Juice the grapes using a centrifugal juicer (the most common type available at appliance stores). Pass the juice through a fine sieve lined with muslin (cheesecloth). Pour the grape juice into a 6 litre (210 fl oz) distillation flask, place in a rotary evaporator*, set the water bath temperature to 45°C (113°F) and the cooling temperature to −25°C −13°F), set the rotation speed to 70 RPM and distil at full vacuum for 2 hours (depending on the strength of the vacuum) or until a thick green syrup forms. Remove from the flask and check the flavour. If it tastes sweet, then add a small pinch of citric acid and stir well — the amount you require depends on the sweetness of the grapes. Store in an airtight squeeze bottle in the refrigerator until needed.