Rivalling your nearest bakery, the best supermarket sourdough loaves still offer that signature tang and airy crumb. From Waitrose premium selection to Sainsbury’s own bloomer , which store-bought sourdough is best? I sliced my way through dozens of loaves to find out. A naturally leavened bread, sourdough rises thanks to the activity of wild yeast rather than added raising agents. You know it’s a good loaf when the bread is soft, springy and the texture inside the bread is crumbly, with air pockets that are evenly spread throughout. Fermentation is key: when the baking process isn’t quite right, it can result in what bakers call ‘tunnelling,’ where overly large, uneven holes form inside the bread.…