摘要
龙眼是风味独特、营养价值高的典型热带亚热带水果之一,随着栽种面积的扩大,龙眼的深加工就显得尤为迫切,其中加工龙眼果汁是其产业可持续发展的途径之一。众所周知,明确果汁香气成分及其在加工中的变化规律是加工高品质果汁的关键。为此,本文在建立龙眼汁挥发性物质提取方法的基础上,通过分析不同品种龙眼汁的挥发性物质,运用气相色谱嗅觉测量法(GC-Olfactometry,GC-O)分析其香气活性成分,明确其主要香气成分,并通过电子鼻分析其整体风味。同时,探讨龙眼汁在p-D-葡萄糖苷酶水解及不同加工单元操作和贮藏过程中主要香气成分的变化,明确主要香气成分之间的转化关系。主要研究结果如下：
1.龙眼汁挥发性物质不同提取方法的比较及其固相微萃取方法的建立。比较了同时蒸馏提取(SDE)、溶剂萃取(SE)、静态顶空萃取(SHS)和固相微萃取(SPME)4种方法对龙眼汁挥发性物质的提取效果,确定SPME作为提取龙眼汁挥发性成分的有效方法。
2.不同品种龙眼的挥发性物质和理化特性分析。测定了8个品种龙眼汁的挥发性物质,共检测出59种芳香物质,其中储良43种,石硖30种,草菇28种、赤叶28种、大果1号21种、青山接种32种、双孖木31种和水眼25种,共同检出的有7种。检测出的芳香物质主要是萜烯类、醇类、酯类化合物。运用主成分分析法发现反式-罗勒烯、D-柠檬烯、γ-松油烯和乙醇4种主要物质是其主要挥发性成分；同时,对其理化特性进行分析,结果表明,不同品种龙眼的理化特性差异较大。
对不同采收期的理化特性和挥发性物质比较分析,采收期120d龙眼的单果质量、可溶性固形物含量、出汁率、果糖、葡萄糖、总糖和总酸含量最高。不同采收期龙眼的果汁中检出挥发性物质共有28种。从主要的4种挥发性物质变化规律来看,采收期120d龙眼的乙醇含量显著增大(P<0.05),柠檬烯和γ-松油烯在不同采收期的含量差异均不显著,采收期110d龙眼的反式-罗勒烯含量显著增大(P<0.05),且采收期110d和120d龙眼的反式-罗勒烯含量差异不显著。综合比较,采收期120d龙眼果实的品质更好,更适合加工果汁。
3.不同品种龙眼的香气活性成分分析。利用GC-O技术对龙眼汁的挥发性物质进行鉴定,确定了8种龙眼汁中共同存在的香气活性成分为乙醇、α-蒎烯、β-月桂烯、D-柠檬烯、反式-罗勒烯、γ-松油烯、里那醇、2种未知化合物。进一步确证了反式-罗勒烯、D-柠檬烯、γ-松油烯和乙醇4种主要的挥发性物质是龙眼中的香气活性物质。风味轮廓分析发现8种龙眼汁的主要香气都属于龙眼／热带水果味、果香／甜香、花香和药草／青味。通过比较8个品种龙眼的香气物质和理化特性,提出龙眼储良比较适合果汁加工
4.龙眼汁的整体香气分析。采用电子鼻对龙眼汁的整体香气进行分析,结果表明,电子鼻可以鉴别龙眼汁的整体香气,有效区分不同品种的龙眼汁,不同采收期的龙眼汁,并判定果汁饮料中龙眼汁的具体浓度,具有良好的灵敏度和重复性。
5.龙眼汁中键合态香气物质分析。采用Amberlite XAD-2树脂吸附和p-D-葡萄糖苷酶水解的方法对龙眼汁中键合态香气组分进行分析,发现水解后其香气物质主要为苯甲酸、苯乙醇、蘑菇醇、诺卜醇、β-里那醇和(E)-里那醇氧化物等6种物质,其中,苯甲酸的含量高达43.956μg/100mL。由此结果表明,龙眼汁中有一部分香气物质是以键合态形式存在的,酶解可使之释放出来以增强香气。同时,对龙眼汁中键合态香气组分的糖基结构进行了分析,结果表明,其糖基主要由鼠李糖、甘露糖和葡萄糖构成。
6.不同加工单元操作对龙眼汁香气物质的影响。通过分析龙眼汁加工过程中灭酶、粗滤、壳聚糖澄清、离心、硅藻土精滤和UHT杀菌等各个加工操作单元中香气物质后表明,加工过程中共检测出29种挥发性物质,其中鲜汁中检测出28种,灭酶汁23种,粗滤汁22种,澄清汁22种,离心汁22种,精滤汁21种,UHT汁16种,主要的挥发性物质是萜烯类化合物,发现反式-罗勒烯含量在灭酶和UHT加工操作单元中显著降低(P<0.05),而D-柠檬烯和γ-松油烯含量显著增加(P<0.05)。由此提出反式-罗勒烯分别转化为D-柠檬烯和γ-松油烯的两条途径：
途径一：反式-罗勒烯转化为D-柠檬烯
途径二：反式-罗勒烯转化为γ-松油烯
分别比较了高压脉冲电场杀菌和普通热杀菌操作单元对龙眼汁香气物质和理化特性的影响。结果显示,高压脉冲电场杀菌操作单元对龙眼汁中酿酒酵母的杀菌作用较明显,对大肠杆菌的杀菌效果不是很明显,普通热杀菌对两种菌的杀菌效果都很明显。高压脉冲电场杀菌操作单元对龙眼汁的理化特性没有显著影响,而普通热杀菌会使龙眼汁的褐变度显著增大,使酒石酸、丁二酸、苹果酸、Vc和总酸含量显著下降。龙眼汁在两种不同的杀菌方式中共检测出47种挥发性物质,其中,鲜汁47种,高压脉冲电场杀菌汁45种,普通热杀菌汁44种。经过普通热杀菌龙眼汁中的主要挥发性物质含量有较为明显的下降,而高压脉冲电场对挥发性物质含量的影响较小。结果表明,与普通热杀菌相比,高压脉冲电场杀菌能有效减少龙眼汁的褐变和减少龙眼汁中有机酸和挥发性物质的散失。
7.龙眼汁在贮藏过程中其香气物质和和理化特性的变化分析。分析经UHT灭菌处理龙眼汁在4℃、25℃和37℃条件下不同贮藏时间的香气物质和理化特性的变化,结果表明,反式-罗勒烯、D-柠檬烯和γ-松油烯的含量都有不同程度的降低,乙醇含量增加；随着贮藏温度的升高和贮藏时间的增加,龙眼汁褐变度增加,可溶性固形物含量和pH值比较稳定,果糖、葡萄糖、蔗糖和总糖含量差异不显著,有机酸含量降低,Vc在贮藏一段时间后检测不出。结果表明,龙眼汁适合低温短时贮藏。
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-limonene
Route 2:trans-ocimene was transformed intoγ-terpinene
Analysis 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.
引文
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