Effect Of Roasting On Flavor And Quality Of Peanut Oil

Special production process of aromatic roasted peanut oil (ARPO) not only can generate its unique flavor but also can retain minor nutrients, which makes it award-winning among consumers. Flavor is a significant indicator of ARPO, and roasting process is the key and important production process for ARPO flavor formation. Therefore, the study on the the effect of roasting process on the formation of flavor components, the impact of roasting on peanut oil quality and the variation of ARPO flavor and quality during frying has great theoretical and practical value for the production of high-quality ARPO. The main contents of this paper were listed as follows.Firstly, an analysis method using HS-SPME technique combined with GC-MS was established for analyzing volatiles in peanut oil. Optimum HS-SPME conditions were as follows: PDMS/DVB fiber, magnetic stirring, extraction temperature of 50℃, extraction time of 40 min and desorption time of 5 min. Then volatiles of ARPO were well separated and identified using the optimized HS-SPME-GC/MS. Among these volatiles, pyrazines accounted for the largest relative percentage area (RPA) of 32.89%, which were major headspace volatile components of ARPO.Seventeen aroma-active compounds of ARPO were identified using GC-MS combined with GC-O. The ARPO aroma could be described as strong roasty/nutty, soft floral/fruity/sweet with light fatty/grassy. Methyl pyrazine, 2,5-dimethyl pyrazine, 2,6-dimethyl pyrazine, 2,3-dimethyl pyrazine, 2,3,5-trimethyl pyrazine, 3-ethyl-2,5-dimethyl pyrazine, 2- acetyl-3-methyl pyrazine were the main sources for roasty/nutty odor; 2-acetyl pyrrole, Pyrrole-2-carboxaldehyde, 3-hydroxy-4,4-dimethyl-γ-butyrolactone, 2,3-dihydro-benzofuran, benzaldehyde, benzeneethanol, guaiacol, 2- methoxy-4-vinylphenol for floral/fruity/sweet odor; hexanal, 2-octenal for fatty/grassy odor.HS-SPME-GC/MS, sensory evauation and electronic nose (EN) were combined to analyze the effect of roasting process on the flavor formation of peanut oil. A total of 95 volatiles were identified using HS-SPME-GC/MS, including N-heterocyclic compounds (20 pyrazines, 5 pyridines and 8 pyrroles), O-heterocyclic compounds (11 furans and 1 pyran) and non-heterocyclic compounds (12 aldehydes, 12 alcohols, 3 ketones, 12 acids, 2 alkanes, 2 alkenes and 7 esters). N-heterocyclic comounds increased significantly and replaced non-heterocyclic compounds to be the major flavor components of peanut oil after 30 min-roasting. Sensory evaluation results showed that peanut oil samples before 30 min-roasting owned much lower total flavor intensity, mainly presenting fatty/grassy odor; peanut oil samples after 30 min-roasting had a sharp increase in total flavor intensity, mainly presenting strong nutty/roasty aroma.? The PCA results showed that pyrazines play a major contribution to the typical flavor generated during the roasting process, presenting high-intensity roasty/nutty flavor. Also, the EN results showed that roasting time had a remarkable effect on the flavor of peanut oil. Flavor of samples before 30 min-roasting were very close to each other; after 30 min-roasting, samples had significant differences in their flavor, which were in accordance with the sensory evaluation results. In summary, it’s clearly shown that roasting time plays a critical impact on the formation of typical flavor of ARPO; 30 min is especially an important turing point during the roasting and thus the roasting stage after 30 min is very important for the formation of pyrazines. Pyrazine compounds generated by the Maillard reaction and the sharp temperature rise after 30 min-roasting provided a well reaction condition for the Maillard reaction. Also, flavor precursors of nutty/roasty flavor of peanut oil were identified as free amino acids (glutamic acid, arginine, aspartic acid, phenylalanine, tyrosine) and reducing sugar (fructose, glucose), which provided nitrogen source and carbon source respectively for the Maillard reaction.A systematic analysis was conducted to analyze the influence of roasting process on the quality of peanut oil. The results showed that peanut oil samples during the roasting process owned high quality. The acid value and peroxide value reached the national standard; minor components (vitamin E, sterols and squalene) contents were not significantly affected; OSI value significantly increased after 30 min-roasting. Also, roasting showed a great impact on oil color, which could be due to the formation of a large number of browning substances (Maillard reaction products) during the post-roasting. It can be speculated that the formation of Maillard reaction products during the roasting process weakened the loss of nature antioxidants (such as tocopherols) in peanut oil. Therefore, overall antioxidant ability can be maintaind or even improved with the roasting time extending.The flavor and quality variation of peanut oil in cooking were studied by using French fries-frying model. Peanut oil quality showed a significant decrease with the frying going on. The acid value, peroxide value, p-anisidine value, oxidative index, polar compounds and trans fatty acid content all showed different degrees of increase; oil become darker in color; vitamin E content showed a sharp decline and disappearance in the early stage of frying; and peanut oil owned very low OSI. Flavor also had great changes. Pyrazines evaporated completely in the initial frying, and peanut oil had no original nutty/roasty aroma from then on. Throughout the frying process, aldehydes were the major compounds in the peanut oil headspace, which mainly from degradation of hydroperoxide generated by autoxidation of unsaturated esters and showed an exponential growth with the frying extending. PCA results showed that E,E-2,4-decadienal showed an important contribution to the p-anisidine value, followed by E-2-decenal, Z-2-heptenal, E,Z-2,4-decadienal, 2-undecenal, nonanal, E-2-octenal and hexanal. Thus, aldehydes can be used as an important indicator of peanut oil flavor and quality during the frying process.According to the analysis above, it can be proved that the formation of typical nutty/roasty flavor and enhancement of quality during the roasting process are dominated by the Maillard reaction. However, flavor change and quality decline during the frying process are mainly affected by the autoxidation of unsaturated esters.