The Study Of The Texture And Microstructure Of Set-plain Yogurt

There are always some problems in the producing, storing and transporting of set-plain yogurt,for example, the separation of whey that may affect the quality of the yogurt intensively. The mainpurpose of this study was to investigate the effect of some factors such as thickeners applied,fermentation temperature, ultrasonic treatment, condition in transportation and storage on thequality of set-plain yogurt. Scanning electron microscopy (SEM) was used here to confirm therelationship between microstructure of set-plain yogurt and its texture.The effect of different thickeners on the quality of yogurt was studied. It was found that when0.1% thickener, such as pectin, gelatin, acid-resistant carboxymethyl-cellulose or sodium alginatewas added to yogurt, the rigidity, viscosity and cohesiveness of yogurt might increase respectively.Meanwhile addition of carob gum, guar gum, konjac gum and carrageenan might decrease theseparameters. Only did gelatin and pectin decrease the whey separation of yogurt. As the additionlevel of thickener was up to 0.15%, the influence of gelatin and pectin on the quality of yogurtbecome much obvious, opposite to that of sodium alginate, acid-resistant carboxymethyl-celluloseand guar gum. If two thickeners were mixed at selected ratio, it was found that mixing ofcarrageenan and carob gum had a beneficial effect to improve the texture of yogurt. Also is themixing of pectin and sodium alginate. Mixing of gelatin and pectin showed an adverse effect on thequality of yogurt, because rheological properties and water retention of yogurt were all.It was found that lower of the fermentation temperature might decrease the rigidity, viscosityand cohesiveness of yogurt at different level, but the improve the water retention slightly. Also, pHchange for yogurt samples with ultrasonic treatment after inoculation had significant differencecompared to control. For samples with ultrasonic treatment at high power level before inoculation,pH might decrease much quickly, and less fermentation time was needed than control. Comparisonof the texture analysis results, the rigidity, viscosity, cohesiveness and water retention of yogurtwith ultrasonic treatment at moderate or high power level were all improved than that of control.On the other hand, yogurt with ultrasonic treatment at low power level had no influence on itstexture.The effect of transportation and storage conditions all influenced the quality of set-plainyogurt. It was found that the rigidity, viscosity and cohesiveness of yogurt sample decreased andsome whey might separate out after continuous shaking. During cold-storage, the rigidity andviscosity of yogurt increased and whey separation decreased, but the cohesiveness increased early and decreased later.SEM was employed to study the microstructure of yogurt. It was found that a continuous,uniform and compact microstructure existed in yogurt might lead to the improvement inrheological properties and whey separation. Otherwise, a discontinuous, friable and loosemicrostructure might adverse to the quality of yogurt. Experimental result showed that the gelatinimproved the microstructure of the set-plain yogurt obviously. On the other hand, locust bean gumhad no beneficial effect on the quality of yogurt.