Banana foams

Thuwapanichayanan et al. (2012) determined the drying curves of banana foams produced with aid from one of three foaming agents, namely egg albumin (EA), soy protein isolate (SPI) and whey protein concentrate (WPG) (Fig. 6.19). The results of these studies showed a strong effect of surfactant type on drying rate due to the development of different foam structures, while the exponential decrease in moisture content with time proved that the drying process of banana foams was controlled by internal diffusion. 

Fig. 6.19 D ing curves of banana foams with different foaming additives. Egg albumin (EA), soy protein isolate (SPI), and whey protein concentrate (WPC). Adapted from Thuwapanichayanan etal. (2012).

Sopomonnarit, S., 2008. Modeling of diffusion with shrinkage and quality investigation of banana foam-mat drying. Drying Technol. 25(11) ... 

Sopomonnarit, S., 2012. Effects of foaming agents and foam density on drying characteristics and textural property of banana foams. Food Sci. Technol. 47 348— 357. 

The thickness of the foam layer appears to be the key parameter for both product quality and process performance. An overview by Zbicinski et al. [Ill] indicates that the foam layer thickness determined the drying time through the moisture evaporation rate but also density and porosity of the solidified structure. Typically, the foam thickness is kept within 1-5 mm, but for some materials like banana, the foam layer can be as deep as 10-20 mm. Foams thicker than 20 mm caused undesirable changes in the final product (e.g., caramelization, nonenzymatic darkening, loss of aromatic components) due to high content of sugars and acids. 

Similar to air velocity, an increase of drying temperature intensified the process of moisture evaporation, but it increased susceptibility of the material to deformations. For bananas, the optimum tanperature was found as 60°C. Besides the layer thickness, tanperature, and air velocity, the drying rate of foamed materials depends mainly on foam characteristics like stability, density, and bnbble size. 

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. 

Sankat, C. K., Castaigne, F., 2004. Foaming and drying behavior of bananas. Lebensm.-Wiss. Technol. 37 518-525. 

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