Alter the meltability of cheese.

Milk proteins provide the structural framework and determine the texture in many dairy foods, including cheese and yogurt. Casein aggregation (caused by rennet activity), gelation, syneresis and particle-curd fusion are the critical steps involved in the structural development of cheese.

Two recent studies by scientists at the University of Wisconsin highlighted the importance of the state of the casein micelles during the initial rennet gelation stage, and show how they affect the functional properties of cheese. The researchers' insights provide cheesemakers with different approaches they can use to alter the initial meltability of their cheese. A greater understanding of the causes of yogurt defects like whey separation will enable processors to develop strategies for improving the quality of their products.

Calcium phosphate is an important structural component in micelles. Its partial removal with calcium-chelating agents results in weak rennet-induced gels. However, cheeses made from calcium-depleted milks have increased meltability. In yogurt manufacture, milk undergoes a very high heat treatment to cause whey protein denaturation and its association with casein micelles. During yogurt fermentation, the presence of denatured whey proteins attached to caseins results in an increase in the gelation pH, as well as increased gel stiffness and viscosity.

Rheological studies indicate that yogurt gels undergo an increase in the loss tangent, or their viscous nature, after measurable gel formation occurs. This transition is related to an increase in the degree of whey separation. This maximum in loss tangent is caused by the solubilization of insoluble calcium associated with caseins after the gel forms. The chelation of insoluble calcium before yogurt gelation can remove this maximum.

Essentially, after the yogurt network is formed, some of the internal cross-links within the milk proteins (caseins) slowly dissolve due to ongoing acid development. These cross-links are made of calcium phosphate. The dissolving of these internal protein cross-links causes the gel to become more flexible and prone to shrinkage (and wheying-off). Removal of some of these calcium phosphate cross-links prior to yogurt fermentation, e.g., by the use of calcium chelating agents such as citrates, can be exploited as a method to modify wheying-off.

Further information. John A. Lucey, Department of Food Science, University of Wisconsin, A203A Babcock Hall, 1605 Linden Dr., Madison, WI 53706; phone: [phone]; fax: [phone]; email: [e-mail].