Part 60

Peanut oil +3.5 to +6.5 Colza ” +17.5 ” +21.0 Cotton ” +18.0 ” +18.0 Linseed ” +47.0 ” +54.5 Lard ” +5.5 ” +5.5 Olive ” +1.5 ” 0.0 Sesamé ” +17.5 ” +19.0 Oleomargarin -15.0 ” -15.0 Butter fat -30.0 ” -30.0 Mutton oil 0.0 ” 0.0 Fish ” +38.0 ” +38.0

In this instrument, therefore, vegetable and fish oils, as a rule, show a right hand, and animal fats a left hand deviation.

The oleorefractometer has been extensively used in this laboratory and the data obtained thereby have been found useful. We have not found, however, the values fixed by Jean to be constant. The numbers for lard have varied from -3.0 to -10.0, and other fats have shown almost as wide a variation from the values assigned by him.

Jean states that the number for lard, determined by the oleorefractometer, is -12, and he gives a definite number for each of the common oils and fats. On trying the pure lards of known origin in this instrument, I have never yet found one that showed a deviation of -12 divisions of the scale; but I have no doubt that there are many such lards in existence. The pure normal lards derived from the fat of a single animal would naturally show greater variations in their chemical and physical properties, than a typical lard derived from the mixed fats of a great many animals. In leaf lard, rendered in the laboratory, the reading of the oleorefractometer was found to be -10°, while with the intestinal lard it was -9°. On the other hand, a lard rendered from the fat from the back of the animal showed a reading of only -3°, and a typical cottonseed oil a reading of +12°. According to the statement of Jean, a lard which gives even as low a refractive number as -9, by his instrument, would be adjudged at least one-quarter cottonseed oil.

After a thorough trial of the instrument of Jean, I am convinced that it is of great diagnostic value, but if used in the arbitrary manner indicated by the author it would lead to endless error and confusion. In other words, this instrument is of greater value in analyses than Abbe’s ordinary refractometer, because it gives a wider expansion in the limits of the field of vision, and therefore can be more accurately read, but it is far from affording a certain means of discovering traces of adulteration with other fats.

=300. Variations in the Instruments.=—In the use of the oleorefractometer, attention should be called to the fact that, through some negligence in manufacture, the instruments do not give, in all instances, the same reading with the same substance. Allen obtained the following data with a sample of lard examined in three instruments, viz., 4°.5, 6°, and 11°. Such wide differences in the scales of the instruments cannot fail to disparage the value of comparative determinations.

The variations in samples of known origin, when read on the same instrument, however, will show the range of error to which the determinations made with the oleorefractometer are subject. Pearmain has tabulated a large number of observations of this kind, covering 240 samples of oils.

Following are the data relating to the most important oils.

AT 22°.

Highest Lowest Mean reading. reading. reading. Name of oil. Degrees. Degrees. Degrees.

Almond 10.5 8.0 9.5 Peanut 7.0 5.0 6.0 Castor 42.0 39.0 40.0 Codliver 46.0 40.0 44.0 Cottonseed (crude) 17.0 16.0 16.5 ” (refined) 23.0 17.0 21.5 Lard oil -1.0 0.0 0.0 Linseed (crude) 52.0 48.0 50.0 ” (refined) 54.0 50.0 52.5 Olive 3.5 1.0 2.0 Rape 20.0 16.0 17.5 Sesamé 17.0 13.0 15.5 Sunflower 35.0 35.0 35.0 Tallow oil -5.0 -1.0 -3.0 Oleic acid -33.0 -29.0 -32.0

AT 45°.

Butter -34.0 -25.0 -30.0 Oleomargarin -18.0 -13.0 -15.0 Lard -14.0 -8.0 -10.5 Tallow -18.0 -15.0 -16.0 Paraffin 58.5 54.0 56.0

=301. Butyrorefractometer.=—Another instrument graduated on an arbitrary scale is the butyrorefractometer of Zeiss. This apparatus, which resembles in some respects the instrument of Abbe, differs therefrom essentially in dispensing with the revolving prisms of Amici, whereby the chromatic fringing due to dispersion is corrected, and on having the scale fixed for one substance, in this instance, pure butter fat. The form of the instrument is shown in Fig. 94. The achromatization for the butter fat is secured in the prisms between which a film of the fat is placed, as in the Abbe instrument. When a fat, differing from that for which the instrument is graduated is introduced, the fringes of the dark and light portions of the field will not only be colored (difference in dispersion), but the line of separation will also be displaced (difference in refractive power). The apparatus is therefore used in the differential determination of these two properties. It must not be forgotten, however, that butter fats differ so much in these properties among themselves as to make possible the condemnation of a pure as an adulterated sample.

=302. Method of Charging the Apparatus.=—The prism casing of the instrument is opened by turning the pin F to the right and pushing the half B of the prism casing aside. The prism and its appendages must be cleaned with the greatest care, the best means for this purpose being soft clean linen moistened with a little alcohol or ether.

Melt the sample of butter in a spoon and pour it upon a small paper filter held between the fingers and apply the first two or three drops of clear butter fat so obtained to the surface of the prism contained in prism casing B. For this purpose the apparatus should be raised with the left hand so as to place the prism surface in a horizontal position.

Press B against A and replace F by turning it in the opposite direction into its original position; thereby B is prevented from falling back and both prism surfaces are kept in close contact.

=303. Method of Observation.=—While looking into the telescope, give the mirror J such a position as to render the critical line which separates the bright left part of the field from the dark right part distinctly visible. It may also be necessary to move or turn the instrument about a little. First it will be necessary to ascertain whether the space between the prism surfaces be uniformly filled with butter, for, if not, the critical line will not be distinct.

By allowing a current of water of constant temperature to flow through the apparatus, some time previous to the taking of the reading, the at first somewhat hazy critical line approaches in a short time, generally after a minute, a fixed position and quickly attains its greatest distinctness. When this point has been reached note the appearance of the critical line (i. e., whether colorless or colored and in the latter case of what color); also note the position of the critical line on the centesimal scale, which admits of the tenth divisions being conveniently estimated, and at the same time read the thermometer. By making an extended series of successive readings and by employing an assistant for melting and preparing the small samples of butter, from twenty-five to thirty refractometric butter tests may, after a little practice, be made in an hour.

The readings of the refractive indices of a large number of butter samples made at 25° are, by means of a table which will be found below, directly reduced to scale divisions and yield the following equivalents:

Natural butter (1.4590-1.4620) : 49.5-54.0 scale divisions. Margarin (1.4650-1.4700) : 58.6-66.4 ” ” Mixtures (1.4620-1.4690) : 54.0-64.8 ” ”

Whenever, in the refractometric examination of butter at a temperature of 25°, higher values than 54.0 are found for the critical lines these samples will, according to Wollny, by chemical analysis, always be found to be adulterated; but in all samples in which the value for the position of the critical line does not fall below 52.5, chemical analysis maybe dispensed with and the samples may be pronounced to be pure butter.

In calculating the position of the critical line for other temperatures than 25° allow for 1° variation of temperature a mean value of 0.55 scale division. The following table, which has been compiled in this manner, shows the values corresponding to various temperatures, each value being the upper limit of scale divisions admissible in pure butter:

Temp. Sc. div. Temp. Sc. div. Temp. Sc. div. Temp. Sc. div. 45° 41.5 40° 44.2 35° 47.0 30° 49.8 44° 42.0 39° 44.8 34° 47.5 29° 50.3 43° 42.6 38° 45.3 33° 48.1 28° 50.8 42° 43.1 37° 45.9 32° 48.6 27° 51.4 41° 43.7 36° 46.4 31° 49.2 26° 51.9 40° 44.2 35° 47.0 30° 49.8 25° 52.5

If, therefore, at any temperature between 45° and 25° values be found for the critical line, which are less than the values corresponding to the same temperature according to the table, the sample of butter may safely be pronounced to be natural, i. e., unadulterated butter. If the reading show higher numbers for the critical line the sample should be reserved for chemical analysis. A special thermometer for use in the examination of butter will be described in the section devoted to dairy products.

=304. Range of Application of the Butyrorefractometer.=—The extended range of the ocular scale of the refractometer, n = 1.42 to 1.49, which embraces the refractive indices of the majority of oils and fats, renders the instrument applicable for testing oils and fats and also for examining glycerol.

By reference to the subjoined table the scale divisions may be transformed into terms of refractive indices. It gives the refractive indices for yellow light for every ten divisions of the scale. The differential column Δ gives the change of the refractive indices in terms of the fourth decimal per scale division. Owing to the accuracy with which the readings can be taken (0.1 scale division) the error of the value of n rarely exceeds one unit of the fourth decimal of n.

TABLE OF REFRACTIVE INDICES.