I always thought it quite weird that somehow by virtue of being in beer, that the water somehow becomes sterilised.

If you've ever tried home brewing, you'll know that non-sterile conditions lead to foul rancid filth due to all the bacteria etc.

I'd find it odd if the people then knew to sterlise the water and equipment to make beer, but then not do the same to drink it.

> I always thought it quite weird that somehow by virtue of being in beer, that the water somehow becomes sterilised.

The reason that water in beer is sterilised is that beer is brewed — i.e. boiled.

> If you've ever tried home brewing, you'll know that non-sterile conditions lead to foul rancid filth due to all the bacteria etc.

I believe that pre–germ-theory brewing practices tended to discourage unwanted microbial activity, in part through inoculation with large amounts of fresh barm. Did they put two and two together and connect those practices in the context of brewing to the broader context of water or food safety? Maybe.

> I'd find it odd if the people then knew to sterilise the water and equipment to make beer, but then not do the same to drink it.

Indeed, the article quotes Paulus: ‘But waters which contain impurities, have a fetid smell, or any bad quality, may be so improved by boiling as to be fit to be drunk.’

You don't really need sterile conditions, yeast just need a head start to outcompete other microbes. Then, as alcohol and CO₂ build up, the brew becomes bacteriostatic. Which is still different from being bactericidal.

The idea is by making it beer you keep safe water safe for longer. It doesn't sound particularly weird to me (I don't know if it was really a common practice in medieval Europe.)

As someone who until recently had believed this myth and had never tried home brewing, the theory was that the alcohol kills the bacteria

I always understood the myth as: "When you traveled somewhere and didn't know about the water-quality, you drank beer instead".

Not that beer is immune against making you ill, but chances are random beer in some random village is better than random water in some random village, since the village people would use the good water for the beer.

I think it's a traveling tip like "always ask for halal option - they have better ingredients" or sth.

People know beer needs specific conditions or it doesn't sell. So they are careful about that.

If you ask for water you might get the water they use for cooking soup or cleaning the mugs and they might not boil it beforehand.