Introduction

This paper is principally about maximising hop aroma but it should be understood from the start that it is not possible to do this without some impact on flavour and bitterness too. Although there are papers analysing the influence of hops going back over 100 years, it is safe to say that it is only within the last 10 years or so that use of technology (e.g. spectrophotometry) and, to be fair, a great deal of research carried out by our American cousins, that has really started to lay bare the secrets of the humble hop. This work is by no means finished, back in 2008 it was determined that there has been 485 hop compounds currently identified and evidence to suggest there are upwards of 1,000 hop oil compounds that may be present! A great deal of work still needs to be carried out before we can really know what in particular about particular hops and their combinations and how to utilise them will allow us to achieve consistent results of the type we are looking for. Until then there will be many years of “glorious” hitting and hoping.

I will touch on the effects of some techniques and how they affect flavour and bitterness but much more is beyond the scope of this undertaking and would become too bloated and frankly a tad boring. The purpose is to relay an overview of the literature, with an emphasis on the science available, but inevitably, given the current level of research, it will have mixed in anecdotal evidence, not least my own impressions and the results of trying some of the techniques outlined.

Process

Liquor Adjustment

Guess what? I’m going to mention water treatment! But not because water treatment has an effect on hop aroma, but because in order to achieve a high level of hop aroma you need to use a lot of hops. A high level of alkalinity in mash and sparge liquor is going to drag out of the hops higher levels of polyphenols i.e. astringency and a harsher perception of bitterness. Mash pH is key, reducing liquor alkalinity to the right range means you can use a high level of hopping to achieve the aroma you are looking for whilst reducing the otherwise negative effects of higher hopping rates. As for adjusting the flavour ions one paper found a statistically significant negative correlation between the intensity of hop flavour and level of sulphate in the brewing water. So it may be better to have a neutral or even chloride forward water, like Timothy Taylor’s Landlord, if hop flavour is an important desirable characteristic.

Hop Schedule in the Kettle

Heavier hopping requires an adjustment to how hops are used in the kettle. It is now well established that hop oil volatility increases over boil time, meaning aroma is driven off and bitterness utilisation increased. This explains why there has been a major move towards late hop additions in an attempt to get the more of these oils into the fermentor. Experiment with adding enough hops at the start of the boil to obtain 80-90% of the IBU’s you are looking for. The rest will come from late additions. Research is demonstrating that bitterness and its perception is obtained from hop additions that have not undergone isomerisation from boiling. This is a key finding for aroma, because it means you can achieve aroma, bitterness and flavour, without destroying it in and after the boil. Experiment with adding hops very late in the boil, after 15 minutes to go, or omitting completely and only adding after the boil has finished.

Whirlpool & Hop Back’s

Techniques such as these are designed to take advantage of the above findings. Steeping hops once the boil is over might be a starting point if you can’t whirlpool or don’t have a hop back. Quite simply after you switch off add the hops replace the lid and let them soak, anywhere from a few minutes up to 90. The other variation other than time is temperature. Keeping in mind the volatility of hop oil at higher temperatures, waiting or cooling to lower temperatures is said to preserve the more volatile aromas. Be careful though as temperatures below pasteurisation run a small risk of introducing infection, though I’m unaware of anything in the literature suggesting this is much of a problem and certainly not one I’ve encountered even as low as 60c. Whirl pooling is based on the assumption that increasing the hops exposure to the wort through agitation will extract more hop oil in a shorter time, although I haven’t found any evidence in scientific papers to confirm this, which is not the same as there isn’t any. It’s a reasonable assumption. A poor man’s whirlpool can be achieved through the simple matter of vigorously stirring the wort then leaving it to spin down over time.

A hop back is a familiar tool to traditional English brewing. It comprises of a “container” stuffed with usually leaf hops through which the wort is passed, sometimes steeped, before being put through chillers and pumped to the fermentor. A number of home brewers have made such devices to sit between their kettle and chiller. Arguably an alternative to whirl pooling designed to achieve the same results rather than an additional or complimentary component. I am about to experiment with one and will report back the results once I have them.

Homemade Hopback

A comparison of both whirl pooling and hop backs suggests that the longer residence of hops in whirl pooling is a superior to the shorter contact time of the hop back, though recirculating through the hop back would presumably equate, again watch this space. Combining this technique with dry hopping in the fermentor was considered to be the optimum for increasing bitterness, flavour and hop aroma, but we are getting a little ahead of ourselves. It therefore clearly pays to delay cooling until there has been sufficient contact time with the hops. So a combination of long post-boil kettle hop residence, we’re talking up to 90 minutes, and dry hopping seems to maximize combined hop aroma and flavour, but it’s worth noting that if you want to develop hop flavour, kettle residence is the key, if you only want aroma then dry hopping only is the key. A footnote to this is it appears that hop aroma and flavour are closely correlated to perceived bitterness. Together, they might be a much better predictor of perceived bitterness than IBU. Which brings into question the usefulness of using IBU as a method of measuring the hop character of say very hoppy IPA style beers.

Dry Hops in the Fermentor

Dry Hopping

Traditionally dry hopping meant a “handful” of hops bunged into a cask before or after it was filled with Beer and the shive banged home. Whilst that still may hold with some cask Beer it is much more likely these days to be referring to hopping in the fermentor or a holding tank before packaging.

I’m going to restrict comments to dry hopping in the fermentor.

Brewers often have to consider several factors in relation to dry-hopping in order to produce beers with consistent hoppy flavour and aroma. These include, hop variety selection and harvest date, rate of addition, oil content of selected hop, dry hopping temperature, and whether to adopt a dynamic or static dry-hopping process i.e.-agitate or not. Even the alcohol content of the beer to be dry hopped must be considered, in order to prevent the extraction of unwanted hop vegetative materials into the finished product due to the solvating power of ethanol. The duration of dry-hopping (contact time) is another crucial factor to control the balance of compounds extracted from hops into beer. I can only recommend certain broad brush strategies rather than specifics, so bear with.

Which Hops, How Many, When, What Temperature & How Long?

It won’t surprise you to learn that there are quite a few views on this. The first and obvious point to make is choose hops appropriate to the style you are making, does the style require a big hop aroma in the first place? Dry hopping Stouts and Porters is a waste of hops but late hopping in the kettle, particularly after the boil, might be considered for developing hop flavour.

There is a school of thought that suggests hops with a low cohumolone level should be chosen but there is no reliable evidence for that so for now the more aromatic the better. On that point only use fresh hops that are still sealed in their original bag, if they are not pungent when opened they are not going to deliver in the fermentor, the stickier the better. Pellets or leaf? Either works, just remember that leaf can float and introduce more oxygen and are a bit of a pain to clear up afterwards and can interfere with racking or syphoning. On balance I prefer pellets as they also break up quicker and expose more surface area to the Beer, there is also evidence suggesting faster extraction rates than leaf too. A recent innovation you really should try, if you can get them, are Cryo hops. These are hops that have had the lupulin glands separated from the leaf material at very low temperatures. They deliver the most intense aroma per gram than any thing else I’ve tried.

The quantity used to achieve high aroma is a minimum of 2g per litre, 4g would be my recommendation. Dry hopping isn’t cheap if you want big aroma. Whether this is as a single variety or a blend is again one for experiment. Leaf will absorb more Beer than pellets but only of concern to commercials. Cryo hops can be used at a rate of 50% of leaf or pellet. Higher gravity worts can take higher levels, low gravity beers tend to expose grassiness. There is evidence that bagging hops can reduce extraction by 50%, so let them go in loose and work round the mess.

Timing of dry hopping has come in for some attention. NEIPA brewers swear by dry hopping early in the fermentation, 24 hours in. Two advantages quoted are that the oxygen that is inevitably introduced is scavenged by the yeast. The second is something called biotransformation. This is where some of the aroma compounds transform from one thing into another e.g. conversion of geraniol to citranellol, which can lead to more of a rose/citrus/fruity aroma. Linalool is converted into terpineol (which is described as having lilac like aromas). However, there is at least anecdotal evidence, mine included, that says the yeast actually strip aroma and CO2 gases it off. Just give your airlock a sniff for proof. If yeast biomass does strip terpenoids (collective word for the aromatic oils) over pitching should be avoided too. Fermenting cooler is offered as a solution but I don’t really buy it. My recommendation would be to purge the fermentor with CO2, if possible, after you’ve dry hopped and to add them at the end of fermentation.

On the subject of temperature for dry hopping, most recommendations are for the same as fermentation temperatures. Perplexingly it is different for different hops linked to levels of alpha acid, but for our purposes better to stick to the usual range. It does appear that polyphenols increase at higher temperatures, particularly if high alpha hops are used, but if contact time is short this is less of a problem. A particularly interesting observation is that at the usual temperature range alcohol is increased by around 7%. My experience is typically 1 gravity point.

There has been a lot of Forum debate about contact time for dry hops, most of it ill informed, anecdotal and the repetition of others anecdotes. All the studies I have seen suggest that most of the uptake of aroma is achieved in very little time, one study suggests as little as 6 hours. Moreover, there is also evidence to suggest that concentration of oils decreases after day 4 with the exception of geraniol; myrcene levels drop off after day 1. Agitation, pumped recirculation, speeds up extraction but at the risk of oxidation. Given the speed of normal extraction it is arguably not worth it, with the downsides confirming it. What is indisputable is that long contact times are unnecessary, a waste of time, and as I’ve already said likely to increase the extraction of undesirable elements such as polyphenols and consequent perception of higher bitterness. Less really is more.

Hop Oil Extracts & Other Chemicals

There is little information I’ve found on the effectiveness of these. I have used them, Cascade and Goldings in particular, and neither have transformed the Beer in the way I would expect in order to recommend them. I have acquired an enzyme that is said to break apart some of the chemical bonds that release more of the aroma compounds, but have so far no reliable evidence to suggest this works too but need to experiment further. There a lot of new hop products on the market designed to reduce the leaf component, source of polyphenols, making the lupulin glands the focus. Bittering extracts to do the same. So called Cryo hops that are an intense version of T-90 pellets. All these are worth experimenting with. I would like to hear of your experiences with them.

Ashley Carr

Sources:

Maximising Hop Aroma & Flavour Through Process Variables – Van Havig

Examination of Studies: Hopping Methods and Concepts for Achieving Maximum Hop Aroma and Flavor – Scott Janish

The Secret to Big Hop Aroma and Flavor – Jamil Zainasheff

Dry-Hopping: the Effects of Temperature and Hop Variety on the Bittering Profiles

and Properties of Resultant Beers – O. Oladokun et.al. Brewing Science November / December 2017 (Vol. 70)

Effect of mash pH on flavour profile – Table 2. Cam Grover (The Drunk Alchemist) 2014