Pour the mandarin juice into a 6 litre (210 fl oz) capacity distillation flask, place in a rotary evaporator*, set the water bath temperature to 40°C (104°F) and the cooling temperature to −20°C (−4°F), set the rotation speed to 45 RPM and distil at full vacuum for 1 hour (depending on the strength of the vacuum) or until a thick syrup forms. Alternatively, although it will not achieve the same result, you could reduce the juice in a small saucepan until a thick syrup forms.
In a very small saucepan, combine the isomalt, sugar and water over low heat, stirring occasionally until the isomalt has dissolved, and heat to 80°C (176°F). Cool to 40°C (104°F), then whisk into the mandarin distillation.
Pour the mandarin distillation into a 500 ml (17 fl oz/2 cup) capacity jug with straight sides. Add the mandarin oil to the mandarin distillation, a little at a time — as you would add oil to egg yolks to make mayonnaise — while blending with a hand-held stick blender until completely emulsified. The result should be an orange-coloured syrup with no visible trace of oil. Add a small pinch of salt and taste. Depending on how sweet the mandarins are you may need a very small pinch of citric acid to balance the sugars. Store in an airtight bottle in the refrigerator until needed.