Drying characteristics and quality of banana foam mat

Abstract

The combination of foaming and drying is an alternative method to produce crisp banana chips. The influences of whipping time and egg albumen concentration on the foam density were studied. The influences of foam density, drying temperature and egg albumen concentration on the drying characteristics and qualities of the final products in terms of shrinkage, texture and microstructure were subsequently evaluated. Banana puree with egg albumen, which was used as the foaming agent, was foamed to the densities of 0.3, 0.5 and 0.7 g/cm³. Banana foam mats with 5 mm thickness were then dried to the moisture content of 0.03 kg/kg db at 60, 70 and 80 °C and superficial air velocity of 0.5 m/s. The experimental results showed that the extensive porous structure of foams with lower densities resulted in higher drying rates, moisture diffusivities and shrinkage. Dried banana foams with lower foam densities also had lower hardness and crispness values. The drying temperature and the egg albumen concentration did not influence the textural properties of the final products, however. To produce banana chips, the initial foam density of 0.5 g/cm³ and drying temperature of 80 °C were recommended.

Introduction

Bananas generally deteriorate rapidly after harvesting. The production of banana chips is an alternative way to preserve the quality and also to add value to bananas. Banana chips can be produced by various conventional methods, e.g., frying and hot air drying. However, there are some limitations of each method. Reduction of nutritional values is observed in dried products (Matz, 1976). The longer drying time is also required for low-temperature air drying (Sankat et al., 1996, Demirel and Turhan, 2003, Nguyen and Price, 2007). Also, air drying may yield the non-crisp product since its moisture content cannot be reduced to the desired value, approximately 0.04 kg/kg db, by low-temperature drying. Although high-temperature drying can produce crisp products, browning of the products, which is accelerated by high-temperature may occur (Tsami and Katsioti, 2000).

Vickers and Bourne (1976) demonstrated that dry crisp foods probably consist of cells or cavities, which are usually filled with air and a structural phase or cell walls that are formed by a brittle matrix. The combination of foaming and hot air drying is thus a feasible option to produce crisp banana chips because banana foams are very porous. In addition, the products can be dried rapidly to 0.04 kg/kg db, even at low-temperature, due to the less dense structure of the foams (Garcia et al., 1988, Sankat and Castaigne, 2004).

Foaming is a process by which liquid or semi solid foods are whipped to form foams. Many foods, e.g., egg white, beef extract and milk, naturally contain soluble proteins, which can be converted into stable foams when being whipped (Hart et al., 1963). Soluble proteins, known as foaming agents, contribute to the formation and stability of the foam structure. Proteins move through the aqueous phase and are spontaneously adsorbed at the air–aqueous interface where the viscoelastic films are subsequently formed. The outcome of proteins adsorption is a reduction in surface tension, which improves the foam formation (Prins, 1988). Moreover, the viscoelastic films are generally resistant to rupture and coalescence of gas bubbles dispersed in the liquid phase (Cherry and Mcwatters, 1981, Karim and Wai, 1999a).

Banana purees need to be incorporated with a foaming agent in order to be whipped to produce foams. Several foaming agents such as modified soybean protein (Gunther’s D-100) (Bates, 1964), soy protein isolate (ICN Biomedicals) (Sankat and Castaigne, 2004) and dried egg albumen (Garcia et al., 1988) can be used to produce foamed bananas. Foams must also retain the open structure during drying. If foam breaks or drains excessively, increase in the drying time and poor product qualities, e.g., extreme shrinkage, may occur (Hart et al., 1963).

As mentioned earlier, the foam structure plays a major role in moisture movement during drying and also on subsequent product quality. Most prior works have emphasized on the drying characteristics of foamed foods (Cooke et al., 1976, Garcia et al., 1988, Karim and Wai, 1999b, Sankat and Castaigne, 2004) and Fick’s law of diffusion, assuming constant moisture diffusivity, was used to describe the moisture content evolution during drying. In spite of its importance, the information on the quality of dried foamed products has received much less attention, however.

The objectives of this work were therefore to study the effects of whipping time and egg albumen concentration on the foam density. Subsequently, the effects of foam density, drying temperature and egg albumen concentration on the drying characteristics and the qualities of the final products in terms of shrinkage, texture and microstructure were evaluated. The effective moisture diffusivity of the foams was also determined by the method of slope.