New and improved methods for measuring acid insoluble ash

Authors: Liu, Keshun

Submitted to: Animal Feed Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/24/2022
Publication Date: 4/25/2022
Citation: Liu, K. 2022. New and improved methods for measuring acid insoluble ash. Animal Feed Science and Technology. 288. Article 115282. https://doi.org/10.1016/j.anifeedsci.2022.115282.
DOI: https://doi.org/10.1016/j.anifeedsci.2022.115282

Interpretive Summary: In animal nutrition research, markers are also used for determining nutrient digestibility of feed and feed ingredients as well as studying digesta kinetics, rumen protein synthesis, herbage intake and species selection. For years, acid insoluble ash (AIA) has been used as a natural marker. AIA is a part of total ash, representing siliceous compounds in herbs, food, feed, and biomasses, due to the natural presence of siliceous compounds and/or contaminations with dirt and sand. Therefore, besides serving as a marker for animal nutrition studies, AIA in these materials is also measured as an index for siliceous impurities. Yet, the current method for AIA determination is rudimentary, time-consuming, energy inefficient, prone to errors, and variable in steps and conditions among reports. This has made AIA a less reliable marker for animal nutrition studies and a less reliable quality parameter for herbs and certain food, food, and biomass. A researcher at USDA-ARS, Aberdeen, Idaho, systematically investigated effects of various factors at several functional steps on AIA measurement and developed a new and a significantly improved methods for measuring AIA in various materials of biological and geological origins. Both the new and improved methods are less time consuming, easier to master, and less prone to analytical errors, and can be considered standardized. The improved and standardized methodology should make AIA a much better and more reliable marker for animal nutrition studies. It also facilitates quality control and characterization of medicinal plants, certain foods and feed, and biomass (such as algae) as an index for silicious impurity.

Technical Abstract: Acid insoluble ash (AIA) is a part of total ash, representing siliceous compounds in herbs, food, feed, and biomasses, while serving as a simple marker for animal nutrition studies. Yet, the method for AIA determination is rudimentary, time-consuming, energy inefficient, prone to errors, and variable in steps and conditions among reports. This has made AIA an unreliable marker or quality attribute. With 16 samples of algae, grains, forage, soy meal, and sand, the present study systematically investigated effects of various factors at several functional steps on AIA measurement. These included ashing temperature and duration, conditions for HCl treatments (concentration, heating temperature and time, washing water temperature and cycles), and methods and conditions to recover AIA quantitatively. Results show that almost all factors investigated had significant effects on AIA values measured for given samples. Consequently, one new method and one significantly improved method were developed, featuring 1) dry ashing samples at 600°C overnight, 2) mixing a portion (not the entire lot) of ash with 2N HCl for dissolving acid soluble ash, 3) recovering AIA by centrifugation (new) or filtration with slow speed ashless paper followed by half-hour re-ashing (improved), and 4) having a reagent blank. The two methods were easier to master, less energy intensive, less time-consuming, and more repeatable than reported methods, making AIA a reliable marker or quality parameter. The study was also the first to document AIA content in microalgae. For 12 selected algae, AIA content varied from 0.01 to 22.86% dry matter, representing 0.29-61.48 % total ash.