Saturday 29 November 2014

How to make malt

I've been doing some more malting at work recently. This is the process in which starchy barley grains are partially germinated so that when used for brewing the starch can be broken down to fermentable sugars. It's very, very, very, very, very, very, very, very, very, very interesting (sorry about that, you'll need to have been on twitter to get it).

The first stage is screening. I like this bit:


This separates the wheat from the chaff, or barley in this case. Though the grains have been mostly separated out there are still some bits of straw you want to get rid of, and a series of sieves and a blower do the job.

A barley grain

After that the barley's ready for steeping.The grains will have been dried for storage and they need to be rehydrated before they will start to germinate. At work we have cylindrical steeping and germination vessels. 

Here's a look inside:

The vessels tilt up for loading, and down for unloading:

Adding the grains

When loaded the barley sits on the sieve plate at the base of the vessel. The barley will then be completely immersed in water, following a carefully controlled programme of steeping alternating with air rests. Having air rests during the steeping of barley came directly from research carried out here. Well maybe not exactly here, but this workplace. It's now universally used as it makes malting much faster and more efficient.

Steeping alternated with air rests means yeast and bacteria on the grain are washed away, as well as phenols from the husk, oxygen can get to the grain so it can respire, CO2 is removed, and so is ethanol produced by the grain during anaerobic respiration. Yes, that last one disturbed me too. What a waste. Air will be blown through the grain bed and fans remove the CO2.

Getting the correct steeping schedule can have a huge effect on how well the grains germinate. Here's a picture of the same barley sample that's had different steeping schedules:

Example schedules may be water 8 hours/air 16 hours/water 24 hours or water 6 hours/air 10 hours/water 6 hours/air 6 hours/ water 6 hours. Tests for germinative capacitiy (and similar tests for germinative energy and water sensitivity) can help determine the best steeping schedule to use.

Here's a grain that has just chitted i.e. the rootlet is just showing

After approximately two days the steeping will be completed and grain moisture 42-46%. The germination stage starts now, and the plant hormone gibberellic acid can be sprayed on at this point. This is where our cylindrical vessels come into play as they will periodically rotate quite rapidly to break up the grains rootlets and stop them forming a big tangled mat.

Here's the rotating:

And here's some tangled grains all stuck together:

Most big malting plants have vessels with rakes or screws running through them to break up grain tangles. And it can also be done by a bloke with a rake, as most pictures of floor maltings show, though I have heard they have something like a lawn mower they can move through the grain bed too which must be a lot less effort.

The germination stage starts next. During steeping the embryo will produce gibberellic acid, and as I've said it can also be sprayed on to help things along their way. The gibberellic acid is transported through the aleurone*, a thin layer of cell surrounding the grain, which produces or activates enzymes which will being the modification of the grain. These include amylases, proteases and β-glucanases. These enzymes are necessary for converting the starch in the grain to fermentable sugars. Though we don't want any more than is necessary at this stage as over modification means the grain will use for growth sugars the yeast could ferment.

Here's a grain after rootlets have grown

The modification breaks down the structure of the cells surrounding the starch granules in the grain and provides the enzymes that will be used during brewing. β-glucans (and pentosans) are polymers that can cause serious problems during the brewing process if their levels are too high. They increase wort viscosity making making it difficult to separate the liquid from the grains at the end of mashing, and can lead to hazes forming in beer. Protein needs to be broken down to make the starch granules it surrounds accessible to the amylases, and to provide raw materials the yeast will use during its own growth.

Germination is allowed to continue for around four days. Rootlet growth is vigorous but a bit erratic. The growth of the acrospire, which would become the shoot, is of more interest to maltsters. Unlike the rootlets which go the easy (proximal) way out of the grain it goes the long way round from the embryo and works its way inside the husk aiming for the far (distal). When it's about 75-80% of the way there germination as gone as far as we want it to.

Here are some grains at the end of germination:

This is moist green malt, and it tastes a bit like bean sprouts at this stage. Which perhaps explains why bizarre as it now sounds beans were once used for brewing. It is possible to brew using green malt, and I've heard of a grain whisky distillery that does this. It has a very short shelf life though, as it will keep growing wasting all that valuable sugar that could be turned to alcohol, and will rapidly go mouldy.

Usually this is prevented by the next stage, kilning. This is when the grains are heated to halt germination at the optimum stage of enzyme production and grain modification, and dry the grains to the moisture content at which they can safely be stored without going mouldy. The degree of kilning will also to a large extent determine which type of malt is made and will have a big effect on the flavour of the beer.

Malt kiln

At first the grains will 'free dried' by heating gently with an air temperature of 50-60°C (though the grains themselves will be at a much lower temperature than this) and the air will be vented away. This will continue for around 12 hours, when the moisture content will be about 24%. The free drying stage will have removed the surface moisture, and that in the outer layers of the grain. We now move on to forced drying, where moisture will diffuse from deeper in the grain to the surface for removal and the grain will start to shrink. This would slow the rate of water removal so the temperature will be increased slightly to 70-75°C and the fan speed reduced. The grains will not get as much evaporative cooling as they did during free drying, and their temperature will start to rise. After perhaps 10 hours the moisture will be down to 10-12%.

Malt in the kiln

Now the curing stage begins, as the most difficult to remove water, that which is bound to large molecules inside the grain, such as the starch, is removed. To achieve this the temperature is increased again, and the air is substantially recirculated. Curing will generally continue for three hours or so until the moisture content is below 5%.

Temperature and humidity probes

As well as reducing the moisture content of the malt kilning also drives off unwanted volatiles, particularly sulphur compounds, and adds colour and flavour due to Mailard reactions between sugars and amino acids. The higher the final kiln temperature the more of this will occur. A lager malt might have a final kilning temperature of 80°C, whereas for a pale ale malt it might be 100°C.

This is a major reason why you get the reek of brimstone during lager fermentations (a sure sign it's the devil's work), and the unpleasant vegetable taste of dimethyl sulphide and watery yellow colour of many lagers. To which we can contrast the pleasant malty flavours and rich golden colour that will be found in an ale made with a pale malt grist.

After kilning the malt will be cooled rapidly but we're not finished yet. Oh no. The rootlets need to be removed, or as we say in the trade the malt needs to be deculmed. We have a machine for that too:

Here's the rootlets or culms:

They're high in protein and not good for beer so are best used as animal feed.

When the malt has been deculmed it's still not over, as freshly kilned or 'fiery malt' is not good for brewing and need to be stored for around a month before use.

Then it's over, the production of 'white malts', the ture enzymic malts (lager, pale, vienna, mild, munich) that can be used as 100% of the grist has come to an end and I can bring this #beerylongread to an end. Crystal or caramel malts, along with the various types of roasted malts are another, though closely related, story so you'll have to wait for the #beerylongreadappendix for that.

*I follow the true path of Palmer, not the false trail of Briggs.


  1. So if green malt can be used to make beer and early brewers malted on an as needed basis is it possible that kilning is a separate innovation that might have been separately created and unevenly distributed? If unnecessary, kilning seems to be an odd stage given the resources required, time and fuel.

  2. If you're just malting the amount you're going to brew with you could potentially cut out the kilning stage, there might be problems with milling and it might not taste very nice though.

  3. Hmm... think I need to start playing at malting and ale brewing. I may enlist our mutual friend, Dan M. I have a plan. Where's your email?