Joe, from Republic Brew Pub helps set me straight on some of the things I’ve found during my brief research thus far. It turns out that much of stuff I wrote yesterday could be filed under ‘general misconceptions about brewing’.
Hey Kate, nice job on the beer writing! You have a great grasp of the general concepts, but I have a few nit-picky points for you (I’m a professional brewer). I’m not sure how detailed of a discussion you’re looking for on the weblog, so I’ll err on the side of not bogging down your comments.
First off, top- and bottom-fermenting are pretty archaic ways of describing yeast. I’m not sure why people (including brewers) insist on using them, but both ale and lager yeasts follow the same basic sequence of rising into suspension, raising a frothy head above the beer during fermentation, then settling to the bottom of the fermenter. I think the terminology came from traditional methods of yeast harvesting, i.e. removing yeast to reuse in subsequent fermentations. English ale brewers harvested from the top “froth” during the most active part of fermentation while German lager brewers harvested from the bottoms of their fermenters after their yeasts had settled. These days, bottom-harvesting is the norm for both ale and lager brewers.
It’s also a myth that lager yeasts are better at converting sugars into alcohol and CO2 than ale yeasts. You’re correct that different yeast strains ferment sugars to varying degrees, but the distinction between ales and lagers has very little to do with it. Some ale strains ferment very thoroughly while other ale strains leave a lot of residual sugar, and the same can be said about individual lager strains. This also means that the amount of sugar left over has very little to do with the shelf lives of ales vs. lagers. When you’re talking about standard-strength beers, the fact that lagers last longer is just a coincidence: the megabreweries with the best staleness-preventing technology all happen to brew lagers. They brew lagers because of a shared German heritage, not a difference in yeast efficiency. When it comes to strong beers, both ales and lagers can age beautifully for years.
From a drinker’s perspective, the big differences between ale and lager yeasts are the flavors they produce. Most of these flavors are byproducts of fermentation, such as esters, and not from the residual sugars themselves. In general, ale yeasts produce a lot more of these flavors than lager yeasts. The core reason why is because lager yeasts can ferment at cool temperatures that would force ale yeasts into dormancy. Metabolic processes are faster at warm temperatures than cool ones, but they tend to be more “sloppy” as a consequence. Because of the temperature difference, ale yeasts eat like sharks in a feeding frenzy while lager yeasts eat like Victorian aristocrats. It’s not a totally accurate metaphor, but it’s an easy way of visualizing the differences between ales and lagers. Metabolic slowness is also why lagers generally take longer to ferment than ales. If you wanted to oversimplify things for the sake of explanation, you could say that lager flavors = malt + hops while ale flavors = malt + hops + fermentation byproducts.
I hope this isn’t too confusing! Feel free to give me a holler if you have any questions about beer or brewing.
Y’see…This! This is why I love the Internet.