Effect of solvent and temperature on the size distribution of casein micelles measured by dynamic light scattering

Abstract

The objectives of this study were to investigate the effect of the solvent on the accuracy of casein micelle particle size determination by dynamic light scattering (DLS) at different temperatures and to establish a clear protocol for these measurements. Dynamic light scattering analyses were performed at 6, 20, and 50 degrees C using a 90Plus Nanoparticle Size Analyzer (Brookhaven Instruments, Holtsville, NY). Raw and pasteurized skim milk were used as sources of casein micelles. Simulated milk ultrafiltrate, ultrafiltered water, and permeate obtained by ultrafiltration of skim milk using a 10-kDa cutoff membrane were used as solvents. The pH, ionic concentration, refractive index, and viscosity of all solvents were determined. The solvents were evaluated by DLS to ensure that they did not have a significant influence on the results of the particle size measurements. Experimental protocols were developed for accurate measurement of particle sizes in all solvents and experimental conditions. All measurements had good reproducibility, with coefficients of variation below 5%. Both the solvent and the temperature had a significant effect on the measured effective diameter of the casein micelles. When ultrafiltered permeate was used as a solvent, the particle size and polydispersity of casein micelles decreased as temperature increased. The effective diameter of casein micelles from raw skim milk diluted with ultrafiltered permeate was 176.4 +/- 5.3 nm at 6 degrees C, 177.4 +/- 1.9 nm at 20 degrees C, and 137.3 +/- 2.7 nm at 50 degrees C. This trend was justified by the increased strength of hydrophobic bonds with increasing temperature. Overall, the results of this study suggest that the most suitable solvent for the DLS analyses of casein micelles was casein-depleted ultrafiltered permeate. Dilution with water led to micelle dissociation, which significantly affected the DLS measurements, especially at 6 and 20 degrees C. Simulated milk ultrafiltrate seemed to give accurate results only at 20 degrees C. Results obtained in simulated milk ultrafiltrate at 6 degrees C could not be explained based on the known effects of temperature on the casein micelle, whereas at 50 degrees C, precipitation of amorphous calcium phosphate affected the DLS measurement.

Changes in particle size, calcium and phosphate solubilization, and microstructure of rehydrated milk protein concentrates, prepared from partially acidified milk

“…The apparent diameter of casein micelles in control skim milk was 169.2±0.9 nm. No significant changes of diameter were detected in reconstituted control MPC samples (UFC and DFC) compared to skim milk, in agreement with other research reported in the literature (Beliciu and Moraru 2009,). However, the acidified samples (UFG and DFG) exhibited a significant increase in the apparent diameter of the reconstituted casein micelles.…”

Comparison of nanofiltration and evaporation technologies on the storage stability of milk protein concentrates

“…As the rehydration proceeds (from wetting to complete solution of powder particles), the values of viscosity, turbidity, and particle size gradually decrease and tend to be stabilized at the end (Donnell and Butler 1999;Ma 2012;Beliciu and Moraru 2009). It needs to be noted that all the MPC samples stored at different temperatures in this study were rehydrated within 10 min.…”

“…Researchers have adopted various techniques and parameters to study the rehydration properties of food powders, such as viscosity (Gaiani et al 2006), turbidity (Gaiani et al 2009), particle size (Beliciu and Moraru 2009), and microscope technology (Mimouni et al 2010a). The dissolution of mineral elements, such as calcium, phosphorus, magnesium, sodium, and potassium from powder to water could effectively affect the rehydration phase of food powders (Augustin and Clarke 1991).…”

Impact of Microfiltration on Particle Size Distribution, Volatile Compounds and Protein Quality of Pasteurized Milk during Shelf Life

“…However, when milk is subjected to processing treatments, the fat globules are disrupted and their average diameter decreases dramatically [6,9]. Changes in particle size and particle size distribution (PSD) are important contributors to surface area, opacity, reactivity, rheological properties, aggregation or dissociation phenomena of dispersed systems [8,10], which offer valuable indicators to predict the sensorial quality properties and stability of milk systems [6,11].…”