Ultrastructure Studies of the Lupulin Glands of Different Hop (Humulus lupulus L.) Varieties Observed by Scanning Electron Microscope

Abstract

The cones of the female hop (Humulus lupulus L.) plants are of importance to the brewing industry. Only the 
cones of the female plants are able to secrete the fine yellow resinous powder known as lupulin glands. The 
lupulin glands are technically termed glandular trichomes; secondary metabolites are synthesized in 
copious amounts by the glands. The resins and essential oils, synthesized and accumulated in the lupulin 
glands, impart the typical bitter taste and aroma to beer. To date, no clear distinctions have been made among 
the ultrastructure features of the glands in different hop varieties. In this study, the glandular trichomes of nine 
hop varieties, with different properties (e.g., hop class, country of origin), were observed using scanning 
electron microscopy (SEM). It is intended to compare the hop varieties and determine if the environmental 
factors have an effect on the ultrastructure of the gland. For each hop variety, 50 micrographs were visually 
assessed and compared based on their surface topography, degree of fullness, and volume. To determine 
if there are any similarities among the examined varieties, the gland features were correlated and clustered 
against the chemical properties of the corresponding variety. For the data analysis, mean centered values 
were used to maximize the variation between the clusters. The cluster data confirmed larger gland volumes in 
the bittering hop varieties independent of the country of origin. The collected data indicates that t