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
[Illustration: FIG. 94.—BUTYROREFRACTOMETER.]
=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:[254]
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_.