In drying, the object also is to remove water with minimum damage to the food.
Whereas evaporators will concentrate foods twofold or threefold, driers will take foods very close to total dryness in many cases less than 2% or 3% water. Driers are used to prepare such well-known products as dried milk powder and instant coffee. Although food traditionally has been dried to preserve it from spoilage and to reduce its weight and bulk, some foods are dried as convenience items and for their novelty appeal; an example has been freeze-dried fruits for cereals. Drying, as well as several of the other unit operations, is treated in detail in later chapters, and so only a few brief comments are needed here.
Liquid foods such as milk and foods in chunk form like shrimp or steak may be dried. It is generally much easier to dry liquid foods because these are easier to subdivide, either as a spray or a film, and in a subdivided form, the moisture can be removed more quickly.
Subdivision of a liquid is the principle behind the widely used spray drier (Fig. 5.10).
Liquid food such as milk, coffee, or eggs is pumped into the top of the large tower, at which point the liquid is atomized by a spray nozzle or equivalent device. At the same time heated air is introduced to the tower. The heated air in contact with the fine droplets of food dries the droplets, and dehydrated particles fall to the bottom of the
Tower and are drawn off into collectors. The moisture removed during drying is exhausted separately. Most commercially dried liquid foods are made this way.
Drying by subdividing food as a thin film is commonly done on a drum or roller drier of the kind illustrated in Fie. 5.11. The drum is heated by steam from within,
and the applied laver of food flashes off its moisture on contact with the heated drum.
The dried food is then mechanically scraped from the drum with long knives. Mashed
Potatoes, tomato puree, and several milk products are frequently dried this way.
Small food pieces such as peas and diced onions can be dried by moving them through a long tunnel-oven, and many types are in use. However, overheating and shrinkage in the course of water removal may give poor quality products in the case of particularly many kinds of vacuum freeze-driers. In all types the food pieces first are frozen and then dehydrated under vacuum from the frozen state. The ice dos not melt but under the conditions of high vacuum goes off directly as gaseous water vapor, a process known as sublimation. This very gentle kind of drying protects all food quality attributes such as texture, color, flavor, and nutrients. Freeze-drying is by no means restricted in its use to solid and particulate foods. Brewed coffee and quality juice products are dehydrated by freeze-drying.