PHYSICAL PROPERTIES OF FATS AND OILS 249 



simplified method of Timmermans^^'^ can be emploj'ed to ascertain the 

 melting point of any phase without the sample undergoing a transforma- 

 tion during the course of the estimation. In the above procedure, the 

 solid fat is placed in a thin-walled capillary tube without melting, and this is 

 immersed in a bath of known temperature. If the sample melts, new 

 unmelted samples are tested with baths of varying lower temperatures 

 until the exact temperature at which melting just takes place is deter- 

 mined. If the sample fails to melt on the first trial, new samples are em- 

 ployed at each new higher temperature tested. 



The different modifications of a triglyceride can be obtained by the fol- 

 lowing procedures. The most inistable form, called the glass form (or 7- 

 modification) can be prepared by rapid cooling of the melted fat, but it 

 cannot be produced from any of the solid forms without melting. Lut- 

 ton*" is of the opinion, however, that the amorphous glass form is an arti- 

 fact and that only three polymorphic modifications exist. The a-poly- 

 morph may be formed by the slow cooling of the molten fat or by main- 

 taining the 7-preparation near its melting point. The /3-form, which is the 

 stable modification, can be prepared, also, from the molten fat by very 

 slow cooling, or from the a-form by maintaining it near its melting point. 

 The fourth form, called the /3'-isomer, occurs only in the case of the mixed 

 triglycerides. It is thermodyiiamically unstable, and has a melting point 

 between the a- "and the stable |S-compound. 



That the variation in melting point is related to differences in crystal- 

 line structure can be readily demonstrated. The gross appearance of the 

 7-form is that of a vitreous translucent product, while the stable /3-form is 

 an opaque powdery substance. On microscopical examination with the 

 polarizing nicol prisms, the 7-form appears as tiny uniaxial crosses which 

 are indicative of spherulite formation. The a-form gives a similar picture 

 except that the uniaxial crosses are much larger. When a conversion to 

 the i8-polymorph occurs, the uniaxial crosses disappear and a multitude of 

 parallelograms of varying sizes and shapes appear. 



The x-ray diffraction patterns also help to explain the divergency in 

 properties between the different polymorphic forms. Such examinations 

 are of value not only in establishing the polymorphic pattern which any 

 given compound represents, but also in determining when one type of 

 crystal has changed to another one. Although quantitative data on the 

 x-ray spacings are not available, and thus the investigation of mixtures of 

 phases cannot be interpreted accurately, the results of Malkin have been 

 of great importance in establishing qualitative differences between the 

 several forms. 



In the 7-polymorph, it is believed that the hydrocarbon chains of the 



^^ J. Timmermans, Chemical Species, Chemical Pub. Co., New York, 1940, p. 26. 

 ^' E. S. Lutton, J. Atn. Chem. Soc, 67, 524-527 (1945). 



