Study Of The Quality Of Soy Protein And Lipid As Affected By Soybean Heat Damage During Storage

At present, with the rapid growth in demand, soybean has become China’s largestagricultural imports. But there is a big problem in soybean storage. Both improper storagecontrol and a lack of temperature and humidity control devices cause quality deterioration ofsoybean, resulting in serious economic loss. The existing research mainly concentrates on thestorage under mild conditions, with temperature in the range of4-35°C and relative humidityin the range of55%-88%. In fact, storage temperature is far more than this range. On the onehand, temperature is higher in summer. On the other hand, China’s importing soybeans almosttransported by shipping, which may make the cabin temperature exceed50°C. Therefore, thissubject focuses on the quality of soy protein and lipid as affected by soybean heat damageduring storage, laying the foundation for controlling the storage conditions, providing thebasis for further utilization of slight damaged soybeans.In this paper, we mainly investigated the influence of heat damage storage by differenttemperatures, humidities and time on structural characteristics and functional properties ofSPI, as well as the lipid qualities. The study had selected two kinds of high temperatures(40°C,50°C) with three different relative humidities (60%,75%,90%). Storage time was0.5-8months. The SPI was characterized by extractability, structural characteristics and functionalproperties. Structural characteristics included denaturation degree, HPLC, SDS-PAGE,surface hydrophobicity, free amino groups, carbonyl groups and sulfhydryl groups, whilefunctional properties were presented by solubility, emulsifying and foaming properties, aswell as gelation. The lipid was characterized by extractability, fatty acid analysis, iodine value,hexanal content and lipoxidase activity.Firstly, the results showed that heat damage storage had a great influence on theextractability of SPI and lipid. Under40°C, storage made protein extractability graduallydecrease from53.35%to10%-30%within4-8months, while no significant decline in lipidextraction rate. The storage under50°C made the rate of SPI rapidly decreased to20%after1month, lower to10%after2months, while the data of lipid decreased3-4percentage points.The higher humidity, the more significant change.Secondly, heat damage storage had a significant impact on the structural and functionalcharacteristics of SPI. Under40°C, protein had gradual degeneration, accompanied by theaggregation and dissociation of subunits. Storage resulted in an increase of carbonyl content, adecrease of surface sulfhydryl content, and a downward trend after first rise of free aminocontent, total sulfhydryl content and surface hydrophobicity. The storage under50°C, whichintensified the protein denaturation and aggregation, made molecular surface properties havea bigger change. The higher humidity, the more significant change. Meanwhile, storagecaused a gradual decline of protein solubility, emulsifying and foaming properties, as well asgelation. The decline was directly proportional to temperature and humidity.Thirdly, heat damage storage also had an obvious impact on the lipid quality. The contentof polyunsaturated fatty acids decreased, along with the reduce of iodine value. Storageaggravated the oxidation of lipid, showing an increase of n-hexanal content and worse beany flavor. The higher temperature and humidity, the more significant changes. Moreover, storageunder50°C caused a sharp decline in the activity of soybean lipoxygenase, resulting in andecrease of n-hexanal content.In conclusion, storage under40°C and60%RH had a minimum heat damage on soybean.SPI can be kept good emulsifying properties within2months, good foaming and gelproperties within4month and good solubility within8months. Changing storage condition to40°C and75%RH, emulsifying properties maintained a month, foaming and gel propertiesmaintained2months, solubility maintained4months. The most adverse storage under50°Cand90%RH caused a significant decline on all aspects of functional properties, only the initialstorage of50°C still having good gel strength.