During March 2010, a super-strong blend called Marmite Extra Old, abbreviated as XO, was initiated in the UK. Normal Marmite contains a mix of lager, bitter and ale varieties of yeast, sourced from breweries throughout the UK. However, as lagers have a lighter, sweeter taste, residue from this product is not used in Marmite XO. Only residue from traditional bitters and ales are blended to ensure the stronger taste. Marmite XO is matured for 28 days - four times longer than usual. Continuing the tradition of different coloured caps for special editions, Marmite XO's cap is black.