12 
BULLETIN 454, U. S. DEPARTMENT OF AGRICULTURE. 
Table VI. — Comparison of the yields, the physical properties, and the chemical com- 
position of peppermint oils distilled from shade-grown and from unshaded plants. 
1910 
1911 
Items of comparison. 
Shade-grovm plants. 
Unshaded plants. 
Shade-grown plants. Unshaded plants. 
Yields and physical 
properties: 
Yield, per 0.13 
cent. 
Color Palp vpllrm- 
0.13 
Pale yellow 
Mild, agreeable, 
minty. 
Fatty, strong, cool- 
ing, pungent. 
0.9273 
0.15 0.9. 
Odor 
Very mild, minty.. 
Fatty, cooling, 
slightly bitter, 
and pungent. 
0.9343 
Mild, agreeable, 
minty. 
Strongly bitter, 
pungent, minty. 
0.9192 
low. 
Strong minty, char- 
acteristic. 
Bitter, puneent, 
strong, cooling. 
0.9233. 
Taste 
. Specific gravity 
(at23°C.). 
—2.24 
-5.7 
-3.4 
—7.3. 
mm. tube, de- 
crees. 
Refraction 
Solubility in 80 
per cent alco- 
hol,, volumes. 
Chemical ' composi- 
tion (per cent): 
Free acids (cal- 
culated as ace- 
tic acid). 
1.4724, at 22° C 
0.7; turbid in H vol- 
umes or more. 
10 96 
1.4820 at 22° C 
0.6: turbid in 2 vol- 
umes or more. 
0.03 
11 67 
1.4708, at 23° C 
0.7: very turbid in 2 
volumes or more. 
13 08 
1.4722, at 23° C. 
0.75: very turbid in 
\h volumes or 
more. 
0.20. 
13 79. 
Free menthol 
Total menthol... 
24.43 
33.07 
30.94 
40.13 
31.20 
41.50 
34.11. 
44.97. 
Compared with the oils from unshaded plants, the oils obtained 
from the shade-grown plants in 1910 showed a slight increase in the 
specific gravity and in 1911 but very little difference. The rotation 
and refraction of the oils from the shade-grown plants were lower in 
both years than in the oils from the unshaded plants. In solubility 
but little difference was noticed. In general appearance, however, 
the oils from the shade-grown plants were somewhat lighter in color 
and showed a slight difference in odor and taste. The marked dif- 
ference in rotation of the two oils during both seasons is especially 
significant and denotes differences in composition. 
The free acidity of the oils from the shade-grown plants is seen to 
be lower in both seasons than that of the oil from the unshaded 
plants, as is also the content of men thy 1 acetate, free menthol, and 
total menthol. The results indicate that both esterification and the 
formation of menthol are retarded by the absence of light, possibly 
because photosynthetic activities are greater in the presence of light. 
EFFECT OF FROST ACTION. 
From the considerations presented in the foregoing paragraphs it 
seems probable that in the peppermint plant the formation of certain 
compounds and the breaking down of others is dependent upon a 
number of factors, some of which stimulate while others retard, or 
possibly inhibit, the reactions taking place in the plant tissues. The 
changes which take place in a volatile oil as a result of the action of 
