Pixian, now part of greater Chengdu, has made doubanjiang since the 1600s: broad beans are moulded into a koji, chillies are chopped and salted in high summer, and the two are combined in huge open clay vats left in the courtyards to sun by day and breathe by night, stirred every morning for a year or more. The paste darkens from bright red to a glossy red-brown and becomes salty, savoury, deeply fruity and only moderately hot. Fried in oil until the oil turns red, it is the foundation of mapo tofu, twice-cooked pork and boiled fish in chilli oil. This home version takes about three months. Use koji spores for a clean bean ferment, and do not cut the salt: homemade bean pastes are low-acid, and the high salt (about 15%) is what makes them safe. Common mistake: sealing the jar airtight in the warm months; it needs air and daily stirring, so cover it with a cloth or loose lid. Normal: darkening colour, a winey-savoury smell, a little liquid. Discard if fuzzy mould spreads or grows coloured (green, black, pink) patches, or it smells putrid. It is ready when the paste is red-brown and tastes rounded, not raw. Chef's tip: fry it slowly in oil until it smells toasty.