Process Development for Soymilk Concentration

Author: Ekumankama, Ekuma Onu

Supervisor: Kehinde A.

Studies were conducted on soybeans to determine the pre-processing conditions of soaking and blanching which inactivate the trypsin inhibitor and lipoxidase enzyme but maximize the protein nutritional value of soymilk. Studies were also conducted on the concentration of soymilk to obtain the processing condition that will maximize its acceptable solids content. Finally, mathematical models were developed to predict the trypsin inhibitor activity in soymilk as a function of blanch time, blanch temperature and pH of blanch liquour and to predict the viscosity of soymilk as a function of solids content, pH and evaporation pressure. In the pre-treatment state, the soaking and blanch liquours employed are water, sodium bicarbonate and sodium carbonate solutions of varying concentrations and pH. Soaking time was varied from zero to 21hrs. While the blanch time and temperature were varied from zero to 30 minutes and 800 to 1000C respectively. The evaporation pressure for concentration of soymilk was varied from 250 to 760 mmHg. The pH was varied from 4.5 to 9.0. Chemical additives used included sodium bicarbonate, sodium suplphite and potassium phosphate. Experimental results showed that soaking and blanching conditions that maximized the protein nutritional value also inactivated the trypsin inhibitor and lipoxidase enzyme. The protein nutritional value varied with blanch liquour in the order H2O blanching > Na2CO3 blanching > Na2CO3 blanching. It also varied with soaking liquour in the order Na2CO3 soaking > H2O soaking > Na2CO3 soaking. The peak of nutritional value was found to increase with soaking time. This work observed that by destroying the trypsin inhibitor and other toxic substances in the blanching stage instead of heat sterilization of soymilk, higher solids content can be obtained during thermal evaporation of soymilk. Results obtained show that the viscosity as well as the maximum acceptable solids content to which soymilk can be concentrated improved with decrease in evaporation pressure and increase in pH. The use of chemical additives and defatted soybeans were found to improve the maximum acceptable solids content. Kinetic and empirical models that satisfactorily fit the inactivation of trypsin inhibitor were developed. The activation energy and frequency factor were found to be pH dependent. The viscosity of concentrated soymilk was well simulated by the empirical model developed. Mixed models from some phenomenological equations did not satisfactorily fit the viscosity data. However, close fit was obtained with the mixed model based on Roscoe's equation (145) when defatted soybeans were used to prepare the soymilk. This confirms that fat clustering contributes appreciably to the viscosity rise.