Estimating volatile fatty acid concentrations in rumen samples by Fourier transform mid-IR transmission spectroscopy

This paper investigates the possibility of determining concentrations of acetate, propionate and butyrate in rumen fluid by Fourier transform mid-infrared (FTIR) transmission spectroscopy. A total of 308 samples were used for calibration of the FTIR instrument, normally used for milk analysis. The samples consisted of semi-artificial rumen fluids, spiked with acetate, propionate and butyrate with and without added bicarbonate and phosphate. The Test set consisted of 123 rumen in vitro samples with a mean value for the sum of the three volatile fatty acids of 5.5 g/L (84 mM), determined by high-performance liquid chromatography (HPLC). Stepwise regression was applied to find suitable wavelengths for predictions, mainly in the region of 6.3–10 μm. A total of 12–13 wavelengths were used for the prediction equations and resulted in mean prediction errors (MPE) ranging from 0.031 to 0.043 g/L (approximately 0.5 mM) for the three VFAs and ratios of population standard deviation (SD) to SD of the regressions > 11.6. The calibration equation predicted the Test set well, with a MPE of 0.074, 0.066 and 0.119 for acetate, propionate and butyrate, respectively. More than 0.5 of the mean square prediction error was due to non-random errors which it should be possible to adjust for by use of calibration samples. A rough estimate also indicated that 0.09–0.16 of the mean square prediction error was due the HPLC method. The FTIR method is recommended for applications were required measurement accuracies are in the range of 1 mM.