Drying

Throughout history, people have prepared dried foods, including meat, vegetables, herbs and fruits, through evaporating the water they contain in the hot sun or over fires. Dried foods have been especially important to explorers and long-distance travellers facing the problem of how to preserve and carry enough food to sustain them on their journeys. Drying foods has also created novel food products with longer shelf life. For example, drying a plum turns it into a prune and grapes become raisins.

More recently, refrigeration and canning have provided new ways to preserve foods, although we still use drying processes. These have moved on since the days of drying under the hot sun and include techniques like spray-drying and freeze-drying to efficiently remove water while maintaining the quality of the fresh food. (Words and phrases in bold are defined in the Glossary at the end of the Step.)

Why do we dry foods?

The answer lies in what we remove. We need water to live and this is the same for the bacteria, yeasts and moulds that cause foods to spoil. Drying foods inhibits their ability to grow. Food scientists use a term called water activity when thinking about the shelf-life stability of foods. If a product is kept below a certain water activity, then bacteria, yeast and mould cannot grow. This results in a longer shelf life. Another benefit is that the end product is light and compact, which is an advantage for storage and transport of ingredients that can then be rehydrated for use in further food manufacture.

Taking food into space

Space travel created the need for fresh thinking about how to preserve food. Foods taken into space must be light-weight, compact, tasty and nutritious, and cannot rely on heavy and energy-hungry refrigeration. The Apollo missions to the moon carried food that had been preserved through the process of freeze-drying. In freeze-drying, first the food is frozen, very quickly, to a temperature below its triple point, which is the lowest temperature at which the food material can exist as a solid, liquid and gas at the same time. This is usually between −50 °C (−58 °F) and −80 °C (−112 °F). This is VERY cold. It is −60 °C at the South Pole in winter! The food is then placed into a vacuum chamber where the vacuum removes all moisture. Finally, the food is packaged while still in the vacuum chamber. Freeze-drying provides foods that will keep their nutrition and taste qualities almost indefinitely. They are extremely light and compact.

Assorted bags of snack food and dehydrated food, as served on the ISS. Source

Food aid

Developing energy-dense nutritional foods that can be packaged and stored for extended periods of time in harsh environments (either extreme cold or tropical climates) is an enormous challenge. Not only must these foods have the right nutritional profile, they must also be palatable as they may be the only food consumed by victims of an emergency for up to two weeks. To be really useful in such circumstances, products require careful consideration and selection of ingredients, processing techniques, and packaging materials. Drying techniques to reduce water activity are critical to producing emergency food which is stable for long periods and transportable.

Spray-drying to make milk powders

Spray-drying works by dispersing a liquid or slurry as tiny droplets into a stream of hot air, which vaporises the water and leaves behind a dry powder that collects in a drum. Spray-drying is commonly used to produce milk powders and powdered spices and flavourings that are used later as food ingredients. It can also be used for making instant coffee powders, although you will more commonly find that instant coffee is sold as granules that have been produced by freeze-drying, which is more expensive but results in a better quality product.

Glossary

spray-drying

This is a common drying method to produce, for example, milk powder. By applying hot gas such as air or nitrogen, a dry powder is rapidly created from a liquid or slurry. The first step is ‘atomisation’ where an atomiser or a spray nozzle creates small liquid droplets. The droplets exit the nozzle/atomiser and are dried by the hot air or nitrogen. The powder is then recovered from the exhaust gas by a filter or cyclone.

freeze-drying

This is a common but relatively cost-intensive drying method for high quality products. The product is frozen and by reducing the pressure around it, the ice in and on the product is sublimated. This means the ice transforms from a solid state directly into a gas state without passing through a liquid state. In this way the water is removed from the product while limiting the exposure to high temperature. Coffee and fruits are often freeze-dried.

water activity

Water activity takes into account the fact that some of the water molecules in a product are ‘bound’ to other substances. It is defined as the partial vapour pressure of water in a product divided by the standard state partial vapour pressure of water. In food it is a measure of the available, free or active water, that is not bound to any other substance. So it’s different from a measure of water content. The water activity influences microbial growth as well as the chemical processes in food and its physical characteristics.

triple point

This is the temperature at which a substance can exist in a solid, liquid and gas state at the same time. At this point the three phases are in equilibrium.

vacuum

A vacuum is created by removing air from a vessel so that the pressure inside the vessel is lower than the atmospheric pressure outside it. A total vacuum is a space totally devoid of all matter, which is a theoretical rather than a practical concept.