| Perilla oil is rich in α-linolenic acid.Long-term consumption of perilla oil can not only effectively improve the current status of insufficient intake of n-3 polyunsaturated fatty acids in Chinese residents,but also prevent cardiovascular system diseases and chronic diseases related to inflammatory response.Currently,commercially available perilla oil is mostly produced by cold pressing,and heat treatment before pressing can generally increase oil yield,improve oxidation stability of oil and extend shelf life.Microwave technology is widely used in the field of food processing due to its advantages such as fast heating speed,strong selectivity,and convenient control.In this paper,the perilla oil was pressed from perilla seeds treated by microwave.The effects of microwave pretreatment on the oxidative stability of perilla oil were systematically studied from two aspects: the natural antioxidants contained in perilla oil and the antioxidant products of Maillard reaction.The structure of the final product of Maillard reaction produced from microwave-treated perilla seeds was characterized.The main research contents and results are as follows:Firstly,the effects of different microwave pretreatment conditions on the basic physicochemical indexes of perilla oil and the main components of perilla seed meal were studied.The results showed that microwave pretreatment had little effect on the composition and content of fatty acids in perilla oil.With the increase of microwave pretreatment intensity,the color of perilla oil darkened,the acid value decreased at first and then increased,and the peroxide value increased at first and then decreased.Microwave pretreatment could significantly improve the oxidation stability of perilla oil(p<0.05).The oxidation induction time of perilla oil increased from 6.21 h to 12.08 h with the increase of microwave pretreatment intensity.The changes in the content of soluble sugar and free amino acids in the defatted perilla meal obtained under different microwave pretreatment conditions showed that the main precursors of Maillard reaction during microwave pretreatment were rhamnose,xylose,arabinose,and glucose in the soluble sugar,as well as proline,arginine,glutamic acid,aspartic acid,and alanine in the free amino acid.Secondly,the changes in the content of antioxidant substances in the oil extracted from different microwave-treated perilla seeds was analyzed.And the relationship between the content of antioxidant substances in perilla oil and the oxidative stability of perilla oil was investigated to determine the key components that affect the oxidative stability of perilla oil.The results showed that microwave pretreatment significantly affected the content of natural endogenous antioxidants(tocopherols,phytosterols,total phenols,total flavonoids)and antioxidant product melanoidins from the end of Maillard reaction in perilla oil(p<0.05).Among them,melanoidins had the greatest contribution to the oxidative stability of perilla oil,and there was a highly significant positive correlation between them(p<0.01,r=0.96).With the increase of microwave pretreatment power and time,the content of total tocopherols,total phytosterols,total phenols and total flavonoids in perilla oil increased at first and then decreased.The contents of total tocopherols and total phenols in perilla oil were the highest when the microwave radiation was carried out for 6 min at 380 W,and the contents were104.53 mg/100 g and 118.76 mg GAE/kg,respectively.The content of total phytosterols in perilla oil was the highest when perilla seeds were irradiated by 700 W microwave for 6 min.The content of total flavonoids in perilla oil reached its maximum value(218.24 mg RE/kg)when the microwave treatment power was 380 W and the pretreatment time was 10 min.The content of melanoidins in perilla oil increased with the enhancement of microwave treatment intensity,and reached the maximum value when the perilla seeds treated at 700 W for 10 min.At this time,the non-enzymatic browning index A420 nm which characterizes the content of melanoidins was 0.401.Thirdly,the effects of microwave pretreatment on the structure of melanoidins in perilla oil were analyzed and characterized by ultraviolet-visible spectroscopy,fluorescence spectroscopy,Fourier transform infrared spectroscopy,matrix-assisted laser desorption ionization time of flight mass spectrometry and ultra-performance liquid chromatography tandem quadrupole time of flight mass spectrometry.The results showed that the melanoidins in perilla oil exhibited a typical “tail pattern” in the ultraviolet-visible spectroscopy,and its absorption intensity in the UV region increased with the increase of microwave pretreatment intensity.It can be seen that microwave treatment promoted the formation of melanoidins.Fluorescence spectroscopy showed that the fluorescence characteristics of melanoidins in perilla oil were consistent with those of typical melanoidins.Fourier transform infrared spectroscopy showed that melanoidins in perilla oil formed under different microwave pretreatment conditions had significant amide I bands and sugar characteristic absorption bands,indicating that melanoidins in perilla oil might contain amino compounds,carbonyl compounds and phenolic compounds.The results of matrix-assisted laser desorption ionization time of flight mass spectrometry showed that glyoxal molecules with a mass charge ratio of 58 Da were the structural units of melanoidins in perilla oil.A total of 9 soluble free phenolic compounds contained in melanoidins derived from perilla oil were detected,including four phenolic acids(protocatechuic aldehyde,caffeic acid,ferulic acid and rosmarinic acid)and five flavonoids(esculetin,quercetin,luteolin,apigenin and chrysoeriol).Based on these results,it was inferred that melanoidins in perilla oil induced by microwave pretreatment might be a high molecular polymer formed by the condensation polymerization of amino compounds and carbonyl compounds through aldol reaction.This kind of polymer took glyoxal as the basic structural unit,and phenolic compounds were bonded to the skeleton of melanoidins through non-covalent bonds.Finally,the changes in the content of the main soluble free polyphenols contained in melanoidins derived from perilla oil under different microwave conditions and the free radical scavenging ability of the melanoidins in perilla oil were measured.And the relationship between the above two was explored to preliminarily determine the antioxidant mechanism of the melanoidins derived from perilla oil which pressed from different microwave-treated perilla seeds.The results showed that microwave pretreatment significantly increased the content of the total soluble free phenolic compounds contained in melanoidins derived from perilla oil(p<0.05).When the microwave treatment power was 700 W and the pretreatment time was 10 min,the content of the total soluble free phenolic compounds contained in melanoidins derived from perilla oil was the highest,reaching 113.22 μg/100 mg M,which was 2.2 times of the content of the total soluble free phenolic compounds contained in melanoidins derived from perilla oil extracted from seeds without microwave pretreatment.Microwave pretreatment had a positive effect on improving the antioxidant capacity of melanoidins in perilla oil.The improvement of antioxidant capacity under microwave pretreatment might be caused by the attachment of some soluble free phenolic compounds to the skeleton of melanoidins in the form of non-covalent bonds.The DPPH radical scavenging activity of the melanoidins in perilla oil was the strongest when the microwave treatment power was 700 W and the pretreatment time was 10 min,and the scavenging activity was142.35 μmol TE/g by this time.When the microwave pretreatment time was 8 min,the ABTS radical scavenging activity(159.03 μmol TE/g)and the total reducing power(0.594)of the melanoidins in perilla oil reached the maximum at 540 W and 700 W,respectively.In conclusion,the enhancement of oxidation stability of perilla oil prepared by microwave pretreatment was related to the increase of the contents of natural endogenous antioxidants and melanoidins in perilla oil.Melanoidins,end products of Maillard reaction,played a leading role in improving oxidation stability of perilla oil.The intensity of microwave pretreatment could affect the structure and composition of melanoidins in perilla oil,thereby altering the antioxidant capacity of the melanoidins in perilla oil.The increase of antioxidant capacity of the melanoidins in perilla oil might be related to the increase of soluble free phenolic compounds bonded on the structures of melanoidins by non-covalent bond. |
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