With Helpline, the readers of Process Heating have a chance to solve problems that crop up in their heat processing operations.

I am trying to dry cheese using a convection dryer. With the present system, the cheese dries too quickly, skinning over and locking in moisture. How can I prevent this? What should I adjust on the dryer? -- C.J.

The cheese is skinning over because moisture is leaving the surface faster than it can migrate from the interior. A crust is formed and can severely impede moisture loss. Your three choices are reduce air speed, reduce the temperature or raise the relative humidity.

The water activity (AW) of the cheese should be checked. AW is a measure of a substance's vapor pressure relative to the vapor pressure of pure water and will be between 0 and 1. AW times 100 equals equilibrium relative humidity, or the humidity at which moisture will not move into or out of the product. A relative humidity about 5% below this will help produce the fastest rate of moisture loss. Drying rooms for producing dry sausage (which have a similar problem with crusting) generally use temperatures in the range of 50 to 60oF (10 to 15.6oC); relative humidities of 85% at the start of the process to 65% at the end; and air speeds of 4 air changes/min. - Tim Crawshaw, GoodMark Foods, (919) 772-1511, ext. 235.