| The convectional hot air roasting process of sesame seeds at high temperature gives sesame oil a strong aroma,however,on one hand it might result in heat damage of sesame protein and sesame oil quality.In comparison,studies have shown that microwave roasting is generally fast,highly efficient and in a way a form of green heating,thus it has attracted increasing attention in oil processing.At present in available literature,the mechanisms by which microwave pretreatment affects the structure of sesame seed and subsequently the oil yield is still not very clear,furthermore there appears to be no consensus among researchers on how microwave roasting influences the tocopherol and lignin content in sesame oil.In addition,the influence of microwave pretreatment on the aroma active components and sensory quality of sesame oil is also yet to be clearly understood and effects of microwave pretreatment on the formation of heterocyclic amines and polycyclic aromatic hydrocarbons in sesame oil has not been studied to detail.In light of the above,this research work comprehensively investigates the effects of microwave roasting parameters(time,power and water modulation of sesame seeds before microwave)on the structure and composition of sesame seeds and subsequently,sesame oil quality.The main research contents and conclusions are as follows:(1)The effects of microwave pretreatment on the surface,cross-sectional area,and subcellular structure of sesame seeds were revealed.The surface morphology of raw sesame seeds is intact,with no obvious pores on the transverse section.The oil and protein bodies are structurally intact and dispersed in the cytoplasm in a spherical structure.With the gradual increase of microwave time(2-10 min,900 W)and power(180-900 W,6 min),the damage degree of sesame seed surface gradually increased,the internal pore structure gradually increased,the rupture of cell wall gradually increased,the oil body visco Elastin membrane broke,and the oil droplets gathered,and the protein body denatured.Extending the microwave pretreatment time to 10 minutes increased the oil yield of sesame seeds to 34.36% compared to raw sesame seeds(23.84%);Increasing the microwave power to 900 W increases the oil yield of sesame seeds to 31.88% compared to raw sesame seeds.Before microwave irradiation(900 W,12 min),sesame seeds were subjected to moisture modulation(5%-30%).It was found that only a small amount of oil overflowed on the surface of sesame seeds with an initial moisture content of 5%,and the oil yield of sesame seeds was 31.01%;When the initial moisture content of sesame seeds is appropriately increased to 10%,the internal pores of sesame seeds increase after microwave treatment,and the degree of oil structure damage intensifies,with an oil yield slightly increasing to 32.21%.The oil yield of sesame seed was significantly reduced to 28.71% when the initial moisture content of sesame seeds was continuously increased to 30%.(2)The effects of microwave pretreatment on physicochemical properties and oxidation stability of sesame oil were studied.With the continuous increase of microwave time(2-10 min,900 W),the acid value of sesame oil decreased from 0.45 mg/g(0 min)to 0.30 mg/g(4min)and then increased to 0.96 mg/g(10 min).The peroxide value decreased from 1.32mmol/kg(0 min)to 0.87 mmol/kg(2 min)and then increased to 3.42 mmol/kg(10 min).With the increase of microwave power(180-900 W,6 min),the acid value of sesame oil decreased from 0.44 mg/g(0 W)to 0.36 mg/g(360 W)and then increased to 0.58 mg/g(900 W).The peroxide value decreased from 1.31 mmol/kg(0 W)to 0.85 mmol/kg(180 W)and then increased to 0.93 mmol/kg(900 W).The induction time of cold pressed sesame oil was 5.25 h,and the induction time of sesame oil was increased to the highest 40.16 h(900 W,10 min)with the increase of microwave power and time.With the water content of sesame seeds increased from 5% to 30% before microwave,the acid value of sesame oil decreased from1.18 mg/g to 0.70 mg/g,and the peroxide value decreased from 1.02 mmol/kg to 0.54 mmol/kg.The induction time of sesame oil oxidation decreased from 53.44 h to 11.34 h by increasing the initial water content of sesame seeds.(3)The effects of microwave pretreatment on the contents of tocopherol,lignan,heterocyclic amines and polycyclic aromatic hydrocarbons in sesame oil were studied.The content of γ-tocopherol in cold-pressed sesame oil was 381.5 mg/kg.The pretreatment time(0-6 min,900 W),power(0-720 W,6 min)and water content(5%-15%)of sesame seeds before microwave were moderately increased.The γ-tocopherol content of sesame oil was significantly increased to 408.50,462.92 and 404.27 mg/kg,respectively.However,increasing the microwave time(8-10 min,900 W),power(900 W,6 min)and water content of sesame seeds(20%-30%)significantly reduced γ-tocopherol to 349.30,416.35 and 362.91 mg/kg,respectively.With the increase of microwave pretreatment time(6-10 min)and power(0-900W),the content of sesamolin decreased significantly from 375.64 and 404.97 mg/100 g to83.84 and 365.75 mg/100 g,respectively.The content of sesamol increased significantly from0 to 53.50 and 15.78 mg/100 g,respectively.The results showed that sesamolin was converted to sesamol during microwave pretreatment.With the increase of initial moisture content of sesame seeds,the content of sesamolin significantly increased from 113.02 to 320.99 mg /100 g,and the content of sesamol reached the highest at 15% moisture content(26.42 mg /100g).No heterocyclic amines were detected in cold-pressed sesame oil.With the increase of microwave time and power,the content of Norharman and Harman increased to 1845.18 and 143.88 ng/g,respectively.The content of PAH4 in sesame oil increased with the increase of microwave time and power.The contents of Norharman,Harman and PAH4 in sesame oil could be effectively reduced by increasing the initial moisture content of sesame seeds before microwave.(4)HS-SPME(Headspace solid phase microextraction)and SAFE(Solvent assisted flavor evolution)combined with GC-O-MS revealed the effect of microwave pretreatment on the composition of aroma active substances and sensory quality of sesame oil.With the extension of microwave pretreatment time(0-10 min,900 W),the total concentration of heterocyclic aroma active substances(pyrazine,pyridine and furan)in sesame oil increased from 22 μg/kg(0 min)increased to 124,917 μg/kg(8 minutes),followed by a decrease to98,666 μg/kg(10 min),and the roasted sesame flavor,nut flavor and caramel flavor of sesame oil reached the strongest at 8 min.With the increase of microwave pretreatment power(0-900 W,6 min),the total concentration of pyrazine,pyrrole,pyridine,thiazole,thiophene,furan and other heterocyclic substances in sesame oil gradually increased.At the same time,the sensory intensity of sesame oil’s roasted sesame flavor,nut flavor,burnt flavor,smoked flavor and bitter flavor gradually increased;The total concentration of carbonyl substances such as aldehydes,ketones and esters increased first and then decreased.At the same time,the sensory intensity of sesame oil’s earthy,woody and green flavor continued to decrease.With the increase of the initial moisture content of sesame seeds before microwave,the concentration of pyrazine,pyrrole and pyridine in sesame oil first increased and then decreased,and the concentration of furan,thiazole and thiophene gradually decreased,resulting in the sensory intensity of roasted sesame flavor,nut flavor,burnt flavor and sweet flavor of sesame oil first increased and then weakened.To sum up,moderate microwave pretreatment time(900 W,2-8 min),power(6 min,180-720 W)and moisture modulation of sesame seeds before microwave(5%-20%)can effectively destroy the cell structure of sesame seeds,the combination of oil body and protein,thus significantly improving the oil yield of sesame seeds and improving the γ-tocopherol and sesamol content of sesame oil and their oxidation stability endow sesame oil with good flavor and sensory quality.It is necessary to avoid the heat loss of sesame oil flavor and nutrient components caused by excessive microwave energy(such as 900 W,≥10 min),and avoid the excess of harmful components such as heterocyclic amines and polycyclic aromatic hydrocarbons in the oil.This paper provides a comprehensive evaluation and theoretical basis for the application of microwave pretreatment technology in the production of sesame oil. |
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