Catalytic Impurities, Habit Modification and Crystal Structure in Fats

1. Catalytic Impurities, Habit Modification and Crystal Structure in Fats Lipid Structural Properties Symposium, Unilever Research, Colworth, 2002 Malcolm Povey University of Leeds

2. Collaborators • Scott Hindle – University of Leeds • Paul Smith – Unilever and SIK • Kevin Smith – Unilever Colworth

3. The Importance of Catalytic Impurities • They are generally essential to crystallization – homogeneous nucleation is very rare in oils • The physics of crystal nucleation is very different to that of crystal growth. So an effective catalyser can also poison the growth of individual crystal faces, dramatically altering crystal morphology and even prevent growth altogether.

4. Quantifying crystal nucleation • Determine the rate of increase of crystal material at constant undercooling • Use ultrasound velocity to determine crystal solids • Obtain Gibbs free energy, upper limit for critical nucleus size,

5. The emulsion crystallization technique • There are great advantages to dispersing material as an emulsion in a non-crystallizing continuous phase in order to study crystal nucleation. • It permits the use of the very accurate and convenient ultrasound technique for the determination of the low quantity of solids evident during the initial stages of nucleation. • The concentration of nuclei may be controlled through droplet size. • Isothermal crystallization studies may be conveniently carried out this way, permitting the identification of nuclei, through the direct determination of the bulk melting point.

6. Determining catalytic impurities • Emulsify oil as a hot oil-in-water emulsion in which all components of the oil are liquid. • Measure psd (Light scattering - dilute and ultrasound scattering - concentrated) • Crash cool to a given undercooling at which the rate of crystallization is appropriate for our measurement technique • Measure the temporal evolution of crystal solids at constant temperature. Repeat at 4 other undercooling. • Find the best fit model (Homogeneous, heterogeneous, surface, volume – 4 models in all) using psd. Determine J, n, G and d50

7. Plot of ultrasonic velocity against temperature for 20.75% (v/v) WACB-in-water emulsions (0.8% Tween 20 and 1.0% sodium caseinate) cooled at 5°C/hour. Plot of solids against temperature for 20.75% (v/v) WACB-in- water emulsions cooled at 5°C/hour (0.8% Tween 20 & 1.0% sodium caseinate).

8. Plot of solids against time for 20.75% (v/v) WACB-in-water emulsions (0.8% Tween 20) crystallised isothermally at 14.2, 15.0, 15.5 and 15.8°C. Heterogeneous volume particle size distribution models are fitted. Plot of solids against time for 20.75% (v/v) WACB-in-water emulsions (0.8% Tween 20) crystallised isothermally at 14.2, 15.0, 15.5 and 15.8°C. Heterogeneous volume particle size distribution models are fitted

9. Characterising catalytic impurities • Determine the temperature dependence of the concentration of catalytic impurities

10. Cocoa butter seed crystals • There are few seed crystals whose size exceeds 0.28 m at 80 C. • From our isothermal crystallization experiments, with between 3 x 1016 and 3 x 1017 seed crystals per m3 of cocoa butter, we infer their average size to be less than 0.09 m. • The melting point of the nucleating layer in the seed crystal is 14 oC. • There is strong evidence from X-ray diffraction that it is the alpha form of POS that comprises the nucleating layer in the seed crystal.

11. Characterising surfactant

12. Crystal growth and catalytic impurities • Paul Smith, “The molecular basis for crystal habit modification in tryglycerides”, University of Leeds Thesis, 1995 • Chain length matching

13. Trilaurin

14. Trilaurin, catalytic impurities and chain length matching

15. Trilaurin crystal growth

16. Conclusions • Catalytic impurities play a role both during nucleation and during crystal growth • Catalytic impurities may both catalyse nucleation and prevent crystal growth of one or more crystal facets • Identification of catalytic impurities is one of the most important outstanding issues in crystallisation

17. References • www.food.leeds.ac.uk/mp.htm • Povey, M. J. W., Hindle, S. and Smith, K. H. (2001) Crystallization in Food Emulsions, in "Food Colloids - Fundamentals of Formulation" - RSC Special Publication Number 258 eds Eric Dickinson and Reinhard Miller, pp 152 -162 • Povey, M. J. W. (2001), Crystallization of Oil-in-Water Emulsions, in Crystallization Processes in Fats and Lipid Systems 1 , Editor N. Garti and K. Sato , Marcel Dekker, NEW YORK, Chapter 7, pp 255-288 • Hindle, S., Povey, M. J. W. and Smith, K. (2000), Kinetics of Crystallization in Polydisperse n-hexadecane and Cocoa Butter Emulsions Accounting for Droplet Collision-Mediated Nucleation, J. Coll. Interface Sci. 232, 370-380 • Smith, P. R., and Povey, M. J. W. (1997), The Effect of Partial Glycerides on Trilaurin Crystallization, J. Am. Oil. Chem. Soc. 74, 169-171 • Dickinson, E., Ma, J., and Povey, M. J. W. (1996), Crystallization Kinetics in Oil-In-Water Emulsions Containing a Mixture of Solid and Liquid Droplets, J. Chem. Soc. -Faraday Trans. I 92, 1213-1215 • Smith, P. R., Povey, M. J. W., and Cebula, D. J. (1994), The Application of Temperature Gradient Microscopy Techniques to Trilaurin Crystallisation, Crystallisation and crystal growth: An interdisciplinary perspective. • Smith, P. R., Cebula, D. J., and Povey, M. J. W. (1994), The Effect of Lauric-Based Molecules on Trilaurin Crystallization, J. Am. Oil. Chem. Soc. 71, 1367-1372 • Povey, M. J. W. (1993), Analysis of Lipid Structure by Neutron Diffraction in Developments in the Analysis of Lipids Editors: J. H. P. Tyman and M. H. Gordon, RSC, Cambridge, UK • Coupland, J., Dickinson, E., McClements, D. J., Povey, M. J. W., and de Rancourt de Mimmerand, C. (1993), Crystallisation in Simple Paraffins and Monoacid Saturated Triacylglycerols Dispersed in Water, Food Colloids and Polymers: Stability and Mechanisms, RSC • Cebula, D. J., McClements, D. J., Povey, M. J. W., and Smith, P. R. (1992), Neutron Diffraction Studies of Liquid and Crystalline Trilaurin, J. Am. Oil. Chem. Soc. 69, 130-136 • Smith, PR (1997) The Molecular Basis for Crystal Habit Modification, PhD Thesis, University of Leeds • Hindle, SA (2000) Investigation of Cocoa Butter Crystallization Using Ultrasound Velocity Measurements, PhD Thesis, University of Leeds