Kadam, Patil, Kaushik - Foam Mat DryingProcess of foam mat drying as developed by Morgan involves drying of liquid or
semi liquid food concentrate in the form of a stabilized foam prepared by the addition of
a stabilizer and a gas to the liquid food in a continuous mixer and drying it in heated air
at atmospheric pressure. Stable gas-liquid foam is the primary condition for successful
foam drying. For natural foams, such as egg whites, pineapple juice etc., there is no need
to add any foaming agents. Vegetable proteins (e.g., solublized soya protein), gums and
various emulsifiers (e.g., glycerol monostearate propylene glycerol monostearate, car-
boxy methyl cellulose, trichlorophosphate) are typically added to juices, pulps, purees,
or concentrate as foaming agents. Mixtures are whipped to form foams using suitable
blender or specially designed device. The foam thus formed is spread as a thin mat or
sheet and exposed to stream of hot air or hot water conduction surface method until it is
dried to required moisture level. The dehydrated product is milled and converted into
powder. Foam-mat drying of the foamed product in the form of thin layer (0.1 to 0.5mm)
is generally carried out between 65 and 85°C for a very short duration, as the foam re-
duces drying time many fold. A continuous belt tray dryer as well as a slightly modified
spray dryer can also be used for this process. As such no complete foam-mat drying sys-
tem is readily available in the world. Many organization / researches are working on
this aspect and Central Institute of Post Harvest Engineering and Technology (CIPHET)
an ICAR institute, Ludhiana India is one of them.
Good quality tomato powder can be produced using this technique. The optimal ini-
tial concentration of food solids is in the range of 39% for tomato pulp. The cost of such
a drying process will be less than spray or drum drying, vaccum and freeze-drying.
Three methods are generally used in the formation of food foams. In one method gas is
bubbled through a porous sparger such as sintered glass into an aqueous solution of low
protein concentration (0.01-2% w/v). The liquid may be completely converted to foam if
a large amount of gas is introduced.
Secondly, foams can be formed by whipping (beating) an aqueous solution con-
taining a foaming agent in the presence of a bulk gas/ air phase. Whipping can be carried
out in a variety of devices that vigorously agitate the liquid and its interface with a bulk
gas /air phase. The method has been preferred for most of the “functional tests” of pro-
teins, as it is the standard means of gas introduction in most aerated products. The
process of bubble formation and the history of a single bubble are not defined. The
whipped foam is well mixed throughout its formation, so the stratifications often found
in bubbled foam column are not observed. Compared to sparging whipping results in
more severe mechanical stress and shear action and a more uniform dispersion of the
gas/air. The severe mechanical stress affects both the coalescence and formation of
bubbles. The volume of air included usually goes through a maximum with increasing
intensity of beating (severe mechanical beating is a standard method of foam breaking).
Hence, the observation of maximum levels of gas incorporation during whipping reflects
a much more real dynamic equilibrium between mechanical formation and destruction
of bubbles. In addition, mechanical stresses can break up bubbles formed earlier into
several smaller ones.
A third procedure for forming foam termed as shaking has been used only rarely.
Foam formation by shaking tends to be slower than by bubbling or whipping under
similar conditions, which is due to the relative efficiency if the process in producing gas