Kadam, Patil, Kaushik - Foam Mat Drying
juice, mango juice etc. is a major researchable issue, subsequently several basic forms of
foam-mat dryers including the laboratory type adiabatic dryer. A belt-type foam-mat
dryer consists of an endless steel or Teflon-coated belt which alternatively passes over
the heating and cooling drums. The foamed material is spread over the moving belt and
temperature of air flowing across the material through the ports is controlled in the
range of 65º C to 21º C. The drying in such process takes about one hour time depending
upon the characteristics of the material being dried. The difficulty with such dryers,
however, is the removal of the product from belt in certain cases.
Tray and crater-type foam-mat drying have been used by various researchers for the
production of fruit/vegetable powder. The tray dryer is an improved design over the
belt dryer. In this, a layer of foam is spread over metal trays and dried. In the crater
technique, perforated metal trays are used on which the foam is spread. A controlled air
blast is directed nozzles direct thin flat jets of air up through the tray perforations to
pierce the foam and pile it in volcano-shaped craters around each hole.
Foam-mat drying using crater technique has several added advantages. Because of
the large surface exposed to the air, most of the water is removed from the material in a
very short time. However, the capacity of such dryers is very low. Tray drying of foams
as mats is simple and cheap in comparison to the crater technique. Foam-mat drying of
tomato paste upto 2% moisture content using crater technique can be done within 2 to
18 minutes depending upon the paste characteristics and drying conditions. About 70-
90% of the moisture can be removed in the first few minutes.
The effect of concentration of foam stablizers significantly reduces the drying time
on drying characteristics of citrus concentrates and soymilk powder. Physical characte-
ristics and reconstitution properties of the powder prepared from foamed citrus juice,
soymilk etc. were found to be better than that from the unfoamed. The final moisture
content attained in the foamed samples is lower than that of the unfoamed sample.
Foam-mat drying of tomato, mango, pineapple, lemon, banana and guava etc.
juice/pulp can be done to produce easily reconstitutable powder. The foams are spread
on plain aluminum trays in the form of thin sheet and dried in a cross-flow hot air dryer
at 80ºC for 30 min followed by 30- 90 min drying at 65-70ºC The dried foams are cooled
to room temperature in a dry atmosphere to prevent caking due to highly hygroscopic
nature of powder prepared by this novel method. Color flavor and texture of the dried
product are good and uniform.
5.6. ADVANTAGES OVER TRADITIONAL METHODS
This method offers wide scope for drying of other difficult-to-dry sticky, viscous and
heat sensitive materials like glue, gelatin, pigments Varnishes etc. Any fluid material that
needs to be dried gently could be dried by the foam-mat method provided it is capable of
forming stable foam. The main advantage of foam is that the structure provides a large
surface at which water may rapidly evaporate. Water vapor produced below the already
dry outermost portions diffuses through the thin, warm walls of these outer parts, and
not through the thick, dense skin, which would result from shrinkage of an unfoamed
mat. The same structure ensures rapid rehydration of the dry product. The water travels
along the web of solid material, like ink onto blotting paper. Formation of a gummy cap-