GB2209114A - Dehydration of foodstuffs - Google Patents

Abstract

A method of dehydrating water-containing foodstuffs including fruit, vegetables, meat, fish and the like, comprising the steps of (a) partially freezing the foodstuff until a proportion of the water included therein has frozen to produce ice in sufficient quantity to create a rigid structure in the foodstuff and (b) applying a microwave drying technique under a selected degree of vacuum to remove the ice by sublimation and the unfrozen water by evaporation.

Description

Title: Dehydration of foodstuffs The invention is concerned with improvements in or relating to the dehydration of foodstuffs including fruit, vegetables and the like, for the purposes of preserving the same from deterioration.

It is known that the storage life of foodstuffs is greatly increased if they are dried to a low moisture content.

However, conventional and traditional methods of drying result in a loss of texture and the product is not easily rehydrated to an acceptable standard.

It is an object of the invention to disclose a method of drying wet materials and, in particular but not exclusively, foodstuffs such as fruit, vegetables, meat and fish, in which the water is evaporated at low temperature with the cellular structure of the material being retained in such a way as to form a rigid porous sponge which is capable of rapid rehydration.

The invention provides a method of dehydrating water containing foodstuffs including fruit, vegetables, meat, fish and the like, comprising the steps of (a) partially freezing the foodstuff until a proportion of the water included therein has frozen to produce ice in sufficient quantity to create a rigid structure in the foodstuff and (b) applying a microwave drying technique under a selected degree of vacuum to remove the ice by sublimation and the unfrozen water by evaporation.

In an example of the invention, the method comprises the following steps: (i) Freezing the product partially so that a mixture of ice and liquid exists such that sufficient ice is formed to create said rigid structure to the product.

The proportion of ice for optimum results will vary for different products but generally will be in the range 5% to 30% ice. However this range is given for illustration only and a lesser or higher proportion of ice is within the scope of the invention.

(ii) Using microwave vacuum drying to evaporate the liquid in the partially frozen product, and to evaporate the ice by sublimation.

It will be appreciated that the liquid will freeze in the same manner as a brine solution, that is that the ice formed will be of pure water whilst the concentration of the dissolved salts will rise in the remaining liquor as more ice forms. Ultimately all the liquid would freeze at the freezing point of the saturated solution, but it is not intended in this method to freeze the product to this low o Q temperature which would generally between -10 C and 40 C.

It will also be appreciated that from the partially frozen state, the ice will not melt until the temperature rises to o O C and the appropriate latent heat is injected unless the pressure is below the critical value at which ice sublimes.

The vacuum pressure may be set to a level at which the o liquid solution boils at less than 0 C. For example the o concentrated solution may freeze at -25 C, but its boiling o point, which is a function of pressure, may be set to -5 C by choice of suitable vacuum pressure. By this means the boiling point of residual liquor after partial freezing as in step (i) may be at a lower temperature than the melting point of the ice.

The residual liquor can therefore be evaporated without melting the ice, simultaneously ice in close thermal contact with the liquor will sublime.

It will be appreciated that the partial freezing only of the product is very advantageous for the subsequent microwave drying because the microwave loss factor of the liquor is high, so that the electric field intensity required to give a satisfactory power dissipation density is low, within practical limits. Typically the dielectric properties at the start of drying would be e' = 5, e" = 0.8. At 2450 MHz with a field intensity external to the product of 100 V/cm the power density would be 0.7 watts/cc, giving a drying time in the order 50 minutes in a product with 15% solids.

The voltage stress applicable is a function of vacuum pressure and can be raised to give more rapid drying by -l reducing the pressure to less than 10 torr. However, the rate of drying is limited also by the diffusion rate of steam through the product, which must not create such a pressure gradient within it that the internal pressure exceeds the critical value at which ice melts instead of sublimes in the central region.

In summary, the salient features of the example may be expressed as follows: (a) The partial freezing of the material to form at least sufficient ice to create a rigid structure with a residual liquid content to provide a microwave loss factor e" of at least 0.2.

(b) Reduction of pressure to a level less than that at which ice sublimes, or to lower the boiling point of o the residual liqor below 0 C whichever is the lower.

(c) The application of heat energy at least in part as microwave energy to evaporate the residual liquid and sublime the ice so as to reduce the moisture content to the required final value. Some of this energy may be of heat-conduction or infra-red radiation source.

It is to be noted that the present invention provides an improvement over the usual freeze-drying technique for three important reasons: (1) The residual liquor from partial frezing provides a microwave loss factor making microwave energy a practical heat source.

(2) Energy is saved in avoiding (a) total freezing and (b) requiring less energy for drying.

(3) The use of microwave energy speeds up the whole process with the usual consequent advantages.

Various modifications may be made within the scope of the invention.

Claims (8)

1. A method of dehydrating water-containing foodstuffs comprising the steps of (a) partially freezing the foodstuff until a proportion of the water included therein has frozen to produce ice in sufficient quantity to create a rigid structure in the foodstuff and (b) applying a microwave drying technique under a selected degree of vacuum to remove the ice by sublimation and the unfrozen water by evaporation.

2. A method as claimed in claim 1 wherein the partially frozen foodstuff contains 5-30% ice

3. A method as claimed in claim 1 or 2, wherein the degree of vacuum is selected such that any unfrozen liquid in the foodstuff boils at -50C.

4. A method as claimed in any preceding claim, wherein the microwave frequency is 2450 MHz and the field intensity external to the product is 100 V/cm.

5. A method as claimed in any preceding claim, wherein the vacuum pressure is 10 1 torr.

6. A method as claimed in any preceding claim, wherein the partial freezing is sufficient to provide a microwave loss factor e" of at least 0.2.

7. A method as claimed in any preceding claim, wherein heat is also provided by heat conduction or infra red radiation.

8. A method which is substantially as herein described.