Study On Environmental Conditions Of Imported American Soybean "Oil Soaking" And The Effect On Quality

In order to ensure a safe supply of soybeans,China imports and stores a large amount of American soybeans.Soybeans are rich in fat,have a high heat capacity and are highly absorbent,which makes them less stable for storage,while American soybeans have a higher fat content and are more prone to"oil soaking"during storage,which seriously affects the quality and resource utilisation of soybeans.In this paper,we used American soybeans as the research object,and carried out simulated storage under different temperature(35°C,45°C and 55°C),moisture content(12.5%,13.5%and 14.5%)and gas environment(conventional gas and 98%nitrogen)combinations to classify the degree of"oil soaking"of American soybeans,and clarify the degree of"oil soaking"of American soybeans.The following results were obtained:the environmental storage conditions for"oil soaking"of American soybeans,the quality of US soybeans,the microstructure of the seeds and the changes in volatile substances during the process of"oil soaking":According to the"Rules for Determining the Quality of Soybeans in Storage",the degree of"oil soaking"in American soybeans was classified into three levels,combining colour(L*and b*values)and the rate of"oil soaking"grains.Lightly"oil soaking":crude fatty acid value of 3.48～4.18 mg/g,L*value of 48.15～51.78,b*value of 16.26～18.79,"oil soaking"grain rate of 5%～25%;Moderately"oil soaking":crude fatty acid value of 4.19～5.08 mg/g,L*value of45.79～48.14,b*value of 15.15～16.25,"oil soaking"grain rate of 26%～50%;heavy"oil soaking":crude fatty acid value greater than 5.08 mg/g,L*value less than 45.78,b*value less than 15.14,"oil soaking"grain rate greater than 50%.Changes in storage conditions can affect the extent and timing of"oil soaking"in American soybeans.Temperature can significantly affect the timing of"oil soaking"in soybeans,with light"oil soaking"occurring as early as 10 d,moderate"oil soaking"occurring within 15 d and heavy"oil soaking"occurring within 25 d at 55°C.At 55°C,the earliest"oil soaking"in American soybeans occurred within 10 d,moderate within 15 d and severe within25 d.At 45°C and 35°C,the earliest"oil soaking"occurred within 15 d and 60 d,respectively.Changes in moisture content had a lesser effect on the"oil soaking"of soybeans,with little difference in the degree and duration of"oil soaking"of soybeans with different moisture contents at 55°C,with"oil soaking"occurring within 10 to 15 d.In contrast to conventional gas storage,nitrogen-filled storage can delay the"oil soaking"process by about 10 d.Protein quality indicators,functional properties and secondary structure of American soybeans deteriorated during the"oil soaking"process.The water-soluble protein content,crude protein content,water-holding capacity,emulsification activity index and emulsion stability decreased and the oil-holding capacity increased,with the water-soluble protein content decreasing by 8.83%and the crude protein content decreasing from 30.20%to 29.10%when the soybeans were heavily"oil soaking"at 14.5%moisture content.The water-holding capacity of the protein decreased by 2.65 g/g from the initial value of 5.38 g/g;the oil-holding capacity increased by 1.80 g/g from the initial value of 2.68 g/g;the emulsification activity index decreased from 38.68 m~2/g to 19.56 m~2/g and the emulsion stability decreased from 246m~2/g to 187.4 m~2/g.The surface hydrophobicity and free amino acid content both increased first during the"oil soaking"process.Both surface hydrophobicity and free amino content showed an increasing and then decreasing trend during the"oil soaking"process.Theβ-folded protein secondary structure was converted intoα-helix andβ-turned angle,and theβ-folded content of the three water contents decreased by 0.70%,1.77%and 2.61%respectively during heavy"oil soaking".This indicates that both the quality and quantity of protein in soybeans changed significantly during the"oil soaking"process,with the direction of change basically being deterioration.The"oil soaking"process is accompanied by hydrolytic and oxidative rancidity of oils and fats,and the quality of oils and fats in soybeans is reduced.At the end of storage,the crude fat content of the three types of American soybeans reached the level of heavy"oil soaking",with crude fat content decreasing by 2.145%,2.84%and 3.60%respectively from the initial value of 22.32%,while crude fatty acid values increased from the initial value of 1.06 mg/g to5.12,5.25 and 5.62 mg/g,respectively.The iodine value decreased from 133.83 g/100 g to124.32,124.59 and 121.30 g/100 g respectively;the saturated fatty acid content increased and the unsaturated fatty acid content decreased in the fatty acid composition of the crude fat,and the moisture content of 12.5%American soybeans reached heavy"oil soaking"at the end of storage,the crude fat The total amount of polyunsaturated fatty acids linoleic acid and linolenic acid decreased from 67.41%to 65.60%,and the oxidation of linoleic acid into oleic acid increased from 18.13%to 18.49%;the content of sterols and tocopherols,which are trace components of crude fat,gradually decreased,and the content ofγ-tocopherol,which has a strong antioxidant effect,decreased by 29.36%,and the content ofδ-tocopherol decreased by35.81%.This indicates that the fat in the soybean undergoes significant changes during the"oil soaking"process,and all of them are of deteriorating quality."Oil soaking"had a negative impact on the surface microstructure and cellular ultrastructure of the soybean grain.Scanning electron microscopy showed that as the"oil soaking"process increased,the surface structure of the soybean seeds became sparse,changing from regular to disordered,losing its original smooth cross-sectional network structure,with the tissue structure becoming severely crumpled,irregularly curled,and gradually blurring in outline.The cell structure of the seeds was observed by transmission electron microscopy,and it was found that the normal soybean seeds had a dense cell structure,with uniform elliptical protein bodies,tightly bound protein and lipid bodies,and a tough and smooth cell wall;when the soybean seeds were heavily"oil soaking",the protein bodies were dispersed in a flocculent manner,and the boundaries with the lipid bodies disappeared completely,with the number of lipid bodies reduced and arranged The number of liposomes is reduced,the arrangement is loosened,and the cell wall is separated and loses its integrity.During the"oil soaking"process,the relative content of volatile substances in soybeans changes.In normal soybeans,there is no bad odour and the flavour is normal,with the largest proportion of alkanes detected at 31.66%.The isovaleraldehyde(3-methylbutyraldehyde),which has a pungent and irritating odour,and 2-methylbutyraldehyde,which has a suffocating,spicy and musty odour,and theγ-butyrolactone,which has a slightly ammoniacal odour,are associated with lipid peroxidation,and their increased value indicates serious deterioration in the quality of the crude soybean fat.