Research On Identification And Control Methods Of Key Off-flavor Components In Thermal Treated Watermelon Juice

Watermelon juice has properties of low acidity and heat sensitivity,and the flavor is an important indicator to evaluate its quality.At present,the processing technology depend on the thermal treatment,but the thermal treatment will lead to the deterioration of aroma,forming the “cooking flavor”.All these affect the industrial production of watermelon juice seriously.This study aims to identify and verify the key off-flavor components of watermelon juice after thermal treatment on the molecular level by using modern sensory science technology,and explore the controlling methods for off-flavor.Firstly,the flavor compounds of watermelon juice were extracted by solid phase microextraction(SPME)and solvent assisted flavor evaporation(SAFE),and the method of SPME was optimized from the extraction coating,temperature,and time.Then the instrument of gas chromatography-olfactometry-mass spectrometry(GC-O-MS)and sensory evaluation for quantitative descriptive analysis(QDA)were used to qualitify and quantify flavor components.Through the significant analysis of odorant concentration between thermal treated and fresh watermelon juice,seven off-flavor components in thermal treated watermelon juice were identified preliminarily,and sensory evaluation indicated that the formation of off-flavor was the result of a combination of multiple substances.Secondly,dynamic headspace dilution analysis(DHDA)and aroma extraction dilution analysis(AEDA)were performed to identify the key off-flavor compounds.A total of 56 flavor substances were detected,of which 18 were aroma active compounds.There were little difference between the types and numbers of compounds in thermal treated and fresh watermelon juice,the main differences were concentrated in the content of compounds and the intensity of smell.By analyzing and comparing,seven key off-flavor compounds were finally identified in thermal treated watermelon juice:(E)-2-heptenal(soapy,almond;FD=81),decanal(soapy,pungent;FD=81),(E)-2-decenal(mechanical,soapy;FD=27),hexanol(bitter,floral;FD=81),octanol(metal,burnt;FD=81),(E)-2-octenol(plastic,soapy;FD=3),isopropyl disulfide(garlic,sulfur;FD=81).These key compounds were accurately quantified by external standard method with SIM mode.At the same time,the odor activity value(OAV),recombinant experiment and omission experiment were respectively used to verify the efficacy of the off-flavor compounds.The results showed that octanol,isopropyl disulfide and(E)-2-decenal were the three most critical compounds in the seven key compounds.In addition,partial least-square regression analysis(PLSR)was used to correlate the content of compounds with sensory properties.It was found that(E)-2-heptenal,isopropyl disulfide,(E)-2-decenal were significantly negative related to overall aroma and they were the most important compounds leading to the formation of cooking flavor.Finally,using physical control methods with the changes in content of off-flavor compounds and sensory scores,principal component analysis(PCA),we found that xanthan gum,CMC-Na,?-cyclodextrin,and sugar-acid ratio were more effective treatment methods.The optimum condition by response surface optimization was: the additive amount of xanthan gum,CMC-Na and ?-cyclodextrin were 0.19%,0.15%,and 0.2% respectively,and the ratio of sugar-acid was 80.Among them,the most effective factor was ?-cyclodextrin.By comparing the effects on ?-cyclodextrin to key off-flavor and aroma components,it was speculated that ?-cyclodextrin had a greater effect on key off-flavor components than key aroma components.The most critical off-flavor components,octanol and(E)-2-decenal,were used as the representative for off-flavor.The structure of the inclusion complex were verified by infrared and nuclear magnetic resonance,indicating that the mechanism of ?-cyclodextrin was to form inclusion complex in order to eliminate off-flavor in thermal treated watermelon juice.