Crystallization behaviour of fats and lipids — a review

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Abstract

Recent progress in research of crystallization processes of fats and lipids is reviewed with an emphasis on kinetic properties of polymorphic crystallization in pure and mixed systems. It is shown that physical techniques using Synchrotron radiation X-ray beams, differential scanning calorimetry and FT-IR measurement have enabled workers to relate the molecular-level structures and crystallization kinetics of fats and lipids. The influences of crystal seeding and shear stresses on crystallization kinetics are also discussed. The polymorphic crystallization properties thus reviewed are relevant to separation phenomena of natural fat resources.

Introduction

Fats and lipids are employed in food, cosmetics, pharmaceuticals, etc., as main bodies of end products, or as matrices in which cosmetic and pharmacological fine chemicals are dispersed (Gunstone & Padley, 1997). The molecular species of the fats and lipids are paraffins, fatty acids, glycerols (mono, di and trio), phospholipids, etc., which are categorized as long-chain compounds (Small, 1986). The crystallization behaviour of fats and lipids has two major industrial implications: (a) processing of the end products made of fat crystals, such as chocolate, margarine and shortening, whipping cream, etc., and (b) separation of specific fats and lipids materials from natural resources. The natural fats and lipids resources are vegetable and animal fats and oils, which contain various molecular species having different chemical and physical properties. It may be worth noting that there is an increasing necessity to develop the fractionation technology of high- and low-melting fats and lipids put forward by the following market demands: (a) to obtain high-melting fats by dry fractionation, instead of hydrogenation which produces trans-fatty acids as bi-products, (b) to cope with new regulation standards of the use of fat materials for confectionery end products, (c) to maintain better functionality of physically refined vegetable oils compared to conventional materials, etc.

This review highlights recent research on the crystallization of fats and lipids, with a special emphasis on the separation phenomena.

Section snippets

Polymorphism and molecular interactions in fat crystals

The specific properties of the crystallization of fats and lipids may be revealed in polymorphism on one side, and molecular interactions on the other. The essence of the two properties, which may be helpful in understanding the following sections is briefly described here.

Polymorphic crystallization

The polymorphic crystallization is primarily determined by the rate of nucleation, being governed by thermodynamic and kinetic factors. The primary concern in a polymorphic nucleation is the so-called Ostwald step rule. This “law” predicts that a phase change may occur step-by-step by the way of successively more stable phases (Mutaftschiev, 1993). Thus, the metastable form nucleates first, prior to the most stable form, when nucleation is induced under large kinetic factors, e.g., supercooling

Phase behaviour and crystallization in binary mixtures of fats

As for the mono-acid TAGs, the polymorphic influence was clearly seen in the PPP–SSS mixture system, where nc of the fatty acid moieties of the two component TAGs differ by two. The solid solution was formed in α and β′, yet β is eutectic as verified by thermal and XRD studies. Eutectic phases are formed when the difference in nc exceeds 4 in the case of the mixtures of the mono-acid TAGs. The same monotectic phase is formed in the mixtures of mono-acid unsaturated and saturated TAGs (Small,

Conclusion

The crystallization of fat materials must deserve to be further exploited from the fundamental aspects as discussed in this review, and from chemical processing aspects as well. In any attempts, it may be worthy to note that the interrelations among the polymorphism, crystallization kinetics and crystal particle network are underlying the apparently complicated solidification behaviour of fats and lipids. In particular, the interrelation must be understood in terms of molecular interactions

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