Study On The Yoghurt With A Fruit Juice Layer

Yoghurt is considered to be a natural, nutritious and health promoting food, therefore it is accepted by the masses for consumption. But different age groups of people have different nutrional requirements. Therefore researchers are developing many kinds of yoghurt products according to the consumers'nutritional requirements. The yoghurt with a fruit juice layer is a product in which some juice is added to the top of the set yoghurt. It does not only possess the advantages of fruit juice yoghurt, but also keeps the fresh and bright color of fruit. The addition of thickeners to the fruit juice can increase the mellow and mouth feel of yoghurt. Using transparent packaging materials is a new way that can attract consumers. Furthermore, the layer can enable consumers to see the thickness of fruit juice layer. Consumers can mix the yoghurt and fruit juice layer as they wish.The best processing parameters for yoghurt making, the addition of fruit juice, the style and quantity of the best thickeners and stabilizers, the rheology and TPA curve of juice with the best thickeners, the effect of different bacteria on the yoghurt, different thickeners added to the yoghurt was studied. The purpose was to make yoghurt with a stable fruit juice layer.Firstly, the best processing parameters for yoghurt making were studied by analyzing the rheology curve of yoghurt. The viscosity of yoghurt could be increased by homogenization. The best homogenization pressure was 20~30MPa. The degree of denaturation of whey protein was mainly influenced by heat treatment. The best temperature/time treatment was 85℃for 20~25min or 90~95℃for 5~10min. The product was uniform, with a good texture and low degree of syneresis. However when the time of heat treatment was prolonged, yoghurt smelt the taste of braise and when the time was shortened, the denaturation of whey protein was less, making the yoghurt frangile.Secondly, the addition of fruit juice and its effect on the yoghurt TPA was studied. Using viscosity, WHC, stability and the degree of the interface as the main indices, the results showed that the concentration of juice 20%,CMC: Gelatin 1.8%(3:1~2:1) or modified starch: Carrageenan 2.5% (10:1~7:1) or xanthan gum:Carrageenan 1.0%(4:1~3:1) in the fruit juice were good,but modified starch and Carrageenan was the best.Finally, the effect of the bacteria and thickeners on the yoghurt was studied. Using WHC, STS and the stability of products as the main indices, the result showed that the best stabilizer was a combination of Gelatin and Modified starch (1.2%) in the ratio 1:2. Compared to the yoghurt without thickeners, hardness and viscosity were improved but cohesiveness and resilience didn't change.