Abstract
The control of crystallization of fats and roles of emulsifiers for oil-in-water (O/W) emulsions are reviewed. First, the meaning of fat crystallization, one significant factor for demulsifying oil droplets in O/W emulsions is described. Secondly, the general mechanisms of fat crystallization in O/W emulsion are explained. Four types of nucleation occur in an oil droplet. The interfacial heterogeneous nucleation which is one of the four is a focus the present review. Two examples using emulsifiers as additives are documented. In the first case, when hexadecane, C16, was employed as the oil phase in O/W emulsions, a new polymorph crystallized when an emulsifier was added. This polymorph was identified to be odd-numbered alkanes such as heptadecane, C17, and even-numbered alkanes never appeared in the normal case. As the additive must be crystallized before oil crystallization, hexadecane crystallized on the surfaces of additive crystals. Since a so-called “template effect” occurred in this system, new crystallization occurred on the template; giving additive crystals. In the second case, when the palm-mid-fraction, PMF, was used as the oil phase, crystallization of PMF would be expected to occur at the oil-water interface. To check this phenomenon and also measure the polymorph of crystals on the interface, the synchrotron radiation microbeam X-ray diffraction (SR-μ-XRD) method was employed. The main result was that when the high melting emulsifier was added to the oil phase of an O/ W emulsion, the high melting fraction of PMF crystallized along the interface. On the other hand, no crystallization was noted without addition of the high melting emulsifier as an additive. Crystallization along the interface could be readily clarified using SR-μ-XRD. Finally, crystallization of the oil phase in oil droplets with no emulsifier was covered using a model sample of mayonnaise. As the oil phase, rapeseed oil and soybean oil were used as solvents. Two thermal thaw processes were performed; cooling from 20 ℃ to -15 ℃ and -30 ℃ , and heating to 20 ˚C after maintaining these temperatures for 10 min. The stability of both samples with cooling to -15 ℃ was retained because no ice crystallization occurred. On the other hand, different phenomena were apparent with cooling to -30 ℃ between the two samples; demulsification with rapeseed oil, and only limited coalescence in the case of the soybean oil sample. We believe that the high melting fraction of soybean oil crystallized along the oil-water interface, and then when the system was frozen during cooling to -30 ℃ , crystals along the interface of oil droplets were blocked from destroying oil droplets caused by volume expansion of frozen water.
