Castle Importing, a company with a cheese plant in Fontana, Calif., chose to use a fluid bed dryer to change how it dehydrated cheese and to maintain quality standards. The dryer, made by Ventilex USA, West Chester, Ohio, was chosen based on production capacity, initial moisture content, drying characteristics and ease of use.
Advantages of fluid bed dryers in food dehydration are that they offer both batch and continuous modes, can operate in large- and small-scale operations, have few moving parts, accomplish rapid heat and mass transfer rates between the product and the drying medium, and achieve rapid mixing of solids -- which leads to nearly isothermal conditions throughout the fluidized layer. They use hot gas, usually air, which passes through the food.
Castle's dryer can process grated cheese at a rate of 4,500 lb/hr. Its mozzarella cheese, for example, has a high fat content and moisture content of 50 percent before drying. It enters the dryer frozen at 18oF (-7.7oC) and is transported by a vibration system using air bellows to control the frequency and amplitude. One problem with this type of product is that if the temperature became too high, the fat would drain from the cheese and it would never dry.
The initial air temperature of the dryer is 100 to 104oF (37 to 40oC). A third of the way into the dryer, the cheese is thawed and begins to dry. Temperature is monitored by RTDs until the cheese reaches between 72 to 74oF (22 to 23oC) while drying takes place in the mid-section of the dryer. The temperature controller is set to only allow a 2oF differential and the Ventilex dryer regulates the air's relative humidity between 15 and 18 percent. This precise temperature and humidity regulation ensures that the fat will not separate from the cheese and that proper drying occurs. The last stage of the dryer is a cool-down section using an air temperature of 42oF (5oC). The final product leaves the machine with approximately 27 percent moisture content and is refrozen.
The residence time on Castle's dryer is adjustable from 15 sec to 2 hr, and the frequency can be adjusted from 100 to 250 rpm. A computerized PLC control system with a graphical user interface allows a technician to easily check on the drying conditions at any time.