Microstructure of Spray-Dried and Freeze-Dried Microalgal Powders

Chiarella and Spirulina algae were spray-dried and freezedried under several different conditions, and the morphological changes induced were studied by scanning electron microscopy. The internal structure of the particles (granules) was revealed by cryofracturing. The cellular composition of the two algae, method of drying, cell concentration, feed conditions, temperature, and drying times all affected the external morphology of the resulting powders. In the case of spray-drying, at a temperature of 160 °C for 6 sec, the particles shrunk or collapsed; if the drying time was over 10 sec at 160 °C, similar results were observed. The cell concentration affected the thickness of the dried particle wall ; a high concentration produced thicker and heavier particles, lower concentrations produced thinner and lighter particles. The rate and pressure at which the liquid was fed into the drying chamber affected the proper atomization of the liquid . When freeze-dried , the algal powder structure was different from particles formed by spray-drying. Spray-dried particles were individual spheres with a void space in the center. Each particle was composed of a few thousand cells in the case of Chiarella and several trichomes in the case of Spirulina. In contrast freeze-dried algae formed sheets of cells that were no longer spherical and which adhered together in a linear fashion. The cryogenic process used in this study to fracture and study the structure of spray-dried microalgal powders should prove to be very helpful in selecting optimal conditions for the production of high quality health food products