Yes, depending on the amount of heat exposure, heat can cause properties of an MPC to change. That does not mean that heat will always cause changes in MPC properties. At standard food processing temperatures and hold times (example: 162°F for pasteurizing), MPC will undergo negligible to slightly perceptible changes. At higher temperatures above 180°F (82°C), however, changes to MPC properties will be more noticeable.
There are two ways in which heat can affect MPC:
- Denaturation of the whey proteins present in the MPC
- Accelerating calcium-casein reactions
Excess heat exposure can cause:
- Decrease of protein solubility
- Decrease in emulsion stabilizing
- Change in organoleptic properties in food and beverage applications—texture and flavor
- Increase in aqueous viscosity
Typically, standard food processing temperatures and hold times, such as standard dairy pasteurization temperatures, will result in a small amount of whey protein denaturation. Such a small degree of whey protein denaturation would have a negligible to slightly perceptible effect on MPC properties. A slight drop in protein solubility and whey protein solubility might occur. It is highly doubtful that even a trained technician could detect significant differences in MPC properties if the MPC is only subjected to usual food processing conditions of less than 165°F (74°C) for seconds.
At higher processing temperatures, increasing percentages of the whey proteins in MPC will heat denature. As the whey proteins heat denature, they become less soluble, even precipitating out under certain circumstances. The loss of increasing whey protein solubility can affect emulsion stabilization and organoleptic properties of the MPC. If the whey proteins become so insoluble that they precipitate out of solution, food texture can be adversely affected. Extreme denaturation of whey proteins is almost always accompanied by a release of sulfur. Flavor and aroma characteristics of food products can be adversely affected as the sulfur is released.
If, on the other hand, a protein application calls for ultra-high heat (ultra-high temperature) exposure, then significant changes will occur with dairy proteins. At typical UHT conditions, the whey proteins in the MPC will be mostly heat denatured. Due to the processing conditions, however, they do not adversely affect texture and have only a slightly perceptible effect on flavor and aroma. While many experts will advise that casein does not denature at temperatures below 400°F, casein micelles will undergo some changes at UHT processing conditions. As an example, in UHT processed ready-to-drink (RTD) beverages dairy proteins are usually processed at temperatures well above 290°F (143°C) and are held at such temperatures for a few seconds to sterilize the beverage. Immediately after UHT processing, it appears that no changes occurred in the MPC (except for the obvious denaturation of almost all of the whey proteins). After several days, however, changes begin to occur in the beverage. The viscosity increases and eventually, the beverage turns into a gel or a precipitate forms. While the heat exposure did not cause a disruption of the casein micelle structures in the MPC, the heat exposure did change the “character” of the casein micelles. In simplistic terms, typical casein micelles in milk will contain about 90% of the total milk calcium. Much of the calcium present in the micelle is bound calcium. A portion of the micelle calcium, however, is not bound (free calcium) but is instead “trapped” inside the structure of the micelle. As heat is added to the system (heat = energy), the chemical bonds of the micelle “stretch” and the micelle expands in size. As the micelle expands in size, the free calcium inside the micelle is liberated to migrate to the outer surface of the micelle. Once at the outer surface, the calcium helps to change the ionic character of the micelle surface. Micelles start reacting with other micelles, agglomerating until their aggregated size is so large that they become less soluble or form a gel matrix. For this reason, UHT processed RTD beverages almost always contain a small amount of complex phosphate salts to prevent the micelles from agglomerating after UHT treatment.