Effects of sample size, dry ashing temperature and duration on determination of ash content in algae and other biomass

Authors: Liu, Keshun

Submitted to: Algal Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2019
Publication Date: 4/17/2019
Citation: Liu, K. 2019. Effects of sample size, dry ashing temperature and duration on determination of ash content in algae and other biomass. Algal Research. 40:101486. https://doi.org/10.1016/j.algal.2019.101486.
DOI: https://doi.org/10.1016/j.algal.2019.101486

Interpretive Summary: Many laboratories routinely measure ash content as a part of analysis for nutritional or compositional evaluation of food, feedstuffs, algae, and other biomass. Therefore, an accurate and reliable measurement of ash content is critical in documenting quality of a biomass, whether it is either a food, feed, industrial material, or renewable fuel feedstock. The ash content in algae and other biomass is commonly measured by burning samples in a muffle furnace at a high temperature for a specified duration. The process is known as dry ashing. However, there has been large variation in ashing temperature and duration, as well as sample size (load), among reported methods. This variation makes it difficult to compare results among studies. The present study provides valuable information on which factors affect ash analysis for algae and non-algae biomass. It is the first to document that the interaction of ashing temperature and duration, rather than temperature alone, influences the ash content measured. It is also the first to show that, for algae samples with higher ash content, sample size can be another factor affecting ash determination. Therefore, for determining ash content, one needs to choose a proper combination of ashing temperature, ashing time, and sample size. Most importantly, the study proposes a standard method of dry ashing that provides reliable and repeatable values for determining ash content in all biomass.

Technical Abstract: Ash content is an important quality parameter for algae and other biomass, and dry ashing has been a primary method for ash determination. Yet reported methods vary greatly in ashing temperature, duration, and sample size, making results incomparable. The present study investigated effects of temperature (550, 600°C), duration (6, 16 h), and sample size (1, 4 g) on ash measurement of 13 algae and 4 non-algae (grains, soymeal, forage) samples under a factorial model. Results show that for most biomass both temperature and duration affected ash measurement significantly (p<0.05). Ashing 6 h at 600°C or 550°C overnight gave values similar to ashing at 600°C overnight, but ashing 6 h at 550°C produced higher ash content than other combinations. Furthermore, for algae having higher ash content, sample size also a significant factor when ashing at lower temperature and/or shorter duration, but the size effect could be alleviated by ashing at 600°C overnight. In conducting dry ashing, one needs to choose proper combinations of temperature, time and sample load. For complete combustion, ashing 1 to 4 g samples at 600°C overnight is recommended for all biomass.