Lipid emulsions nucleation

Nucleation in Lipid Emulsions In many foods, the lipid phase appears in emulsion form, or small droplets of fat dispersed in a continuous aqueous phase. 

Crystallization from the emulsified state may lead to different nucleation processes than observed for the same fat in bulk liquid form. It has been suggested that nucleation often occurs at the interface of the droplet where surface-active agents are located. The general similarity of the lipophilic components of surfactants oriented at the surface may provide some ordering and structure for the lipid molecules within the droplet and enhance nucleation, as found for example by Kaneko et al. (40) for a hydrocarbon emulsion. Walstra (11) also suggests that formation of compound crystals from emulsions of natural fats may be different than the same fat crystallized from bulk liquid. The initial polymorph formed may also be different, with more stable polymorphs more likely to form in the emulsion (38). 

The mechanism oifilm rupture by nucleation of pores has been proposed by Deqaguin and Gutop (99) to explain the breaking of very fliin films, built up from two attached monolayers of amphiphilic molecules. Such are the secondary foam and emulsion films and the bilayer lipid membranes. This mechanism was further developed by Deijaguin and Prokhorov (3, 100, 101), Kashchiev and Exerowa(102—104), Chizmadzhev and coworkers (105— 107), and Kabalnov and Wennerstrom (108). The formation... 

The process of crystallization proceeds via two distinct processes crystal nucleation and growth (Garside, 1985). The nucleation kinetics in fine droplets is often different from nucleation in the same liqnid in bulk. In a fine emulsion, the number of droplets exceeds the number of potential nncleation catalysts (impurities) present in the liquid oil. Thus, a proportion of the lipid is effectively catalyst free and must nucleate by other mechanisms. This may either be completely spontaneous homogeneous nucleation or, more probably, some catalytic effect of the droplet surface (Coupland, 2002). In either case, the crystallization temperature is greatly reduced below the melting point and depends both on particle size and on the nature of the emulsifier selected. For example, Higami and co-workers (2003) showed that the crystallization temperature of trilaurin molecules decreased from 18.9 °C, the crystallization temperature of the bulk lipid, to -9.5 °C when emulsified into droplets smaller than 100 nm. 

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