Compressed freeze-dried food offers numerous benefits, such as reduced transport and storage volume as well as less strict requirements connected with packaging materials compared to non-compressed products. In order to prevent fermentation during compression it is necessary for the product to retain appropriate plasticity. It is currently obtained by re-moistening freeze-dried food before compression.

Freeze-drying is considered one of the best methods of food de-hydration – very high quality of such food is preserved at minimal nutrient loss compared to fresh food. Importantly, the process has very little impact on product shape as well. Retaining its physical shape has been considered a desired characteristic for years. Despite that, freeze-dried food does not fully retain its original properties as it has porous structure – the product is light and brittle, with low packaging density.

Disadvantages of low packaging density:

  • porous structure easily absorbs moisture, so special packaging is necessary
  • large packages are necessary for small product amount (weight)
  • freeze-dried products crumble easily – particles are formed during reloading and packaging
  • products require large storage space

Various studies are being conducted in order to select the method that will eliminate these inconveniencies. It can be obtained for example by compressing such food, but in order to do so, freeze-dried products should retain their plasticity. It can be achieved when water content ranges from 5% to 20% (depending on product type). When it comes to products with high sugar content, for example fruit, plasticity can be obtained by temperature manipulations.  

Freeze-dried food products (slightly moistened or heated) are placed in a chamber and subjected to high pressure with the use of hydraulic press. The next stage consists in their re-drying or cooling down. More stages can be necessary for the food with low sugar content.

It is believed that maximum degree of compression is directly connected with initial water content in a product. Apart from eliminating the volume of free spaces between subsequent chunks, compression can also result in eliminating free spaces occurring as a result of ice crystals sublimation.  

Final moisture content in compressed samples has always been similar to moisture content in non-compressed samples, freeze-dried at the same time. In the majority of cases, moisture content ranged between 1.5-5%, which proves that compression during freeze-drying does not prevent dehydration (final moisture content did not depend on compression degree).

Re-hydration of compressed freeze-dried food

The re-hydration of compressed samples was fast and in most cases the equilibrium was restored within 15 minutes. The samples in general regained their pre-compression volume after re-hydration.

Vegetables were the most efficiently re-hydrated in cold water, while beef bones in warm and hot water. However, at higher temperatures the colour of the meat turned dark, as if it had been boiled.

Re-hydration in cold water, although slower, resulted in better quality of the meat product. It was observed that freeze-dried, non-compressed vegetables re-hydrated less than compressed samples.

It seems that compressing freeze-dried food constitutes a very good method for reducing its volume and accelerating re-hydration. This method can for sure be successfully applied in all situations when reduced volume brings considerable benefits, for example in civilian or military transport and in food storage.