Influence of fatty acid chain length and unsaturation on mid-infrared milk analysis

Our first objective was to optimize center wavelengths and bandwidths for virtual filters used in a Fourier transform mid-infrared (MIR) milk analyzer. Optimization was accomplished by adjusting center wavelengths and bandwidths to minimize the size of intercorrection factors. Once optimized, the virtual filters were defined as follows: fat B sample, 3.508 μm (2,851 cm^-1), and bandwidth of 0.032 μm (26 cm^-1); fat B reference, 3.556 μm (2812 cm^-1), and bandwidth of 0.030 μm (24 cm^-1); lactose sample, 9.542 μm (1,048 cm^-1), and bandwidth of 0.092 μm (20 cm^-1); lactose reference, 7.734 μm (1,293 cm^-1), and bandwidth of 0.084 μm (14 cm^-1); protein sample, 6.489 μm (1,541 cm^-1), and bandwidth of 0.085 μm (20 cm^-1); protein reference, 6.707 μm (1491 cm^-1), and bandwidth of 0.054 μm (12 cm^-1); fat A sample, 5.721 μm (1,748 cm^-1), and bandwidth of 0.052 μm (16 cm^-1); fat A reference, 5.583 μm (1,791 cm^-1), and bandwidth of 0.050 μm (16 cm^-1). The bandwidth and its proximity to areas of intense water absorption had the largest effect on the intercorrection factors. The second objective was to quantify the impact of fatty acid chain length and unsaturation on fat B and fat A MIR measurements. Increasing the chain length increased the difference (i.e., MIR minus reference) between MIR prediction and reference chemistry by 0.0429% and by -0.0566% fat per unit of increase in carbon number per 1% change in fat, for fat B and fat A, respectively. Increasing the unsaturation decreased the difference (i.e., MIR minus reference) between MIR prediction of fat and chemistry for fat B by -0.4021% and increased fat A by 0.0291% fat per unit of increase in double bonds per 1% change in fat concentration.