Yogurt is a fermented dairy product. It is obtained by fermenting milk, where bacteria convert milk sugar into lactic acid. During this process, the milk coagulates and the characteristic gel structure of the yogurt is obtained. The aim of the diploma thesis was to determine how the production of yoghurt affects its rheological properties. Experimental work was performed on two yogurt samples. The first was obtained from a mass production, while the second one was home-made. We performed rotational and oscillation rheological tests to determine the flow characteristics of the samples, and to determine viscoelastic properties. The differences between the two yoghurt samples were already evident from the flow test, and were confirmed also by oscillatory amplitude and frequency test. It turned out that prior to flowing mass-produced yogurt withstands higher shear stresses compared to home-made yogurt. Both samples exhibited narrow range of linear viscoelastic response, which can be attributed to the internal structure of the material. From the frequency dependence of both dynamic modules, we observed that the elastic contribution dominated over the viscous one in both samples. It has been shown for both samples that they behave as highly structured viscoelastic materials with fluid-like properties. Thus, we can conclude from the results of the measurements that the method of manufacturing affects the rheological properties of the produced yogurt.
|
