Fractional sterilization or tyndallization is a method used to destroy bacteria and endospores in preparation of grain spawn(rye, wheat, millet, birdseed...) and agar, which requires no pressure cooker.
In this case, the jars fitted with a filter disc or a polyfill lid filter
are boiled or steamed at 212°F (100°C) for 30 min in a pot with lid, three days in a row. Between the boiling steps the jars are kept warm, around 30°C(but room temperature will work too), to allow the remaining endospores to germinate.
The basic principle behind this method is that any resistant endospores will germinate after the first heating and therefore be susceptible to killing during the second and third heating.
Timetable of the tyndallization (=fractional sterilisation) process
1) Steam heating to 100 °C for 30 min
Vegetative cells are destroyed but endospores survive
2) Incubate at 30°C-37°C overnight
Most bacterial endospores germinate
3) Second heat treatment, 100 °C, 30 min
Germinated endospores are killed.
4) Second incubation at 30°C-37 °C overnight
Remaining endospores germinate
5) Third heat treatment, 100 °C, 60 min
Last remaining germinated endospores are killed