Lipolysis effect on milk fat and protein analysis by infrared spectroscopy using filter and Fourier Transform Infrared (FTIR) methods

Abstract Text: Normal composition is an important aspect of milk quality. The infrared spectroscopy (IR) is used by most laboratories worldwide to determine milk composition. However, the results obtained by this method may be affected if reactions, such as lipolysis, occurs in milk. The objective of this work was to quantify the effect of lipolysis on milk fat and protein measurement by IR spectroscopy, using filter and FTIR (Fourier Transfor Infrared) methods. Ten liters of high quality raw milk were collected, and immediately pasteurized (LTLT). This volume was aliquoted into 40-mL samples containing bronopol as preservative. Lipolysis was induced using Pseudomonas fluorescens Lipase (Sigma Aldrich^®; 20.000 U/g; EC Number 232-619-9 ) added at three concentrations (100 U, 300 U, 600 U), and followed by incubation at 7°C, 20°C, 30°C, and 40°C. Samples without enzyme addition were used as control. Incubation periods were 0, 3, 6, 24, 48, and 96 hours. IR spectroscopy methods included filter (Bentley® 2000 Combi-System) and FTIR (Combiscope^® FTIR) equipment. Experiment was a split plot design, with treatments as factorial 4 x 4 x 6 x 2 (enzyme concentrations, temperatures, incubation periods, and IR methods). ANOVA with Duncan test (p=0.05) were used to detect differences among treatments. There was a significant effect of milk lipolysis on milk components measured by both IR spectroscopy methods, with reduction of up 27% (2.87 g/100g to 2.08 g/100g) of fat, and increase of up to 15% (3.40g/100g to 3.92 g/100g) of protein readings for the treatments with the highest enzyme concentration and incubation at temperatures of 20°C, 30°C and 40°C. Although component analysis in both infrared methods were affected by milk lipolysis, FTIR spectroscopy was more robust than filter method for fat measurement (p <0.05). It is concluded that lipolysis can have a significant effect on fat and protein contents measured by both IR spectroscopy methods. This is of particular concern in dairy herd improvement programs in tropical regions, where samples are usually transported and stored for long periods without refrigeration before arriving to the official laboratories. These results indicate the need for better sample management in order to obtain compositional results alike the original samples of milk.

Keywords: Lipolysis, infrared spectroscopy, milk quality, lipase