Using heat to pasteurise straw is a commonly used technique by mushroom cultivators. It is a prcoiess of heating the substrate to approximately 70-74c for a period of time, usually about an hour. The 3 methods of heat pasteurisation are hot water bath; steam injection; and dry heat. There are other “pasteurisation” options such as chemical, using hydrogen peroxide, and anearobic fermention of a substrate then using the influx of oxygen to kill off the dominant anaerobic micro-organisms. However, the most common method, and the one detailed here, is the hot water submersion at 70-74c for 1 -2 hr.
Cultivating mushroom such as oysters (the Pleurotus complex) is a good way to start learning about mushroom cultivation, the oyster mushroom is a good teacher. Straw is cheap and easy to obtain and the technique of pasteurisation is simple and doesn’t require specialised equipment. As such, pasteurising a substrate has several advantages over sterilisation. It’s more economical in terms of energy expenditure; it’s less time consuming; and you have no need for expensive cropping containers such as the use of myco/autoclavable bags; and above all it’s easy!
- Straw (shredded or cut into smaller pieces)
- Large pot
- Probe cooking thermometer
- Large plate
- Brick (or other relatively clean weight)
- Plastic bags or other suitable cropping containers
- cut/shredded straw into small pieces 2 – 6cm
- soak in soapy water for 12hrs (optional)
- drain, rinse and drain
- add hot water at 85/90c, this should then drop the temperature down to between 70-74c
- cover with plate and weigh down to keep the straw submerged
- Keep at 74c – 80c for at least 1hr
- drain and cool to below 36c before inoculating
- sieve gypsum onto straw and mix
- mix in required amount of spawn and add to cropping container, or:
- layer in bag or suitable fruiting container adding spawn to each layer as you go.
- incubate for three to four weeks before initiating fruiting
Pre-soaking the straw for 12hours helps to hydrate the substarte. Adding some washing up liquid helps remove grease and dust when you drain the substrate it also begins the process of killing off competitor micro organisms.
Once drained from the pre-soak rinse off again to remove any soap then add your hot water at a temperature of 80-90c. Once added, the temperature of the water will drop hopefully to around somewhere between 70-74c, heat the pot if necessary until the temperature reaches somewhere around 74c. Now you can either simmer the pot to hold it at that temperature or if you want to save energy place insulate the pot by, for example, wrapping it up in old duvets. Leave it for an hour or so then drain and allow to cool before adding gypsum and inoculating.