Milk that has passed its expiry can turn into a thick yoghurt-like formula, and can often smell foul. But spoilt milk actually has a little-known use in the garden, so don’t pour it away just yet.Milk that has passed its expiry can turn into a thick yoghurt-like formula, and can often smell foul. But spoilt milk actually has a little-known use in the garden, so don’t pour it away just yet.