151
THE REFRACTIVE DISPERSION OF GHEE
By K. T. ACHAYA and B. N. BANERJEE, Department of Biochemistry, Indian Institute of
Science, Bangalore
(Received for publication on 27 December 1945)
(With two text figures)
W HILE the refractive index has long been recognized as a useful sorting test in the analysis of
ghee, its usefulness has been strictly limited in that mixtures of oils can be prepared to yield
refractive indices similar to that of ghee. The refractive dispersion, which is the difference in refractive
indices for two specified lines of the spectrum at any constant temperature, was first used for the
analysis of oils by Szalaygi [1914], and a few years later by Fryer and Weston [1918], the latter with
the express intention of detecting the adulteration of one oil with another, if possible, While they
succeeded in
dly differentiating between the drying and the non-drying oils, the variations in
dis ersion were too small to permit the detection of small quantities of sophistication.
In 1938, Athavale and Jatkar took up again the 20 year old work and showed that by plot
ting dispersion against refractive index, oils could be divided into several classes. Since the one
sample of butterfat was indefinitely placed, they studied six new samples of ghee and stated in con
clusion that 'it should be possible to detect the adulteration of ghee by measurement of both refractive
index and dispersion '. The method was commended by Godbole and Sadgopal [1939] who were
also working on similar lines by observing the colour fringes of fats on. an uncompensated
refracto meter ; besides, most oils have a high dispersion unlike that of ghee, and though teniary
mixtures of oils can be made having the same dispersion as that of ghee, the consistency of the
mixture would be a drawback to its use as an adulterant.
EXPERIMENTAL
The refractometer used for the study was of the Pulfrich model, made by Zeiss, and the source
of light was a Hanovia, η shaped mercury arc. The lines used were the green 5460µ and the violet
4358µ, and these were sufficiently sharp for use without light filters. The readings were taken
with the temperature carefully maintained at 40°C.
The samples examined were those of buffalo ghee by reason of its common use in India. A
large variety of products were studied : fresh, low-acid samples made in the laboratory; rancid
samples, and high-acid products before and after neutralization ; sets of ghee of uniformly high and
low R.M. values respectively ; and commonly used adulterants of ghee—both oils and hydrogenated
oils.
RESULTS
The results of the study are given in order below.
TABLE I
Fresh pure ghees
Sample
1
2
3
4
5
6
7
8
9
10
11
12
Scale reading
4358 (violet)
46° 17.6'
46° 12.3'
46° 16.8'
46° 19.3'
46° 20.8'
46° 24.4'
46° 20.8'
46° 14.1'
46° 9.8'
46° 13.4'
46° 10.0'
46° 11.0'
5460 (green)
44° 25'
44° 20'
44° 24'
44° 27'
44° 28'
44° 32'
44° 28'
44° 21'
44° 18'
44° 21'
44° 17'
44° 18'
Refractive index n
4358
1.47310
1.47362
1.47318
1.47294
1.47279
1.47243
1.47279
1.47345
1.47387
1.47352
1.47385
1.47375
n 5460
1.46434
1.46484
1.46444
1.46415
1.46405
1.16365
1.46405
1.46474
1.40504
1.46474
1.46514
1.46504
n 4358—n 5460
.00876
.00878
.00874
.00879
.00874
.00878
.00874
.00871
.00883
.00878
.00871
.00871
Refractive
index
n D
Average R.I,
= 1.4570 with
very
slight
variations