Differences in rheological behaviour and microstructure of yoghurt processed by high isostatic pressure and dynamic high pressure
High Isostatic Pressure (HIP) and Dynamic High Pressure (DHP) are promising non-thermal processes. These processes are able to increase the consistency of yoghurt due to changes in the protein and milk-fat structure. This study aimed to compare the effect of HIP and DHP in the consistency and microstructural of yoghurt. Pasteurized milk (3% v/v fat-milk; 72 °C / 15 sec) was subject to HIP (200 to 600 MPa for 15 min at 50 °C) or DHP (50 to 150 MPa at 50 °C). After the processes, 40 mg/L of yogurt culture (Lactobacillus bulgaricus and Streptococcus thermophiles) was inoculated in milk and incubated at 43°C to reach pH 4.6. The yoghurts were evaluated by rheological small amplitude oscillatory test (1 Hz, 0.1-10 Pa at 5 °C in yoghurt prior to stirred), steady-state shear (100-0.01s-1 at 5 °C in yoghurt after stirred), confocal microscopy and protein desnaturation. The results showed that both the HIP and DHP increased the consistency of yoghurts with increase of pressure. The small amplitude oscillatory test showed more consistency in yoghurt subject to HIP (376 Pa.s¬n’ to 600MPa) than DHP (198 Pa.s¬n’ to 150MPa). However, unlike the previous results, the results of steady-state shear test showed lower consistency in HIP (0.22 Pa to 600 MPa) than DHP yoghurt (0.51 Pa to 150 MPa). This different behaviour in this analysis occurred because in HIP had more protein denaturation than DHP (49% to HIP at 600MPa/15min and 2% to DHP at 150MPa). On the other hand, the confocal microscopy showed that DHP contributed more to the interaction between fat-protein than HIP. Thus it was concluded that the HIP confers more consistency in yoghurt prior to stirring (set yoghurt) and DHP results in greater consistency after stirring (stirred yoghurt).