Lipid Stability In Soyflours Produced From Raw And Processed Soybeans.

ABSTRACT Soybean, a high protcin/oil legume is known (o contain high levels of polyunsaturated fatty acids, which are susceptible to lipid oxidation and hydrolysis. However, suitable processing methods and storage conditions can enhance the stability o f lipid in soyflour. The objective of the project was to investigate the effects of some processing methods and storage conditions on the stability o f lipid in soyflour and to define the parameters for the prediction o f the shelf life soyflour. Some whole commercial soybeans were milled into raw soyflour. Some of the whole beans were cooked for one hour, and milled into cooked-dried soyflour. The final portion was roasted in an open pan and then milled into roasted soyflour. The raw, cooked-dried and roasted flours produced were stored at temperatures o f 5, 16, 30, 42, 68 and 80°C, and at water activities o f 0.15, 0.23, 0.45, 0.68 and 0.75 for 12 weeks. Indices o f lipid oxidation (peroxide value and ihiobarbituric acid number) and an index of lipid hydrolysis (free fatty acids) were determined at time intervals of 0, 2, 6, 10 and 12 weeks. Sensory evaluation was performed on the samples stored at 5 and 30°C at storage times of 4, 8,10 and 12 weeks. Results showed that the rates of lipid oxidation and hydrolysis were higher in raw soy flour than in the heat-processed flours at the same storage temperature and time and at (he same water activity. The rale of lipid hydrolysis was found iiito be minimal at 5°C and maximal at 3-0 °C in both raw and heat-processed soy II ours. The rate of lipid oxidation was also found lo be minimal at water activity corresponding to the average monolayer value o f both raw and heatprocessed soydours. The flavour of raw and heat-processed Hours began to change significantly after 6 and 12 weeks of storage respectively; and this occurred at peroxide value of 4.21 meq/kg and thiobarbituric acid number of 9.76 mg/kg. Hcat-treatment of soybeans prior to processing into Hours yield Hours with lower rates o f chemical reactions, which result in minimal lipid oxidation and hydrolysis. Storage o f soy Hour under cold condition or maintaining (he moisture content of soyllour at or close to the monolayer value of soy Hour results in lower rates of chemical reactions, which increases the stability o f lipids in soyHour. The shelf life of soyllour can be predicted when the peroxide value and the TBA Number of soyHour are known. It is possible to extend the shelf life of soyHour by heat-processing the beans prior to milling into Hour and storing the flour under cold conditions.