Complex sentence: On the long sea voyages, the wines would be exposed to excessive heat and movement which transformed the flavour of the wine as the wine producers of Madeira found out when an unsold shipment of wine returned to the islands after a round trip.
Split into simpler sentences: On the long sea voyages, the wines would be exposed to excessive heat and movement which transformed the flavour of the wine. This was discovered by the wine producers of Madeira found out when an unsold shipment of wine returned to the islands after a round trip.