Heat a circulating water bath (or a saucepan filled with water) to 70°C (158°F).
Pour the olive oil and zest into a small vacuum bag and seal in a chamber vacuum sealer on full to remove any air. Place the vacuum bag in the water bath and cook for 12 hours. Remove and refrigerate until required.
Strain off the oil through a sieve lined with an oil filter when ready to use (see <