Fat washing is the technique of stirring a fat into a spirit, waiting, freezing the fat back out, and keeping what the spirit learned in the meantime. It sounds like it should produce something greasy. It produces the opposite: a spirit that carries flavour it could not otherwise hold, and a texture that coats the mouth in a way a shaken sour never manages. This version washes a rum split with coconut oil, and it is the base of the Tradewinds Old Fashioned.
What the Fat Actually Does
A lot of what we taste as coconut — lactones, mostly, the same family of compounds that make an oaked whisky taste of coconut — is fat-soluble and barely water-soluble. Those compounds will not come out into a spirit by steeping shredded coconut in it, because there is nothing in the alcohol for them to dissolve into. Melted coconut oil is the solvent they are already living in. Shake the oil and the rum together and you create an enormous amount of surface contact between the two, and the flavour compounds redistribute across it.
The alcohol also takes on a small amount of the fat itself — not enough to see, but enough to feel. That is the texture change. A fat-washed spirit has a heavier, rounder body and a longer finish because those trace lipids coat the palate and release aroma more slowly than ethanol does on its own. It is the same reason a full-fat sauce tastes of more than a skimmed one.
Why It Has to Freeze
Coconut oil melts at about 24°C, which is roughly room temperature, and that is precisely why it is the easiest fat to wash with. Left on the counter, the oil separates from the rum and floats — but it is still liquid, so straining it out at that point is messy and incomplete. Put the jar in the freezer and the oil sets into a solid cap you can lift or strain off cleanly in one piece. Filter cold, through two layers of cheesecloth or a coffee filter, and keep the whole operation refrigerated while you do it. Warm the rum back up mid-filter and the last of the oil melts straight through the paper.
The Split, and Why Overproof
Three ounces of an aged Spanish-style rum — Havana Club 7 is the reference — to an ounce and a half of Plantation OFTD. The aged rum brings the caramel and dried-fruit register the drink is built on; the OFTD brings proof and a heavy, funky, molasses weight the Cuban style does not have on its own. The proof matters mechanically as well as for flavour: fat washing dilutes nothing, but the finished infusion has to stand up in a stirred, spirit-forward drink over a large cube, and a 40% rum on its own goes thin under that much ice.
Keeping It
Bottle it and keep it refrigerated. Filtered properly it holds for a couple of months, and the limit is the trace fat rather than the alcohol — residual lipids eventually go rancid, and you will smell it before you taste it. If the surface ever looks cloudy or greasy after a night in the fridge, the filtering was incomplete; run it through a fresh coffee filter cold.