Characterization of ash in algae and other materials by determination of wet acid indigestible ash and microscopic examination

Algae are known for high ash content. It is important to properly characterize their ash for value added utilization of algae as food, feed, and feedstock for biofuels. In this study, 12 algae of different sources were measured for proximate composition and mineral profile. Results showed that the relative difference between ash content by dry ashing and total minerals content by wet digestion increased with ash content. A major cause was soon identified: when using a common procedure of strong attacks for sample digestion before mineral analysis, incomplete digestion existed for most algae samples due to the presence of siliceous materials. It was proposed that algae consist of wet acid indigestible ash (WAIA) and wet acid digestible ash, whereas WAIA is siliceous. Methods to measure WAIA content in the 12 algae, along with oat grain, oat forage, defatted soymeal and fine sand, were then developed based on digestion with nitric acid or sulfuric acid-hydrogen peroxide. For the 12 algae, ash ranged 1.9 to 37.4% dry matter while WAIA by nitric acid digestion varied 0.1% to 25.6%. High correlation between WAIA and ash contents indicates WAIA as an important contributor for algae ash. For identifying what constituted the siliceous materials, all samples in three matrixes (original, ash by dry ashing, and WAIA) were microscopically examined. Because wet acid digestion had an ability to concentrate siliceous materials and maintain their original shape and size, WAIA was the best matrix for microscopic examination. Micrographs of WAIA show three types of siliceous materials in algae: non-diatom cellular structures, diatom cell walls, and sandy particles. It was concluded that high ash content of algae resulted partly from contamination of diatoms and/or sandy particles of geologic origin and that WAIA should be an important quality parameter for algae. Subsequently, several measures are proposed to produce algae with low ash content.