The Quality of Freeze-dried Products

The quality of a freeze-dried product depends on a number of factors and questions of processing technology. The most important are  

The characteristics and importance of these processing steps will subsequently be dealt with employing examples from the spice and flavour industry. A division into 

Finished mixtures for the food sector are often prepared with powdered flavourings. Especially if natural finished flavourings are called for, the use of freeze-dried products should be taken into consideration. The dry matter of a natural extract contains carbohydrates, proteins and other nitrogen compounds, fats and waxes, minerals, vitamins, acids and flavouring substances, which all have an impact on the drying behaviour. 

For the following discussion, we will chose the method of freeze-concentration, as it combines well with the ensuing freeze drying process. A solution of fmctose in water will be selected as a practical example for illuminating the physical principles. 

If a 20% fructose solution is selected. Young [17] shows in diagram (Fig. 2.63), that pure ice freezes out from this mixture at appr. -2.5°C this is a result of the known phenomenon of freezing point reduction in solutions. The freezing-out of pure ice, however, causes an increase in the sugar concentration and thus a further freezing point depression. This continues until a mixture of ice and fmctose dihydrate is present at -10°C. This lowest common solidification point is called eutectic point . 

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