Foam characterization aids processing.

Foam mat drying may be used commercially with a variety of juices. In this process, a small amount of edible foam stabilizer, such as a monoglyceride or a modified soybean protein with methyl cellulose, is added to liquid. A stiff foam is produced by whipping the material. The foam is spread in a thin layer and dried in a stream of hot air. The product separates easily into small particles when cooled.

Foam mat drying is a good way to dehydrate liquid foods in a short amount of time. The porous structure of the foamed materials enhances mass transfer, which leads to shorter dehydration times. The drainage of the liquid phase causes the structure of foams to change over time. This intrinsic instability of foams must be addressed before the foam is processed.

Canadian scientists sought to characterize the stability of apple juice foams as a function of their composition, properties and processing conditions. They prepared apple juice foams using 0.1% to 3% egg white powder or methylcellulose and different whipping times, ranging from 3 minutes to 7 minutes. The foams' stability, rheological properties and physical properties, such as foam density and bubble size, were measured to evaluate the effect of foaming agents and whipping times on the foam's stability and characteristics.

Foams made with egg white powder were less dense than those made with methylcellulose. There was an improved capacity of the continuous phase to include air. However, the methylcellulose foams were more stable, more uniform and had a smaller bubble diameter at equal concentrations and whipping times.

Density was at a minimum as the concentration of foaming agent increased. Preliminary rheological tests showed that the viscoelastic behavior of the foams depended on the range of foam density--higher or lower than the minimum density value. This research on foam characterization and stability is expected to help in the processing of foams and contribute to the production of tailor-made food products through the manipulation of the initial material characteristics.

Further information. Cristina Ratti, Soils Science and Agri-Food Engineering, Universite Laval, Paul-Comtois Building, Room 2407, Ste-Foy, Quebec, Canada G1K 7P4; phone: [phone]; fax: [phone]; email: [e-mail].