The effect of mechanical shear in ambient yoghurt

Sammanfattning: Yoghurt is a fermented product form the milk, well known in the world. The fermentation is carried out by Lactic acid bacteria (LAB). The most common are Streptococcus thermophilus and lactobacillus bulgaricus, these bacteria make the milk acid, generating a disturbance in the casein micelles making a solid mass called coagulum. This specific LAB also produces expopolysaccharides (EPS) that form the characteristic ropy in the yoghurt. There are many types of yoghurt based in its consistency; the most popular are the set and the stirred type. Set yogurt is the solid coagulum where the yoghurt is incubated, cooled and retailed to the consumer in the same container. The stirred yoghurt is fermented in a big container and after fermentation the coagulum is destroy gentle, cooled and pumping to the final package to the consumer. Ambient yoghurt production is the same as stirred type whit the difference of the ambient yoghurt is heat-treated after fermentation in order to inactivate the LAB and so therefore prolong the shelf life of the product; due to this, ambient yoghurt doesn’t needs to be cooled. Heat-treatment and pumping have an impact in the final viscosity and stability as water retention in the product since the yoghurt gel is shear sensitive. The use of different stabilizers is needed to maintain the quality parameters of the yoghurt as the viscosity perceived as smoothness and avoid the syneresis (separation of the whey). The correct mix in the stabilizers added to the yoghurt and to know how yoghurt behaves during shearing in the process presents new challenges in the dairy industry with the finality to give the consumer a product with long shelf life that keeps the smoothness and stability characteristic in the yoghurt. Taking into account these considerations, the purpose of this study was to evaluate the rheological behaviour of the yoghurt after applying mechanical shear to the stirred yoghurt. The procedure of the experiment consist in 3 parts: 1) the setting of the rig to simulate the shear rates in the industry (called as low, medium and high shear rate), from low shear rates, medium and high 2) Yoghurt production based in two different standard ambient yoghurt formulations with stabilizers: a) Formula 1 (F1) and its control (F1C) and Formula 2 (F2) and its control (F2C). F1 and F2 had modified starch and pectin, with the difference that F1 had gellan gum and F2 gelatin 3) applying the shear rates in different times (30 and 60 seconds) to the yoghurt in the rig and in the rheometer. Before and after the shearing of the product in the rig and in the rheometer, it was measured the percentage of water retention (%WR) and the viscosity in a rheometer to see how were the changes of F1 and F2 after shearing in rig and rheometer. The results showed that both formulas and controls were sensitive to shear but behave different due to the stabilizers used. F1 viscosity after shearing shows better stability, it can be attributed to the gellan gum, on the other hand it doesn´t have good %WR even though with the addition of stabilizers and had a continuous decreasing after medium and high shearing. F2 has higher viscosity and better higher retention than its own control but does not have a good viscosity stability having a constant decreasing when shear was applied, however the %WR was higher and more stable at high mechanical shear; gelatin could be the answer for this behaviour. The different times that the same shear rates were applied to the samples (30 and 60 sec) doesn´t show a significative difference in the viscosity and the %WR. Finally the viscosity results of the yoghurt which run in the rig are not the same as the ones that run in the rheometer, even though the formulas used in the development of the rig were correct, the prove is the %WR in both samples which run in different systems (rig and rheometer) has not significative difference.