You cannot escape rice wine when on vacation in Vietnam, it is staple tourist stuff but also cheaper than beer. It is mass-produced by local farmers despite government attempts to have it banned (link). Rice wine is a topic in serious scientific research! see here end here. Question is: how is it made?
The specialists agree you ferment rice with a special kind of yeast to a rice/water/ethanol mix and then you can either distil the brew to get to the wine or you can separate rice from wine by filtration. Problem is that the installation I have happened to witness (on the Sapa hiking trail in northern Vietnam) does not do either. In image 1 a large vat containing water and rice is heated in a wood-fueled kiln. Cold water is constantly added from the top and removed to the left. On the right a metal pipe leads away from the vat into a plastic drum. It produces clear rice wine! No distillation involved or filtration. Strange. The second vat by the way behind vat number one with the wooden stick in it contains fermented rice. The local rice wine is sold in second-hand PET water bottles. But what is the alcohol percentage? The local experts (fellow Australian and Canadian hikers) estimated the alcohol content was at least 20%. The scientific studies already mentioned explain that by simple filtration the percentage is 10% at the most (distillation will get you to 30%). The worrisome thing is that this amount of alcohol is not enough to preserve the wine so in an artificial way additional ethanol must be added. You may hope then that the local farmer (not rich) has a reliable ethanol supplier.
So the question as what the original alcohol percentage is, remains open, chemists should of course always carry a basic chemistry kit with them while on vacation but I forgot mine. So can the next chemist venturing into the Sapa region report back some accurate numbers? Thanks!
By the way, deep down south in the Mekong delta even plastic bottles are too fancy a container for storing rice wine. A plastic bag and a chopstick will do just as well! To dispense the wine just briefly raise the chopstick to allow the wine to flow out. Remarkably accurate. Something for the laboratory?