In Harbin, Changchun and Shenyang, late October is when families buy napa cabbage by the cartload, scald it, and pack it head to tail into big glazed crocks on the balcony, weighed down with a smooth river stone. A month later, as winter sets in, it has become suan cai: pale, almost translucent leaves with a clean, lactic sourness and a soft crunch at the rib. It is shredded into suan cai bai rou (sour cabbage with pork belly), stuffed into dumplings for Spring Festival and simmered in hot pots through the long winter. Common mistake: fermenting somewhere warm; above about 20 °C the cabbage softens and can go slimy, so use the coolest spot you have. Normal signs are fine bubbles, a cloudy brine and a sharp sour smell; a flat white film (kahm yeast) can be skimmed off. Fuzzy coloured mould, pink or black patches, or a putrid smell mean discard the lot. Do not eat it before 3 weeks: it is ready when the leaves are uniformly pale yellow-green and taste fully sour. Then keep it in the brine in the fridge for 2 months. Chef's tip: rinse and squeeze the shreds before cooking to soften the sourness.