The answer to your question is no, you cannot use lactose to backsweeten sour beer.Lactose is milk sugar and as a sugar it is subject to fermentation as long as you give it to an organism that has the enzymatic capability to ferment it. Good friend saccharomyces cerevisiae, responsible for fermenting clean beers, cannot produce the enzymes that break down lactose into smaller sugars it can metabolize. That's why your milk stouts are milk stouts: the lactose doesn't ferment. However, brettanomyces and souring bacteria do produce the enzymes that break lactose down into sugars they can metabolize. If you're not so sure I'm right, just smell some old milk. It's sour because lactobacillus and other common bacteria get in the milk and ferment out the lactose into lactic acid (among other things). If you're still not sure, just smell some sour milk alongside a sour beer. You'll notice the same lactic acid aroma in both. In milk the lactose is fermented into lactic acid. In sour beer the starches and sugars are fermented into lactic acid.
Steps you can take to backsweeten your sour beerIn this post I discuss three methods to blend clean and sour beer but you can adopt for backsweetening sour beer instead of blending:
- use heat to pasteurize the beer, then backsweeten to your preference and add fresh yeast to bottle;
- filter your beer and then backsweeten and re-yeast to bottle;
- use a combination of gelatin, cold crashing and campden to stabilize the beer, then backsweeten and re-yeast to bottle.
The simplest method of backsweetening is to just add some sugar to your glass and pour the beer in with it. It isn't as glamorous as the other methods since you can't just crack open a bottle and pour but at least you don't have to worry about using any funky process or screwing up your beer. If you backsweeten too much in the bottle you can't take the sweetness back out but you can always dose your glass to taste and get it right every time. It's also a good option if you want to be able to see how your sour beer evolves and share it with people who may not like their sours backsweetened.