Determination of the mass fraction of milk fat in kefir by IR spectroscopy and digital colourometry methods

A non-contact method for determining the mass fraction of milk fat in bottled kefir using diffuse reflection of radiation from LEDs with different wavelengths (365, 390, 850, and 880 nm) using a smartphone and a special homemade colourimetric device was proposed. To record the analytical signal we used a smartphone OnePlus 10 Pro, iPhone 14 with PhotoMetrix PRO®, ColorGrab, RGBer, and FTIR spectrometer for the near infrared region (4000 – 10000 cm–1). The experimental data were processed using specialised programs: TQ Analyst, The Unscrambler X, XLSTAT. It was found that the intensities of RGB channels change depending on the fat content of kefir, which indicates that there is a correlation between colour characteristics and milk fat content. The influence of protein and carbohydrate content on the analytical signal was found to be minimal, as their concentration in kefir with different fat content varies insignificantly. These data confirm the possibility of using digital RGB values as an analytical signal for determining the fat content of kefir. It is shown that polyethylene terephthalate packaging has minimal effect on diffuse reflection, which allows analysis without opening the packaging. The mass fraction of milk fat in kefir was estimated using the method of multivariate data calibration — partial least squares regression. The relative standard deviation of the results did not exceed 0.11%, which confirms the high accuracy of the method. The best results were obtained using LEDs with emission wavelengths of 390 and 850 nm, when RMSEC and RMSEP reached maximum values of 0.0901 and 0.887%, respectively. Equivalence of the results obtained was confirmed by FTIR spectroscopy.