emulsions-creaming phenomena

ELS EVI E R
Current Opinion in Colloid & Interface Science 5 (2000) 265-272
www.elsevier.nl/locate/cocis
Emulsions - creaming phenomena*
Margaret M. Robins”
creaming behaviour of emulsions in order to assess the value of the various techniques for measuring creaming, and to put the published data into context. The composition of an emulsion dictates its creaming behaviour. In the simplest case, if the droplets are monodisperse and are highly charged or coated with an effective surfactant, they repel each other and they cream as individual particles. At infinite dilution, the creaming rate is described by the well-known Stokes' terminal velocity V,, proportional to the square of the droplet diameter and density difference, and inversely proportional to the background viscosity. However, where there is a significant the number of other droplets of volume fraction presence of neighbouring droplets and the consequent backflow of continuous phase against the droplet movement causes a reduction in the terminal velocity of the droplets, so that they cream at a velocity I/= V,.f(+). The hindrance factor f(+) < 1 and, in the ideal case, depends only on the local concentration of droplets. When the droplets reach the top of the emulsion they form a uniform cream layer of volume fraction 4,. Fig. l a shows a schematic diagram of oil volume fraction profiles during the creaminenoted type IM. Ini-
*Late article, Emulsions, Gels and Foams, Curr Opin Colloid Interf Sci 5/3-4 (2000). * Tel.: + 44-1603-255209; fax: + 44-1603-507723. E-mail address: margaret.robins@ (M.M. Robins).
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Institute of Food Research, Nonvich Research Park, Colney, Nonvich NR4 7UA, UK
Abstract
Recent years have seen advances in the monitoring of emulsion stability, particularly multiple-point techniques using optical, ultrasonic and NMR sensors. In parallel, there has been interest in the mechanisms of creaming and phase separation, leading to progress in the theoretical interpretation of the process. 0 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Emulsions; Creaming; Stability; Flocculation; Phase separation
1. Introduction
Creaming is the process by which buoyant emulsion droplets tend to rise to the top of a container. It is the same process as sedimentation, but in the opposite direction. The commercial importance of creaming is very high, and it is estimated that 40% of the cost of developing a new food emulsion is incurred in shelflife testing, where the major criterion is the onset of visual non-uniformity. Despite its commercial importance, emulsion creaming does not feature highly in food science literature. Many of the reported studies on emulsions are addressed at formulation issues where the performance of specific components, usually proteins, is examined empirically as an aid to commercial application. Yet there are questions still to be answered on the basic mechanisms of creaming in food emulsions. In order to obtain insights into the physics of the processes, particularly where droplets are flocculated by polymeric ‘stabilisers’, one must access the colloid science and chemical engineering
literature. Here, at least, there are models of phase behaviour and mass-balance, offering quantitative analyses that can be used as a framework for empirical studies. Emulsions can undergo a number of different instabilities. Those that involve the changes in primary droplet size, viz. coalescence, disproportionation (ripening), inversion and breaking, are not considered here, although they have knock-on effects on the creaming behaviour. Flocculation (aggregation) and creaming both involve the spatial rearrangement of droplets with respect to each other and with respect to an external frame of reference. In many food emulsions, the creaming behaviour is dominated by the flocculation of droplets, and both aspects are considered in this article. This review is intended to highlight the published research on creaming over the past 3-4 years. In particular, advances have been made in the instrumental measuring of creaming (Section 2), and in the elucidation of the physical mechanisms involved in the process (Section 3). In Section 4, some specific applications are given to demonstrate the systems of current interest to food scientists. Finally, I have attempted to identify the aspects where future work is needed. The remainder of this section aims to classify the