Study On Preparation Of Gel Yoghurt And Its TPA And Rheological Characteristic

Gel yoghurt is made of stir yoghurt with gelatin, and the bacteria in it are viable, so the nutritional value of yoghurt remains the same. The stable gel structure can avoid the products'texture breaking during transport. The process technology of gel yoghurt was studied, and the gelatins that are suitable for producing gel yoghurt and their addition amount were selected. The TPA, rheological characteristic and shelf life of the gel yoghurt was studied, and a method which can evaluate the quality system of gel yoghurt was established.Firstly, the proportion of bacillus and coccus addition (1:2) was determined by activity measurement and sensory analysis. Influence of several key-processing parameters on sensory quality was studied by orthogonal experiment. The sequence of key-processing parameters from major to minor (for adding gelatins before fermentation) was degree of sterilization, fermentation end-point and speed of mixing; the optimum values of above mentioned processing parameters were 95℃/5min, 800T and 6000r/min respectively. Similarly, the sequence of key-processing parameters (for adding gelatins after fermentation) was temperature of adding gelatins, ferment end-point, degree of sterilization and pressure of homogenization; the optimum values were 50℃, 800T, 95℃/5min and 20MPa respectively. The technology of adding gelatins after fermentation is determined by sensory analysis at last.Secondly, the action of hydrocolloids on yoghurt gel was studied. Index on gel time and gel strength - the effect of single-factor test indicated that agar, gelatin, sodium alginate and low methoxyl pectin could gel yoghurt effectively. Index on gel strength and water holding capacity - adding gelatin and agar or low methoxyl pectin and high methoxyl pectin could improve the quality of gel yoghurt. The amount of gelatin and agar were 1.2% & 1.3% and their proportions studied were 7/1, 6/1 and 5/1 & 7/1 and 6/1 respectively were chosen as the better ones. The amounts of low methoxyl pectin and high methoxyl pectin were 1.4% & 1.6% and their proportions were 6/1 and 5/1 & 7/1 and 6/1 respectively were chosen as the better ones.Thirdly, the TPA, rheological characteristic and microstructure of gel yoghurt were studied. The TPA parameters for various gel yoghurt changed with different types and amount of hydrocolloids. The elastic modulus G'and viscosity modulus G''of gel yoghurt increased when time was longer, but as G'went faster; G'increased and G''decreased when frequency was high. The system viscosity decreased when shear stress reinforced, and it appeared thinned when sheared. The more compact texture was formed in gel yoghurt, seen by SEM photo, and it made the system more stable.Finally, the weight for color, flavor and texture of gel yoghurt were 0.312, 0.347 and 0.341 through combination weight method by statistical analysis of sensory evaluation data. A method was established to evaluate the quality system of gel yoghurt by using subjective index and objective indices: gel strength, susceptibility to syneresis and water holding capacity. Gel yoghurt should be conserved in low temperature, and its acidity is 133.30T and viable count is 1.1×106cfu/ml after 3 weeks. The sample first week's STS and WHC changed small just like gel strength, but they changed more quickly after this.