The structure formation of yogurt gels made from bovine pasteurized milk of various fat contents (0%, 1.5% or 3.5%) and prepared using natural yogurt as a starter was monitored by measuring the rigidity modulus (G) development by means of a dynamic U-tube rheometer of novel design. The results indicated that both the heating pretreatment of the milk and its fat content significantly affect the rate of gelation as well as the final rigidity attained by the gel. That is, the higher the heating temperature employed and the higher the fat content was, the faster the onset gelation appeared to be and the more rigid was the gel network formed. The shape of the gelation curves of yogurt samples made from milk, either with or without fat, heated at 80, 85, 90 or 95 degrees C for 1 min and fermented at 45 degrees C, showed three distinctive parts indicating that the yogurt gelation process took place in three consecutive stages. The results are discussed in the light of a proposed mechanism which could explain the above mentioned findings. (c) 2007 Elsevier Ltd. All rights reserved.