Particulate Products

Most foods are freeze-dried in particulate form. Heat and mass transfer during the freeze drying of particles is more complex than for the freeze drying of slabs. Sublimation occurs beneath all exposed surfaces, but is most rapid where temperature is greatest—at ice-vapor interfaces closest to the heating plate. Although sublimation takes place over a greater depth of product, the required drying time remains proportional to L2 (the product bed depth squared). Because vapor flows readily through spaces between particles, trays with solid bottoms can be used without interfering with drying in lower parts of the load. Because ts depends on L2, thicker parts of particulate loads can take markedly longer to dry than parts of average thickness. Therefore, care must taken to load trays uniformly.

Excessive amounts of fines were produced when slabs of freeze-dried extract were ground to provide granules for consumer use. To prevent this, frozen coffee extract is ground to produce granules of desired size before drying and freeze-dried in granule form in shallow beds in trays. Fines produced during grinding are removed by screening and recycled by mixing them with extract about to be frozen. Vapor-flow resistance during freeze drying of extract granules is much smaller than for slabs, but heat-transfer resistance is greater. In drying extract granules, heating plates are often maintained at 130°C while sublimation occurs. Yet, because of low pressure drop, Ts is only slightly higher than the condenser temperature during this period. After sublimation is complete and desorption alone occurs, plate temperatures are gradually reduced to 50°C.

Excessive product loss caused by granule entrainment in high-velocity vapor used to occur when coffee extract granules were freeze-dried, but now, in modern, properly operated, well-designed dryers, losses are now less than 0.1%.

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