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 quality 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 liquid to stable foam follows air drying at relatively low temperatures to form a thin porous 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.
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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.
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 dehydrated; or by hot water conduction method.
(d) Conditioning and grinding of the dried porous mass into an easily reconsitituble powder in water.
Process 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.
The most desirable characteristics of a foam honey comb is that it should consist of a large number of small and uniform bubbles which should retain their honeycomb structure during drying. Foam bubbles usually burst because of air and thus surface energy of the resulting drops is smaller than that of the original system. Adequate amounts of sta bilizers are added to provide stability to the foam, which would otherwise collapse before all the moisture has been removed during drying. Foam stabilizers might be surface-active agents, hydrophilic colloids or a mixture of both.
The characteristics of the foam are measured in terms of two parameters i.e. foaming 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.
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.
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 concentration 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 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 significant 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.
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 different levels to varying composition of the food concentrate, and their performances could be measured by their foaming, reconstitution, and sensory attributes.
DRYING OF FOAMS
In order to dehydrate the foams, different drying techniques like vacuum foam drying, foam spray drying, foam-mat drying including crater techniques are in practice worldwide. Development of a belt-type foam-mat dryer and production of free flowing, rehydratable powders from tomato juice, milk, coffee extract orange juice, pineapple juice, mango juice etc.
is a major researchable issue, subsequently several basic forms of foam-mat dryers including the laboratory type adiabatic dryer.
A belt-type foam-mat dryer consists of an endless steel or Teflon-coated belt which alternatively passes over the heating and cooling drums. The foamed material is spread over the moving belt and temperature of air flowing across the material through the ports is controlled in the range of 65º C to 21º C. The drying in such process takes about one hour time depending upon the characteristics of the material being dried. The difficulty with such dryers, however, is the removal of the product from belt in certain cases.
ADVANTAGES OVER TRADITIONAL METHODS
This method offers wide scope for drying of other difficult-to-dry sticky, viscous and heat sensitive materials like glue, gelatin, pigments Varnishes etc. Any fluid material that needs to be dried gently could be dried by the foam-mat method provided it is capable of forming stable foam. The main advantage of foam is that the structure provides a large surface at which water may rapidly evaporate. Water vapor produced below the already dry outermost portions diffuses through the thin, warm walls of these outer parts, and not through the thick, dense skin, which would result from shrinkage of an unfoamed mat. The same structure ensures rapid rehydration of the dry product. The water travels along the web of solid material, like ink onto blotting paper.
Over the past decade, this relatively old technology, known as foam mat drying, received 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 materials.
Foam-mat dried vegetables or fruit powders have less heat induced changes in color and flavor as compared to conventional spray-dried or drum-dried products. A product 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