Green, fruity, dank … Tom Shellhammer On hops

For a hop scientist, a months-long summer holiday coinciding with the annual hop harvest in New Zealand is a dream sabbatical.

Tom Shellhammer is a Professor of Fermentation Science at Oregon State University where he leads the brewing science education and research programmes. His brewing research investigates hops, beer quality and the origins of hop aroma and flavour in beer.

He’s in New Zealand for around four months and will speak at the Hāpi Symposium in Moutere on March 21.

His talk is titled: “Understanding the Influence of Terroir and Harvest Timing on Hop Quality and Beer Characteristics.”

The first month of his travels involved trekking around New Zealand in a campervan.

“We arrived on December 30 and started travelling on the 1st of January. We picked up a 2006 VW T5 campervan from some Slovenian surfers on Facebook Marketplace and we’ve been driving it around the country and camping, until we settled in Ruby Bay for a few months,” Shellhammer says.

My first question is, naturally, what did he think of the beer?

Given that the Hāpi Symposium is a Garage Project event, he’d been drinking some of their beers, but also name-checked Altitude Brewing, Townshend and Hop Federation.

He’s visited the brewing school at Massey University, been to Garston Hops in Southland, and hooked up with the godfather of hop breeding in New Zealand, Dr Ron Beatson, hand-delivering Dr Ron a gift of some New Zealand-hopped beer from the legendary Russian River.

“Ron has been saying, ‘hey Tom, you gotta come down to New Zealand and see what’s going on down here’. So here we are.

“I’m going to spend the three months here in Nelson, connecting with Ron and Kerry Templeton [of Plant & Food Research] and the hop growers and hopefully meeting brewers.

“I work closely with hop growers in the US on agronomics and how it influences hop quality, and with brewers on how brewing practices influence hop expression in beer.”

With one of his main interests being the terroir of hops he was excited to visit Garston because it’s so far removed from the traditional Tasman growing area.

“James [MacNamee] and his son Owen took us around the farm and it’s pretty amazing — right in the middle of a sheep farm. That was the first time I’d seen sheep running around a hop field.

“It’d be interesting to know the degree of differences that regional identity has in New Zealand relative to the US because certainly that latitude change from the Nelson region around at 41 [degrees] down in Garston at 45 — the magnitude of difference is pretty interesting.”

More than terroir, Shellhammer is an expert on hop maturity — basically working out how the flavour and aroma of hops change the longer they stay on the bine.

When I suggest that understanding hop maturity is a relatively new lane on the hop highway, Shellhammer does a good job at self-effacement.

“It sounds big-noting to say, ‘oh, it’s because of my research that people are focused on it’. But I do think in some cases when people start paying attention to something, then other people pay attention to it as well.

“Hop maturity in my opinion has a bigger influence than regional differences do. Harvest timing for some varieties can really change the expression of that hop flavour.”

From green to OG

In general terms, hops picked too early have a “green, almost vegetative aroma”. Those picked in the sweet spot will have those fruity aromas many brewers and drinkers love, while hops left too late will tend to develop “stinky onion, garlicky aromas” — or OG for short.

“Citra, for example, start off with almost bell pepper, green aromas when they’re young, two weeks or three weeks before they’re going to be harvested. And then they just blossom into these beautiful smelling tropical-fruity and citrus aromas. Then, as the hops stay on the bine longer and longer, what gets layered on top of this are intense onion garlic aromas.

“We’re seeing the biochemistry changing, changes in oil content and alpha acid content, and really dramatic changes in the sensory perspective.

Shellhammer says these changes can happen quickly, with thiol-driven varieties such as Nelson Sauvin and Nectaron more prone to the OG character.

On paper, onion-garlic sounds off-putting but anyone who loves their beer verging into “dank” territory will appreciate that characteristic.

“What this provides is both opportunities and challenges for hop growers. The opportunities are that you can create a range of hop aromas. And what I found talking to brewers is that there isn’t one ideal hop expression for any variety, which I think is great.

“Some brewers are looking for cannabis-like qualities in some of their hops, leaning into that sulphury OG character, while others want none of that. They want it very fruit-forward.

“The fact that the hops will change so much over a harvest window provides an opportunity for the grower and the brewer to do this matchmaking.”

With the OG character, Shellhammer suggests that “taken further you can get diesel, jet fuel character”.

Given that this is exactly what can happen with Riwaka hops, which can go from intense grapefruit and passionfruit and then a much-appreciated diesel note, I wonder aloud whether this is to do with Riwaka’s fickle character. It’s a hard hop to grow and therefore, I imagine, hard to judge the best picking time.

The professor doesn’t have an answer to that, but hopefully by the end of his trip he might have an idea!

Story-telling around hops

Curiously, Shellhammer and his students were led into hop maturity studies from a different corner of the field — they started out researching hop creep. That’s a phenomenon of dry-hopped, unpasteurised beers continuing to ferment once packaged, causing beers to gush on opening or even explode.

The research showed refermentation is due to the enzymes in hops and Shellhammer’s team looked at kilning temperatures of dried hops to see if higher temperatures would       reduce hop creep. Short answer: yes.

But in the process of that work, they asked one hop farm if they could study enzyme activity through the harvest period to see if it changed depending on when hops were picked.

“We started getting hops early, like way before they were ready to be harvested. And then we let them hang on the bine way after they should be harvested — almost until the farm shut down.

“And what we saw year upon year, field upon field, is that hops picked earlier had higher enzyme activity and later in the season the hops had lower enzyme activity.

“At the same time, we’re measuring the chemistry on these hops and we’re doing a sensory evaluation of hops — and we found this cool, sensory stuff.”

The science of how hops taste depending on when they’re picked and where they are grown opens doors for what Shellhammer calls “good story-telling” that can enhance a drinker’s experience.

“There’s room for more relationships between individual farms and brewers — it happens in the Northwest of the United States, with breweries featuring beers that are single-farm hopped, which are quite cool.

“It’s about the consumer understanding that it’s not just a factory-built beverage. There’s also the storytelling around sustainability and local. You’re buying a product that’s made with agricultural materials that are grown just down the street.

“And the economic story fits the same way. The beer that you’re buying is paying people who are living in that community and you’re paying people who grow that product in that community, which is pretty cool.”

The upcoming fresh hop season will offer drinkers the chance to do just that.

Hāpi Symposium

The Hāpi Symposium is returning for 2025. Hosted by Hāpi Research partners Freestyle Hops and Garage Project, you can expect a day of talks targeted to professional brewers focused on the latest research around all things hops.

International presenters will focus on hop terroir, hop and haze interaction, along with a hands-on sensory session giving you a first-hand experience of thiols, terpenes and esters.

Alongside all of this, Hāpi Resaerch will present the latest developments and give you a first-hand look at new experimental lots from the Hāpi Research breeding programme.

Come spend a day with like-minded brewers from around the globe sharing the latest techniques and trends, with plenty of opportunities to swap stories and insights, all set against the backdrop of Sunrise Valley and the Freestyle Hops home farm.

2025 Hāpi Symposium Speakers

Dr. Tom Shellhammer – Oregon State University
Dr Laura Burns – Omega Yeast
Dr Jo Stephens – Select Breeding / Hāpi Research
Dr Peter Bircham – Garage Project
Dave Dunbar – Freestyle Hops
Brewers Panel Discussion – Announcing soon
Moderated by Pete Gillespie

Hāpi Symposium 2025

Schedule