Kadam, Patil, Kaushik - Foam Mat Dryingsule is avoided by the small gas inclusions, which do not escape until the solids outside
have dissolved or dispersed. When the foams are sufficiently stable, the dry product is
uniform and has foam structure. When the foam bubbles are sufficiently small, the dried
product may be subdivided fairly finely without losing the advantages of foam structure.
From the point of view of rehydration, foam structure provides the advantages of natu-
ral porosity.
Over the past decade, this relatively old technology, known as foam mat drying, re-
ceived renewed attention because of its added ability to process hard-to-dry materials,
obtain products of desired properties (e.g., favorable rehydration, controlled density),
and retain volatiles that otherwise would be lost during the drying of non-foamed mate-
rials. Foam-mat dried vegetables or fruit powders have less heat induced changes in col-
or and flavor as compared to conventional spray-dried or drum-dried products. A prod-
uct with density less than that in a conventional dryer is obtained. The product density
is about equal to the density of instantized or agglomerated powder.
The dehydrated powder / flakes are superior to drum dried and spray dried prod-
ucts because of its honey comb structure and better reconstitution properties. Selected
fruit pulps such as mango, star fruit, papaya and banana have been dried to produce
flakes by using foam mat drying technique.
The thin layer drying study on foamed mango pulp concluded that the mango pulp
treated with egg albumen (10%) and methyl cellulose (0.5%), dried at 60ºC with one
mm foam thickness retained significantly higher biochemical contents than that of other
foaming and drying treatments. In general, drying of foamed materials is faster than that
of non-foamed ones, although certain foams such as the ones from soymilk or starfruit
exhibit higher drying rates in the beginning of foam mat drying whereas for other mate-
rials such as tomato paste, bananas, and mango drying rates are greatly accelerated at
the end of drying.
Foam-mat drying allows processing of hard to-dry materials such as tomato paste
and a variety of fruit pulps and juices. Preferential product quality stems from accele-
rated drying at generally lower drying temperatures. Reduced density of foamed mate-
rials leads, however, to a decreased dryer load, which has to be compensated for by
shorter drying time to maintain the dryer throughput.
Besides accelerated transport of liquid water to the evaporation front, drying ex-
perts have repeatedly pointed to the increased interfacial area of foamed materials as
the factor responsible for reduced drying time. Because density of foamed materials is
lower than that of non-foamed ones and extends from 300 to 600 kg/m3, the mass load
of the foam-mat dryer is also lower. However, shorter drying time can not only offset the
reduced dryer load but also increase the dryer throughput. Shorter drying time per unit
mass of foamed materials might not always bring about lower energy consumption and
better process economics.
5.7. COMMERCIAL APPLICATIONS
The process has been under study for some time and several commercial operations
are developed. The process has evolved in several basic forms including drying on a Tef-