Estimation of milk fat globules distribution in milk using scattering in near infrared region

The interaction between the electromagnetic radiation in the near infrared region and suspensions of small particles in continuous media such as fat globules in milk generates scattering phenomena. The NIR spectrum of whole milk arises from both absorbance due to molecular vibrations and elastic scattering. The amount of photons that are deviated from theirs straight trajectories depends on the wavelength and on the size of the scattering particle. The extraction of the information relate to the size distribution of scattering particles requires the development of a suitable physical model in order to extract the dimensional information about fat globules from NIR spectra. The milk samples were collected monthly for one year from 50 cows (2 breeds) arranged in 6 different farms with different feeding types. All the cows were half sisters with 6 common fathers for Frisean and 3 for Jersey breed. The reference particle size analyses of fat globules were performed using a Mastersizer 2000 (Malvern instruments Ltd., UK) granulometer (laser=633 nm). NIR spectra were collected using a NIRFlex 500 (Buchi, Switzerland) spectrometer in transmission mode using a quartz flux cuvette (pathlength=0.2 mm) at 40°C. The model was developed using both Visual Basic for Excel (Microsoft, USA) and Matlab (The Mathworka, USA). A model based on the Evans Fournier approximation of Mie scattering was developed using the Weibull distribution for the description of the fat globule distribution in calculating the contribution of scattering to the milk transmission spectrum. The inversion of the model was performed using the conjugate gradient method in order to estimate the particle size distribution from the spectra using the regions 1000-1360 and 1580-1800 nm that are free of absorption bands. The correlation between granulometric data and NIR data was of 0.95 for the median diameter with a SEP of 0.14 microns and of 0.92 for volume-surface average diameter. The samples analyzed showed that the particle size distribution of milk fat globules within the two cow breeds had a bigger dependence on the genetic of the cow than on the farm. The method developed in this work can be useful in monitoring cows and creaming processes: small fat globules are richer in phospholipids than big ones and well suited for new functional foods development, while big fat globules are better for whipping purposes.