Viscosity changes of soymilk due to vacuum evaporation with moderate heating

Abstract Commercial soymilks were evaporated under reduced pressure at various water bath temperatures (55, 65 and 75 °C), and their viscosities were measured by a tuning fork vibro viscometer. A semi-logarithmic plot of viscosity versus solids content gave two separate regression lines, indicating a two-step exponential increase in viscosity. Pairs of lines showed intersections (solids content, 155–199 g/kg) depending on the temperature and varieties (Japanese and Canadian soybeans). The viscosity changes in the higher solids content range, namely above the intersection, were temperature dependent showing a steep rise, whereas in the lower solids content range viscosity was less steep and relatively insensitive to temperature. Furthermore, evaporated soymilk in the lower solids content range exhibited moderate Newtonian flow, but in the higher solids content exhibited non-Newtonian flow. The average diameters of oil droplets (from 0.25 μm (88 g/kg) to 0.30 μm (280 g/kg)) and proportion of floating fraction via ultracentrifugation (from 0.28 (92 g/kg) to 0.68 (270 g/kg) increased in the higher solids content range, showing aggregate formation including protein particle and oil droplet. These interactions between the oil droplets and the protein particles were considered to be resulted in a large increase in the viscosity of the evaporated soymilk samples.