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Flavor and Drying

by Matthias Huhnke, Hamburg, Germany


Preservation of flavor during warm air drying of solid food


With the storage and the production of high-quality food the exchange of material and transport processes are quality-determining . The loss of essential nutrients, the effectiveness of added taste materials and preserving compounds or also the contamination with pollutants from the environment are due to this.

The evaporation of flavor and the modification of the flavor composition is also due to exchange of material and transport processes. It can be extraordinarily high after a warm air drying. Interesting is the transport inside the property and the desorption at the surface.

Frequently flavor is added  during the food processing, in order to improve dried or longer time stored food sensorily. A substantial role plays the trend to " convenience foods ", which are to be served after short preparation time.

The natural flavor is usually not imitateble by aromatizing of the industrially manufactured products.

In spite of food-technological meaning, the evaporation of flavor from solids during warm air drying is not comprehensively described, contrary to those in fluids. With empirical data or relations model conceptions can hardly be derived for the improvement of the retention.

This research will shows, how flavor materials evaporate with different initial conditions and boundary conditions from simple model systems with air-drying. An additional layer differentiation should make also the transport of the flavors recognizable.

In drying experiments it was first determined, which parameters are mainly responsible for the evaporation. The relevant parameters were varied according to an improved retention.

The results of these investigations should enable to calculate the flavor retention also from incomplete analytic data.


During drying flavor escapes as well as water because of their evaporability. The quality of such drying products is of course reduced.

With simple drying experiments should be examined, how flavor can quantitatively be kept better in the gel by a suitable processing. Primary parameters with an air dryer are temperature and humidity of drying air. Apart from these outside measured variables it should be also studied to what extent the polarity of the flavor materials as well as the type of the gel influence the retention.

In preliminary tests suitable drying goods were tested. Gels should be used, which are comparable in type of solid and water content to solid food.

It should be ensured that production and drying of the gels is reproducible. In addition a drying method should be developed, where the gels dried under quasi balance conditions.

The criteria for the selection of suitable flavor compounds are not excluding their occurrences and their flavor effectiveness in food. Also physical characteristics (e.g. the polarity) in connection with preservation are of importance.

The models should be dried under defined and constant outside conditions. The drying process should be made both in the warm air dryer (open system) and by salt solutions (closed system), whereby the drying process parameters temperature and humidity are to be varied.

Except the desorption also the transport inside the property should be observed. In addition the flavor content in different layers is to be measured. Beyond that, structure-changing influences by chemically inert additions - e.g. glass balls - should be examined.

In the last step the results should be checked at food matrices.

Summarized objectives:


Simple model systems were developed, in order to be able to vary the drying process parameters regarding an improvement of flavor preservation. To the model systems belonged gels from potato starch and gelatine with defined compositions and dimensions.

These gels contained in each case one flavor compound. It were used: 2-Acetylpyrrol, Ethanal, Ethanol, 4-Ethylphenol, n-Hexanal, Methanol, 3-Methylbutanal, 1-Octen-3-ol, Phenol and n-Propanol.

These compounds are easily quantifiable by GC/FID and they are in sensory relevance in food. Different versions were used with the following determined parameters:

  1. Slow drying: The gels were in a separate sample space, which had a passage to the dryer space. The free gas exchange with the dryer atmosphere was strongly reduced by narrowing of this opening. Thus a quasi equilibrium was achieved. The temperature was varied (70, 80, 90 and 96 °C).
  2. Quick drying: The relative humidity in the dryer space amounted to 70 %. The temperature was varied (80 and 90 °C).
  3. Ball model: Alternatively glass balls were added to the gelatine gel, which should cause a surface enlargement inside the gel and a reduction of the front surface.

The received results should be verified at the models potato, tomato and champignon.

The flavors remaining during the drying process were analytically determined.

In regular time intervals gels were taken, extracted with Diethylether and quantified by GC.

The reduction of the flavor retentions as a function of the mass of the property was linear or not linear. Thereby the preservation was best with linear retention.

The flavor was particularly much preserved with the quick drying, in particular at high temperatures. This result could be observed in all hot dried (> 90 °C) model systems.

The addition of balls enabled likewise an improved preservation.

A layer differentiation of the model systems should make recognizable the transport of the flavors to the front of the gel. A concentration shift showed up in the individual layers.

Like that the taking off of polar methanol is clearly larger in the proximity of the surface than in the deeper layers. The nonpolar n-Hexanal remains after the drying process evenly over the whole body.

The drying experiments with different salt solutions did not represent a practicable drying alternative. Additionally no differences between the selected salt solutions could be recognized in the behavior.

Sum up, many possibilities exist to improve flavor preservation by a suitable processing at warm air drying.

Thereby a fast and hot drying is particularly favourable. Other quality parameters as for example colour, consistency, ability for swelling and the content of vitamine are worsened with these drastic conditions.

Therefore further differentiated investigations are necessary, in order to offer high-finished food as flavor-native as possible to the consumer.

March 15, 1998

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