If the volume of milk you are expressing is still relatively low (10-20ml for instance), the milk may foam somewhat during expressing. This may be the case during the first few days after giving birth, when the milk supply has not got going properly yet. Or when you have just started expressing and still need to get used to it and as such, supply is temporarily limited. Frothy milk is not harmful and will rectify itself once your let-down reflex improves and the milk is flowing well. If a foam layer appears on the milk after expressing, this can be addressed by swirling the bottle around until the bubbles dissolve.