Kadam, Patil, Kaushik - Foam Mat Drying
5.1. INTRODUCTION
Foam- mat drying is a promising new development in the field of drying aqueous
foods. This method offers a wide scope for application in vegetable puree and fruit juice
processing industry as it enables the dehydration of heat sensitive foods or which are
difficult-to-dry, sticky, and viscous under relatively mild conditions without undue qual-
ity change. This technology is finding an increasing application and importance on
commercial scale mostly in the drying of liquids that tender a high quality concentrate
such as milk, fruit, juices, soluble coffee, etc.
The foam-mat drying is a process in which the transformation of products from liq-
uid to stable foam follows air drying at relatively low temperatures to form a thin por-
ous honey-comb sheet or mat which is disintegrated to yield a free-flowing powder. The
dried product obtained from foam-mat drying is of better quality, porous and can be
easily reconstituted. Concentration of the material prior to conversion into stable foam
may or may not be an essential requirement and will depend on surface tension and
consistency of the product. In spite of the fact that a large volume of gas present in the
foamed mass impedes the higher rate of heat transfer, drying rate is comparatively high
because of enormous increase in liquid-gas interface. The advantages of the foam-mat
drying process include relatively fast drying, easy reconstitution and better quality
product. The renewed process is of potential interest in foam-mat drying for developed
and developing countries for its simplicity, rapid drying at lower temperature, suitable
for all type of juices, retention of nutritional quality and cost effective for producing easi-
ly reconsitituble juice powders.
5.1.1. History
In 1950 Eddy reported that process of production of free flowing powders of fruits
and vegetables by spray drying or drum drying is facing great difficulties. To overcome
these problems he recommended the use of methylcellulose for the preparation of spray
dried grapefruit and orange juice powders.
Foam mat drying originally was developed by Morgan et al. in 1959 at the Western
Regional Research Laboratory of the U.S. Department of Agriculture. The foam-mat
process involves drying thin layers of foamed material in heated un-dehumidified air at
atmospheric pressure and is reported to be considerably cheaper than vacuum, freeze
and spray drying methods.
5.1.2. Process
The essential steps in foam-mat drying are as follows:
(a) Pre-treatment of the raw material and preparation of liquid concentrate;
(b) Conversion of the concentrate into a stable foam by incorporating air or other
gases and a suitable foaming agent;
(c) Exposing the foam in the form of a thin sheet to a current of hot air until it is de-
hydrated; or by hot water conduction method; and
(d) Conditioning and grinding of the dried porous mass into an easily reconsitituble
powder in water.