Bread can be made with a single rise, but longer periods of fermentation give better flavour and texture. Particularly when using baker's yeast, fermentation is quite vigorous and the dough will expand quickly. Deflating the dough and leaving it to rise a second time means the yeast can continue to ferment for longer. The two risings also give the gluten in the dough longer to develop or mature, giving the final loaf better structure.
Some bakers refer to bulk fermentation as the first prove and the prove as the second prove.
Want to find out more? Look at the chapter on rising, proving and fermentation in the book flour and water.
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