Rheological characterization of coffee-flavored yogurt with different types of thickener/Caracterizacao reologica de iogurte sabor cafe com diferentes tipos de espessante

Yogurt is a functional food that has great demand due to the consumer's search for a healthier diet. In order to expand the consumer market of this product, many flavors are available, satisfying the most varied preferences. Besides the taste attribute, consistency and viscosity of yogurt are some of the main factors involved in product quality and acceptance. Therefore, this work is a study of the influence of concentration of thickener in coffee-flavored yogurt. The thickener agent used was gelatin. The rheological behavior (flow and viscosity curves) of yogurts with and without addition of gelatin was compared with commercial yogurt, which contains another type of thickener (locust bean gum) in its formulation. The flow and viscosity curves were obtained from rotational rheometer Thermo Haake Mars, with a range of shear rate from 0.02 to 100 [s.sup.-1] (rising curve) and 100 to 0.02 [s.sup.-1] (descendent curve) at a total time of 20 minutes. Hysteresis was determined as the area between the curves and adjusted to the models of Bingham, Casson, Herschel-Bulkley and Ostwald-de-Waele. Were also carried out tests of thixotropy, by measuring the viscosity as a function of time at a constant rate of 100 [s.sup.-1] for 10 minutes. These curves were adjusted by the Weltman model. All samples showed pseudoplastic and thixotropic behavior. The Herschel-Bulkley model was the best fit to the three samples tested. The Weltman's model well described the thixotropy tests, except for the sample of commercial yogurt. The use of gelatin as a thickener showed protective character, reducing the structural break of the gel. KEYWORDS: Rheological models; pseudoplastic behavior; gel stability; viscosity. Iogurte e um alimento funcional que apresenta grande demanda devido a busca por uma vida e alimentacao mais saudaveis. De forma a ampliar o mercado consumidor, diversos sao os sabores existentes, satisfazendo as mais variadas preferencias. Alem do sabor, a consistencia e viscosidade do iogurte sao uns dos principais fatores envolvidos na qualidade e aceitacao do produto. Este trabalho e um estudo da influencia do tipo de espessante em iogurte sabor cafe. O espessante utilizado foi a gelatina. O comportamento reologico (curvas de fluxo e de viscosidade) dos iogurtes com e sem adicao de gelatina foram comparados entre si e com iogurte comercial, o qual utiliza outro tipo de espessante (goma alfarroba) em suas formulacoes. As curvas de fluxo e de viscosidade foram obtidas em re