Journal of Colloid and Interface Science 253, 35–46 (2002) doi:10.1006/jcis.2002.8452 Structure, Dynamics, and Optical Properties of Concentrated Milk Suspensions: An Analogy to Hard-Sphere Liquids M. Alexander, ∗ L. F. Rojas-Ochoa, ∗ M. Leser,† and P. Schurtenberger ∗,1 ∗ Physics Department, University of Fribourg, CH-1700 Fribourg, Switzerland; and †Nestl´ e Research Center, Department of Food Science and Process Research, Vers-Chez-Les-Blanc, CH-1000 Lausanne 26, Switzerland Received October 15, 2001; accepted April 30, 2002 A systematic study of the effects of volume fraction increment on the optical properties, the structure, and the dynamics of the casein micelles and fat droplets in milk was performed using dif- fusing wave spectroscopy. Four types of milk were investigated, NIDO full fat milk, fat-free milk, whey and fat-free milk, and fi- nally lactose and fat-free milk. Independent measurements to cal- culate the dependence of the viscosity and the index of refraction of the milk serum and casein micelles as a function of the volume fraction were also performed. We compare the experimentally de- termined quantities photon transport mean free path (l ∗ ) and self- diffusion coefficient D s with the predictions from theoretical calcu- lations using classical colloidal models such as a hard-sphere fluid. We demonstrate that all types of milk with and without fat content behave, structurally, like colloidal hard-sphere systems up to vol- ume fractions well over 45%. In the case of dynamic measurements, both lactose- and fat-free and whey- and fat-free milk behave also like hard-sphere systems whereas fat-free milk and fat-containing NIDO milk deviate slightly at volume fractions over 35%. Finally, a comparative measurement and theoretical calculation of the casein micelle’s size was performed. C  2002 Elsevier Science (USA) Key Words: Diffusing wave spectroscopy; casein micelles; hard sphere; milk. I. INTRODUCTION Milk, as it comes from the cow, is a rather dilute suspen- sion of highly hydrated colloidal particles, the casein micelles, and fat. These are dispersed in a continuous phase made up of mainly water with salts, lactose, and whey proteins (1). It is highly polydisperse and prone to phase separation by the fat globules. Since this is an “undesirable” effect from a consumer’s point of view, cow milk is homogenized in different ways but all being rather violent processes. This homogenization produces casein-micelle-covered fat globules of around 400 to 600 nm in diameter and casein micelles of around 280 nm. Homoge- nized milk is a very stable dispersion; it can withstand boiling and large quantities of salt and can be dried or frozen, but milk 1 To whom correspondence should be addressed. Physics Department, Uni- versity of Fribourg, P´ erolles, CH-1700 Fribourg, Switzerland. Fax: +41-26-300 9747. E-mail: Peter.Schurtenberger@unifr.ch. properties remain practically unaffected. However, as the casein micelle is an association colloid, the milk is very sensitive to pH changes in its medium (2). One of the most common dairy products consumed in the world is milk powder. It is therefore important to fully under- stand the process by which this powder is formed. In the com- mercial process of producing milk powder most of the water content of the milk (around 87%) must be removed. It is con- ceivable to think that both the dynamic and static properties of the emulsion might change as it goes from a mostly dilute sys- tem to one of very high solid volume content. This likely change in properties will have an impact in the outcoming product. The aim of the study presented here is to follow the phy- sical and/or dynamic changes that may appear in milk as the fraction of solids, i.e., the proteins, fat, sugars, and salts are increased with respect to the dispersing medium. The process by which we will monitor this progression is diffusing wave spectroscopy (DWS). DWS allows us to monitor both static and dynamic properties of the dispersion: statically, we can follow the changes in the structure of the scatterers, through a mea- sure of the so-called photon transport mean free path length, l ∗ . Dynamically, we can follow the average particle dynamics on very short length scales, thus giving us a measure of the time dependence of the mean square displacement. We will make an attempt to compare these quantities with the behavior found in well-defined colloidal model systems. To profit from the ad- vanced understanding of colloidal suspensions, we will use sev- eral modified model systems to independently study the effect, if any, that increasing total volume fraction might have on the different colloidal components: the fat globules and the casein micelles. Any attempt to understand milk behavior by making analogies with colloidal model systems places a particular important role on the casein micelles. This protein is the second most common dispersed element in milk after fat. Milk proteins are present in two forms, casein micelles, about 80% of total proteins, and whey proteins, the remaining 20%. The casein micelle is an association colloid made up of four types of proteins, α s1 -, α s2 -, β-, and κ-casein (3). A typical micelle may be considered as a roughly spherical aggregate and they are polydisperse, ranging in size from 50 to 500 nm with an average radius of about 100 nm. 35 0021-9797/02 $35.00 C  2002 Elsevier Science (USA) All rights reserved.