There seems to be some confusion here. You might want to double check your beer recipe program, because if that’s what it says I’m pretty sure it’s wrong.
Both Table Sugar (Sucrose) and Corn Sugar (Glucose) are fully fermentable. However, Table Sugar will produce slightly more CO2 and alcohol for a given weight than Corn Sugar. This is just because a molecule of Table Sugar has a lower Formula Weight but just as many carbons as two molecules of Corn Sugar, so Sucrose has more carbons for and given weight.
I was just rereading Ray Daniels’ “Designing Great Beers” and in his chapter on extracts, he sites a study done in 1991 showing “several worts would not ferment lower then 1.020, while others fermented to less then 1.006”. The study also shows false labeling of some extracts as " all malt", when in fact they had adjuncts.
Now that was in 1991, you used many brands, and have added adjuncts to your recipes, so the 1.020 you are seeing is interesting.
Daniels suggest doing a fermentability test on your chosen extracts, making a 1qt beer with 5oz of extract, take the OG, add usual yeast, ferment completely, chill to drop yeast, take FG measure and calc apparent attenuation, and of course taste it.
Anyway, I’d be interested in any changes in your next brew, as I’m mostly an extract brewer.
No doubt, 70% is not bad at all. I used to sub about 5-10% of my LME/DME for cane sugar to help dry out my beers before I switched over to all grain brewing. Another option is to perform a “mini mash” with your LME/DME and about a pound of 2-row or pilsner or other base malt. This can help convert some of the longer chain sugars in the extract and make them more accessible to the yeast (hopefully I said that correctly.)
I’m an extract brewer also. I rather like the idea of fixing the attenuation issue before (instead of ;)) going AG. You may still end up with the same issues. You may make a more fermentable wort, but the job of turning wort into beer is up to how you handle your yeast and fermentation process, and that ain’t gonna change when you go AG.
One thought- when you are oxygenating your chilled wort are you letting the oxygen bubble gently into the wort? If you do it too vigorously you may just be sending the O2 right through the wort and into thin air. Slow it down and go for 2 minutes.
Also, the higher percentage of dark grains in your grist, the higher your FG will be. And big beers with really high OG’s will also tend to finish a little high. But you did say that your problem batches tend to taste too sweet.
Several people suggested subbing out some extract for sugars. Personally I would only ever do this if that ingredient is appropriate for the style. Yes, sugars are more fermentable than malt extract, but is that part of the flavor profile you are after?
You can easily make a more fermentable wort by going all grain and controlling the mash schedule. I found that my main issue when I first went all grain was that my worts were more fermentable than I had planned for based on my previous experience with extract brewing. In my experience 80% attenuation is more the rule than the exception unless you mash at pretty high temperatures.
A healthy fermentation will ensure that all of the fermentable sugars are eaten up but that won’t improve your attenuation if you have a higher amount of unfermentable sugars. And large amounts of extract will give you more of those.
You know, its interesting on my system as I’ve found that I typically attenuate per the recipe or slightly less leaving my FGs a touch high…except for one batch that overattenuated which was just enough to keep me from drastically changing my mash temps. I have just started to tweak my mash down 1-2 deg F to see if that is enough to adjust my results. It just goes to show you how every system/process is a little different. I moved to 10-gallon AG last summer and am still refining my process and learning my equipment (I brew about every 6-weeks).
You know, I typically do just fine and target 152F with minor variances as I mash in a direct-fire kettle with a recirculation pump attached and ready. I’ll have to look at my last year of batches (since I changed all my equipment) and see if I have a trend or just minor ups and downs due to process variability (which is what I expect)…my only real suprise was my Oktoberfest which only attenuated 67% with WLP820 and WLP833.
One thing that jumped out at me from your third batch (and may be a factor in your earlier batches) is the Amber Extract. The darker the extract, generally the low amount of fermentables within. Plus since you don’t know what darker grains were used to make the darker extract, flavor profiles may end up inappropriate (or at least unexpected) for the style.
I had the best luck with my extract batches by using the lightest extract possible and getting my color and character from steeping grains. I regularly got final gravities in the 1.014-1.018 range with this method and pitching plenty of healthy yeast (which is sounds like you’re doing)
If you are using air to oxgenate, there is no reason to not oxgenate for at least 5 or 10 minutes. One study suggested that you need to run an aquarium pump for 30 minutes to adequately oxygenate wort through a stone.
Some studies say that aerating the wort while racking into the fermenter, e.g., fan the wort on the side of the fermenter as you rack and occasionally shaking the fermenter, is better.