Product Surface Temperature

T0 is greatly affected by Tp, the heating plate temperature; higher Tp cause higher T0. Usually, Tp is rapidly ramped up to a set value and then held there until sublimation is complete. If so, T0 initially will be only slightly higher then the condenser temperature. Then, as drying proceeds and the sublimation interfaces recede, that is, Z increases, T0 progressively rises. T0 can be calculated as a function of Tp during sublimation by relating the rate of radiant heat absorption by the food to the rate at which heat is conduc-tively transferred through the ice free layers. Equation 2 and the vapor pressure versus temperature relationship for ice also have to be used in carrying out this calculation.

To prevent thermal damage, T„ must be kept below a specified upper limit or limits that depend on the product being dried. A fixed upper limit, such as 40°C for coffee extract, is usually used. Upper limits that depend on exposure time are used occasionally; higher T0 is allowed if the exposure time is short. Plate temperatures and drying rates must be reduced if excessively high T0 develop as Z increases. During the desorption phase of drying, plate temperatures are usually progressively reduced to prevent T0 from rising excessively.

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