Kadam, Patil, Kaushik - Foam Mat Dryingbilizers are added to provide stability to the foam, which would otherwise collapse be-
fore all the moisture has been removed during drying. Foam stabilizers might be sur-
face-active agents, hydrophilic colloids or a mixture of both.
The key step in foam-mat drying is the preparation of stable foam. Whip ability, the
capacity to form stable foams with air, is an important functional property of proteins
for applications in food products. The term “whip ability” and “foam ability” are used
interchangeably in the literature although whip ability usually suggests that tests were
conducted using a household-type mixer with a whipping arrangements, while foam
ability refers to the intensive shaking/agitation of a protein solution in a cylinder or the
bubbling of air through a protein solution via a fritted disc. Foaming methods require
less protein and are measured under milder and more controlled conditions. The mixing
method for whipping is a simple one and gives more reproducible results, which can be
easily translated from laboratory bench to pilot plant.
The characteristics of the foam are measured in terms of two parameters i.e. foam-
ing ability and foam stability. Foaming ability is the initial height of foam in the column
immediately after formation. Foam stability is the rate of subsidence of these foams. In
other words, stability of a foam or whip is its ability to retain its maximum volume and is
usually measured by the rate and/or amount of leakage of fluid from the foam.
As explained above the primary requirement for the process of foam mat drying is
preparation of stable foam. Two important characteristics required for a stable foam
preparation are consistency. The film forming components, used in the drying of fruits
and vegetables are glycerol monostearate, soluble soya protein, polypropyleneglycol
monostearate etc. Most suitable stabilizers are egg albumin, fatty-acids, monglycerides,
mix of mono & diglycerides fatty esters of sucrose at levels of about 1% of the dry solids
in the products helps in forming the stable foam.
In foam-mat drying of tomato paste and apple sauce certain methylcellulose such as
methecol is the most effective stabilizer, as Methecol develops very stable foam and at
faster rate due to its compatibility characteristics and low gel point.
5. 3. FACTORS AFFECTING THE FOAM CHARACTERISTICS
The factors which greatly influence are two parameters, viz., foaming power and
foam stability are the total solids contents of sample, temperature during foaming,
whipping time and type and concentration of foaming agent and foam stabilizer. Foam
stability is greatly influenced by soluble solids content of the sample, and type and con-
centration of the added foam inducer. Pulp content of the sample and mixing time has
negligible effect on stability of the foam. In general, when the content of soluble solids in
the sample is low, more amounts of foaming agent and stabilizers are required to be
added. If the sample food is relatively free of pulp, the foam must be whipped to a very
low specific gravity in order to acquire the necessary stiffness.
The concentration of the additive necessary to produce a given density of the foam
depends on the characteristics of the material being foamed in addition to the other ex-
ternal variables. The foam-mat drying of some tropical fruits using two types of foam
inducers, namely, modified soybean protein (D-100) and glycerol monostearate (GMS)