At the end of the germination time the green malt has to be dried in order to keep it. It is generally accepted that malt should be kept below then 6% moisture. All base malts however will be kilned and cured between 4 and 4,5%. After transfer to storage and/or bagging, a maximum of 5% will not be exceeded.
Malt is very hygroscopic: it will take water up if it is exposed to free air. To understand this, one has to look to the kilning process of green malt (germinating barley). During drying greenmalt will at normal drying temperatures of 60° reach an equilibrium at 10-12% moisture. In order to drop below 5% , temperatures have to increase in order to make water migration possible from in the very inside of the kernel to the outside where it escapes from the kernel. Once the malt has been cooled down it has the tendency to go back to its earlier equilibrium at higher moisture: that is what we call hygroscopic.
There is one exception to the max 6% moisture rule and that are low coloured caramel malts. Caramel malts have a different process. The greenmalt will be saccharified between 60 and 80°C and then dried. During caramelisation water is produced and in order to produce the low colours extra heating is not possible. The water content will vary from 8% to 6% depending on colour (10 to 60EBC). However this water will have a chemical bounding with the caramel once it has cooled down and the kernels become very hard.
In order to prevent the natural tendency to pick up moisture malt has to be kept in closed “containers”. At the malthouse we use vertical high cylindric bins. Only the very small top layer of a couple of cm/inches is exposed to air. That is why flat storage is never used to keep malt.
Malt is packed in bags with a polyethylene inside layer which prevent water up take. However one should store bags always in a dry environment. Once the bag opened malt should be used.
In case one need only small quantities e.g. special malts like Chocolate or Black malt, it is better to keep the remainder of the malt from a bag in a plastic container which can be closed airtight. This has a second very important advantage: volatile aroma compounds from special malts (caramel and roasted) will not be lost in the surrounding air.
Malt can be kept healthy several years and will not lose its enzymatic power. One condition: keep water and air away!