My dh was browsing the ready made formula products in Boots yesterday. As we give our 7 month old ds a bottle only occasionally at night and we have tended to buy the ready made cartons as we use so little. He noticed that the SMA gold 250 ml cartons can only be kept in the fridge for 24 hours when opened but the 1 litre cartons can be kept for 48 hours. Is there special stuff in the 1 litre cartons that makes it last longer or are they taking us punters for mugs?