Study On The Degradation Of Sesame Kernel Cell Wall Polysaccharides During Roasting

Sesame（Sesamum indicum L.）is an oil crop with high nutritional value,which is commonly used in the food industry to produce sesame oil,sesame paste,etc.Roasting is a key step in the production of sesame products.The roasting process involves many complex physicochemical reactions,which often change the flavor and organization of sesame products.Among them,sesame kernels are rich in proteins and sugars,which are capable of caramelization and Maillard reactions,both of which are essential for the formation of aroma and color of sesame products.It has been shown that polysaccharides can be degraded under heating conditions to reduce sugars and volatiles,which in turn affect the rate of the Maillard and caramelization reactions and the distribution of their products.Sesame kernels contain about 15.33%of cell wall polysaccharides.However,there is a lack of studies on the degradation of cell wall polysaccharides in sesame kernels under heating conditions.Therefore,in this study,cell wall polysaccharides were extracted from defatted sesame kernels（sesame meal）without roasting and after roasting at different temperatures,and then the structural changes involved in the cell wall polysaccharides were analyzed in order to provide a theoretical basis for the study of the Maillard and caramelization reactions during roasting.Below are the primary research contents and findings:（1）Sesame meal was treated with subsequently extraction method,and the obtained cell wall polysaccharides were named as WSP（water-soluble pectin）,CSP（chelator-soluble pectin）,NSP（sodium carbonate-soluble pectin）,KSH-1（1 mol/L KOH-soluble hemicellulose）,KSH-4（4 mol/L KOH-soluble hemicellulose）,and cellulose.The six polysaccharides differed markedly in structure and content in the cell wall of sesame kernels.Cellulose was the most abundant and CSP was the least abundant.The most abundant sugar units in WSP and cellulose were mannose and glucose,respectively,while the most abundant sugar unit in the remaining four polysaccharides was arabinose.The molecular weights of the six polysaccharides ranged from 13270 Da-811491 Da.CSP,in addition to having the highest thermal stability,also had the highest DMPD⁺radical scavenging capacity with an EC₅₀ of 1.45 mg/m L.KSH-4exhibited the highest DPPH radical scavenging capacity with an EC₅₀ of 0.54 mg/m L.（2）The structural changes of sesame kernel cellulosic polysaccharides during roasting were investigated.Six cellulosic polysaccharides,UC,C-140,C-160,C-180,C-200,and C-220,were isolated from sesame meal unroasted and roasted at different temperatures（140°C,160°C,180°C,200°C,and 220°C）,respectively.Structural analysis of the six cellulosic polysaccharides showed that the galacturonic acid and rhamnose units contributed more to the occurrence of the Maillard reaction and the caramelization reaction than other sugar units.The molecular weight of cellulosic polysaccharides decreased from 163270 Da to 125477 Da when the roasting temperature was increased from 180°C to 220°C.The crystallinity of cellulosic polysaccharides increased from 16.4%to 32.5%during roasting,and the volatiles produced by the degradation reaction in the amorphous region were rich in aldehydes（10.17%-33.33%）and furans（12.84%-41.55%）compounds,which can affect the flavor formation of sesame products.（3）The structural changes of sesame kernel hemicellulose during roasting were investigated.Six hemicelluloses（UH,H-140,H-160,H-180,H-200,and H-220）were obtained from sesame meal roasted at different temperatures（140°C,160°C,180°C,200°C,and 220°C）and unroasted,respectively,and the percentages of arabinose and xylose in hemicelluloses decreased gradually with increasing roasting temperature.The side chain structure is more susceptible to degradation reactions than the main chain.The highest heavy average molecular weight of hemicellulose was observed at roasting temperature of 140°C.Roasting caused changes in some functional groups of hemicellulose and in the form of links between monosaccharides.The degradation of hemicellulose occurred mainly at roasting temperatures above 140°C,and the degradation products could be used as precursors for the Maillard and caramelization reactions,thus affecting the sensory quality of sesame products.（4）The structural changes of sesame kernel pectin during roasting were investigated.Six types of pectins,namely UP,P-140,P-160,P-180,P-200,and P-220,were obtained from sesame meal roasted at five different temperatures（140°C,160°C,180°C,200°C,and 220°C）and unroasted,respectively.The percentages of rhamnose,arabinose,galactose,galacturonic acid in pectins gradually decreased in the roasting temperature range of 140°C-220°C,and the molecular weight of pectins gradually decreased from 1404 KDa to 35 KDa,as well as the microstructure was more fragmented in this temperature range.The side chain structure and the RG-I region of pectin are more susceptible to degradation reactions than the main chain structure.Roasting had no significant effect on the functional group species of pectin.Comparing the signals in the UP and P-220 HSQC spectra,it can be known that roasting caused the disappearance of some of the glycosidic bond NMR signals and the appearance of some of the glycosidic bond NMR signals.（5）The oligosaccharides formed by the degradation of cell wall polysaccharides of sesame kernels during roasting were investigated,and the results showed that the cell wall polysaccharides（pectin,hemicellulose,and cellulose）underwent degradation reactions during roasting leading to cell wall structure wrinkling and fracture.The oligosaccharide produced by the degradation of cell wall polysaccharides were complex in composition and varied in functional group types.The molecular weight of oligosaccharides tends to decrease,then increase and then decrease as the roasting temperature increases from 140°C to 220°C.The arabinose and glucose units in the cell wall polysaccharides contribute to the occurrence of the Maillard and caramelization reactions.By studying the structural changes of cell wall polysaccharides of sesame kernels during roasting,it can be known that the cell wall polysaccharides are degraded to different degrees during roasting,with pectin being the most easily degraded,followed by hemicellulose and finally cellulose.The degradation products of cell wall polysaccharides during roasting can be used as precursors to participate in the Maillard and caramelization reactions,which in turn affect the flavor,taste and color of sesame products.