In the next in our series on taste, Sybil Kapoor and Anna McNamee are cooking with sour foods. While the idea of sucking on a lime might not be particularly appetising think of how nice it would taste if you squeezed a bit on to a ripe piece of melon. Or what would hot salty chips be like without a splash of malt vinegar. When it comes to tastes sourness is one of the great accessories - used judiciously it is the ultimate appetiser. Eaten alone it's a little more intense as an experience as Anna and Fred Foster, owner of Turnips in Borough Market found out when they nibbled on some fresh sorrel.