Influence of nitrogen fertility practices on hop cone quality

Authors: ISKRA, ANNE - Oregon State University; LAFONTAINE, SCOTT - Oregon State University; Trippe, Kristin; MASSIE, STEPHEN - Washington Hop Commission; Phillips, Claire; TOWNEY, MEGAN - Oregon State University; SHELLHAMMER, THOMAS - Oregon State University; Gent, David - Dave

Submitted to: Journal of the American Society of Brewing Chemists
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/5/2019
Publication Date: 6/11/2019
Citation: Iskra, A., Lafontaine, S., Trippe, K.M., Massie, S., Phillips, C.L., Towney, M., Shellhammer, T., Gent, D.H. 2019. Influence of nitrogen fertility practices on hop cone quality. Journal of American Society of Brewing Chemists. 73(3):199-209. https://doi.org/10.1080/03610470.2019.1616276.
DOI: https://doi.org/10.1080/03610470.2019.1616276

Interpretive Summary: Nitrogen fertilization is an important aspect of producing high quality hops profitably. We conducted field experiments over a five-year period to understand how the rate and timing of nitrogen fertilization influenced multiple aspects of hop cone quality, including levels of bittering acids, oils, cone color, cone aroma, nitrate levels in cones, as well as yield. There was year-to-year variation in most cone quality metrics evaluated. However, over all years the average effect of increasing nitrogen rate was to reduce levels of bittering acids and oils, whereas the degree of greenness of cones and nitrate content was increased with nitrogen rate. The highest yields were not consistently associated with the highest nitrogen rates evaluated. Similarly, when nitrogen fertilizer was applied after bloom bittering acids were decreased and nitrate concentration increased. We found only relatively small differences in hop cone aroma between the various nitrogen treatments. The whole of this research suggests that cone quality attributes important to growers and brewers can be maximized by avoiding unduly high rates of nitrogen fertilizer and late season applications. Such practices also will reduce nitrate content of cones, which is considered a negative attribute in hops.

Technical Abstract: A multi-year field study was conducted in Oregon and Washington to evaluate the influence of nitrogen fertilization rate and timing on cone quality and nitrate accumulation in cones. The impact of nitrogen rate on cone yield, levels of bittering acids, total oil content, color, and nitrate level were year dependent. However, when data were aggregated over years and analyzed using a mixed effect model, a-acids, ß-acids, and total oil volume decreased linearly with increasing nitrogen rate; while cone color, expressed as the degree of greenness of cones, and nitrate content of cones increased linearly with nitrogen rate. Yield was not improved with the highest nitrogen rate. In one of four studies, panelists using triangle tests to evaluate hop aroma of ground hop cones detected a difference among treatments. Alpha- and beta- acids decreased and nitrate concentration increased when nitrogen was applied after bloom. One harvest showed that fertilizer timing led to differences in the aroma of the hop cones although this difference was within the standard aroma variation for the variety. Overall, this research indicates that applying the lowest feasible nitrogen rate and ceasing nitrogen applications before or at bloom may optimize certain cone quality factors while minimizing nitrate accumulation.