Dehydration as a means of preservation

Dehydration as a means of preservation

All micro-organisms need moisture to multiply. Dehydration reduces the amount of available water and thus prevents growth. However, some bacteria spores will germinate on the reconstitution of the dried product. Although yeasts and moulds usually grow at lower moisture levels than do bacteria, mould spoilage is also prevented, as is enzyme activity. Provided dried foods are stored in suitable airtight packs, and kept dry, they will keep for a considerable period of time.

Sun drying was the earliest method of dehydration and is still practised in hot climates, for example, for drying currants, raisins and figs. Artificial drying is quicker and normally more effective than natural means. Unfortunately, food undergoes irreversible changes to the tissue structure during drying, which affects both texture and flavour.

Artifcial drying techniques include the use of hot air, for example in tunnel drying, fluidised-bed drying, roller drying and spray drying. Changes in protein structure and flavour can be reduced by using warm air, as in accelerated freeze drying. The choice of technique often depends on the type of foodstuff and the degree of dehydration required. Blanching of vegetables must be carried out before drying to obviate enzyme activity during storage.

In spray drying, a solution, paste and slurry is dispersed as small droplets into a stream of hot air. The small droplets result in a rapid loss of moisture and a large proportion of the colour, flavour and nutritive value of the food is maintained. However, because the evaporative cooling effect keeps the temperature of the droplets low, a pasteurisation process is usually required prior to spray drying.

In roller drying, the food is turned into a paste which is dried on a heated drum and scraped off. Generally, product quality is inferior to spray-dried product. For fruits and vegetables; a tunnel drier, 10 to 15 meters long, is used. Trays of product are passed though it while hot air is blown across the trays. This contentious process leads to a gradual loss of moisture. Total removal of moisture may not be necessary. The remaining water forms a strong solution with salts and soluble proteins. This water which remains is not available to micro-organisms.

Accelerated freeze drying

Food is frozen quickly and then lightly heated under vacuum. Ice in the food is then extracted as water vapour, a process known as sublimation. The process minimises the effects of drying and the product reconstitutes better.