ABSTRACT

The effects of milling (wet and dry) and coagulants (lime, Epsom salt and tamarind) on the viscoelastic properties and growth dynamics of selected food pathogens of tofu from blends of soybean and sesame were studied. The soybean and sesame blended in different ratio (100:0, 82.5:17.5, 65:35, 47.5:52.5, 30:70, and 0:100) were processed into flour by dry and wet milling method. The physicochemical properties of the flour samples were determined. The wet and dry milled flour blend in different ratio were further processed into tofu using three coagulants – lime, Epsom salt and tamarind. The physical, proximate and mineral compositions of the tofu samples were determined. The texture profile analysis, stress strain test and stress relaxation test were also carried out. Furthermore, the texture profile analysis of the samples were subjected to different mixture models using the D-optimal mixture model. The growth dynamics of selected bacteria (Escherichia coli, Salmonella enteritidis) and fungus, Aspergillus flavus were also observed in the fresh tofu samples. Finally, the best tofu sample was identified using the optimization test result. The result from the flour analysis revealed that functional properties of 100% dry milled soybean (D100:0) was the highest. The fat and ash content were highest in 100% dry milled sesame (D0:100) while the protein content was highest in 100% dry milled soybean (D100:0). The mineral composition of the flour was highest in (D0:100). The physical characteristics of the fresh tofu revealed that 100% soybean dry milled tofu coagulated with tamarind (TD100:0) had the highest initial weight and coagulation time. Wet milled tofu coagulated with lime and tamarind was fitted in linear mixture model and cubic mixture model respectively. In texture profile responses, the wet milled tofu from soybean/sesame blend coagulated with Epson salt (EW65:35) had the highest value in hardness (569.86), chewiness (154.92N) and gumminess (181.98). The stress strain test revealed that the dry milled tofu from soybean/sesame blend coagulated with Epson salt (ED82.5:17.5) had the highest stress strain yield (0.948N/mm²) while the dry milled tofu from soybean/sesame blend coagulated with Epson salt (ED47.5:52.5), had the lowest value(0.165N/mm²). The internal stress and stress modulus had the highest values (1.206N/mm²) and (3.02N/mm²) respectively   in ED47.5:52.5 while the lowest values (0.032N/mm²) and (0.66N/mm²) were found in the dry milled tofu from soybean/sesame blend coagulated with lime (LW47.5:52.5), and the dry milled tofu from soybean/sesame blend coagulated with tamarind (TD47.5:52.5) respectively. The growth dynamics of Escherichia coli, Salmonella enteritidis, Aspergillus flavus revealed that the lowest growth was found in dry milled Epson salt coagulated tofu with 30% soybean and 70% sesame mixture (ED30:70) using the optical density growth reading. Finally, the optimization test revealed that dry milled tamarind coagulated tofu with 30% soybean and 70% sesame mixture (TD30:70) had the overall best among the 30 samples based on nutritional, textural and microbial constraints. This suggests that soft tofu with high protein, fats and mineral composition is preferred for all ages and can be processed by dry milling of soybean and sesame, coagulated with tamarind.