Course:FNH200/Lessons/Lesson 08/Page 08.1

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8.1 Introduction

Terms to remember

  • Water activity
  • Dehydration vs. Concentration
  • Case hardening
  • Mass transfer
  • Water soluble components
  • Sublimation
  • Hygroscopic
  • Atmospheric pressure & vacuum
  • Different dehydration methods (sun drying, tray (air) drying, freeze drying, etc.)

Some reasons for dehydrating foods are:

  • preservation of the food (dried milk, juices, fruit);
  • retention of the size and shape of the food while imparting storage stability (freeze dried steak, vegetable pieces);
  • reducing weight and bulk of food for easier storage and transportation; and
  • production of convenience items (instant coffee, instant mashed potatoes, vegetables that rehydrate in instant soup preparations).

Food preservation by dehydration is based on the principle that microbial growth, chemical and enzymatic reactions occur only if sufficient free water is present. When the water activity of foods is lowered there is a direct impact on microbial growth as well as chemical & enzymatic reactions.

Recapping from Lesson 2:

  • Water activity (aw) defines the proportion of water in a food that is in the free, unbound form
  • Microbial activity, enzymatic activity and chemical reactions can occur only in the free water phase of foods
  • Water activity of foods ranges from 0 to 1.0
    • Water activity of dehydrated foods is in the range of 0.2 to 0.6
    • Microorganisms cannot grow at aw below 0.6
    • Chemical reactions (e.g. Maillard browning) can begin to occur at aw of 0.3
Want to know more?
Please visit this website from the Cole Palmer Instrument Company for an overview on water activity, including examples of water activity values for several food products

It is important to remember that with dehydration, microorganisms are not readily killed. Once the food is rehydrated, microorganisms resume growth if favourable conditions exist.

It is also important to distinguish between food dehydration and concentration, both of which involve the removal of water from foods:

Dehydration implies removal of as much water from the food as possible in order to impart a long storage life.

Concentration, on the other hand, implies that some of the water is removed from the food in order to concentrate the food constituents. Concentrated foods are not inherently shelf-stable and require the use of other forms of food preservation (e.g., refrigeration, freezing, dehydration, thermal processing) to extend storage life.