Hop oast

Malt / hops oast, food dehydrator

Kilning malt is an important part of the malting process, and drying hops preserves them – really useful if you grow your own. I plan to use my oast to dry all sorts of stuff that I grow myself, not just for brewing ingredients. Any food dehydrator needs to be able to maintain a temperature to within a few degrees accuracy, and sustain a flow of air over the food to carry away moisture. During malting, after the barley has sprouted to the optimum modification, the grain must be dried to halt its germination. Usually this means holding it at a temperature of 40C for around 24 hours, then ‘finished’ at around 80C for a shorter period. I’m not going to explain malting properly in this post, but that example illustrates what this machine has to achieve.

Hop oast

Some people successfully use ovens combined with various thermostatically-controlled heating elements and fans, or commercially available food dehydrators. My solution does take up a lot of space – it’s about 90cm long by 40cm wide by 40cm tall – but the capacity of the shelves is over 7 square feet.

So – the design brief is:

  1. Air flow
  2. Heat
  3. Accurate temperature control between 40-80C
  4. Shelves to hold the malt / hops / food
    • mesh, to allow air flow on both top and bottom
    • must be food-safe material – especially when heated

I used a fan, constantly switched on, to create air flow. A small electric heater provides the heat. My mains power thermostat module regulates the temperature. The thermostat module is regulated by a thermometer linked to it by a short wire. I placed the thermometer a short distance in front of the heater. I made the shelves to hold the malt / hops / food myself. I made 9 rectangular wooden frames with outer dimensions of 30 x 40cm, to which I stapled food grade steel mesh. A 1m x 1.2m roll of this should cover 9 shelves of those dimensions with a bit to spare. I found it listed as ‘vivarium vent mesh’ on eBay. All of this needs to be enclosed in a tunnel or case (open at both ends) to force the air from the fan over the heater and then the shelves. The end of the tunnel with the fan is your air inlet, and the end with the shelves is the air outlet. The tunnel should be metal, because I might want to heat this to 80C or over depending on what I use it for. I built my metal tunnel from turkey tins and baking tins from the pound shop, but there are many other better ways to do it. The important factor is that it must fit the fan, heater and shelves snugly, otherwise the air won’t be channeled so efficiently.

There were a lot of words in that paragraph, but here’s a diagram of the setup inside.

Concept diagram

If you connect this up with my mains power thermostat module, the easiest way is to use a 4-way extension socket. The fan plugs directly into the extension, and the heater plugs into the thermostat module, which is plugged into the extension. The thermostat module also has an AC/DC adaptor (black plug in the diagram below), which plugs into the extension.

Electrical diagram

MESH SHELVES

These shelves might come in useful even if you kiln malt in a normal oven.

I didn’t take any photos while I was building the frames! The wood was a pack of 2100x32x12mm planed stripwood from B&Q - cheap and cheerful. I cut lengths of 30cm and 40cm with a chopsaw set to 45 degrees. 4 staples on each corner, 2 above the joint and 2 below, hold the frames together surprisingly well. I stapled spacers onto the top of each shelf to keep the stack separated and let the air flow between the shelves:

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Put on some leather gloves. When you cut this mesh, it leaves razor sharp edges. I laid the frame on the steel mesh, and drew around the inside with a permanent marker. You’ll want to cut around that line leaving about a centimetre margin. I used metal shears to cut the mesh.

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Next, fold along the lines.

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You’ll get a sort of box shape.

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Insert the box into the frame..

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..and staple the folded-up edges of the mesh into the inner edges of the frame.

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My oast has a 30 x 22cm opening to accommodate the shelves:

Malt oast Food dehydrator

The rear of the oast – the fan blows inwards, towards the heater (which isn’t visible here).

Fan

My mains power thermostat module, with temperature control knobs. My module can control two plug sockets independently, but for the oast, I only needed to use one.

Thermostat Module

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