Research On The Changes Of The Trypical Physical And Chemical Properties In Soybean Oil During The Different Types Of Frying System

The changes of color, viscosity, fatty acids and volatile compounds composition and their amounts, the infrared spectra and the component molecular distributions of the soybean oil (SO) heated or fried with wheat dough (WD) and chicken breast meat (CBM) at180℃were comparatively analyzed in this study. The proposed production reaction mechanisms of these typical products found during the treatment were also investigated according the yeilds and the molecular structures of these products.The color and viscosity of the treated oil samples were determined by colorimeter and rheometer, respectively. The color of the heated, WD-fried and CBM-fried SO were changed from pale yellow to deep yellow, reddish yellow and aterrimus, respectively, as the processing time increased. After the treatment of7days and8h/day, the viscosities of the heated, WD-fried and CBM-fried SO sample were increased from0.052Pa.s of the fresh SO to0.142,0.312and0.146Pa.s, respectively.The fatty acid (FA) profiles were detected by GC-MS. The changes in amount of the FAs were different under the different frying ways. The introduction of the food components increased the intensity of the oxidation occurred in the polyunsaturated FAs. The impact on the changes of FAs resulted from the constituents of CBM was the most significant, which was related with the relative high content of fat contained in CBM.SPME-GC-MS was used to isolate and investigate the volatile compounds in the headspase of the oil samples. A series of alkanes, alkenes, alkynes, alcohols, aldehydes, ketones and the other type of compounds were found in the heated SO samples. Aldehydes were the main volatile compound from the viewpoint of relative concentration. The number of the volatile compounds was reduced and the relative conteration of the volatile compounds were lower in the WD-fried SO samples compared to those in the heated SO samples. The concentrations of the detected typical volatile compounds (especially the aldehydes) found in the CBM-fried SO samples were the lowest among the three frying ways. However, the nitrogen-containing volatile compounds were only found in the CBM-fried SO samples. Phthalates which might be the reaction products of the oxidative decomposition products derived from the polyunsaturated FAs were found in relative low concentration in all the treated SO samples.The contents of the basic components with O-H, C=C, C-H, C=O, and C-O bonds were well descriptively analyzed with the proceeding of the heating and food deep-frying by thoroughly proceesing the FTIR spectra of the treated oil samples. By combining the whole imformation contained in the FTIR spectra and the cluster and discriminant analysis, the qualified corn oil, peanut oil, rapeseed oil and SOs adulterated with the used frying oil were all well quanlititively analyzed. Furthermore, by using the varying patterns of the area ratio (A19/A20) and the wavenumber shift of the typical bond (band19) along with the increace of the mixed proportion of used frying oil, the limit of detections (LODs) of the four adulteration situations were6.6,7.3,5.1,4.3and8.3,9.1,7.4,5.7%under the two parameters, respectively.MALDI-TOF-MS was used to study the molecular distribution of the constituents of the treated SO samples. Three districts of the molecular distribution named as District A, District B and District C were clearly shown from m/z600to3000for the constituents of the treated SO samples. Constituents in the District A (m/z853-1001) included the triacylglycerols (TAG) and their oxidation monomers (oxidized TAG). Constituents in the District B (m/z613-853) included the diacylglycerols (DAG) and their oxidation monomers (Oxidized DAG) and the combined compounds between the oxidized DAG and the small-molecular-weaght decomposition compounds produced from the thermaloxidation of the unsaturated TAGs. Constituents in the District C (m/z1001-2907) included the TAG dimers and their oxidation products (oxidized TAG dimers), the combined compounds between the oxidized TAG dimers and the small-molecular-weaght decomposition compounds, the TAG trimers and their oxidation products (oxidized TAG trimers) and so on. Different molecular distribution patterns were shown under the condition of different frying systems.The constituents of SO were changed significantly during the heating and food frying process. The content of the polyunsaturated TAGs was significantly decreased. Consequently, the concentrations of the small-molecular-weight acids, volatile compounds, DAGs, oxidized DAGs, oxidized TAGs, TAG dimers, and TAG trimers were correspondingly increased. The introduction of the food components intensified the chemical reactions occurred under the conditions of high temperature and prompted the decrease in content of the TAGs and the increase in amount and concentration of these products. More products and more complex reactions were found in the CBM-fried SO samples than those in the WD-fried SO samples.According to the types and structure features of the products, the main chemical reaction occurred during the heating and food frying of SO is oxidation decomposition of TAGs. In addition, hydrolysis, thermal polymerization, Maillard reation and caramelization were also found during these frying ways. The specific chemical reaction type include free radical action, Location of heterogeneous reaction and stereoisomerism of the functional groups (such as C=C bonds), epoxidation, cyclization, concerted reaction, and condensation reaction and so on.