Complex sentence: After opening, wine in a bottle it is oxidised by air in the bottle which has displaced the wine poured; wine in a bag is not touched by air and thus not subject to oxidation until it is dispensed.
Split into simpler sentences: After opening, wine in a bottle it is oxidised by air in the bottle which has displaced the wine poured. Wine in a bag is not touched by air and thus not subject to oxidation until it is dispensed.