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Title: Optical Characterization of Milk
Other Titles: Optische Karakterisatie van Melk
Authors: Aernouts, Ben
Issue Date: 3-Oct-2014
Abstract: Cow’s milk and derived products are important components in the Western diet. The composition of raw milk and the size distribution of suspended fat globules and casein micelles largely determine the nutritional, physicochemical and functional characteristics of the milk. Moreover, as these properties have a considerable impact on the general perception of the milk and the properties of derived products, they are important quality parameters for the dairy industry and the consumer. Additionally, the composition of the extracted raw milk and the size distribution of the suspended particles comprise valuable information on the metabolic, nutritional and general health status of the cow as there is a strong interaction between the cow’s blood circulation and the milk production. Therefore, techniques to measure the milk quality can help to improve cow health monitoring and enhance the efficiency and animal welfare on a dairy farm. Further on, the use of similar sensing-technology in a dairy plant could enable online monitoring of production processes to further reduce the variability and promote the development of new food products with improved properties.Optical measurement techniques are frequently used to monitor processes in industry and agriculture. Among them, spectroscopy studies the interaction between electromagnetic radiation and the product of interest. As the molecules of the product absorb this electromagnetic radiation at specific wavelengths, spectral analysis gives insight in the composition of the measured product. The molecular absorption of visible (Vis) and near-infrared (NIR) radiation is sufficient to be used for the analysis of the main sample components, while the penetration depth of Vis/NIR radiation is still adequate to reduce the need for extensive sample preparation. Nowadays, spectral detectors for this wavelength range are fast, robust and cost-efficient. Hence, it is widely used for non-destructive and online quality control in industry, food processing and agriculture. Several researchers have shown the potential of Vis/NIR spectroscopic analysis of raw milk in the lab. However, recent studies indicate that this technology might not be robust against changing scattering properties that are typical for on farm raw milk samples. Scattering is a process where the radiation is forced to deviate from a straight trajectory due to local non-uniformities. In milk, the fat globules and casein micelles are the main cause of Vis/NIR light scattering. This increases the travelling path of the Vis/NIR radiation to an unknown extent and seriously complicates the prediction of the sample composition from measured spectra. Accordingly, powerful and advanced techniques are needed to separate and extract pure absorption and scattering information from the obtained spectra. The pure absorption spectrum relates directly to the sample composition according to the Beer-Lambert law. Accordingly, prediction of the milk composition from the pure absorption spectrum with multivariate statistics would be more robust and independent of the effects of light scattering. The scattering properties, on the other hand, are determined by the physical microstructure properties (particle size distribution) of the sample. For milk, this primarily relates to the quantity and size of the suspended fat globules and, to a smaller extent, the casein micelles. Therefore, these scattering properties could be employed to extract this microstructure information, creating an added value for Vis/NIR spectroscopy on milk.Knowledge on the Vis/NIR optical properties of milk is essential for the design and optimization of a measurement configuration and model that would result in accurate and robust estimations for the composition and microstructure of milk. Accordingly, the measurement and study of these properties is the main objective of this dissertation. To this end, two sub-objectives were set: (1) the development of a measurement setup for accurate optical characterization of turbid media in the Vis/NIR wavelength range; and (2) to study the effect of quantity and size distribution of the suspended particles on the scattering properties of milk.The full optical characterization of turbid media is not straightforward and can only be achieved through an indirect method where multiple measurements, reflectance and/or transmittance, are combined with an inverted theoretical light propagation model. The sample’s total reflectance and total transmittance, as measured with double integrating spheres, together with an unscattered transmittance measurement are generally accepted as the ‘golden standard’ method to estimate bulk optical properties (bulk absorption coefficient, bulk scattering coefficient and scattering anisotropy factor). Therefore, a dedicated measurement setup was designed and built to acquire these measurements with high signal-to-noise ratios for turbid and absorbing media in the Vis/NIR wavelength range. This setup consists of a flexible high-power light source, which produces a pre-dispersed narrow collimated light beam and allows for fast and automated wavelength and waveband selection, two integrating spheres with detectors and an unscattered transmittance measurement path. The bulk optical properties of the sample can be extracted from these measurements with an inverse adding-doubling algorithm adapted from literature and optimized for the setup. The measurement and estimation procedure to obtain the bulk optical properties for turbid media was thoroughly validated on a set of 57 liquid optical phantoms. The phantom set was designed to cover a wide range of absorption and scattering properties by mixing intralipid (scattering agent), methylene-blue (absorption agent) and water (dilution agent) in different ratios, similar to the phantoms often used to validate measurement systems in the field of biomedical optics. Intralipid is an oil-in-water emulsion which is, except for the much smaller fat globule size, very similar to raw milk. It was found that the followed approach resulted in very accurate estimation of the sample’s pure absorption and scattering propertiesThe obtained dataset was further explored to investigate the effect of an increasing concentration of scattering intralipid particles on the phantom’s scattering properties. Furthermore, it was tested when the particle density is so high that individual scattering events start to influence each other, a phenomenon known as dependent or correlated scattering. It was found that dependent scattering has a significant impact on the scattering properties of intralipid-dilutions for particle volume concentrations above 2%. Additionally, semi-empirical equations were derived, describing the scattering properties as a function of the radiation wavelength and the volume concentration of scattering intralipid particles, taking into account dependent scattering.To study the effect of the size distribution of suspended spherical particles on the sample’s scattering properties, a simulation algorithm was developed that relates them. A generalization of the Mie solution for Maxwell’s equations was used to calculate the optical properties for a single spherical and scattering particle in an absorbing host medium. Accordingly, the optical properties were combined for multiple particles and polydispersity was supported by discretization of the provided particle size distribution. The number of discrete intervals is optimized automatically in an efficient iterative procedure. Finally, the developed microscale light propagation algorithm was validated by simulating the bulk optical properties for two aqueous nanoparticle systems and intralipid in the Vis/NIR wavelength range, taking into account the representative particle sizes. The simulated bulk optical properties matched closely with those obtained by the ‘golden standard’ method.In a next step, the gathered knowledge, measurement techniques and models were employed to study the effect of the fat globule size distribution on the Vis/NIR bulk scattering properties of milk. Ultrasonic homogenization was performed on raw milk to create milk samples with different fat globule size distributions. Next, the Vis/NIR total reflectance and total and unscattered transmittance spectra of these samples were measured and their bulk optical properties were estimated as described earlier. Additionally, the actual particle size distribution of fat globules and casein micelles was measured for each sample and the obtained distribution was used as an input for the microscale algorithm, described earlier, to simulate the sample’s bulk optical properties. Consequently, the validity of the developed microscale algorithm could be tested for milk by comparing the measured and simulated bulk optical properties. The simulated values were very close to the measured ones as long as scattering was independent. Moreover, it was found that a reduction in the fat globule size results in a higher wavelength-dependency of both the Vis/NIR bulk scattering coefficient and the scattering anisotropy factor. Therefore, these scattering parameters are very suitable to estimate the fat globule size distribution from. However, this requires inversion of the microscale model.Finally, the Vis/NIR bulk optical properties were determined for a diverse set of 60 raw milk samples that are representative for individual milkings on a dairy farm. The observed variability was discussed and the relation between the obtained bulk optical properties and the raw milk composition and fat globule size was extensively studied. The bulk absorption coefficient spectra were found to mainly contain information on the water, milk fat and casein content, while the bulk scattering coefficient spectra turned out to be primarily influenced by the quantity and the size of the fat globules. Moreover, there was a strong positive correlation between the fat content in raw milk and the measured bulk scattering coefficients in the 1300 – 1400 nm wavelength range. The variation in the scattering anisotropy factor turned out to be rather limited as it is mostly affected by the size of the fat globules, which is relatively constant between raw milk samples.Overall, it was found that the Vis/NIR bulk optical properties of emulsions can be accurately extracted from multiple spectroscopic measurements, after taking into account the theoretical aspects of light propagation. A strong relation, following the Mie solution, was found between the obtained bulk scattering properties and the amount and size distribution of suspended particles. Consequently, inversion of this Mie-model would clear the way for the estimation of the amount and size distribution of suspended particles from relatively easy spectroscopic measurements. Nevertheless, for dense media, a prior correction will be needed to account for the effect of dependent scattering. Additionally, the obtained insights and data can be used in simulation studies to efficiently optimize and validate different optical sensor designs and estimation-models for quality control of raw milk.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Division of Mechatronics, Biostatistics and Sensors (MeBioS)

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