thin layer of the food product on the heated drum dries very quickly into thin layers. These layers
are further ground to obtain powders. This method is not typically used for milk b/c the solubility
of milk powder tends to be low if drum dried.
freeze-drying: food is first frozen, then is placed in a vacuum where the ice is evaporated via
sublimation (passes directly from a solid to a vapour w/o passing through liquid state). This is
the most effective method of drying food b/c food is not exposed to high heat and
therefore more vitamins remain intact. The food also retains its original shape. However, this
method of drying is also more expensive.
Problem with heat drying:
• a problem with drying by heat is the loss of food’s heat-sensitive nutrients such as vitamin C,
riboflavin and niacin. Heat can also alter the foods flavour, colour and structure.
Pretreatments: some foods need to be pretreated before drying.
1. Physical: some fruits, such as pumps have their skin exposed to lye or hot water to
crack the skin before drying. This shortens the drying time and improves the texture of the end
product (in the case of plums, the prune).
2. Blanching: some fruits and vegetables (e.g.: carrots and potatoes) are blanched
prior to drying to deactivate enzymes that cause browning.
3. Chemical: some vegetables and fruits (e.g. apricots and bananas) are dipped in
sulphate solution or exposed to sulfur dioxide gas prior to drying to prevent browning and to
protect against the loss of vitamins A and C. However, these products destroy thiamin and can
cause allergic reactions in some people.
2. THERMAL PROCESSING
the important heat treatment methods used in processing include sterilization, pasteurization,
blanching and canning. Thermal processing can cause a loss of heat-sensitive nutrients.
complete destruction of microorganisms through exte