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It’s a Hazy, Hazy World

The use of oats in brewing beer has risen sharply over the last few years due to the new craze for haze. In the past five years at Crisp Malt, we’ve seen oat sales increase tenfold. That’s a lot of haze! Traditionally, the addition of oats had mainly been reserved for stouts since they add a silky smoothness to the finished beer due to their high polyphenol. However, the high fibre, protein, beta-glucan and fat content was thought detrimental to beer quality, causing processing issues and poor shelf-life stability. They continue to cause issues today, but since unfiltered beers are now in vogue, the main issue with oats is the slow mash filtration time.  The same issues are applicable to high rye addition rates.

Naked Oat Malt | Oats in brewing


The Science Bit!

The simple rule that governs mash filtration is Darcy’s Law. Henry Darcy was a French hydraulic engineer who first published this work in 1856. He developed his ideas throughout his career when he worked on using sand filters to improve the public water supply of Dijon.

From this calculation, the raw materials you select influence the bed permeability and wort viscosity. which in turn influences the run-off time. In simple terms, to increase the flow rate through the bed you can either increase the factors on the top of the formula or decrease the factors on the bottom of the formula.



Let’s take a look at each of the factors…

Filter Bed Permeability

Permeability is just a fancy word for how easily liquid passes through a solid. Think about water passing through a big bag of pebbles = very permeable. Water passing through clay = not so permeable. It all comes down to the particle size of the solid bits. In a grain bed, the particle size is determined by what is in the grist and also how fine the grind is. For a regular barley malt grist, a course grind results in larger particles of starch and also nice big chunks of husk. These both act to provide plenty of gaps for the wort to pass through. I.e. high permeability. In a fine grind, you end up with a higher percentage of flour and fine particles. This is good for extract but makes for low permeability.

The problem with naked malted oats (or porridge oats for that matter) is that they are huskless, so when ground up they tend to increase the percentage of flour and fine grits in the mash tun so we recommend not using more than 10% addition – this will reduce the filter bed permeability.  On the other hand, torrefied oats are rolled flat and act like a husk and will increase the bed permeability. Rye and wheat are also huskless and so high addition rates will give more flower and slower run offs.

Compressed Bed = Low permeability = Low flow
Lose Bed = High permeability = High flow


Viscosity is basically how thick a liquid is.  Water has a low viscosity and flows freely, syrup has a high viscosity and is thick & sticky.

Wort viscosity can be affected by many factors such as OG and temperature but beta-glucans are the main problem, they significantly increase wort viscosity and gum up the filter bed, if you make porridge you can see how it sticks together.

The beta-glucans originate from the cereal cell walls; they are present in barley and wheat in manageable levels, but oats and rye have particularly high levels that significantly increase wort viscosity and hinder the run-off.  Indeed, we add oats to make the final beer thick and creamy and rye to improve mouth feel, so it should come as no surprise.  In naked malted oats, these compounds are more readily available and dissolve easily in the wort significantly increasing wort viscosity.

Our search for thick and full-bodied beers has resulted in greater use of adjuncts in the mash, along with high mashing temperatures to restrict fermentation and raise the final gravity of the beer.  The reduced malted barley addition to the grist dilutes the enzymes so there are fewer of them to do the job and the high mash temperatures restrict the enzyme activity making them slow and lazy.  When combined it can result in incomplete starch conversion, increasing wort viscosity and resulting in a stuck mash.

The mash temperature can also affect the kind of beta-glucans we extract.  Mashing above 65oC extracts beta-glucans that are more prone to gel formation, luckily the presence of maltose does inhibit gel formation, but high-temperature mashing restricts maltose production.

The Equipment

While the choice of ingredients has an effect on bed permeability and viscosity, the equipment we use also has a part to play. With a mash tun you don’t have the advantage of the rakes of a lauter tun which means you can’t cut the bed if you run into trouble. For mash tuns it’s better to keep the additions relatively low or else add rice or oat hulls to open up the bed with using huskless grains. With a lauter you can be a bit more generous with huskless additions, safe in the knowledge you can cut the bed. If you’ve plumped for a mash filter then fill your boots! We’ve seen customers doing 100% oat or rye mashes on a mash filter and the flavour is amazing.

Constant & Bed Depth

This is the loading on the mash or lauter tun.

The ‘constant’ is the vessel diameter, you can’t change it but it helps to explain why a wider a mash or lauter tuns give better filtration.  If you spread the grain over a wider area, you get a thinner bed and faster filtration.

The ‘bed depth’ can also be controlled by how much grain you are adding.  A beer with an OG of 1030 has a lower grain loading and will have better filtration performance than one with an OG of 1050.  You can adjust the bed depth by OG or by shortening the brew length.

Differential Pressure

Differential pressure = the pressure above the bed – the pressure below the bed.

At the start of filtration, the pressure at the top of the bed is the same as that below the bed, there is no differential pressure.  As filtration continues the bed begins to drop and compress.  The run-off is slightly reduced and the differential pressure increases.  The bed has not collapsed and there are still channels for the wort to run through.  As the bed compresses and blinds, the differential pressure increases.  The design of the equipment has an overriding effect on differential pressure.


How to Improve Filtration

Brewing with oats will never give good wort filtration results, but there are a few things to try.

·         The first thing to ensure is full starch conversion, test the mash with iodine.  If there are conversion issues, the mash regime needs to be modified.

·         Use a proportion of Torrefied Oats in the grist to add to the husk fraction.

·         Reduce the grist loading – (Bed Depth from Darcy’s equation) it’s inefficient but it will improve filtration.

·         Use enzymes.  But this may be counterproductive.  You are adding the grains for a reason, the enzymes may improve mash filtration but reduce the effect in the glass.

Below you’ll find a handy summary table of the raw materials we’ve talked about, alongside their recommended addition rates and flavour attributes.

And if all that was just a tad confusing, then we’re written up some recipes for your perusal. Here are some brews you could use oats in…

As you can see, there is a lot to consider when adding these grains to your mash. Our basic advice, start low and build from there to better understand what your brewery can handle. Appreciable haze can be formed with low addition rates. When it comes to understanding haze we’ll be covering this in the near future.

For now, check out the datasheets for Naked Oat Malt and Torrified Oats, or if you need some advice on recipes or grist composition, don’t hesitate to get in touch with us on the link below. If you’re not yet a customer and would like a price list, do fill out the enquiry form below.

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