Milk Gelation Studied with Small Angle Neutron Scattering Techniques and Monte Carlo Simulations

Abstract

The sol-gel transition of fat-free milk by acidification was studied with neutron scattering experiments and Monte Carlo simulations. Spin-echo small angle neutron scattering (SESANS) and ultrasmall angle neutron scattering (USANS) experiments were performed to measure the static structure of milk and yogurt, as well as the aggregation kinetics. Colloidal gelation was simulated from a reaction limited domain (RLCA) to the diffusion limited regime (DLCA) as cluster-cluster aggregation of adhesive, hard spheres on a three-dimensional lattice. Comparisons were drawn between experimental and numerical correlation functions. Milk was modeled as a suspension of casein micelles in water, and its structure was described with a dilute system of solid spheres with a log-normal distribution of sizes. The structure and formation of yogurt were described with a self-affine model, used for systems containing heterogeneities with a wide range of sizes. Simulation speed was increased by 1 order-of-magnitude using a new algorithm to eliminate dead time. Observations by SESANS and USANS of milk particle sizes and yogurt length scales were consistent and agreed well with literature. Kinetic USANS data yielded reliable information about the growth of typical length scale during aggregation. The simulation model predicted the measurement data qualitatively best staying close to the RLCA regime until large structures had formed. Correlation lengths were in good quantitative agreement, but longest simulated length scales were a of factor 2(1)/(2) below experimental findings. We conclude that small, mobile aggregates are formed during the first 3 h, mostly influencing the dimensionality of the system and that large, inert structures are formed from 2 up to 8 h, which determine the typical length scale.