Anecdotal evidence of freeze-drying to preserve foods dates back to the ancient Incas. Today, this technology is used commercially to improve shelf-life whilst maintaining flavour and nutritional quality of a variety of products such as coffee, spices, or trekking meals.
The ancient Inca population in Peru was aware of it: freeze-drying of foods worked well. The potatoes and other crops they stored on the mountain heights above Machu Picchu lasted longer than other foods and were light and thus easy to carry. The Indians used the mountain climate – with temperatures at night dropping below zero – and the low air pressure at the high altitudes to freeze the foods and slowly vaporise the ice and water inside.1
Solid foods and medicines
Freeze-drying – also known as lyophilisation or cryodesiccation – was first turned into a commercial technique in the late 1930s. It was used to preserve blood plasma without the need for refrigeration, and to make instant coffee. Since then the technique has been applied for the preservation of hundreds of different types of foods and medicines.1
Freeze-drying ‘modern style’ requires the use of a special machine: the freeze-dryer. This machine consists of a large chamber for freezing and a vacuum pump for removing moisture. The treatment consists of four steps: 1) Freezing to provide conditions for low temperature drying, 2) Vacuum application to allow frozen water/solvent in the product to vaporise without passing through the liquid phase, i.e. sublimation, 3) Heat application to accelerate sublimation, and 4) Condensation to remove the vaporised solvent from the vacuum chamber by converting it back to a solid.1,2
As the freezing process is fast, only small ice crystals are formed. Slow freezing would result in much larger ice crystals that can damage the product structure by penetrating through cell walls. In the vacuum stage, the low pressure prevents the frozen products from melting and accelerates the next stage of the process, the primary drying stage.3 Sublimation of the ice ensures the product structure stays intact.3 During the first drying step, about 95% of the water is removed from the product. In the second stage of drying – that sometimes takes place at a higher temperature – the water that is bound to proteins and carbohydrates inside the product is removed as well.1-3
High quality
Freeze-drying can result in an extremely low moisture content of 1-4%, preventing bacteria and moulds from growing and enzymes from inducing product-deteriorating chemical reactions. Freeze-dried products have a long shelf-life: in a sealed package-protected against humidity, light and oxygen-they can be stored at room temperature for many years.1,2
After rehydration, freeze-dried products may have a better taste, texture and appearance compared with some other preservation techniques.1,2 For example, air-drying of fruits causes shrinkage, a phenomenon that does not happen with freeze-drying.
Compared to air- or spray-dried products, freeze-dried products can be rehydrated quickly, as the process leaves behind microscopic pores. The pores are created by the ice that disappears upon sublimation.1-3
There are however also disadvantages of freeze-drying: it is about 4-8 times more expensive than a technique like hot air- or spray-drying and it consumes 2-5 times more energy. Moreover, as freeze-drying is a batch process it requires a lot of handling, and drying times are considerably longer than for other ways of drying.2 There can also be problems with oxidative rancidity in freeze-dried products due to the low moisture content. As such some of these products, e.g. meat products, may require the addition of antioxidants. For these reasons, freeze-drying has only been used for products where quality is of highest importance, such as nutraceuticals, vaccines, antibiotics, instant coffee, vegetables, herbs and spices, trekking meals, fruits for breakfast cereals, foods for astronauts, luxury instant soups, high value chemicals and pigments.1-3,5
Cutting-down costs
The food industry is investigating how to increase the number of applications in a cost-effective way. For example, an approach called active freeze drying has been developed that reduces handling and drying times. There are also developments towards freeze-drying under atmosphere conditions instead of vacuum, thereby saving energy. Another direction is focused on the combination of conventional pre-drying, followed by freeze-drying for the final drying step. This reduces drying time and energy use.2,4,6
References
- Phase Technologies Inc. (1999). Lyophilization: Introduction and Basic Principles. Jennings TA.
- Ratti C (2001). Hot air and freeze drying of high-value foods: a review. Journal of Food Engineering 49:311-9
- Oetjen GW, Haseley P (2004). Freeze Drying. Wiley-VCH.
- TNO-report V 8441 (2009). Behoud waardevolle natuurlijke inhoudsstoffen met innovatieve (vries)droogprocessen. Authors: Van Deventer H. and Oostrom W.
- Chan EWC et al (2008). Effects of different drying methods on the antioxidant properties of leaves and tea of ginger species. Food Chemistry 1:166-72
- Nawirska A et al. (2009). Drying kinetics and quality parameters of pumpkin slices dehydrated using different methods. Journal of Food Engineering 1:14-20
FOOD TODAY 07/2009
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