Mathematical formulae for accurate estimation of in vitro CH4 production from vented bottles

A widely used method for the in vitro measurement of fermentation parameters, total gas and methane (CH4) production (mL/g) from feed samples employs Ankom(RF) Technology with vented bottles, initially flushed with carbon dioxide (CO2). The volumes of gas accumulated between ventings, and the volumes vented, are calculated from the headspace pressure data. By using these data, it is possible to infer mathematically the quantity of CH4 produced, given just a single measurement of the CH4 mixing ratio measured in the headspace at the termination of fermentation. However, this calculation requires an additional assumption. Two possibilities for this are considered. We present mathematical formulae arising from each, and describe their sensitivity to violations of their respective assumptions. Also considered is a formula based on using N-2 instead of CO2 as the flushing gas. It appears that, in practice, all three formulae are suitable for estimating CH4 production, with errors less than 3%.