New and improved methods for measuring acid insoluble ash

Abstract

Acid insoluble ash (AIA) is a part of total ash, representing siliceous compounds in herbs, food, feed, and biomasses, while serving as a marker for animal nutrition studies. Yet, reported methods for AIA determination are rudimentary, time-consuming, energy inefficient, prone to errors, and variable in steps and conditions. 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 of AIA measurement. These included ashing temperatures, 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 (p < 0.05). 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 2 N 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 use less HCl, energy and time (eco-friendly), and are easier to master 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 the 12 selected algae, AIA content varied from 0.01% to 22.86% dry matter, representing 0.29–61.62% total ash.