The sublimation heat is not needed at the product surface, but at the boundary of the ice core that is withdrawing into the centre of the product as drying proceeds. Whilst the flow of water vapour is from within the product to the outside, the transfer of heat must be accomplished in the opposite direction from the outside to the inside. Due to the low thermal conductivity of the dried product layers, the temperature gradient required for heat transfer steadily increases. To avoid damage to the product, the maximum admissible temperature for the dried product must not be exceeded. On the other hand, care must be taken to maintain the required sublimation temperature throughout drying, keep the heat supply to the ice-core boundary in equilibrium with the heat requirement at that particular location, and avoid any overheating of the sublimation zone. The primary drying phase continues until all the ice contained in the product has been sublimated.