Changes Of Volatile Compounds In Chinese Bayberry(Myrica.rubra)Juice During Processing And Storage

Bayberry(Myrica.rubra Sieb.Et Zucc),as a characteristic fruit in China,is rich in nutritional values and has a unique and strong aroma.Due to the short ripening period of the fruit and its intolerance to storage and transportation,clarified bayberry juice is a common processed product in recent years,but the escape and deterioration of aromatic substances during the processing and storage of fruit juice would degrade the product quality.Therefore,this thesis focused on the aroma changes of bayberry juice,using sensory evaluation,solid phase microextraction,gas chromatography-mass spectrometry(GC-MS)technology and gas chromatography olfactory measurement technology(GC-O)to analyze the changes of aroma in bayberry juice under different pressing conditions,different clarification methods,different heat treatment conditions and different storage conditions.The bound aroma compounds in bayberry juice were also studied.The main conclusions were showed as follows:1.Study on the aroma differences of bayberry juice under different pressing conditionsThe aroma differences of bayberry jucie under four pressing conditions were compared:fresh bayberry pressed at room temperature,fresh bayberry pressed at 4?,frozen bayberry thawed and pressed at room temperature and frozen bayberry thawed and pressed at 4?.First,the SPME conditions were optimized,which was adding 1.5g of sodium chloride to 4 mL of juice,equilibrating for 10min at 50 ? in water bath and extracting 40 min.A total of 66 volatile components were extracted.Combined with GC-O,25 compounds were identified as the key aroma components in bayberry jucie,such as hexanal,linalool,ethyl acetate,?-caryophyllene,and the odor attributes contributed were mainly "green","floral","fruity"and "woody".According to the sensory evaluation results,it was found that the overall scores of the two bayberry juice directly squeezed by fresh bayberry were higher,and the two groups of samples squeezed at room temperature had unpleasant "fermented"off-flavor.The contents of volatile components of the two groups of freshly pressed bayberry samples in aldehydes,alcohols,esters and total contents were significantly higher than those of the two groups of samples squeezed after thawing frozen bayberry.The total content of volatile components in bayberry juice pressed under room temperature after thawing frozen bayberry was the lowest,and the content of key aroma components such as ?-caryophyllene and humulene was significantly lower than other groups.The bayberry juice obtained by pressing fresh fruit at 4?had better aroma.2.Study on the aroma differences of bayberry juice under different clarification methods:The differences in aroma of clarified bayberry juice after clarification(xanthan gum,sodium alginate,chitosan),enzymatic hydrolysis,and plate and frame filtration were studied.The content of volatile components in fruit juice was significantly different from that before clarification,the content of volatile components in terpenes decreased significantly,and the key aroma component ?-caryophyllene,which had the highest content in bayberry,was lost.The aroma of fruit juice after enzymatic clarification had the biggest difference compared to the juice before clarification,and the "fermented" smell was obvious.The clarified fruit juice had a "musty" smell,and the aroma of bayberry juice obtained by the three clarifiers was not much different.The aroma of the juice filtered by the plate and frame was the closest to that of the juice before clarification.Therefore,the clarification method of the plate and frame filtration had less influence on the aroma of the juice.3.The effect of different heat treatment conditions on the aroma of bayberry juice:The effects of different heat treatment temperatures of 60?,70?,80?,90?,and 100? for 15min and 80? for 5min,10min,15min,30min,and 60min on the aroma of bayberry juice were studied.With the increase of heat treatment time and temperature,aldehydes content in bayberry juice increased and terpenes content decreased.Combined with the principal component analysis,it could be concluded that samples with different heat treatment temperatures and times can be distinguished.The components that contributing the difference were mainly linalool,1-nonanol,methyl hexanoate,3-nonen-1-ol,etc.The fragrance and floral substance content decreased with increasing temperature and time.After heat treatment,the content of volatile components in fruit juice decreased significantly.With the increase of heat treatment temperature and time,the key aroma components of bayberry gradually decreased,while caprylic acid,?-terpineol and other components that contributed bad odor gradually increased.4.Study on the changes of aroma of bayberry juice during storage:The aroma changes of bayberry juice during storage at 25? for 35 days and 4?for 90 days at refrigerator temperature were studied.During storage at 25?,the volatile components in bayberry juice began to change significantly at 28d;during cold storage at 4?,the volatile components began to change significantly at 75d.With the increase of storage time,key aroma components such as linalool,3-nonen-1-ol,?-phellandrene,etc.in the Myrica rubra juice showed a downward trend.Components that contribute bad odor,such as caproic acid,tend to increase.5.Isolation and identification of bound aroma compounds in bayberry juice:Amberlite XAD-2 resin was used to separate bound aroma compounds in bayberry juice under different enzymatic(pectinase,?-glucosidase)and acidolysis(pH 1.0,pH 2.0)conditions.A total of 22 components were detected after enzymatic hydrolysis,including key aroma components of bayberry such as 3-nonen-1-ol and?-caryophyllene.Compared with the results of the hydrolysis of the two enzymes,the esters were mainly digested by the pectinase,and the alcohols were mainly digested by the ?-glucosidase,and the aroma content was higher,which was more suitable for flavor enhancement of the bayberry juice.A total of 19 components were detected after acid hydrolysis,mainly alkanes.Compared with the results of acid hydrolysis at two pH conditions,there were more types and higher contents of bound aroma substances detected after acid hydrolysis at pH 1.0.However,because the products after acid hydrolysis,such as 1-terpineol,had strong irritating and odorous components,the components obtained by enzymatic hydrolysis were closer to the flavor of the fruit itself.Therefore,?-glucosidase was a more suitable method for releasing bound aroma compounds in bayberry juice.