A new generation of beers with a more pronounced and aromatic hop flavour are getting more and more interest from a broad beer drinking audience, led by the IPA revolution. Since specialty beer volumes are increasing, current dry-hopping techniques face challenges coping with bigger hop volumes. A safe, controlled and automated solution will augment efficiency and lower the cost of the process and hop usage while improving beer quality and flavour, explains Kim Verschueren, team manager for Process Automation, Food & Beverage and Fine Chemicals at Agidens

Hops are crucial and expensive ingredients in the brewing industry. When dry-hopping a beer to add more aroma, hops are added to the green beer at the end of the fermentation stage to avoid wasting the precious, more volatile compounds found in specific hop varietals.

This technique is typically used in beer styles like IPAs (India Pale Ale) or Imperial lPAs; it gives them more specific aromas (herbal, spicy, citrusy etc.).

Dry-hopping has a lot of parameters to take into account. Besides beer and yeast type, the quality of the hops as raw material has a major impact, i.e. varietal, harvest and amount of alpha- and beta- acids, essential oils etc.

However, to get the right hop flavour into the beer, a lot of physical parameters come into play as well to boost the diffusion of the necessary compounds from the hops into the beer, such as surface area of hop particles exposed to beer, contact time beer/ hops, temperature (both of hops and beer) and agitation or flow over the hop’s surface area. 

From a quality point of view, oxygen pickup should be avoided after first fermentation to protect the beer from oxidative reactions and diacetyl (re)formation, but also to avoid oxidative decomposition of the hop’s sensitive beta-acids or lupulones, known to be detrimental to the taste of beer. 

Depending on batch size and desired aroma level, dry-hopping implicates getting certain amounts of hops in contact with the fermented beer according to the recipe used. Concentrations range from mildly hopped (0.5 g/l) to extremely hopped beers (up to 30 g/l).

The current techniques give a lot of brewers a headache and looking at the volumes of beer in bigger fermenters, the amounts of hops to be used are getting even harder to handle.

For a batch of 500 hl beer at a hop addition range of 2 g/l, you should use about 100 kg of hop pellets. It speaks for itself that these amounts of hops only get bigger with the volume of beer and degree of dry-hopping.

The well-known fact that hop pellets swell, expanding to seven times the initial pellet volume in combination with huge quantities of these pellets cause major concerns regarding blockages and upsetting the normal beer production Homogenisation by circulating the contents of the fermenters should be taken into account as well to get an even distribution of the flavour compounds. 

These challenges push brewers towards dosing and dissolving hop pellets into beer instead of circulating it over packed amounts of hops in smaller secondary vessels next to the fermenter (so called hop guns).

Getting the hop pellets into these kinds of complex systems is a lot easier than getting the massive green sludge of vegetable matter out and losing precious beer along the way. 

Furthermore, by dissolving hop pellets into beer, the amounts of hops needed and the total contact time can be drastically reduced since surface contact and diffusion rates are several times bigger.

This boosts efficiency and reduces the overall grain-to-glass time. Putting hops in a solution with beer is overall preferred over keeping the hops in a closed vessel with filter plates when handling big amounts of hops.

Manually adding the hops to the fermenter vessel top is currently an unsafe, time consuming, potentially beer spoiling and complex task. The classical procedure to bring the bags of hops up to the fermenter top and dosing – unaerated – hops to fermented beer via the top plate is not the most elegant way. 

Another available technique which is derived from dairy processes is dosing and dissolving additives using high shear pumps. But dosing from open containers – even with CO, blanketing – does not ensure dosing of fully deaerated hop pellets.

The use of high shear pumps is often not favourable since it is not the objective to make an emulsion nor to pulverize the organic hop material. 

Grinding the hop pellets will contribute to higher levels of polyphenols in the beer coming from the vegetable matter, generally considered as a non-desirable flavour. In addition, reduced particle size will not help getting the hops out of the beer, for example by partial decanting.

Blowing the pellets with pressurized gas to the top plate is another way to get hops into the fermenter, so called hop-to-the-top systems. This technique implicates some quite expensive hardware adjustments, including top plate modifications, to get a complete solution in place and ensure a correct cleaning procedure.

Getting the hops back out of the beer can be a rather complex process as well: using a centrifuge to separate the hops from a homogenous beer/hop mixture is the most effective way.

Some green beer centrifuges can also be used to remove suspended hops, keeping the dissolved hop flavours intact and avoiding additional expensive investments. 

Besides the centrifuge, decanting the hop particles is also possible, considered that the heavier fraction sinks to the fermenter cone while the fractions lighter than beer will float to the top. 

Depending on the dissolution process and type/quality of hops, the relative percentage of heavy/light fractions can vary significantly between 95/5 and 70/30. Existing methods to dry-hop large volumes of beer in fermenters can be optimised with new techniques of dry-hopping, focusing on faster and more qualitative methods. This implicates new hardware and another level of automation to get safe, reproducible, and flexible dry-hopping methods in place. 

A major brewery set up a masterplan to rationalize their existing cold block. The basic engineering study was done by Agidens, a Belgian process solution provider. An automatic and reproducible dry- hopping method was part of the requested solution. 

As the conventional dry-hopping methods did not fully comply to the requirements of a fully automated process, Agidens proposed a solution with a new designed hop dosing unit called AHOP and the use of an IMXD agitator from Alfa Laval to disperse and homogenize the contents of the fermenter vessel. 

The goal of the AHOP design was an economical dosing solution, tackling some of the challenges of existing dry-hopping systems. Based on the Siemens TIA portal platform, a recipe-driven fully automated and flexible system was developed to cope with the variety of beers, hops and other additives and to avoid the manual intervention currently needed in commercial dosing systems. 

In practice, testing included both raiser pipes and the use of mixing systems, bottom- built into the existing fermenter to avoid complex and expensive adjustments to the existing fermenter tank park. The systems are fed by an external circulation loop using a centrifugal pump, to keep hops in suspension and optimize diffusion of the aroma compounds. 

Two tank mixer types were investigated, including a motor (Alfa Laval Iso-Mix) and a nonmotor driven solution (AJET eductor) for recirculation and homogenisation of the fermenter contents.

The proposed solution was extensively tested, starting with water tests for fine-tuning of the hop dosing system up to tests on commercial beer types (table 1).

A special focus was laid on the disintegration time of the pellets after dosing, decantation time after recirculation and removal of the hop residues. In some cases, additional hop dosing was done after a certain circulation time, when sensory tests prior to product release resulted in adding just a little bit more hops.

The system proved to be very flexible, fast, and easy to use. Furthermore, tasting tests of the beers led to a considerable better overall flavour profile using the AHOP module over hop gun type dry-hopping methods.

Additional tests were performed with dosing of finer granulates being coffee grains and liquid additives and witnessing the homogenisation of the fermenter contents with these additives. Dosing of this type of additives proved to be easier than dosing hop pellets.

The applications of dosing powders, granulates and liquids are limitless, offering the brewer a playground where only his own creativity is limiting the creation of new beer recipes.

Homogenizing the fermenter’s content with the additives went very fast, using both the IMXD (Iso-Mix external Drive) and an inline tank jet mixer, the AJET (features are described in table 2).

The jet mixer has the advantage to be non-motor driven and the in-tank mixing flow is up to 5 times bigger than the external circulation flow, where the IMXD is more effective in avoiding sedimentation on the fermenters cone when using very oily hops.

The mixers are adjustable to be built in any fermenter vessel without disturbing the yeast harvest process. One needs to be careful putting restrictions on the fermenter tank outlet, thus reducing the available outlet pipe section when harvesting yeast. 

As an extra, the solutions including the recirculation over the fermenter offer a brewer the possibility to investigate the advantages of keeping the fermenter’s contents during fermentation or maturation in optimal suspension.

This is known to possibly speed up fermentation and/or deep cooling, thus boosting the efficiency of the entire brewery and further helping to reduce the grain-to-glass time. 

Cleaning cycles followed all water and product tests to make sure the mixing systems were cleanable with standard brewery CIP recipes.

The AHOP dosing module, including its dosing chamber, were cleaned cold, together with the CIP of the fermenter tank. Visual inspection of the closing unit and fermenter tank as well as disassembly of the hop dosing unit showed that the installation was perfectly CIP-able. 

Both the Alfa Laval Iso-Mix and the non-motor driven jet mixer AJET were also proven to be fully CIP- able. A positive effect using the Alfa Laval Iso-Mix is the additional cleaning jet action in the bottom part of the fermenter. 

In conclusion

The ever-evolving brewing market demands new processes and flexible solutions to produce high quality and interesting beers in a controlled way. Current developments in dry-hopping techniques make it easier to handle the dosing of hard-to-handle ingredients like hop pellets.

Upscaling dry-hopped beer production is no longer an issue with the variety of solutions that some suppliers can provide. Other applications might be interesting, like dosing spices in the cold wort during transfer instead of adding them into the hot wort in the brewhouse, or dosing dry yeast and sugar into the finished beer.

Testing the AHOP dosing module and mixing processes confirmed their unique features: a fully automated solution for unlimited dosing of deaerated hops and other additives under safe and hygienic conditions.

About the author: Kim Verschueren: Team Manager Process Automation, Food & Beverage and Fine Chemicals at Agidens

Kim manages a team of engineers and provides sales and project management support for food and brewery applications and the standardisation of working procedures for our Food & Beverage, Oil & Gas and Life Sciences customers.