Drying may bring about changes in food: Color Flavor Texture Viscosity Reconstitution Rate Nutritional Value Storage Stability

The level of water necessary to preserve food is establish by:

Water activity, aw defined as:

aw = pw /pws (0 - 1)

Equilibrium relative humidity (%ERH) %ERH = [pw /pws ]*100 = [xw /xws]*100 where: xw-mole

fraction of water vapor and x ws -mole fraction of saturated water vapor

Factors controlling the rate of drying: Surface area Temperature employed Relative humidity of air Velocity of air Drying time Atmospheric pressure vs vacuum

Surface area

Subdivision of food provides more surface area for contact with heating medium as well as the moisture escape

The smaller particles the shorter distance the heat must travel to center of food and the moisture must travel to reach surface.

Temperature and humidity

The greater the difference in temperature between the food and heating medium the greater the rate of drying

As water is driven from food it creates saturated atmosphere near the food surface

The dried and hotter the air the more moisture it may take up from the food surface- the more rapid is food drying

Each food has its own equilibrium relative humidity Equilibrium relative humidity is the

humidity at a given temperature at which food will neither lose moisture to atmosphere nor pick moisture from the atmosphere.

Air velocity and pressure

Moisture must not only be taken up by the heating medium but also sweep away from the drying surface in order to prevent the moisture from formation of saturated atmosphere.

Pressure- evaporation of water depends on temperature. Boiling point is affected by the pressure. The lower the pressure the lower is the boiling point and the shorter is the drying time. Drying under vacuum is important for heat sensitive food products

Drying curve:

Drying curve consists of

Constant Rate Period - As long the amount of water reaching the surface is equal to the amount of water evaporating from the surface - the evaporation is quite rapid and this cools the food surface

Falling Rate Period - When the amount of water reaching surface decreases - evaporation decreases and surface temperature increases

The falling rate of drying may be explained as follows: Outside drier layer of food forms insulation barrier

against rapid heat transfer (evaporated water leaves voids filled up with air).

As the water evaporates the concentration of solute in water increases. This elevates the boiling point.

The increase in the distance the water has to travel to reach surface.

The shape of drying curve depends on: Food product Type of drier Temperature Air velocity Air humidity Thickness of food