Drying

Drying is a unit operation in which moisture is removed from solids, liquids, or pastes through heat and mass transfer, typically by evaporation. The aim is to reach a target equilibrium moisture content that inhibits microbial growth and deterioration while preserving product quality.

Moisture removal occurs as heat increases the vapor pressure of water, causing it to move from the interior to the surface and evaporate. The rate is governed by temperature, humidity, air flow, contact area, and the physical properties of the material.

Drying methods fall into natural (ambient) drying and mechanical drying. Ambient drying includes sun drying and wind drying, which require energy inputs mainly from the environment and are common for agricultural products with thin layers. Mechanical drying uses heated air or other media to accelerate evaporation, examples include hot air convection dryers, spray and drum dryers for liquids and pastes, belt and tray dryers for solids, vacuum dryers to prevent oxidation, and freeze-drying (lyophilization) that removes water by sublimation at low temperature.

Applications include food preservation (fruits, vegetables, grains), wood processing, textiles, ceramics, paper, and pharmaceutical powders.

Key factors include drying temperature, relative humidity of the drying medium, airflow rate, surface area, and the diffusivity of moisture within the material. Drying kinetics describe how moisture content declines with time and can be influenced by product thickness and initial moisture.

Quality and energy considerations: improper drying may cause case hardening, cracking, nutrient loss, or flavor changes; insufficient drying invites spoilage; excessive energy use can be wasteful. Efficient drying aims to minimize energy while achieving the required dryness and maintaining product quality.