Kadam, Patil, Kaushik - Foam Mat Drying
as foam inducers was less than 0.25% of D-100 adequate to produce foam of a density
300 - 400kg m-3 for most of the systems studied with or without the use of foam –
stabilizer. Similarly, 0.5% GMS is able to produce foam of density 330-360 kg m -3 in
some system (papaya puree), but, on the other hand, it is impossible to foam this puree
with normal quantities of soya protein. The foam density and stability was reported to
be increased with the increase in the soluble solids content of the juices.
The effect of whipping time in the preparation of grapefruit foam using GMS as the
foaming agent increasing the whipping time from 6.5 to 15 min did not result in signifi-
cant change in bubble size. However, after 15 min some break down occurs at a very
slow rate and even after 30 min of whipping, foam still posed a good bubble size. The
concentration of surfactant (foaming agent) in the range of 1 to 3% showed a decrease
in bulk density till 8 min of stirring time after which the foam structure collapsed result-
ing in an increase in the foam density. A criterion for good foam stability was its unifor-
mity and the lack of fluid drainage in 60 minutes after its preparation. These characters
can be observed in foams with bulk densities below 300 kg m-3. In general, best results
can be obtained when the smaller amount of surfactant is added, and for the shortest
stirring, to obtain a bulk density of 260 kg m-3. Foams prepared under these conditions
will show stability to drainage for over 1 hour.
The influence of mechanical mixing, gas input rate and a gas sparger (hole diameters)
dependent on the gas flow rate under steady state condition. Also rotational speed from
1000 to 2000 rpm improves the foaming capacity without any adverse effect on foam
density. Foam expansion is not influenced by changes in pH of the material but stability
decreased with its increase during functional properties of sunflower seed protein. An
increase in protein concentration increases both the expansion as well as stability of the
foam. Similarly, higher stirring time did not appreciably affect foam expansion but in-
creased the foam stability, whereas higher stirring speed increases foam expansion sig-
nificantly but foam stability only slightly.
5.4. TYPES OF FOAMING AGENTS
Foam-mat drying requires stiff stable foams which are not readily made from many
food concentrates especially food with low protein content, unless there is an addition of
a small quantity of foaming agents or foam stabilizers. Foam stabilizers are added at dif-
ferent levels to varying composition of the food concentrate, and their performances
could be measured by their foaming, reconstitution, and sensory attributes.
Many types of agents are used for production of foams and each of them offers cer-
tain distinct advantages. One of the most commonly used is the monoglyceride, or fatty-
ester type. This agent has the capacity to produce extremely fine foams which enable
thorough and efficient moisture removal under milder conditions than could be used
with some other agents. Hence this agent is particularly useful with the more heat-
sensitive materials. It is probably for this reason that most commercial foam-mat dried
products have utilized monoglyceride agents. Orange, lemon and tomato powders of this
type have been produced commercially. By adding a small amount of edible foam stabi-
lizer such as monoglycerides or a modified soyabean protein with methyl cellulose to
liquid stiff foam is produced by whipping. Monoglyceride agents, however, impart to the