पाश्चुरीकरण

01What the heat is for

Compost arriving from Phase I carries fly eggs, mites, nematodes, and the spores of competitor moulds and weed fungi. A controlled heat treatment kills them. It is not meant to kill everything: the thermophilic fungi and bacteria that convert ammonia in the next stage have to survive, or the compost will never become selective.

That is the whole balance of the operation. Too little heat and the competitors ride into the growing room with the compost. Too much and you sterilise away the organisms that make Phase II work at all.

02A narrow window

DMR is specific: compost is pasteurised properly when held at 58 to 59 degrees Celsius for 4 to 6 hours. The temperature is reached by narrowing the dampers and letting the mass heat itself at about 1 degree per hour, which takes roughly 10 to 12 hours.

The ceiling is firm. Above 60 degrees, DMR warns, all kinds of organisms are killed, including the thermophilic fungi essential for Phase II. The same manual lists temperature above 60 degrees during pasteurisation, together with poor aeration, as the trigger for olive green mould.

03Where you measure decides everything

A hold is only real at the coldest point of the mass. Air temperature in the tunnel reaches the target long before the compost does, and a probe near the door reads differently from one in the middle.

DMR keeps the difference between air, compost and plenum within 3 degrees Celsius, which is itself a check on whether any single probe can be trusted.

• Probe the compost, never the air, for the hold.
• Both ends and the centre, at two depths.
• Start the clock on the coldest probe.
• Log every probe every hour through the hold.

04When there is no steam line

Seasonal growers using long method compost often have no boiler. DMR publishes a chemical alternative applied at the last turning, after which the compost is heaped and covered with a polythene sheet for two days. Products and approvals change, so read the current label before using anything and never scale a published figure yourself.

DMR trial data on that treatment is useful context: 18.1 kg of mushrooms per 100 kg compost against 10.5 kg for a water sprayed control. Casing soil is a separate job, steamed at 60 to 65 degrees for 6 to 8 hours.