Crystallisation is the solidification process of atoms or molecules into a highly structured form with long-range periodic order called a crystal. The crystallisation of components in food, cosmetics, and pharmaceutical formulations has significant implications on the industrial processing, structure, texture, and stability of manufactured products [1,2]. Of particular relevance to this review is the crystallisation of food lipids. Lipids are a class of compounds which are characterised by long-aliphatic hydrocarbon chains often with carboxylate headgroups or esterified onto glycerol or phosphatidylglycerol. Although food lipids can be produced and sold as homogeneous products such as cooking oils, they are commonly formulated into emulsion- or foam-based formulations whereby their crystallisation properties are critical in the manufacturing of products such as whipping cream [[3], [4], [5]], ice cream [6], cheese [7], spreads [8] and confectionery [9].

The properties of any crystallised lipids in emulsions, and any interaction with other endogenous or added compounds, will determine the location of the lipid crystals, i.e., whether they are positioned in the bulk oil or at the oil-water interfaces. Interfacially-active lipid crystals, which can be described as amphiphilic crystals, that have formed at the oil-water interface or have migrated from the bulk oil phase to the interface, play a critical role in the stability, structure, texture and function of emulsion systems in food formulations [10]. Although triglycerides (TGs) are the primary constituent of lipid products, the dominance of the three non-polar fatty acid chains precludes their interfacial activity. Modification or influence from other components are required to ensure TG crystals are present at oil-water interfaces [11,12], and hence, while briefly discussed due to their industrial relevance, are not the focus of this review.

Monoglycerides (MGs) and diglycerides (DGs) are naturally occurring in crude lipid products, whilst also commonly introduced as product stabilisers due to their ability to influence the crystallisation properties of bulk lipids as well as their propensity to form interfacially-active lipid crystals [13]. Adding to the complexity is the presence of other amphiphilic stabilisers (so-called emulsifiers), which will compete for space at the oil-water interface. As emulsion droplet size decreases, the specific interfacial area occupied by interfacially-active species increases dramatically. Therefore, it can be expected that the composition of the interface and the properties of any adsorbed species will influence MG and DG crystallisation at the oil-water interface and, subsequently, the stability, sensory and functional attributes of an emulsion.

While research into emulsion stability has significantly broadened the current knowledge over the last century, there remain uncertainties surrounding the influence of interfacial lipid crystallisation on the subsequent stability, structure, and function of these emulsions. Most research has been conducted on emulsion systems, where distinguishing lipid crystallisation behaviour in the bulk and at the interface is not possible [10,14,15]. However, the emergence of new interfacially-sensitive techniques, experimental geometries and methodologies has provided new insights into lipid crystallisation at oil-water interfaces. This review will highlight these studies while discussing the current understanding of the interfacial crystallisation of MGs and DGs.