109-4 |
|
J. A. LUCEY, Dept. of Food Science, Univ. of Wisconsin, Madison, 1605 Linden Dr., A-19 Babcock Hall, Madison, WI 53706-1519 Yogurts are a growing dairy segment in the US, especially for children as well as health conscious consumers. Having the correct textural properties of yogurt is critical for consumer acceptability. There has been substantial progress on understanding yogurt texture especially using dynamic oscillatory rheology to monitor the gelation process. During fermentation casein micelles aggregate and form a weak gel network. The main factors that determine the rheological properties of yogurt include: milk heat treatment, incubation temperature, acid development, total solids (protein and fat) contents and type and concentration of stabilizers. High milk heat treatment causes whey protein denaturation and denatured whey proteins interact with casein micelles. This results in a higher gelation pH and a stiffer network. Loss of colloidal calcium phosphate from casein particles, that are part of the gel network, increases the susceptibility of the network to rearrange. Yogurt gel networks are very dynamic and rearrangements continue during and after gelation; excessive rearrangements of the network are the cause of defects such as wheying-off. The common industrial practice of using high incubation temperatures result in weaker gels and more wheying-off. A possible mechanism to explain the causes of this wheying-off defect will be discussed. Optimum gel stiffness is attained around pH 4.6 and increasing the solids content leads to increased firmness and viscosity. Excessive usage levels of whey proteins can contribute to grainy defects and result in a shorter yogurt texture. Breakdown of gels by shearing is a key part of the manufacture of stirred yogurt. Yogurt gels exhibit shear thinning during stirring but rebuild structure to some extent during storage. The breakdown process is poorly understood and this hampers our understanding of factors that control the final viscosity of stirred yogurt products. Some approaches to studying the breakdown of yogurt structure during shearing will be discussed.
Session 109, The rheology of dairy foods
|