Dairy Pipeline What's Inside: University of Wisconsin—Extension College of Agricultural and Life Sciences Wisconsin Center for Dairy Research A Technical Resource for Dairy Manufacturers Winter 2000, Volume 12 Number 1 by Mark Johnson, CDR Melted cheese— it’s the essential ingredient in nachos and pizza and the soul of a grilled cheese sandwich. But what do we really mean when we say that cheese melts? A chemist will tell you that a substance melts when it changes from a solid or crystalline state to a liquid. A food scientist will describe melt as the ability to flow or spread. Generally, we think of cheese melting from high heat, perhaps during baking or grilling. However, some cheeses, like ripened Camembert or Limburger, actually melt or flow at refrigeration temperature. Whether or not a cheese melts or stretches, and how well it melts and stretches, depends on the chemistry and thermodynamic properties of its casein network. Sound complicated? Well, it is! But most cheesemakers are already influencing the chemistry and physics of cheese melt and stretch when they adjust the following factors: milk heat– treatment, pH during cheese manufacture, cheese composition, the lowest pH obtained in the cheese, and proteolysis, or the breakdown of intact casein. Stretch is the ability of the casein network to maintain its integrity and not break when pressure is applied to the cheese. Observing the elasticity, or stringy behavior, of Mozza- rella baked on a pizza is a simple way to see stretch. Stretch is also responsible for the blisters that form when cheese is baked. Stretch is most often associated with high temperatures but, like melt, it can occur at much cooler temperatures. Stretch is necessary for developing round eyes in Swiss cheese. Whether you produce a Swiss cheese with round eyes or slits will depend on the stretch or pliability of the casein network, the rate of gas formation during the warm room treatment, and the temperature of the cheese. Similarly, the tempera- ture of the cheese and the rate at which the cheese is stretched determines the elongation of cheese on a baked pizza. Casein interactions The melt and stretch properties of cheese are based on the number of interactions between casein molecules, the major milk protein. The fewer the interac- tions, the greater the melt. Stretch requires an intact, interconnected casein network and is lost as the interactions between casein molecules (or aggregates) decrease. Stretch is the result of casein–casein interactions that are broken easily but also readily reform at different locations in the casein network. Think of holding a piece of warm Mozzarella, take one end in each hand and gently pull it apart. The casein molecules are grabbing and releasing each other while sliding past as you pull the cheese. There are several factors we need to review to understand the casein to casein interactions. Each The Melt and Stretch of Cheese next page The Melt and Stretch of Cheese ................................... 1 Mechanics of Membrane Processes .................................... 6 News from CDR ................................................................... 8 Curd Clinic ......................................................................... 10 Calendar ................................................................ Back page