Non-destructive assessment of milk quality using pulsed UV photoacoustic, fluorescence and near FTIR spectroscopy

Abstract

Abstract The work describes the application of photoacoustic (PA), fluorescence spectroscopy, and Fourier transform near-infrared spectroscopy as non-destructive optical techniques to examine the quality of milk. The amplitude of the acoustic wave was linearly proportional to the absorbed fluence. The acoustic velocity and the fluence threshold for onset of non-linearity were decreased as the fat % increased. Initially, the PA pressure was increased with fluence but it exhibited non-linearity and occurred earlier i.e. faster as the fat % was increased. The peak pressures of 120, 160, and 180 kPa were determined for 1%, 2%, and 3.5% respectively. The corresponding acoustic transient times of 0.5, 0.44, and 0.36 µs were calculated for 1%, 2%, and 3.5% milk respectively. The absorption coefficient of milk samples was determined using the pressure-fluence slope and Grüneisen constant, which increased with fat %. The bandwidths between 350–450 nm and 450–550 nm correspond to tryptophan or valine, and Methionine amino acids respectively, and the peak at ≈315 nm is thought to be due to tyrosine. The fluorescence intensity of the sample day 1 (D1-open) decreased with time more significantly due to variations in the environmental condition. The bands between 4000 and 4500 cm−1 correspond to CH-stretch, and day 4 (D4-closed) showed the highest peak amplitudes compared to the others. Combination of N–H and O–H stretch was mainly observed between 4500 and 5000 cm−1, and the bands at 4581, 4655 cm−1 in fresh sample disappeared in D1-open and D1-closed. New bands of 4717, 4792, and 4829 cm−1 were observed.