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

Authors: Liu, Keshun

Submitted to: Algal Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2017
Publication Date: 5/12/2017
Citation: Liu, K. 2017. Characterization of ash in algae and other materials by determination of wet acid indigestible ash and microscopic examination. Algal Research. 25:307-321.

Interpretive Summary: Algae have been used as animal feed (including aquafeed), human food, fertilizer, and in recent years as a biomass for biofuel production. Yet, algae is known for having high ash content, some can have ash up to 70% of dry matter. High ash content not only reduces the amount of ash-free matter, the valuable portion of algae biomass, but also diminishes inclusion levels for feed use. It also poses major operational problems in biomass conversion systems including slagging, fouling, high temperature corrosion, and bed agglomeration. The present study is among the very few to simultaneously achieve several important objectives, including 1) to document that silica-containing materials are important contributors of the ash component for algae, particularly those of high ash contents, 2) to develop a reliable and easy to adapt methodology for quantitatively measuring silica content in biological (including algae, forage, grain and grain by-product) and geological (sand) materials, 3) to characterize the chemical nature of algae ash, and 4) to provide evidence and explanation on why some algae have high ash content. With 12 algae and 4 non-algae samples, the study is the first to show that there are two types of ash: wet acid digestible and wet acid indigestible, based on their digestibility in a wet acid digestion system containing strong acids. The latter is silica in nature. There is a high and positive correlation between wet acid indigestible ash content and ash content, strongly indicating that silica is an important component of the algae ash. Microscopic examination of wet acid indigestible ash show three types of silica materials in algae: cellular structures of non-diatoms, diatom cell walls and sandy particles of geologic origin. Among them, contamination of diatoms and contamination of sandy particulates are the two major contributors towards high ash content of algae. Therefore, the present study basically addresses several important questions facing algae and feed industries. These include: 1) why algae have higher ash contents, some extremely so, compared to other biological materials, 2) why algae have a large variation in ash content, 3) do ash components in algae differ from other biological materials, 4) what is the chemical nature of algae ash, and 5) how to prevent ash built-up during algae production. The study also proposes several measures to produce algae with reduced ash content and recommends that WAIA content be established as an important quality parameter for algae.

Technical Abstract: 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.