Study On Key Flavors In Peeled Walnut Kernels And The Pattern Of Flavor Release During Mastication

Peeled walnut kernels(PWK)are widely popular and highly valued for their nutritional health and good flavor,however,their key flavor substances and the release pattern of them in oral processing are still unclear,which seriously affects the flavor-oriented variety screening,process improvement and consumer applicability.In this project,the flavor evaluation method of PWK was constructed at the level of odor,taste and oral processing;the key flavor substances of PWK under the combination of three major varieties,traditional and novel processes were systematically studied;the change pattern of key flavor of PWK in in vitro mastication was clarified.The main conclusions are as follows:1 The sensory flavor evaluation method of PWK was established to identify the key flavor substances in different varieties of PWK.The headspace solid-phase microextraction gas-mass spectrometry(HS-SPME-GC-MS)was used to detect 23,17 and 15 volatile flavor substances in "Wen 185","Qingxiang" and "Dapao",respectively.The correlation between solvent-assisted flavor evaporation combined with sniffing(SAFE-GC-O)and sensory attributes by partial least squares regression(PLSR)revealed that nonanal,hexanal,2-heptanol,2,3-butanediol and 1-pentanol were the key substances responsible for sensory differences.There were significant differences(p<0.05)in milk,fat and sweet sensory attributes among varieties of PWK.The Taste Activity Value(TAV)yielded glutamic acid,alanine,histidine,arginine,valine and sucrose as typical sources of taste-presenting amino acids and sweetness.Combined with the sensory and key flavor substances analysis,"Wen 185" showed overall fatty,sweet and creamy flavors with low bitterness compared to the other two varieties.2 With "Wen 185" as the target,the effects of two-stage processing process combining traditional and new processes on the aroma presentation of walnuts were systematically investigated.39 volatile compounds,mainly aldehydes,alcohols,ketones and heterocyclic compounds,were identified by the new two-stage process of hot air-microwave/RF(RW/RS),microwave-hot air/RF(WR/WS)and RF-hot air/microwave(SR/SW).Compared with the traditional process,the content of aldehydes(46.27-118.86 μg/kg)and alcohols(11.24-137.39 μg/kg)were increased in the samples with the novel process,and the content of heterocyclic compounds increased from 1.18 μg/kg to 15.30-89.9 μg/kg.SAFE-GC-O and PLSR identified 11 flavor substances that were the critical substances for sensory differences.The sensory and key flavor substance analysis revealed that RW was rich in key flavor substances such as 2-ethyl-5-methylpyrazine and sucrose,which were the main reason for the high intensity of roasted flavor,creamy,fatty and sweet characteristics,and was the best two-stage processing process for peeled walnuts.3 A PWK oral processing model was constructed and validated for the in-vitro mastication process of PWK.After screening the professional training of 6participants,the physical properties of the food mass of PWK at different stages of oral processing were evaluated by using PCA and OPLS-DA to evaluate the effects of the physical properties of the food mass at different stages of oral processing,and a three-dimensional oral processing model of PWK was established by combining the dry base water content and chewing time.According to the in vivo oral processing model,the in-vitro chewing model with different saliva addition and differentα-amylase was constructed by PCA,and it was found that SSA+M was the most suitable condition for PWK in vitro chewing.Four stages of PWK oral processing(25%,50%,75% and 100%)could be simulated when the saliva addition amount was450 mg/g and the agitation time was 5 s,10 s,15 s and 25 s.4 The change pattern of key flavors during in-vitro mastication of PWK under traditional RR and new processing was revealed.The in-vitro model was used to simulate the changes of PWK flavors,and 50 and 61 volatile compounds were found in RR and RW.Most of the volatile flavor substances in RR and RW samples showed an increasing content after chewing,and the release of volatile substances increased in the 0%-50% stage,and reached the maximum in the 50%-75% stage until the release gradually decreased before swallowing.The odor activity value(OAV)and PLSR were screened for 5 and 8 key odor active compounds in the RR and RW models,respectively;The dynamic sensory revealed that RR was mainly creamy in chewing,RW was mainly roasted aroma,and 2,5-dimethyl-3-ethylpyrazine(OAV=1.19-1.29)in RW food mass was the key component to distinguish from RR.Sweetness was the main taste sensation during PWK oral processing,and combined with TAV worth to glutamic acid,alanine,arginine,and sucrose were the compounds that played an important role in taste perception during PWK mastication.The results of this experiment provided a comprehensive flavor evaluation method for flavor research of peeled walnuts,and also offered new ideas for variety screening,process optimization and consumer perception,which was beneficial to flavor quality breakthrough and high quality development of walnuts and other nut processing.