| Wheat germ contains high concentration of polyunsaturated fatty acids and fat-soluble active compounds such as tocopherols.However,widespread utilization of wheat germ is limited by its rapid rancidity issues.The rapid rancidity mechanism of wheat germ and its key influencing factors is unknown till now.The focus of existed stabilization studies mainly based on inactivating lipase and lipoxygenase of wheat germ with high temperature heating,which is very harmful to its quality.Besides,the storage stability of wheat germ heated with high temperature is relatively poor and prone to re-rancidity again,and the corresponding mechanism is not clear.Thus,after exploring the fast rancidity mechanism of wheat germ and its key Influencing factors,the stabilization research was carried out by controlling the water activity?Aw?of wheat germ with mid far infrared?flameless catalytic infrared,FCIR?and short-wave infrared?SIR?radiation at low temperature,so as to reduce the damage to its fat-soluble nutrients and determine its optimum storage Aw.The moisture adsorption law,rancidity features and oil quality degradation features were studied in order to reveal the possible mechanism of re-rancidity phenomenon of infrared-stabilized wheat germ during storage.The main purpose of this investigation was to provide theoretical and data support for industrial application of infrared stabilization technology.The main research contents and the corresponding results were listed as follows:1.For exploring the mechanism of rapid hydrolytic rancidity of wheat germ and its key influence factor during storage,the changes of molecular mobility of water and lipids in wheat germ,the changes of morphology and composition of liposomes membrane in wheat germ,the effect of liposygenase activity,oxygen content,and visible light radiation and Aw upon the hydrolysis rate of wheat germ lipids were studied.The results indicated that phosphatidylcholine in wheat germ was hydrolyzed prior to the hydrolysis of triglyceride?TAG?during storage.The phospholipid membrane of wheat germ liposomes rapidly hydrolyzed,and TAG was released from the liposomes and coalesce to oil droplets.The molecular mobility of lipids and wheat germ lipase accelerated at high Aw,and TAG can be directly catalyzed by lipase.The lipoxygenase acticity,visible light radiation and oxygen content had no significant influence upon the hydrolysis rate of wheat germ lipids during storage.With the increase of Aw,the content of tightly bound water and less tightly bound water in wheat germ significantly increased,and the molecular mobility of water and lipids gradually increased.The maximal enzymatic hydrolysis rate of fresh wheat germ was observed at the Aw of 0.75.Thus,the crucial influence factor of the rapid rancidity of wheat germ lipids lay in the Aw of WG.2.FCIR radiation was used to reduce the Aw of wheat germ in minutes so as to inhibit its rapid rancidity.The storage stability and lipid quality of lipids were used as indicators to study and optimize of the stabilization thecnology.The results indicated that at the optimized technological parameters?radiation distance 35 cm,duration time 6 min?,the final surface temperature of wheat germ samples reached 109.7°C.The residual lipase and lipoxygenase activity were 7.94%and 14.33%,respectively.The free fatty acids?FFA?content,peroxide value?PV?,conjugated diene acid?CD?content and p-Anisidine value?pAV?of this wheat germ sample only increased by 1.67%?1.18 meq/kg?0.09%and 0.52 at 40°C for 60 days,respectively.The hydrolysis and oxidation rancidity have been effectively controlled at this condition.No significant changes in main fatty acids and color but significant decrease in tocopherols content and oxidative stability were observed with compared to raw samples.Thus,FCIR treatment is a fast stabilization technology for wheat germ.However,the surface temperature of wheat germ rose too fast to control during FCIR radiation,which made its lipids even more susceptible to autoxidation once its Aw was lower than 0.1.Meanwhile,high surface temperature of wheat germ during processing caused great harm to tocopherols,the retention rate of total tocopherols in lipids from FCIR-treated samples at the optimal processing condition was only 74.25%.3.In order to retain a maximum level of tocopherols,SIR radiation technology was adopted to reduce the Aw of wheat germ in order to retard its rapid rancidity at low temperature.Meanwhile,the monolayer moisture content of wheat germ was determined,and the relationship between the Aw of wheat germ and its lipids hydrolysis and oxidation rate was investigated,so as to make sure the critical storage Aw of wheat germ for inhibiting both hydrolytic and oxidative rancidity.The results indicated that at the optimal heating condition?processed at 90°C for 20 min?,the FFA content and PV of this wheat germ sample increased by 1.42%and 2.26 meq/kg at 40°C for 60 days,respectively.And CD content and pAV were effective controlled too.The monolayer water content of SIR-stabilized wheat germ was 3.92%at 40°C corresponding to the Aw of 0.18.The hydrolysis reaction rate of wheat germ lipids during storage was positively correlated with its Aw.The minimal lipids oxidation rate was observed in SIR-stabilized wheat germ with residual Aw of 0.15.So the optimal storage Aw of wheat germ was close to the Aw corresponding to its monolayer water content.The results of lipids quality analysis revealed that no significant change in main fatty acid and total tocopherols content was observed.After heated at the optimal processing condition,the retention rate of total tocopherols in SIR-treated samples remained up to 96.43%,which increased by 22.18%compared with the FCIR-treated germ at optimal condition.Thus,the rapid hydrolytic and oxidative rancidity of wheat germ can be effectively retarted with SIR irradiation at low temperature,and the tocopherols of wheat germ can be effectively protected.However,the FFA content,PV and Aw of SIR-stabilized germ samples increased slightly during storage.4.The moisture sorption characteristics of the SIR-stabilized wheat germ were studied with the static gravimetric method during storage,so as to develop moisture adsorption isotherm model for wheat germ.The results indicated that the water adsorption and desorption isotherms for wheat germ were of type II in the storage humidity of 11.2%-94.6%at 20-40°C.Compared with desorption,a hysteresis loops existed in germ adsorption process.Water was further released from SIR-stabilized germ samples in relative humidity?RH?of 0-11%.While water vapor could be adsorbed to SIR-stabilized samples once the storage RH was beyond11%,with the water content of dried WG samples increasing gradually.The monolayer water content of wheat germ during adsorption and desorption process fitted with BET multilayer adsorption model were in range of 3.92%-4.46%and 3.81%-4.11%,respectively.The optimal adsorption and desorption model for wheat germ was the modified Halsey model,and its adsorption and desorption isotherm equation were Aw=exp[-exp?4.569-0.007T?*EMC-1.442]and Aw=exp[-exp?5.652-0.008T?*EMC-1.616],respectively.5.The lipase and lipoxygenase activity,hydrolysis and oxidation extent were used as indicators to explore the mechanism of re-rancidity phenomenon of lipids in SIR-stabilized wheat germ.The results revealed that the hydrolysis reaction rate of TAG in SIR-stabilized wheat germ was positively correlated with its storage humidity.The impact sequence to the hydrolytic rancidity of SIR-stabilized germ samples during storage were the storage humidity,storage time and storage temperature.The minimal PV and hexanal content were observed in SIR-stabilized germ samples during storage at the storage humidity of 33%.The oxidation rate of lipids in SIR-stabilized wheat germ significantly increased once the storage humidity was higher or lower than 33%.The maximal hydrolytic and oxidative rancidity rate of SIR-stabilized wheat germ were observed at the storage humidity of 75%.After SIR-stabilized germ was stored at 40°C and RH 75%for 28 days,its lipase and lipoxygenase activity increased from 0.08 U/g and 0.33 U/mg to 1.82 U/g and 2.17 U/mg,respectively.At this storage condition,the TAG content of SIR-stabilized germ reduced from 78.41%to34.20%,its PV increased from 2.33 meq/kg to 21.23 meq/kg,and the hexanal content increased from 1.29%to 40.73%,respectively.Therefore,the main mechanism for re-rancidity of SIR-stabilized wheat germ may be attributed to the regeneration of lipase and lipoxygenase activity after moisture adsorption at high storage humidity,and the enzymatic hydrolysis and oxidation rate of lipids significantly increased.On the other hand,autoxidation rate of lipids in SIR-stabilized germ significantly increased after moisture desorption at low storage humidity.6.The PUFA,?-tocopherol,polyphenols and phytosterols content were used as indicators to evaluate the effects of storage conditions on lipid quality of SIR-stabilized wheat germ.The minimal oxidation rate of PUFA,?-tocopherol,polyphenols and phytosterols in SIR-stabilized wheat germ were observed at RH of 33%during storage.The maximal oxidation rate of PUFA,?-tocopherol,polyphenols and phytosterols of lipids were observed in SIR-stabilized wheat germ at the storage humidity of 75%.After SIR-stabilized germ was stored at 40°C and RH 75%for 28 days,the PUFA,content of SIR-stabilized wheat germ reduced from 61.62%to 58.55%,the?-tocopherol content reduced from 2968.02 mg/kg to880.47 mg/kg,the polyphenols content reduced from 245.51 mg GAE/kg to 149.32 mg GAE/kg,and the phytosterols content reduced from 17138.92 mg/kg to 9773.48 mg/kg,respectively.The oxidation of?-tocopherol in SIR-stabilized wheat germ was found to follow first order kinetics at different temperature and humidity conditions.Therefore,the results indicated that oxidation rate of PUFA,?-tocopherol,polyphenols and phytosterols in SIR-stabilized wheat germ were significantly correlated with storage humidity. |
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