Information from the Defence Nutrition Research Centre, DSTO
Send any comments to the maintainer Roger Caffin
Meals which used to be available from DSTO were in principle one-man meals, but when carbohydrate and some freeze-dry vegetables are added they were fine for two. What with all our troops being overseas for the last few years, these DSTO meals are no longer available. Pity - a private enterprise would simply have doubled capacity.
Freeze-drying is based on the sublimation of ice. That is the food dries by the direct evaporation of ice to water vapour, rather than the evaporation of liquid water. This is achieved by freezing the food and then placing it in a strong chamber under a high vacuum, typically about one thousandth of air pressure. Under these conditions liquid water can not exist, only ice and water vapour. By application of heat, and removal of evaporated ice to a very cold surface, ice continues to evaporate until only a very dry, porous solid is left. This has several advantages for the quality of the dried food.
Evaporation of water, as in conventional drying, occurs at the surface of the food. To evaporate, the water must move to the surface and as it does so it carries dissolved solids such as sugars and proteins. These solids can form a layer on the surface that effectively slows rehydration during preparation of the food before eating.
Freeze-drying is done at low temperatures. The food need not rise above ice temperature if this is essential for product quality. The product quality is therefore higher than with other drying methods which raise the temperature of the food to speed the drying process.
The freezing that is carried out before drying concentrates solids, particularly sugars in small areas of the product. These form a "glass" that is a very good trap for volatiles. This results in a good retention of volatiles, which are the flavour components of food. Freeze-dried foods therefore have good flavour.
The ice crystals that form during freezing, evaporate during drying leaving a porous material. This allows rapid uptake of water when the food is rehydrated. If done carefully, a 1 cm steak can be rehydrated in minutes, compared with the hours that would be required for a similar sized hot air dried steak.
These pores are one reason why the freeze-dried food is packed under vacuum. The pores allow oxygen in the air to get to sensitive components of the food, particularly fats in the cell membranes. These are very rapidly oxidised in the presence of air to produce unpleasant rancid flavours. Special precautions are taken with freeze-dried foods to protect them from air from the moment they are removed from the drier.
The food typically has a moisture content of less than two percent at completion of drying. This may be compared with the more than 90% of water in many vegetables, 75% moisture in fresh meats, and the 15 to 20% moisture in dried fruits. This very low moisture means that the food has a very long shelf life even at elevated storage temperatures. This is of great importance to the military and one of the major reasons freeze-dried foods are used for military rations.
The low moisture also means that microorganisms are not able to grow in the product during storage.
History of freeze-drying at Scottsdale
The first freeze-drier, a small experimental unit was installed at Scottsdale in 1958. Research work was conducted on this unit until a larger unit capable of drying 250 kg of wet food at a time was installed in 1970. It is this unit that has been used since then to produce all the freeze-dried food requirements of the Australian Defence Force.
Spare capacity existed for some time so the current line of products were introduced to the Australian camping market in 1995. Sadly, they are no longer available.