抄録
The relations between the crystallization behaviour and the mechanical properties of fats were investigated. The process of crystallization was followed by a falling temperature differential thermal analysis. The physical state of solidified fats was modified to a great extent by varying both the crystallization temperature and the melt temperature. The hardness was measured by means of a micropenetrometer which was commonly used in fat industry. The dynamic elastic modulus E* and loss factor tan δ were measured at 20c/s by a dynamic viscoelastometer. The static viscosity η was determined by a modified penetrometer which was also used as a parallel plate plastometer. Observation of the texture of crystal grains was made by a polarized microscope. The measurement of X-ray diffraction was also carried out.
The falling temperature DTA revealed the distinct variation in the crystallization behaviour among a variety of margarine and fat. Higher values in hardness was found to be accompanied with higher temperature required for initiation of crystallization, larger amount and more uniform distribution in latent heat, and a higher degree of crystallinity determined by X-ray diffraction analysis. This difference has mainly been caused by the difference in triglyceride components of the fats.
As a standard specimen of fat, refined tallow was employed. After melting at 80°C, the fat was crystallized at different temperatures from 30°to -30°C. With the decrease of crystallization temperature (Tc), the temperature initiating crystallization becomes lower and the latent heat becomes larger. The hardness measured by a micropenetrometer is maximum at Tc=20°C and minimum Tc=0°C. The hardness of Tc=-30°C is slightly higher than that of Tc=0°C. Gomparison of E*, tan δ and η for specimens of Tc=20°, 0°, -30°C showed the same tendency. In the specimen crystallized at 20°C, which seemed to be the most favorable temperature for crystallization, large spherulitic crystals developed in the texture. And a rigid network connecting large crystal grains has been developed, hence the hardness is largest. In the specimen crystallized at 0°C, the size of crystal grain becomes smaller and irregular, and the network of crystal grains is intervened by the amorphous boundaries, resulting in the weaker network structure. The viscous flow takes place at grain boundaries and the structure is softest. In the specimen crystallized at -30°C, a number of small crystal grains are formed and packed closely, resulting again in a firm network structure of small crystal grains. Therefore the texture becomes more stable.
In order to investigate the effect of crystalline nucleus remaining in the molten fat, refined tallow was melted at various temperatures from 40°C to 200°C and crystallized at 0°C. With the increase of melt temperature (Tm), the temperature initiating crystallization and latent heat fall gradually and become nearly constant above Tm=80°C. The specimen of lower Tm is found to be softer. As crystallization takes place from the nucleus remaining in the molten state, the glycerides with higher melting point contribute preferentially to the growth of nucleus. As the result, the texture consists of crystallites with glycerides of high melting point, and of grain boundaries with glycerides of low melting point. Therefore the viscous flow in grain boundaries can take place more easily than in the specimen crystallized from completely molten state where the separation of glycerides of high and low melting point is not so clear during the crystallization process.
