Lipid Stability in Soy Flours Produced From Raw and Processed Soybeans

ABSTRACT Soybean, a high protein/oil legume is known to 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 of lipids in soy flour. The objective of the project was to investigate the effects of some processing methods and storage conditions on the stability of lipids in soy flour and to define the parameters for the prediction of the shelf life of soy flour. Some whole commercial soybeans were milled into raw soy flour. Some of the whole beans were cooked for one hour and milled into cooked-dried soy flour. The final portion was roasted in an open pan and then milled into roasted soy flour. The raw, cooked-dried and roasted flours produced were stored at temperatures of 5, 16, 30, 42, 68 and 80°C, and at water activities of 0.15, 0.23, 0.45, 0.68 and 0.75 for 12 weeks. Indices of lipid oxidation (peroxide value and thiobarbituric 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 the same water activity. The rate of lipid hydrolysis was found to be minimal at 5°C and maximal at 30°C in both raw and heat-processed soy flours. The rate of lipid oxidation was also found to be minimal at water activity corresponding to the average monolayer value of both raw and heat processed soy flours. The flavour of raw and heat-processed flours 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 (TBA) number of 9.76 mg/kg. Heat-treatment of soybeans prior to processing into flours yields flours with lower rates of chemical reactions, which results in minimal lipid oxidation and hydrolysis. Storage of soy flour under cold condition or maintaining the moisture content of soy flour at or close to the monolayer value of soy flour results in lower rates of chemical reactions, which increases the stability of lipids in soy flour. The shelf life of soy flour can be predicted when the peroxide value and the TBA number of soy flour are known. It is possible to extend the shelf life of soy flour by heat-processing the beans prior to milling into flour and storing the flour under cold conditions.