Drying and Dehydration
What this domain tests
This domain tests the difference between sun drying and controlled dehydration, the drying curve, pretreatments, and calculations of final weight and water removal.
Methods and pretreatments
Sun drying is cheap but slow and weather-dependent. Controlled dehydration uses tray, tunnel, belt, fluidized-bed, spray, drum, vacuum or freeze dryers; spray drying suits milk powder and drum drying suits purees. Pretreatments include washing, blanching, sulphuring by dipping in potassium metabisulphite solution, brining and osmotic dehydration in sugar or salt syrup.
The drying curve
After a short initial period, drying proceeds at a constant rate while the surface is wet. When the critical moisture content is reached, the falling-rate period begins and internal diffusion controls the rate. Raising the temperature too early hardens the surface (case hardening), trapping moisture inside. Other changes are shrinkage, browning and loss of vitamin C.
Quality targets
Dried vegetables are usually 5-10% moisture and dried fruits 15-20%; water activity should be below 0.6. Rehydration ratio measures quality. Dried food is not sterile and must be packaged against moisture pickup.
Worked example
100 kg of mango pulp at 85% moisture contains 15 kg solids. Drying to 12% moisture means the solids become 88% of the final mass, so final mass = 15 / 0.88 = 17.05 kg. Water removed = 100 - 17.05 = 82.95 kg. The solids balance is the key step, because solids do not evaporate.
Traps that cost marks
Do not begin with the moisture lost; begin with the constant solids. High initial temperature causes case hardening, not faster drying. Sulphur dioxide residues are legally limited, and low moisture is not the same as low water activity.