I’m studying in earnest for my exam on “Cereals, Malting, and Mashing” on Thursday. I’m taking the tried and tested route of looking at old exam papers and writing answers to them in the vague hope that writing answers under test conditions will somehow come easier. I want my mock answers to be informative enough and factually correct so I don’t practice test conditions at home but instead look things up when I want to. And now my hand is cramping and I haven’t posted today, so I think I’ll post something on malting as a way of killing two birds with one stone.
Malting barley or any cereal is all about getting the moisture content right. When you malt you don’t so much do anything to the grain as attempt to create the conditions necessary for it to do what nature has programmed it to do but at a time and manner of our choosing. Barley comes off the field at a moisture level of about 15% by weight (it can be more). This is too high. Not all the harvest can be malted at once. So it must be dried to a level safe for storage, which means below 12% by weight. Above that level, it is susceptible to fungal and microbiological infection that could make it unsuitable for use in brewing. Barley comes off the field teeming with micro-organisms mostly bacteria but also mould, and fungi. There are two moulds the maltster needs to be especially wary of. Extensive infestation by aspergillus can produce aflatoxins which are hazardous to health and there are legal maximum levels that have to be observed. The other mould is fusarium which can cause gushing problems in the finished beer if its byproducts are able to survive through the process. So barley is first dried.
When it is ready to be malted, the barley is steeped in large steeping tanks for 2 days to bring it up to around 45% moisture (43-48%). It is alternately steeped and drained/allowed to drip dry (termed an ‘air rest’) to allow the barley to breathe during this steeping phase. The steeping regime might be 10 hours steeping followed by 13 hours draining and then repeat or 6 hours steeping 18 hours resting, six hours steeping 16 hours resting, two hours steeping and two hours resting. That sort of thing. The steep tanks are usually aerated as well during steeping. In some maltings the barley is spray steeped rather than entirely submerged as this saves water. The steep water washes off lots of pesticides and microorganisms and contains leaching from the grain itself so it is very expensive to dispose of (having a high biological and chemical oxygen demand (BOD, COD). Phenols, tannins, sugars and amino acids, silicates and other minerals all come out of the steep water. The steep water should be around 16’C. And the relative humidity during the air rests has to be 100% to avoid the grain drying out. Some maltsters add gibberellic acid, a plant growth regulator (i.e a hormone) to the steep water to promote germination, and this is perfectly safe as far as we know. But the Scotch whisky industry won’t accept malt that has been treated in this way for precautionary reasons.
After steeping the barley is turned into large germination vessels. One type of germination vessel is the so-called saladin box pictured below.
The grain is piled about 4 feet deep in Saladin boxes. This is a departure from the traditional floor maltings where it was, as you might expect, spread all over the floor. The barley is turned to stop the rootlets matting together. During the time in the saladin boxes the barley chits, that is, the little root starts to pop out
That happens in the first couple of days of germination. Malt is in the saladin box for five days. Towards the end, the rootlets really show themselves, like this.
There’s a temperature gradient of about 3’C between the top and the bottom of these saladin boxes. The grain at the top is drier, warmer, and has more CO2 and less O2 than the grain at the bottom. This can be countered by spraying the top with water but that is not very effective. There is a net dry weight loss of 4-8% during the germination phase as the plant converts its reserves of starch into more young plant. When this starts to happen, it is imperative that the grain is quickly kilned dry to arrest the process of germination/endosperm modification.
The last stage in the malting process is kilning. During Kilning the moisture level is brought down from ca. 45% to 3-5%. It happens in stages. The maltster has to dry the malt off without damaging the enzymes inside, which the brewer needs to use later. The combination of moisture and high temperature is absolutely fatal to enzymes so the initial drying phase, called free drying, has to be relatively cool. The ‘air-on’ temperature is 50-60’C. During free drying large volumes of air are blown through the malt. This is necessary to stop the air becoming saturated with moisture from the malt. When the relative humidity is 100% moisture can condense on kiln surfaces and drop back onto the malt. That would cause the malt to stew, which damages the enzymes. The moisture baring capacity of air can be increased by raising the temperature of the air, but early in the kilning process we can’t risk damaging the enzymes. The blast of air also helps drive off volatile compounds. Green malt has a grassy smell that’s not desired in the finished beer. It also has lots of s-methylmethionine which is the precursor of a compound called dimethyl sulphide (DMS), which has a noted cabbage or cooked corn aroma which is sometimes desired in lager beer but not in other kinds. The free drying stage can take around 12 hours and during this time the moisture level is reduced to 20-25%. The ‘air-on’ temperature is gradually ramped up through the free drying period and might be as high as 70’C by the end. There is a large difference between air-on and air-off temperatures during free-drying on the order of 30’C (although this difference reduces towards the end of the period).
After free drying comes force drying that takes the malt down to 10-12% moisture. The temperature is again raised gradually but the air volumes drop (the increased moisture capacity of the hotter air combined with the drier malt mean large volumes are not so needed.
‘Curing’ takes the malt moisture level down to 3-5%. Here the air-on temperature is determined by what kind of malt is desired. A pale malt with lots of enzymatic power, such as a lager malt, will be cured at the comparatively cool temperature of 85’C. An ale malt will be darker. It will be cured at 110’C. During curing the air volumes are reduced as far as possible and the air is mostly recirculated. During the curing phase the remaining water in the grain is ‘bound’ to molecular starch and beta-glucans by hydrogen bonds. As this is driven off the air-off temperature quickly converges with the air-on temperature. This is called break through. Curing takes about two hours. after which the rootlets are ‘de-culmned’ broken off, for instance, by dropping the malt from a height and the malt is transported away for storage. Different colours of base malt, for instance Munich or Vienna malt are created at the curing stage by adjusting the temperature. The warmer the air, the more maillard reactions occur in the malt creating flavour and colour compounds.
So that’s a basic summary of the process of malting anyway. In reality there’s a lot more to say, particularly regarding the biochemical processes that are going on while all this is happening. But I’m calling it a day now.