected, of which 43 compounds were present in Chuliang,30 in Shixia,28 in Caogu,28 in Chiye,21 in Daguo No.l,32 in Qinshanjiezhong,31 in Shuangmamu and 25 in Shuiyan.7 volatile components were in all Longan varieties. The aroma was comprised of terpenes, alcohols and esters. With the principal component analysis, it was found that the major volatile components were trans-ocimene,D-limonene,γ-terpinene and ethyl alcohol. Meanwhile, the study on their physicochemical properties showed that there was significant difference in eight longan varieties.Analysis of volatile components and physicochemical properties in different harvest periods of Longan juice. The weight, soluble solids content, juice yield and the content of fructose, glucose, total sugars and total acid were highest in the Longan juice which picked 120 days after full blossom.28 aroma components in different harvest periods of Longan were detected. From the variation of the 4 major volatile components, it can be seen that the content of ethanol had significant increase (p<0.05) in the Longan juice which picked 120 days after full blossom, the content of D-limonene andγ-terpinene had unremarkable differences in different harvest periods. The content of trans-ocimene significantly (p<0.05) increased in the Longan juice which picked 110 days after full blossom, while unremarkable differences in the Longan juice which picked 110 days and 120 days after full blossom. The results showed that Longan juice was better at the picking period of 120 days after full blossom.3. Analysis of active aroma of different varieties of Longan. Method of GC-O was used to determine volatile components of Longan juice. The co-existing volatile components were detected in eight kind of longan juice including ethanol,α-pinene,β-myrcene,D-limonene,trans-ocimene,γ-terpinene,linalool and two unknown components.Results further proved that trans-ocimene,D-limonene,γ-terpinene and ethanol were the most important active aroma compounds in Longan juice. The flavor profile analysis showed that eight Longan juice's main aroma belonged to the smell of Longan/tropical fruit,fruit/sweet,flower and herbal/grass. With the comparison of aroma and physicochemical properties of these Longan juice, Chuliang was choosed to be the suitable species for juice processing.4. The overall analysis of aroma compounds in Longan juice. Application of electronic nose was used in analysis of Longan juice and result showed that electronic nose had great effect on identification and concentration of a variety of Longan juice, it can also determine Longan juice's specific concentration because of its excellent sensitivity and repeatability.5. Analysis of bonding aroma components in Longan juice. The bonding aroma components were isolated by adsorption of Amberlite XAD-2 resin and enzymatic hydrolysis ofβ-D-glucosidase. Six kind of bonding aroma components were detected, including benzoic acid, phenylethyl alcohol, 1-octen-3-ol, nopol, linalool and (E)-linalool oxide respectively. The content of benzoic acid reached to 43.956μg/100mL. The results showed that there were relatively a amount of bonding aroma components in Longan juice, which could be released to enhance flavor through enzymatic hydrolysis. Meanwhile, it was found that glycosyl was comprised of rhamnose, mannose and glucose.6. The effect of Longan juice's aroma components in different processing units. Changes of Longan juice's aroma components after killing enzyme, rough filtration, chitosan clarification, centrifugal, diatomite essential filtration and UHT sterilization were studied. There wre 29 volatile components detected during prosessing that 28 were for fresh juice,23,22,22,22,21,16 were respectively for juice treated by killing enzyme, rough filtration, chitosan clarification, centrifugal, diatomite essential filtration and UHT sterilization. Terpene was the most significant substance, meanwhile the content of trans-ocimene sharply decreased in killing enzyme and UHT sterilization units, but the content of D-limonene andγ-terpinene remarkably increased. Two paths for being transformed into D-limonene andγ-terpinene from trans-ocimene were as follows:Route 1:trans-ocimene was transformed into D-limoneneRoute 2:trans-ocimene was transformed intoγ-terpineneAnalysis of volatile components and physicochemical properties in High Pulsed Electric Field (HPEF) and ordinary thermal sterilization processing units.the result showed that HPEF had obvious bactericidal effect on Saccharomyces cerevisiae but unobvious bactericidal effect on Escherichia coli, while the ordinary thermal sterilization had obvious bactericidal effect on both of them. The study on their physicochemical properties showed that there was unremarkable differences in the HPEF processing unit. However, browning index was significantly increased (p<0.05) and the content of tartaric acid, butanedioic acid, malic acid, Vc and total acid were significantly decreased (p<0.05) in the ordinary thermal sterilization processing unit.47 aroma components were detected in the fresh Longan juice while 45 were detected in the HPEF juice and 44 in the ordinary thermal sterilization juice. The content of major aroma components had significantly decreased in the ordinary thermal sterilization juice but unremarkable differences in the HPEF juice. The results showed that Longan juice was better in the HPEF processing unit.7. Through the analysis of aroma and physicochemical index change of Longan juice treated with UHT on three different temperature storage conditions (4℃,25℃and 37℃), the result showed that the content of trans-ocimene, D-limonene and y-terpinene all declined in different degree, while the content of ethyl alcohol increased. Browning index was directly proportional to storage time and temperature. Soluble solids content and pH valve were more stable. The content of fructose, glucose, sucrose and total sugars had unremarkable differences. The content of organic acid decreased, and vitamin C was little detected after several weeks storage. The results showed that Longan juice was better at low temperature in short time.
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