n6 
Journal of Agricultural Research 
Vol. II, No. 3 
According to Sykes and Ling (1907), it is well known that hops grown 
in different parts of the world differ in aroma. It is further stated that, 
since the oil from California hops yields the same compounds as the oil 
from Bavarian or English hops, the difference in odor must be due to the 
different proportions in which the constituents exist. 
While it can not be said with certainty that the aromatic constituents 
of hops are useful industrially, yet they are of sufficient importance not 
to be overlooked. Concerning the use of the volatile oil in determining 
the value of hops, Chapman (1898, p. 233) observes: 
It is, in the first place, perhaps, the surest ^uide to the general value of a sample 
of hops, when reliance is placed upon a physical examination alone * * *. in the 
selection of hops * * * the essential oil is certainly the constituent to which 
chief attention should be paid. 
Hops with a fine, agreeable bouquet are usually preferred to those 
with a poorer odor. If, therefore, differences in odor are clearly per¬ 
ceptible in different hops, a comparison of the volatile oils should be 
most important in determining possible differences in the aromatic 
quality of the hops. Since the volatile oil is the carrier of the aroma 
and since the aroma is a factor in judging hops, it should be possible to 
compare the aroma of one sample of hops with another by means of the 
volatile oil from each. 
ESTERS AS THE PRINCIPAL ODOR BEARERS 
Since esters are important factors in determining the odorous quality 
of volatile oils, it was thought that perhaps this would be a good point 
of attack for the comparison of the various hop oils. The very agreeable 
odor of the oil of hops would indicate the presence of esters, and a pre¬ 
liminary test showed them to be present in considerable quantity. 
The ester content is easily measured and may be expressed by the 
ester number. While this value does not express the exact percentage 
of any particular ester, yet for comparative purposes it answers equally 
well. For the purpose, therefore, of making a logical chemical com¬ 
parison of the several oils, the ester value was adopted as being possibly 
the constant most likely to show any fluctuation which would have a 
direct bearing on the aromatic quality. 
PHYSICAL AND CHEMICAL PROPERTIES OF OILS AS A BASIS FOR 
COMPARISON 
The physical properties of volatile oils often show variations sufficient 
to enable comparisons to be made. The specific gravity of most volatile 
oils is an important factor in their investigation and is influenced to a 
considerable extent by the source and condition of the plant from which 
the oil is distilled, as well as by the nature of the constituents of the 
oil. Optical rotation is a property of volatile oils which is very important, 
