What is pasteurization? Pasteurizing beer is basically filtering the beer from the sediments that float in the beer and make it cloudier. There are people that are very against drinking a cloudy beer, but pasteurization also means removing some of the flavour that beer is meant to have. That cloudy look comes from the yeast floating after the fermentation process and not only this isn’t dangerous but it keeps the beer fresh and “alive” with all the flavour and aroma.
Why would people want to remove the flavour from their beer? Basically because big beer companies have to make money out of it, so they need to mass produce, mass store and ship all over the world.
A standard beer’s shelf life, let’s say a simple Pale Ale, is between 6 months and a year (sometimes more, sometimes less, due to the amount of hops and the freshness it can preserve). The longer you keep it stored, hops lose freshness, so the beer loses flavour and aroma. And obviously these big mass companies want to produce a lot, store a lot and send a lot. What pasteurization does is it extends that shelf life, so they can store for much longer before the beer starts tasting funny. Also, they created all this “you have to drink the beer ice cold” (especially in mass lager beers) , that already doesn’t taste much, so it doesn’t really matter if the beer is actually fresh or it’s been stored in a warehouse for a year and a half – it’s not going to taste or smell like anything.
There would be two exceptions to this: the first one is if you have a keg of craft beer in your cellar with yeast floating in it during a certain time (24 – 48h+) that yeast is going to settle at the bottom of the barrel. So the longer it’s sitting undisturbed, the clearer it’s going to become and will leave the last pints cloudier and “more dirty”.
The other exception is the traditional cask ales that involve a conditioning process before serving. They have a very short shelf life (around a week or two after being tapped). This conditioning process means that you leave the beer to rest and settle for 48h before you can serve it to allow the yeast to settle in the form of the sediments at the bottom of the barrel.
So as a conclusion:
Pasteurized beer: Crystal clear, transparent. That means flavours and aromas have been removed to allow the company to keep it in a warehouse for longer without spoiling.
Unpasteurized beer (hazy): Beer in their natural state. Unfiltered, hazy and with all it’s aromas and flavours (according to each beer style). The fresher the better.
Source of the image: https://southherts.camra.org.uk/2016/12/beer-cloudy-or-clear/