For purposes of this discussion, ready-to-drink (RTD) beverages will refer to that group of protein-fortified beverages that are shelf stable, i.e., do not require refrigeration for storage and transportation. RTD beverages all have one thing in common no matter how they have been processed. Microbiological assays on RTD beverages yield values of zero or none detected. A properly manufactured RTD will, for all intents and purposes, be commercially sterile until the package is opened. Obviously, the common method used to achieve these minimal microbiological levels is heat treatment. It is safe to say that RTD beverage pasteurization heat treatments surpass the heat thresholds required to denature whey protein fractions, whether the RTD was manufactured via retort or Ultra High Temperature (UHT) pasteurization followed by aseptic packaging. Therefore, whey proteins (WPC and/or WPI) are not the best protein ingredients for RTD beverages because they will denature and lose solubility when exposed to the rigorous heat treatments required to manufacture a shelf-stable RTD.
The first wave of high protein RTDs that came on the market were all manufactured using a high level of WPC or WPI because the technology for stabilizing casein micelles against high heat conditions wasn’t as advanced as it is today. Initial trials to manufacture RTDs containing casein ended with the RTDs forming irreversible gels just days after processing. To avoid the casein gelation, early RTD formulators only used WPC or WPI for protein fortification. The intense heat treatment, however, would cause the whey proteins to denature and lose solubility. When whey proteins denature, they form extremely fine flocculants that are not readily observable without sophisticated analysis. It takes days or weeks before one can visibly detect a settling out of denatured whey proteins. Such was the case with WPC RTD beverages; they initially appeared to have survived the heat treatment but days to weeks later, the whey proteins were settling out to the bottom of the RTD packaging or floating to the top of the beverage. Formulators, therefore, had to add stabilizers to maintain the fine particle denatured whey proteins in suspension in the beverage. Even with the addition of stabilizers, RTDs made with whey protein had poor shelf life, with most falling apart in just a few months.
MPC, on the other hand, is heat stable, even at UHT pasteurizing temperatures and one can manufacture RTDs with excellent shelf life using MPC for protein fortification. Since MPC is primarily casein based, it possesses excellent heat stability compared to WPC. The casein in MPC, however, is in micellar structure and exposure to high heat conditions can initiate casein micelle aggregation which, if unchecked, will lead to the irreversible formation of a gel in the RTD. In many cases, these gels appear within days or weeks after processing. Fortunately, there is a known way to retard casein micelle aggregation caused by heat exposure. Addition of a small amount of complex polyphosphate to the RTD will slow down heat stimulated casein micelle aggregation to the point where RTD beverages containing MPC remain stable for as long as 18 months after processing. MPC is now the preferred protein fortification ingredient for RTD beverages.