14 
BULLETIN 454, U. S. DEPARTMENT OF AGRICULTURE. 
the oil from the unfrozen plants, possibly because of the greater 
proportion of menthol in the first-mentioned oil. 
The physical constants reveal marked differences in the rotation 
and some differences in the solubility in 80 per cent alcohol. The 
higher rotation of the oil from the frozen plants is attributable to 
the presence of a greater quantity of the strongly levogyrate com- 
pounds, menthol and menthyl acetate. 
Table YII. — Comparison of the yields, the physical properties, and the chemical compo- 
sition of peppermint oils distilled from frozen and from unfrozen plants. 
1919 
Items of comparison. 
Frozen plants. 
Unfrozen plants. 
Frozen plants. 
Unfrozen plants. 
Physical properties: 
"Yield., per cent.. 0.065. 
Color I Pale straw . 
Odor. 
Taste. 
Specific gravity. 
Rotation, in 50 
mm. tube, de- 
grees. 
Refraction. 
Solubility in 80 
per cent alco- 
hol, volume. 
Chemical composi- 
tion (percent): 
Free acid (calcu- 
lated as acetic 
acid). 
Menthyl acetate. 
Free menthol . . . 
Total menthol... 
Mild, flowery, char- 
acteristic minty. 
Bitter, pungent, 
and exceedingly 
cooling. 
0.9107, at .24° C... 
-11.5 
1.4592, at 25° C 
0.4; clear in excess. 
0.09. 
24.3. 
40.7. 
67.8. 
0.07 
Pale yellowish 
green. 
Strongly mint v 
Pungent, slightly 
bitter, cooling. 
0.9142, at24°C 
-8.43 
1.4689, at22°C 
0.6; turbid in U 
volumes or more. 
0.086.., 
0.11 
Dark golden yellow. 
Flowery, not 
strongly'minty. 
Bitter, " pungent, 
and cooling. 
0.9155. at 23° C 
-16.5 
1.45S0, at23°C 
0.6; faintly turbid 
in 3i volumes or 
more. 
0.09... 
8.8. 
31. S. 
38.7. 
.! 40.66.. 
. 28.93.. 
.! 60.97.. 
0. 077. 
Pale golden yel- 
low. 
Strongly minty. 
Birter, slightly 
pungent, cooling. 
0.9180, at23°C. 
-11.7. 
1.4621, at 23° C. 
0.5; turbid in 2} 
volumes or more. 
0.12. 
31. c 2. 
9.99. 
35.06. 
The oils from the frozen plants, being readily soluble, remain more 
or less clear when an excess of 80 per cent alcohol is added, whereas 
the oils from the unfrozen plants become turbid in one and one-half 
volumes or more of alcohol. In the oils from the frozen plants, 
therefore, a higher content of the easily soluble menthol and menthyl 
acetate, and possibly a lower content of the more or less insoluble 
terpenes, may be expected. 
In discussing the chemical constants of the oils it is noted that the 
quantity of free acids is nearly the same for all the oils. The ester 
content of the oils from the frozen plants, when calculated as menthyl 
acetate, was 34.3 in 1910 and 40.66 per cent in 1911, which is much 
higher than the ester content of the oils from the unfrozen plants. 
During both seasons the percentage of free menthol in the oils from 
the frozen plants exceeded that in the oils from the unfrozen plants. 
The total menthol content was also much higher in the oils from the 
frozen plants than in that from the unfrozen plants. 
If, as in the ordinary esterincation reactions, the formation of 
esters in the plant takes place by the action of the organic acids upon 
