Evan Elford, OMAFRA New Crop Development Specialist

Originally posted: 25 September, 2018
Updated with additional content: 26 September, 2018

Over the last few days I’ve had enquiries from hop growers regarding lower than anticipated alpha acid levels in the 2018 crop.  One of the first options for any grower concerned with hop resins testing outside of the reported range is to consider sending a second sample for testing at a different facility (hop testing facilities).  This will ensure the initial test results are correct.

After confirmation of test results, the next stage is to reflect on the environmental conditions of the year and the cultural management practices used through the growing season.  A preliminary search of the scientific literature revealed a general consensus that environmental conditions (such as those recorded in many Ontario regions during the 2018 growing season) can affect the formation of alpha acids in some hop cultivars.

As a background, all of the studies summarized below were conducted in Europe on European cultivars.

Point Form Summary (additional information below):

• There is a high positive correlation between increased rainfall /irrigation and the formation of alpha acids (i.e. typically more rainfall or irrigation will result in a higher rate of alpha acid biosynthesis/formation).
• There is a negative correlation between increasing temperature and alpha acid formation (i.e. the higher the daily temperatures, the lower the alpha acid biosynthesis/formation).
• Temperatures during the month of July appear to affect alpha acid formation the most in terms of timing of impact on alpha acid formation (many studies suggest the largest impact of environmental conditions on alpha acid development in the month of July, however, they can also be influenced during the months of May-August).

Additional notes for consideration:

• Many of the studies compared long-term weather records (e.g. 25-35 years) with alpha acid levels and total cone yield in hops.
• High alpha acid levels are attributed to 1) moist summer conditions; 2) below average temperatures; and, 3) average sunshine.
• In some regions, alpha acid synthesis is correlated to temperatures between the dates of May 24 and June 21, other regions documented July temperatures as the most important, while still others showed August. Generally, across studies, July (cone formation) and early August (cone maturation) were the most important times of influence on alpha acid formation.
• Optimal temperatures for alpha acid development occurred in the range of 15-17^o C. Alpha acid synthesis decreased above or below these temperatures.  The negative impact of temperatures above 20^oC on alpha acid formation can be compensated with increased irrigation to some degree.
• A minimum total rainfall or irrigation of 300 mm (~12”) from May through August is required for optimal alpha acid production.

In the short term, I have yet to find a replicated study evaluating the impact of environmental conditions on alpha acid formation in US cultivars such as ‘Cascade’.   Hopefully more information can be found on this topic and will be added to this posting.

In the long term, it appears that monitoring soil moisture levels and being vigilant with irrigation practices, especially during higher temperature days, is important to maximize alpha acid formation in hops (or at least help to minimize the negative impact of higher temperatures on alpha acid formation) under Ontario growing conditions.

Additional Resources:
Best Management Practices: Irrigation Management
OMAFRA FactSheet Monitoring SOil Moisture to Improve Irrigation DecisionsSoil Moisture Monitoring Equipment Suppliers