Ultrastructural studies on microencapsulated oil droplets in aqueous gels and dried films of a new starch-oil composite

Combination of starch, water, and oil by a process utilizing an excess-steam jet cooker produces a new class of stable, oil-in-water dispersions. Examination of aqueous gels by light microscopy (LM) and transmission electron microscopy (TEM) and of dried thin films by scanning electron microscopy (SEM) shows that the oil is microencapsulated in the starch water matrix or in the dried starch matrix as droplets that are typically 1-10 mu m in diameter. The size and distribution of oil droplets in cornstarch-soybean oil composites are determined by a number of factors, such as oil:starch ratio, the number of times the formulation is passed through the steam jet cooker during preparation, and the steam pressure used during cooking. Oil droplet size can be reduced by addition of protein or starch-oil composites from a previous cook to the formulation prior to cooking. Oil droplet distribution in these composites is observable not only in scanning electron micrographs of fracture surfaces, but also by light microscopy and by transmission electron microscopy of aqueous gels. Micrographs suggest the presence of a boundary layer surrounding the oil droplets which prevents them from coalescing.