Characterization of ash in algae and other materials by determining acid insoluble ash: method development and relationship to wet acid indigestible ash

Authors: Liu, Keshun

Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2017
Publication Date: 4/18/2017
Citation: Liu, K. 2017. Characterization of ash in algae and other materials by determining acid insoluble ash: method development and relationship to wet acid indigestible ash. Bioresource Technology. 25:307-321. https://doi.org/10.1016/j.algal.2017.04.014.
DOI: https://doi.org/10.1016/j.algal.2017.04.014

Interpretive Summary: Algae have many uses but high ash content hinders their use. In an effort to characterize algae ash and determine what contribute it, a previous study in our lab introduced a new concept of wet acid indigestible ash (WAIA) and developed methods to quantitatively measure its content based on wet ashing principle. The present study continued this effort by modifying methods for measuring acid insoluble ash (AIA), which have been developed based on dry ashing principle to measure siliceous compounds in herbs, and certain feed and food. For the past few decades, AIA has been extensively used as an internal marker for animal and fish digestibility studies. Yet there is a great variation among reported methods. This makes results difficult to compare. Furthermore, all reported methods are time consuming, difficult to master and prone to errors. There are two significant implications of the present study. First, this study was the first of its kind to make major changes in AIA determinations. It was also the first to symmetrically investigate various factors that might affect the AIA measurement, purely from chemical and physicochemical points of views. As a result, the improved methods are less time consuming, easier to master and less prone to analytical errors than the conventional methods. They will be very useful in several areas, including animal nutrition studies, quality control and characterization of biomass (such as algae), medicinal plants, and certain foods and feed. Second, this study was also the first one to document AIA contents in microalgae. Together with wet acid indigestible ash (WAIA) introduced and measured in a previous study, AIA represents another quality parameter for characterizing algae ash.

Technical Abstract: In a previous study, a procedure based on wet ashing was developed to measure wet acid indigestible ash (WAIA) in 12 algae and 4 non-algae (oat grain, oat forage, soy meal and sand) samples. Unlike WAIA, methods for acid insoluble ash (AIA) determination have long been developed to measure siliceous compounds in herbs, certain feed and food, and more extensively for use as an internal marker for animal and fish digestibility studies. Great variations exist among reports but an AIA method generally consists of dry ashing samples, mixing ash with a dilute HCl and heating, filtering with a paper filter, and re-ashing. In this study, the procedure for measuring AIA was significantly modified, and factors affecting both total ash and AIA determinations were investigated. Major changes included: using a portion of ash instead of the entire ash from a given sample, using centrifuge tubes for mixing ash with HCl, recovering AIA directly by use of a tared glass filter or centrifugation, and inclusion of reagent blanks. Two modified conventional methods using ashless filter paper and re-ashing to recover AIA were also included for comparison. Results show that the condition for making ash had significant effect on total ash and AIA measurements. The improved AIA methods were less time consuming, easier to master and less prone to errors than the conventional ones, with a mean relative standard deviation < 4%. The study was the first to document AIA content in microalgae. Total ash in 12 samples varied from 1.9 to 37.4% while AIA content ranged from 0 to about 22% dry matter, with a mean of 6.5%. This represented 0 to about 60% of total ash with a mean of 22%. Since AIA contents matched well WAIA contents, the two represent the same siliceous substances. It is proposed that algae contain two types of ash. One is acid insoluble and acid indigestible, which comprises extraneous and internal matters that are siliceous. The other is acid soluble and acid digestible, which comprises the rest of ash. WAIA or AIA content should be assessed as another important quality parameter for algae beside total ash content.