Foam-mat drying

Foam-mat drying is a process in which a suspension, slurry, or solution is transformed into a stable foam by inert gas injection. The foam stmcture provides porosity and the mat is dried in trays or on a belt in a tunnel compartment, either under vacuum or with ckculating gas. A free-flowing powder capable of rapid rehydration results. Emit juices (qv) are dried successfully in this manner. 

Berry et al. (54) evaluated foam-mat dried instant orange juice and determined that its flavor was acceptable over 26 weeks when stored at 21.1°C. At 29.4°C flavor changes were observed after two to four weeks with samples in the pH range of 4 to 6. The flavor stability of the instant orange powder was directly related to pH when stored at 29.4°C. Stability was improved by using more acidic juices, addition of acid, or removal of sugar. 

Foams can also be involved in the drying of food products. A wide variety of foods, including fruits and vegetables, can be foamed and then spread out in thin layers on a support for foam-mat drying [633,845], Other foods, such as milk, cream, and cheese can be dried by foaming them, then injecting the foam into a spray-drier. This process is called foam spray drying [633],... 

Source Data from Lovric, R., Sablek, Z. and Boskovic, M. 1970. Cis-trans isomerization of lycopene and color stability of foam-mat dried tomato powder during storage. J. Sci. Food Agric., 21 641-647. Copyright Society of Chemical Industry. Reproduced with permission. Permission is granted by John Wiley Sons Ltd. on behalf of the SCI. 

LOVRIC T, SABLEK z and BOSKOVIC M (1970), "Cis-trans isomerisation of lycopene and color stability of foam-mat dried tomato powder during storage, J Sci FoodAgric, 21, 641-647. 

Aside from aforementioned hybrid technologies, there are two basic techniques for convective drying of foamed materials, namely, drying of bulk foams (foam-mat drying) and drying of dispersed foams (spray drying). 

In continuous systems, the product is spread over a stainless steel belt and passed throngh a vacnnm vessel with the help of rollers. The dried prodnct is collected at the other end. This method is also used in conjunction with foam mat drying, described previously. 

The foam drying process is limited to specific products, such as fruit powders, for preparation of instant drinks. Techniques like vacuum puff drying, foam mat drying, microflake dehydration, and foam spray drying have been described elsewhere in this book. Among these, the foam mat drying process has received considerable attention. 

Foam mat drying, originally developed by Morgan, involves drying thin layers of stabilized foam from liquid food concentrates in heated air at atmospheric pressure. Foam is prepared by the addition of a stabilizer and a gas to the liquid food in a continuous mixer. It can be dried in a continuous belt-tray dryer. Good quality powders capable of instant rehydration were made experimentally from tomatoes, oranges, grapes, apples, and pineapples [22]. 

Foam mat drying has two definite advantages [36]. First, the use of foam greatly speeds up moisture removal and permits drying at atmospheric conditions in a steam of hot air in a short time. Second, though the product may be sticky at drying temperatures, it can be transferred to a cooling zone and crisped before it is scraped off the surface. 

In spray drying, a gas-admixing technique is used to produce a dispersed foam from the feed, whereas in foam-mat drying mechanical methods (e.g., mixing, whipping) are applied to stabilize a bulk volume of the foamed raw material. 

In the foam-mat drying method Uquid foods are whipped into stable foams by using mainly mechanical methods, such as mixing to produce a bulk volume of... 

The main goal of research into foam-mat drying is first, to determine optimal drying conditions and, second, to select an appropriate surfactant in order to obtain high-quality products (e.g., Vemon-Cartera et al, 2001 Rajkumar et al,... 

The foam-mat drying technique was used successfully to dry exotic fruits such as mango (Cooke et al., 1976 Rajkumar et al, 2007), starfruit (Karim and Wai, 1999), banana (Sankat and Castaigne, 2004 Thuwapanichayanan et al., 2008, 2012), tomato paste (Lewicki, 1975, 2006 Kadam et al, 2012), and yogurt (Krasaekoopt and Bhatia, 2012). Because the density of foamed materials is lower than that of non-foamed materials (typically in the range of 300-600 kg m ), the mass load of a foam-mat dryer is also lower, which in turn leads to reductions in both dryer throughput and drying time. 

When Marques et al (2006) compared different methods for drying tropical fruit pulps, they recommended convective foam-mat drying for liquid foods as well as semi-liquid and thermosensitive foods, and stressed that the main advantages of this method were a reduced drying time, quick rehydration, and low drying temperatures. The main drawbacks of foam-mat drying were nonenzymatic darkening, a loss of aromatic components, and problems with foam stability. 

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