The frequency with which one brews determines the type of brewer one becomes.

Since I started homebrewing in the summer of 2014, I have made 21 beers. That is a rate of one batch every two months.

But the average rate does not tell the true story. I tend not to buy ingredients for only one brew as that would be an inefficient allocation of shipping and packaging costs. Instead, I get a bulk delivery of ingredients for two or three brews, and some of those ingredients – chiefly the liquid yeasts – spoil relatively quickly. As a result, rather than brewing one batch every two months, I am more likely to brew two or three batches over the course of 6-12 weeks, and then go perhaps two or three months without brewing at all.

This irregularity and relative infrequency of brewing inclines a brewer, especially in their early years of practice, to create more diverse styles of beer, relative to his or her total output, as opposed to “tweaking” the same recipe numerous times in search of perfection.

Creating recipes is 80% of the fun of brewing. If one brews, in effect, three or four times a year, one is constantly looking forward to the next recipe, the next style. If one brews three different beers every month, as many homebrewers do, one batch can be an experiment while the other two can be used to tweak the best recipes.

Were brewers to be divided up into “tweakers” and “experimenters” on the Day of Judgement, I would certainly find myself categorised as an experimenter.

Having said that, we are all part saint and part sinner, and all experimental brewers indulge in a bit of tweaking from time to time. While I have brewed 21 beers, I have set out to create something genuinely new and different only 12 times.

Every brewer has a certain style that they wish they could achieve and perfect at home. For me, that style is the Belgian Strong Dark Abbey Ale. Regular readers will know that I have attempted this style, with different recipes, on five occasions. Often the resulting beers looked and tasted wildly different, and at least two of them were excellent, but they were all, ultimately, failed attempts to create a beer like Rochefort 10.

There are other reasons why natural experimenters sometimes start tweaking. Inspiration can come from travelling: following a beer-hunting holiday in Wallonia in 2015 I made two attempts, in relatively quick succession, to create a Brettanomyces-fermented pale ale in the style of Orval. Some beers are seasonal: I have twice made a smoked porter for bonfire night and a spiced ale for Christmas. I have also made two attempts at a Bavarian Weiβbier, largely because my brother is a big fan of the style.

This weekend I have been tweaking again.

Five months ago, I enlisted the help of a couple of friends to brew a U.S.-style Double India Pale Ale, “Pacific Northwest”. The beer was a great success, a lovely balance of malt-and-alcohol sweetness, restrained bitterness and lush American-hop aromas. That was the main incentive to recreate the brew.

The other was that something rather intriguing happened to the flavour of the beer during its first few months in the cellar.

When fresh, the beer was a riot of grapefruit and passion fruit juiciness. As it aged, the fruitiness subsided and an earthy, pine-resin pungency came to the fore.

Moreover, this change seemed more pronounced in smaller, 500ml or 330ml bottles than it did in larger, 750ml bottles (all of which were sealed with crown caps).

None of these changes made “Pacific Northwest” a better or worse beer. They simply made it notably different. But they did inspire me to find out more about what might be happening to the chemistry of my hoppy beer – and, of course, to tweak the recipe to discover whether or not I could counteract or slow down these changes.

My aim was to heighten the up-front bitterness of the beer, and to preserve the fruity, citrus and floral hop aromas for longer, keeping the resin aromas somewhat at bay.

My first port of call to find out what was happening to the ageing of the hop aromatics was Patrick Dawson’s Vintage Beer: A Taster’s Guide to Brews That Improve over Time (Storey Publishing, 2014), which I reviewed earlier this year. Unfortunately, this book had little to say on the subject beyond these words, on p.29: “due to oxidation, these [hoppy] flavors and aromas will disappear over time, with the myrcene oil-derived aspects (citrus, floral) being the first to go”.

While this fits with my experience – citrus and floral flavours receding, earthy, resin flavours coming forward – attributing this to degradation of myrcene doesn’t seem right. If anything, myrcene, a monoterpene, is associated with peppery, resinous aromatics.

I looked at some peer-reviewed scientific papers on the subject. But before we turn to those, let’s go back to basics.

From George Fix’s Principles of Brewing Science (Brewers Publications, 1999), pp.54-62, we learn that most of the bitterness in beer comes from the isomerisation of the hops’ alpha acids in the boil, and that its aromatic flavours are derived from the hops’ essential oils. It follows that, for a very bitter beer, hops with a high concentration of alpha acids should be boiled for longer; and, for all the controversy that this excites in the brewing-chemistry literature, the later that hops are added to the boil, the more the volatile compounds that constitute their essential oils will be preserved in the finished beer.

Hops’ essential oils are around 75% hydrocarbon components and 25% oxygen-bearing components. The hydrocarbon components are either monoterpenes (myrcene or beta-pinene, which deliver what Fix describes as intense, “pungent” flavours of citrus rind, pine resin or pepper); or sesquiterpenes (humulene, farnesene or caryophyllene, which deliver what Fix describes as more “delicate” flavours, floral or fruity). The oxygen-bearing components include two alcohols, linalool and geraniol, which combine to deliver floral, sweet-fruit and spicy aromas.

It follows that, for an earthy, pine-resin flavour in your beer, you would increase the proportion of hops with a high concentration of monoterpenes and linalool in their essential oils, especially in the later stages of your boil. If you prefer something fresh, fruity and floral, go for hops with a higher concentration of sesquiterpenes and geraniol.

For “Pacific Northwest”, I chose three hops: Amarillo, Chinook and Simcoe. At the time, the choice was not particularly scientific. I knew I wanted quite a bitter beer, which the high-alpha acid concentration of Simcoe would deliver. Beyond that, I was looking for a nice balance of the orange-blossom and tropical fruit flavours of the Amarillo with the pine-resin and grapefruit flavours of Chinook and Simcoe.

This was before I had properly compared the chemical constituents of each of these hops’ essential oils. When I did this, I found that Amarillo had the highest concentration of sesquiterpenes (and the joint highest of monoterpenes); that Chinook was balanced between monoterpenes, sesquiterpenes and linalool/geraniol; and that Simcoe had the highest concentration of linalool/geraniol (and the joint highest of monoterpenes).

This surprised me. I had expected Amarillo to be dominated by linalool/geraniol, and Chinook and Simcoe to be dominated by monoterpenes.

Dig a little deeper and the chemistry starts to tally more closely with tasting experience, however.

Amarillo and Simcoe may have the same concentration of monoterpenes, but Simcoe has a 25%-bigger dose of the highly pungent beta-pinene. Simcoe also comes with a much lower ratio of humulene to caryophyllene in its sesquiterpenes – and it is the latter that lends black pepper its earthy spiciness. Even its exceptionally high concentration of linalool/geraniol would be associated with a spicy flavour.

By contrast, Amarillo gets its dominant flavours from myrcene, humulene and farnesene, each of which delivers a variation on classic American-hop fruitiness or fruit-rind aroma.

So much for the qualities of these three hops when they are fresh. What about when they begin to age in the bottle? Here we turn to those scientific papers.

The first thing to note is that these papers seem less concerned with what’s happening to monoterpenes than they are with what’s happening to sesquiterpenes and the oxygen-bearing alcohols, linalool and geraniol. This appears to be due to the fact that most beer studies are conducted using noble-hopped Pilsner lagers. Noble hops typically have between two- and four-times the amount of sesquiterpenes than monoterpenes in their essential oils.

For that reason, it is still not clear to me to what extent the conclusions on the effects of ageing on flavour that I draw below are due to changes in the level of sesquiterpenes and linalool/geraniol alone, or changes in the levels of both monoterpenes and sesquiterpenes that are offset by the lower taste threshold of monoterpenes.

The classic study is that by Val Peacock and Max Deinzer of Oregon State University, “Fate of Hop Oil Components in Beer”, in the Journal of the American Society of Brewing Chemists, Vol. 46 (1988). This assessed the rate of degradation of linalool, geraniol, humulenol II, and humulene diepoxide A that the researchers had “spiked” into a “premium American beer”, after the beers were stored at room temperature for two months.

The researchers found that around 10-12% of the linalool and geraniol had disappeared after just two weeks, probably via oxidation, but the depletion rate was much shallower thereafter.

By contrast, a staggering 84% of the humulene was gone after two months, probably through both oxidation and acid hydrolysis, and the rate of depletion was virtually linear.

“Reaction of these compounds may explain the fleeting nature of hop aroma in beer,” the authors concluded.

More recently, Filip Van Opstaele, Gert De Rouck, Jessika De Clippeleer, Guido Aerts and Luc De Cooman, in “Analytical and Sensory Assessment of Hoppy Aroma and Bitterness of Conventionally Hopped and Advanced Hopped Pilsner Beers”, Journal of the Institute of Brewing, Vol.116.4 (2010), carried out a “forced ageing” of Pilsners brewed and flavoured in a variety of ways, by storing it at 30°C for two months.

Their findings agreed with those of Peacock and Deinzer when it came to sesquiterpenes: they degraded rapidly. Interestingly, however, they did not detect any degradation of linalool – in fact, in their dry-hopped Pilsners the researchers detected an increase in linalool, “probably due to the liberation of linalool from the glycosidic precursors that were extracted during the dry-hopping treatment”.

In the light of this 2010 study, a lot of what I experienced with the ageing of “Pacific Northwest” begins to make sense. The sesquiterpenes and humulene that characterise Amarillo are likely to have degraded fastest and most completely; whereas the beta-pinene and linalool/geraniol that define Simcoe’s earthy, pine-resin spiciness are likely to have maintained or even increased their concentrations.

Earlier I wrote that, for an earthy, pine-resin flavour in your beer, you would increase the proportion of hops with a high concentration of monoterpenes and linalool in their essential oils, especially in the later stages of your boil; and that, if you prefer something fresh, fruity and floral, you would go for hops with a higher concentration of sesquiterpenes and geraniol.

If you want to maintain that fresh, fruity and floral flavour over a longer period in the bottle, you would presumably do the same thing to an even greater degree (and perhaps also keep the beer in larger bottles to slow down oxidation).

The chart below shows the amount of the three hops, Amarillo, Chinook and Simcoe, used in the first and second iterations of “Pacific Northwest”, in grams.

The total weight of hops was the same, at 190g. Chinook, the most balanced of the three hops, remained in the same proportion. Amarillo went up from 75g to 85g, and Simcoe was correspondingly reduced from 55g to 45g. The overall effect should be to increase the sesquiterpene proportion and reduce the beta-pinene and linalool proportions of the total essential oils in the wort.

Of course, the point at which these hops are introduced to the boil will also determine how many of their volatile compounds survive into the finished beer. The next chart shows how much of each hop went into the boil at certain intervals, for both iterations of the beer.

Two changes from the first to the second iteration stand out. First, the early hopping, for bitterness, is now dominated by Simcoe and Chinook, and it has doubled from 30g to 60g. Second, a greater proportion of the Amarillo hop has been pushed later into the boil, taking it from 30g to 55g in the final five minutes, and replacing the original Simcoe addition at flame-out.

The aim is to produce a beer with a slightly more robust bitterness than that of the original “Pacific Northwest”, and also one that sustains its zesty, fruity hop character for a little longer in the bottle, keeping the earthy, pine-resin elements as a background note for a long as possible.

Between now and the New Year we should have learned a bit more about how this tweaking of the hop schedule has panned out.

Whatever happens, of course, it will be on to another style for the next brew!

“Pacific Northwest” (November 2017)                              

In the mash:

• 6.0kg of Pilsner malt
• 1.5kg of Maris Otter malt
• 200g of Wheat
• 150g of Crystal 100 malt
• 60 minutes of rest

In the boil:

• 30g of Simcoe hops for 90 mins
• 30g of Chinook hops for 60 mins
• 30g of Amarillo hops for 20 mins
• 15g of Simcoe hops for 10 mins
• 30g of Chinook hops and 15g of Amarillo hops for 5 mins
• 40g of Amarillo hops at flame-out
• 90 minute boil

Fermented with Wyeast Laboratories 1056 “American Ale”

Starting Gravity: 1.064 / 16° Plato

Finishing Gravity: 1.010

ABV: 8.0%