A build is the name given to each stage in a sequence of pre-ferments used to make sourdough breads, often over a long period of time. Some recipes call for two or three pre-ferments to be prepared and refreshed over a period of hours, if not days, each containing some of the previous ferment: each stage is referred to as a 'build' because it increases or 'builds up' the number and activity of the yeast and lactobacilli and promotes the production of acetic and lactic acid. This technique of gradually increasing the activity of the pre-ferment is quite advanced, as well as time-consuming. Nonetheless, an experienced baker can manipulate each build, increasing or decreasing temperature, hydration and fermentation time to control the flavour of the final loaf.
Want to find out more? Look at the chapter on pre-ferments in the book flour and water.
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