Controlled drying of shelf-stable cheese
Marcy is a professor in the food science and technology department at Virginia Polytechnic Institute and State University in Blacksburg, Va. He has worked extensively on issues relating to food packaging and is a former business development manager at Rampart Packaging, now a division of Printpack. Marcy also is involved in food-processing projects and is affiliated with the Center for Advanced Processing and Packaging Studies at North Carolina State University in Raleigh, N.C. Food Engineering recently spoke with Dr. Marcy to learn more about this new aging technique.
FE: What is the conventional method for producing shelf-stable Parmesan cheese?
Marcy: Current drying techniques typically use warm air at about 30 C degrees (86 F degrees) under low relative humidity for a few minutes. The cheese wheels are grated and the cheese placed on a warm a fluidized bed dryer. The moisture content of aged Parmesan is 30 to 32 percent, and water activity (Aw) is about 0.90. Processors typically target a moisture content of 17 to 18 percent and Aw of 0.75. Unfortunately, when they remove moisture, they also remove aroma and flavor compounds during dehydration.
The most resistant pathogen of concern is Staphylococcus aureus, and conventional wisdom holds that the risk of Staph is eliminated by reducing water activity to 0.75. One upshot of the research was that we determined that Aw 0.86 was sufficient to render Parmesan cheese safe, while at the same time improving yield. That finding could likely change conventional drying methods.
FE: How does the alternative drying method work?
Marcy: To prevent loss of valuable flavor and aroma compounds, we sought to develop a very slow moisture-removal process. With most food products, the objective is to get it into a dry state as quickly as possible. With many cheeses, the aging time is specified in the standards of identity. In the case of Parmesan, a 10-month period is specified, and if you speed up the process, you can't call the finished product Parmesan.
We devised a set of experiments to determine the best time to remove moisture from the cheese. Not too surprisingly, that turned out to be the final two- to four-week period. We placed a polymer coating over the wheels of cheese to arrest water loss during the beginning and middle portions of the process, while allowing water to migrate out toward the surface. We then removed the film and placed the cheese in a chamber held at 45 to 50 F degrees.
When we first tried the process, there was some mold growth on the wheels. None of the films we tested gave us the moisture vapor transmission rate we needed. So we filled the chamber with nitrogen. That serves as an inert environment to prevent mold growth.
FE: Are there any notable features to the drying chamber?
Marcy: I built the chamber myself by adapting a polypropylene vessel that we had available. There is no material requirements other than the ability to be able to recharge the desiccant if needed, the ability to control the chill temperature to 45 to 50 degrees, and the ability to control humidity. Those three parameters describe a lot of storage systems for fruits and vegetables. It's up to our industry partners to tell us what is more functional for them: a gas-tight chamber in the case of an artisan cheesemaker, for example, or a large room or holding area that's filled with nitrogen for large-scale commercial processors.
FE: How does a processor know when the optimum moisture content has been achieved?
Marcy: We developed a sorption isotherm chart that documents the relationship between the percent of moisture in the wheel and water activity. Water activity is the vapor pressure in food relative to pure water at that temperature. By referring to the sorption isotherm, you can determine the pounds of water that have to be removed from a given wheel, then make a quick calculation of what the weight should be when it is removed from the chamber. The control mechanism then becomes the scaled weight, which is monitored from outside the chamber until Aw 0.86 is achieved.
FE: What are the economics of this process?
Marcy: Humidity control, the chamber itself and the cost of the nitrogen are the negatives. On the plus side, you don't have the expense of dehydration, which is fairly significant, the quality and aroma of the finished product is as good as fresh Parmesan, and the process results in yields of approximately 12 percent greater than conventional methods, which is significant. You can buy a lot of nitrogen at that rate.
If I can come up with a material that would coat the cheese and prevent mold growth while at the same time allowing moisture to migrate through the cheese and the coating at the desired rate, we could do away with the nitrogen chamber entirely. We haven't found such a coating yet, but I'm still seeking one.
FE: What's the next research challenge?
Marcy: This process would make sense for Romano and other cheeses that currently go through a drier to make them shelf stable. One of the surprises in the early research was that Romano is more pathogen-resistant than Parmesan, which means an even higher Aw could be targeted.
As for the aging process, that is a highly complex process and the reason I needed assistance from my colleagues at the Center for Dairy Research. Proteins, acid bacteria, salts and other components are reacting to one another during the process. We knew that if we removed water early in the process, the end product would be different and that different would not be better; when you change the equilibrium in the cheese's development, you significantly alter the end result. That's why no one knows how to make the perfect cheese. You can only repeat processes that were successful before, and some people control those processes better than others, which is why cheese quality varies.