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. 



Items of comparison. 



1910 



1911 



Shade-grown plants. 



Unshaded plants. 



Shade-grown plants. Unshaded plants. 



Yields and physical 

 prooerties: 

 Yield, per 



cent. 

 Color 



13 



0.13 



0.15 



0.9. 



Pale yellow 



Very mild, minty. . 



Fatty, coo ling, 

 slightly bitter, 

 and pungent. 



9343 . . . 



Mild, agreeable, 

 minty. 



Fatty, strong, cool- 

 ing, pungent. 



0.9273 



Pale golden yellow . 



Mild, agreeable, 



minty. 

 Strongly bitter, 



pungent, minty. 



0.9192 



Dark golden yel- 



Odor 



low. 



Strong minty, char- 



Taste 



acteristic. 

 Bitter, pungent, 



Specific gravity 

 (at 23° C.). 



strong, cooling. 

 0.9233. 



2 24 . . . 



—5.7 



—3.4 



-7.3. 



mm. tube, de- 

 grees. 



Refraction 



Solubility in 80 

 per cent alco- 

 hol, volumes. 

 Chemical composi- 

 tion (percent): 



1.4724, at 22° C 

 0.7; turbid in 14 vol- 

 umes or more. 







10.96 



1.4820 at 22° C 



0.6; turbid in 2 vol- 

 umes or more. 



0.03 



1.4708, at 23° C 

 0.7; very turbid in 2 

 volumes or more. 



0.07 



1.4722, at 2-3° C. 

 0.75; very turbid in 



1§ volumes or 



more. 



0.20. 



culated as ace- 

 tic acid). 

 Menthyl acetate. 



30 94 



31 20 



13.79. 



24 43 . 



34 11. 





33.07 



40.13 



41.50 



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 hut 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 menthyl 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 



