Thermal Degradation Of Sesame Hull Lignin During Roasting And The Effect Of Its Products On The Oxidative Stability Of Sesame Oil

Sesame(Sesamum indicum L.)is an important oilseed crop that is widely grown worldwide.Sesame oil is rich in unsaturated fatty acids(about 85%),has high nutritional value and is the main product processed from sesame seeds.Compared to other vegetable oils,sesame oil is more oxidatively stable than other vegetable oils,and the oxidative stability of sesame oil made from roasted sesame seeds is higher than that of unroasted sesame seeds.It is widely believed that the reasons for the strong oxidative stability of sesame oil include: first,the presence of lignans in sesame oil,which have certain antioxidant activity;second,the presence of tocopherols(γ-tocopherol)in sesame oil,which are recognized as natural antioxidants;third,the roasting operation of sesame seeds before oil extraction,which makes reducing sugars and proteins in sesame seeds undergo the Melard reaction at high temperatures,and the Melard reaction products have strong oxidative stability.The products of the Melad reaction have strong antioxidant activity.It was shown that vanillin,an endogenous substance in sesame,is a natural antioxidant component that can be transferred into sesame oil during processing,and that the vanillin content in sesame seeds increased substantially after roasting.In addition,the flavor and oxidative stability of roasted sesame oil were better than those of peeled sesame,and the roasting of sesame hull produced a large amount of phenolic compounds,which were similar to those found in sesame oil so far.Sesame hull is rich in lignin,which is a natural phenolic polymer and can be degraded to small molecules under heat treatment.Therefore,it is assumed that the phenolic compounds formed during roasting of sesame hull are likely to originate from the roasted degradation of lignin.No reports have been found on the structure of lignin in sesame hull,nor on the source of phenolic compounds in sesame oil.Based on this,this study was conducted to investigate the structure of sesame hull lignin,its structural change pattern during roasting and the influence of degradation products on the oxidative stability of sesame oil,in order to provide a new mechanistic pathway for the roasting of sesame seeds to build a stable quality of sesame oil.The main research contents and conclusions of this paper are as follows:1.Structural characterization of sesame hull lignin and lignin-carbohydrate complexes(LCC): neutral organic solvent extraction was chosen to extract lignin and LCC from sesame hull,and their structural characterization showed that in addition to the three traditional lignin monomer types(G-type,S-type and H-type),C-type lignin monomers were present in sesame hull,of which the largest proportion was G-type,followed by C-type.followed by C-type;the major lignin-linked bonds in MWL and LCC-Ac OH fractions were β-O-4’ bonds(16.6% and15.7%,respectively),β-β’ bonds(45.4% and 46.4%),and benzodioxane linkage(38.0% and37.9%,respectively).Ph Gly was the major lignin-carbohydrate linkage bond in these three fractions.The above results provide some theoretical basis for the later study of the structural changes of sesame hull lignin during roasting.2.Structural changes of sesame hull lignin during roasting: sesame hull was roasted at190,210,230 and 250°C for 30 min,from which mill wood lignin(MWL)was extracted and structurally characterized,the results showed that the carbohydrate content of MWL fraction decreased with increasing roasting temperature of sesame hull;the molecular weight of MWL fraction decreased with increasing roasting temperature of sesame hull.The molecular weight of the MWL fraction showed a decreasing trend with the increase of sesame hull roasting temperature;the proportion of S-and C-type lignin monomers in the lignin monomer type decreased more obviously than that of G-type lignin,while the proportion of H-type lignin gradually increased;during the roasting process of sesame hull,the MWL fraction underwent degradation and condensation reactions,and the proportion of β-O-4’ and β-β’ linkages decreased;with the increase of sesame hull roasting temperature,the lignin More phenolic hydroxyl groups were produced in the structure with the increase of roasting temperature.The above results provide a theoretical basis for further study of the degradation products of sesame hull lignin in the roasting process.3.Analysis of the degradation products of sesame hull lignin during roasting and the effect of degradation products on the oxidative stability of sesame oil: the MWL fractions were heated in an autoclave at 190,220 and 250°C for 5 min,and then the lignin degradation products and the effect on the oxidative stability of sesame oil during storage were investigated by different characterization methods,in vitro antioxidant experiments,and Schaal oven experiments.The effects were studied.The results showed that lignin was partially degraded at different roasting temperatures,and the degree of lignin degradation increased with the increase of roasting temperature;lignin degradation under roasting conditions produced some phenolic substances,such as vanillin(which can be used as an antioxidant and flavor substance),3-methoxy-4-hydroxy-butyrophenone,4-ethyl-2-methoxyphenol,vanillin acetophenone,etc.,and with the increase of roasting temperature The lignin degradation products inhibited the oxidation of sesame oil,and the antioxidant capacity of lignin degradation products increases with the increase of roasting temperature.Among the lignin degradation products,the R250 fraction had the strongest ability to maintain the oxidative stability of sesame oil.The above results revealed the degradation products of sesame hull lignin during roasting and the influence of degradation products on the oxidative stability of sesame oil.The study revealed that the monomer types of sesame hull lignin included G,S,C and H types,and the main linkage bonds between monomers included β-O-4’ bond,β-β’ bond and benzodioxane linkage;during the roasting process of sesame hull,the proportion of β-O-4’ andβ-β’ linkage bonds between lignin structures decreased,and with the increase of roasting temperature of sesame hull,more phenolic hydroxyl groups were produced in the lignin structures.The lignin degradation products inhibited the oxidation of sesame oil,and the antioxidant capacity of lignin degradation products increases with the increase of roasting temperature.This study explains that some phenolic compounds in sesame oil(e.g.vanillin,guaiacol,etc.)originate from the degradation of lignin,and in addition,these degradation products enhance the oxidative stability of sesame oil,thus providing a new theoretical basis for the study of factors affecting the oxidative stability of sesame oil.