Determination of Yogurt Quality by Using Rheological and Textural Parameters Tulay Ozcan  Uludag University, Department of Food Engineering, 16059 Gorukle, Bursa, TURKEY Abstract. Sensory and textural parameters of yogurt are known to have an important impact on food quality and acceptability. The emphasis in this article is to summarize the rheological and textural parameters of yogurt. Viscosity, hardness, adhesiveness, cohesiveness, and springiness are considered the most important descriptors for the textural perception of yogurt. Factors affecting textural properties of yogurt and the possible defects are also discussed in this paper. Keywords: Yogurt, rheology, texture 1. Introduction Yogurt is a very popular functional dairy product which is produced by acid fermentation of milk with the thermophilic homofermentative lactic acid bacteria Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus. The manufacturing processes of yogurt differ depending on the country, but it always comprises a lactic fermentation that brings milk to gelification due to destabilization of the protein system. It is normally retailed in one of the three physical states, namely set (undisturbed gel in the retail pot), stirred (the acid gel formed during incubation in large fermentation tanks is disrupted by stirring) or fluid (drinking yoghurt). Most of the basic plain yogurts are set yogurts, which are fermented in the container that they are eventually sold in, while most fruit-flavored yogurts are stirred yogurts, because flavors and fruit added after fermentation need to be properly dispersed in the yogurt matrix (Tamime and Robinson, 1999; Chandan, 2006; Tamime 2006). 2. Rheological and Textural Parameters of Yogurt Texture is one of the most essential components of yogurt quality. Texture, which comes from Latin textura (cloth), is a vocable used initially to describe the cross-linking style of weave threads. It now defines disposition and arrangement for the different parts of a system. For food products, according to the International Standard Organization, texture represents all the rheological and structural attributes perceptible by means of mechanical, tactile, and, when appropriate, visual and auditory receptors. Rheology and structure of a product evaluated by instrumental methods also give relevant information on its textural properties (Ramaswamy and Basak, 1991; Borwankar, 1992; Foegeding et al. 2011; Fisher and Windhad, 2011). The texture of yogurt or any type of fermented milk product is important with regard to the quality of the products. It is related to sensory perception of food product. The textural characteristics of yogurt are generally studied in the cup, using a spoon, or in the mouth by sensory methods. The most common sensory attributes relating to yogurt texture are thickness /viscosity, smoothness (opposite to lumpiness, graininess, grittiness), and sliminess (or ropiness). Determination of the yogurt texture usually includes sensory,  Corresponding author. Tel.: +90 224 294 1498; Fax: +90 224 294 1402. E-mail address: tulayozcan@uludag.edu.tr. 2013 2nd International Conference on Nutrition and Food Sciences IPCBEE vol.53 (2013) © (2013) IACSIT Press, Singapore DOI: 10.7763/IPCBEE. 2013. V53. 23 118