Ultrastructure studies of the lupulin glands of different hop (humulus lupulus l.) varieties observed by scanning electron microscope

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 the ultrastructure characteristics of the lupulin glands are primarily associated with the hop class.