| Soybean milk is rich in nutrition, and is loved by consumers, the flavor and quality is influenced greatly by the raw materials and the production process, so it is very important to improve and stabilize the quality of soybean milk. This study takes soybean as raw material, and the effects of pretreatment on the quality of soybean milk were studied by using the method of freezing, ultrasonic wave and microwave respectively treated soybean. According to the principle of the combination of sensory evaluation and instrumental analysis, the volatile compounds in the soymilk extracted by simultaneous distillation extraction( SDE) and then analyzed by gas chromatography-mass spectrometry(GC-MS). The separated peaks were identified by mass spectral library searching combined with retention index comparison. Area normalization method was used for quantitative analysis and benzyl alcohol was used as internal standard. The purpose of the analysis is to find a way to effectively improve the quality of soybean milk, and to maintain a favorable flavor, to provide a theoretical basis for soybean milk processing. The results are as follows:(1) The regression model was obtained by response surface: Y(protein c ontent)=3.36+0.25A+0.24B+0.0013C+0.045AB-0.033AC+0.018BC+0.087A2+0.051B2+0.015C2; Y(sensory score)=71.36-0.80A-1.07B+0.075C-0.55AB+0.45AC+0.60 B C-3.63A2-2.68B2-2.98C2.According to the influence of the factors on protein cont ent and sensory evaluation, the optimal process for the production of soybean mi lk was: soak the beans and water ratio 1:3, soaking 10 h, grinding bean water r atio 1:9, boiling temperature of 95℃ to maintain 9 min. Under these conditions,the protein content was 3.5468%, the sensory score was 68.9 and the predictive value was basically in agreement.(2) Freezing pretreatment can improve the quality of soybean milk. The protein content, protein extraction rate and in vitro protein digestibility of soybean milk were increased with the increase of freezing time. The protein content and protein yield of frozen 40 h samples were the highest, which were 3.8658% an d 78.82%, respectively. In vitro protein digestibility of freezing pretreated 30 h s oybean milk was the highest, which was 79.29%. After heating soybean milk sol uble solids content increased, but little change before and after freezing. The par ticle size distribution showed that the size distribution of soybean milk had a si milar trend, and the particle size of soybean milk was decreased after heating. With the freezing time prolonged, the average particle size of the sample was si gnificantly reduced(P<0.05); the stability showed a downward trend; the preci pitation showed an upward trend. The L* value of the frozen pretreated soybean milk was higher than that untreated soybean milk, while the a* value and the b* value were lower than that of the untreated soybean milk. The kinds and con tent of different freezing time milk flavor compounds had certain difference, and the main flavor substances were aldehydes, hydrocarbons, esters, ketones and al cohols. The content of aldehydes was the highest. By simultaneous distillation ex traction and gas chromatography-mass spectrometry(SDE-GC-MS) combined w ith sensory evaluation, showed that the quality of soybean milk could be improv ed and the beany flavor decreased obviously by freezing pretreatment. Comprehe nsive each index selection freezing pretreatment time was 20 h.(3) Soybean milk protein content, protein extraction rate, and in vitro protein digestibility increased significantly after ultrasonic pretreatment(P<0.05), Ultrasonic pretreatment 1.5 h soymilk protein content was the highest, 4.1432%; ultrasonic pretreatment 0.5 h followed was 4.0266%. With the ultrasonic time prolonging, the average particle size of soybean milk was significantly reduced, The average particle size of soybean milk was significantly decreased with the extension of ultrasonic time, cooked soymilk ultrasound 2 h was the minimum, 12.02 μm; raw soymilk ultrasound 0.5 h the smallest, 13.48 μm; the stability showed a downward trend, and the precipitation showed an upward trend. Ultrasonic pretreatment of soymilk L* value is higher than the untreated soymilk, while the a* value and b* value is lower than the untreated soymilk. The kind and content of flavor compounds in different ultrasonic time soymilk the gradually increased, the content of aldehydes were the highest, the main substance hexanal of untreated was 4.8612 μg/g, ultrasonic 0.5 h of 5.3085 μg/g, ultrasonic 1 h of 6.7712 μg/g, ultrasonic 1.5 h of 7.4342 μg/g, ultrasonic 2 h of 9.9756 μg/g. Combined with sensory evaluation, soybeany flavor after ultrasonic pretreatment significantly increased, more full-bodied taste. Considering the selection of ultrasonic pretreatment time was 0.5 h.(4) Microwave pretreatment resulted in protein content, protein extraction rate, and in vitro protein digestibility of soybean milk(P<0.05), but the change of protein extraction rate of microwave 30 s was not significant(P>0.05) compared with untreated. With the increase of microwave time, the average particle size of soybean milk was significantly increased, the stability showed a downward trend, and the precipitation showed an upward trend. Microwave pretreatment L* value is lower than the untreated soymilk, and a* and b* value is higher than the untreated. The contents of flavor compounds in soymilk with different microwave time gradually decreased, the main substance hexanal beany flavor of untreated 4.8612 μg/g, microwave 30 s of 4.3217 μg/g, microwave 60 s of 3.7113 μg/g, microwave 90 s of 2.0968 μg/g, microwave 120 s of 1.9324 μg/g. Combined with sensory evaluation, soymilk after microwave treatment beany flavor decreased obviously, taste lighter. Considering the selection of microwave pretreatment time was 30 s.(5) Comparison of different pretreatment(untreated, freezing pretreatment 20 h, ultrasonic pretreatment 0.5 h, microwave pretreatment 30 s) scanning electron microscope image was obtained: freezing pretreatment 20 h soybean cells was loose, and the intercellular spaces were large; ultrasonic pretreatment 0.5 h soybean cell structure more closely, the diameter of the tube bundle decreased; microwave 30 s pretreatment of soybean with loose structure and rough surface. Ultrasonic pretreatment had little effect on the structure of soybean, soybean could keep better, and make the arrangement more closely; freezing pretreatment had some damage to soybean structure; microwave pretreatment of soybean structure was maximum damaged.(6) A total of 3 principal components were proposed by principal component analysis, the comprehensive score of soybean milk samples with different pretreatment methods was ranked as: microwave pretreatment 30 s > ultrasound pretreatment 0.5 h > untreated group > freezing pretreatment 20 h, so the flavor of soymilk was affected by pretreatment.(7) According to the scanning electron microscopy(SEM) of soybean combined with physical and chemical indexes of soybean milk after different pretreatments, the results of the principal components analysis were obtained: freezing pretreatment 20 h group, ultrasonic pretreatment 0.5 h group, untreated group, microwave pretreatment 30 s group, finally, the optimal pretreatment method was selected as freezing pretreatment 20 h. |
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