I've recently been looking at all-grain brewing and noticed that the recipes tend to have a wide variation in the expected final specific gravity after fermentation. I was wondering how you manage to hit these varying S.G. targets if the yeasts job is to convert sugar to alcohol. Is there some other factor at work here ?
The short answer is that happy and healthy yeast are a critical component to reaching your target S.G.
Of course, this all begins with where your O.G. is based on your grain bill and mash temperatures.
Your S.G. will be determined by the complexity of the sugars that made it to your fermenter, based on the mash schedule used. A slow ramping up of temperatures tends to create a wort with much simpler sugars. The simpler the sugar, the more easily it is consumed by the yeast.
Different strains of yeast can process different types of sugar. Most ale yeasts tend to stop consuming sugars that are bigger than maltotriose. Brett yeasts by contrast, will consume pretty well any sugar source, including the lignin that makes up wood fibers! French Saison Dupont strains are notorious for indefinitely pausing fermentation at 1.020. So yeast selection is important, such that it matches your wort profile, i.e. lager vs berlinerweisse.
Furthermore, different yeast strains will have different attenuation, this is the degree of completion to which they complete the fermentation. If you look at the supplier's website for the yeast for your next brew, you will see this information listed.
The yeast you employ will perform best if they are happy and healthy, with reduced stress. It is therefore important to make sure your wort is well aerated, you are starting with a viable and vital yeast culture and that your wort conditions are favourable for the yeast. Variables such as pressure and temperature come into play, and can be used to tailor the yeast towards making clean or estery (aromatic) beers.
Tools like Brewer's Friend Recipe Calculator can help you get started by putting in your recipe and selecting different yeast strains to see where you might end up.
Mash Temperature is the criticial component when targeting a final gravity. When I'm making a stout I shoot for a higher mash temperature in the 153/154 range. while if i want to dry out a pale ale 149-151 degrees F in increase B~Amylase activity and allow a higher finishing gravity. A couple of other things that could be affecting your final gravity is your pitch rate and/or lack of wort oxygenation pre-fermentation. If you don't oxygenate your wort well enough before pitching, your yeast will set up weak cell walls and reproduce weak hungry offspring that will autolyze early. try using a yeast nutrient like servomyces and a food grade oxygen through a sanitary oxygenation stone. it's always better to over pitch than to underpitch. if your fermentation temperature is too far from where it should be your gravity will suffer. And a less likely cause is a clogged blow off hose. Fermentation will stall and yeast will die if the Co2 levels are too high in your FV. Yeast does not survive well in it's own byproduct. hope that helps. -Ryan Power Head Brewer Geaghan Brother's Brewing Company Bangor & Brewer, MAINE