The effect of thermosonication of milk on selected physicochemical and microstructural properties of yoghurt gels during fermentation

Yoghurt Cultures (0.1, 1.5 and 3.5% fat) were prepared from milk which was preheated to 45 degrees C and subjected to thermosonication (TS) for 10 min at an ultrasound amplitude of 24 kHz. Compared to conventional yoghurts prepared from preheated (90 degrees C, 10 min) milk, cultures from TS milk at similar fat contents had higher gelation pH values, greater viscosities and higher water holding capacities (WHC). On average, yoghurts from TS treated milks with 1.5 or 3.5% fat had almost 2 fold greater WHC and 25% higher G' values than conventionally produced yoghurt. Electron microscopy showed differences in the microstructure, with TS yoghurt having a honeycomb like network and exhibiting a more porous nature. These characteristics are absent in conventional yoghurts. in addition, the average particle size in TS yoghurts was smaller (<1 mu m) than in conventional yoghurts. (C) 2008 Elsevier Ltd. All rights reserved.