In the past month, we’ve noticed that the gravities have been finishing lower than usual when using L28. It usually finishes at 2.5ºP, but has been finishing at 2.0ºP. I’ve tried troubleshooting this, but our process looks pretty consistent. There are no major changes to top-off volumes, mash temperatures, or original gravities etc.
What could be affecting the final gravity? We do pitch cone to cone, but even if a slightly larger volume of yeast were added shouldn’t the final gravities be the same, because it’s not like there are more sugars in the wort?
You are correct. Adding more yeast will not result in higher levels of attenuation, unless what was added before was severely under-pitched or stressed out. I don't think this is the issue here. You'd see a lot of other accompanying signs if this were the case.
You're also correct that FG is typically determined by the amount (and type) of sugars available to the yeast. It might be worth looking into factors affecting wort composition a bit more. Some areas that come to mind are, differences in malt COA’s (protein levels and diastatic power specifically), variations in mash temperature, pH and liquor to grist ratio, mashout temperature and the amount of time it takes the grain bed to reach denaturing temperatures.
For lagers, we typically mash at 152ºF and rest for 60 minutes. We mash out at 168ºF, but I’m not sure how long it takes to get there. Would a +/- 5ºF difference in mash temperature make that big of a difference in final gravity?
The short answer is yes, +/- 5ºF can make a big difference in final gravity. A 60 minute mash rest held at 152ºF is going to be pretty fermentable. You could try increasing the mash temperature slightly, 154ºF may be a good compromise. Another option may be to increase the liquor to grist ratio (L:G) slightly, if it’s pretty thick. If adjusting the mash temperature and/or L:G doesn’t help, then you might try adjusting the time it takes to reach 168-170ºF during sparge by shortening the mash rest time.
Malts are so highly modified and carry such high loads of diastatic power (DP) that enzymes can remain active until fully denatured throughout the grain bed or in the kettle. The goal here is to reach denaturing temperatures as soon as possible. Performing an Iodine test during the mash rest will help determine when the starch has been converted, and when it’s time to move on to Vorlauf (recirculation). If it only takes 30 minutes, then beginning collection and sparge 30 minutes sooner may help decrease further enzymatic breakdown to fermentables by 30 minutes.
Another thing to note is that bagged malts can be pretty inconsistent and may vary depending on lot number and seasonality. If you’re using bagged malt, then it may be helpful to keep track of the lot numbers and the COA’s, if possible. At the very least, when you receive a lot that has higher levels of DP, or has been shown in your brewhouse to produce variations in wort fermentability, you might be able to compensate ahead of time with some of these hot side adjustments.
From a yeast perspective. It’s best to aim for a consistent pitch rate with healthy viable yeast, supply an adequate and consistent level of dissolved oxygen during knockout, and maintain consistent temperatures within the strain’s fermentation range. If these are dialed, then there shouldn’t be an issue with attenuation.
Our mash tun is not jacketed so we only “control” the temperature by the water we put in. After 60 minutes in the mash tun, we Vorlauf for 15 minutes. Then we sparge with 175ºF water but it could take 30-40 minutes for the mash tun to reach 168ºF. While the sparge water is being added, wort is simultaneously transferred to the boil. Some of the wort never reaches 168ºF in the mash tun.
Sometimes, the mash tun is so full we can’t even add enough sparge to get the temp to 168ºF. So the wort never reaches that temperature until the boil. What I’m not clear on is, what is the problem with this? I’m assuming the sugar type and concentration would change, which would affect gravity.
If we are consistently hitting our target OGs (+/- 0.002), would the hot side still be the cause for variance in the FGs? Also, I’m going to research Iodine tests a bit more
It's fine that the MT jackets don't heat up. You would need a mash mixer and separate lauter tun for this. I've brewed on systems like what you're describing and understand the limitations. There's nothing "wrong" with the process necessarily, it might just produce more fermentable wort if/when enzymes are still present and able to do the work.
You're exactly right that the sugar type and concentration can change, which can also affect the FG. This has more to do with the malt's diastatic power (DP), or "enzymatic package" and its capability of breaking down more of the starches into simpler forms of fermentable sugars that the yeast can metabolize.
This can happen in a few different ways. The extract (OG) can be the same, but the % composition may be different depending on how much of both enzymes are present, which are activated, and when they're denatured. Yeast go after sucrose, glucose, and fructose, followed by maltose and then finally maltotriose, but not dextrins (unless it's STA1+).
The mash/collection/sparge controls will help determine the percent make up of each of these. This is what leads to different FG's. Below is an illustration of the compositional breakdown of wort (Kunze, 1996).
An iodine test (starch conversion test) is pretty simple. Use a white plate, or dish, drop a few drops of clean wort from the mash (no husks) onto the plate, then add a drop or two of iodine. If the starch turns blue or black, there's still starch present. If it doesn't turn blue or black, there's no reason to hold the mash rest any longer.
The longer it takes to reach denaturing temperatures, the more fermentable the wort can become. Shortening the mash rest is just one control you have. Below are a few more.
The following controls may result in the following TG's:
High TG (less fermentable)= Thinner mash (high L:G), higher temp (≥156ºF), lower pH (≤5.3), lower DP malt (Munich, Vienna, oats, wheat, cara/cryst, DME etc.), shorter mash rest time (~30 min), early denaturing of enzymes (~170ºF)
Low TG (more fermentable) = Thicker mash (low L:G), lower temp (149ºF), higher pH (5.5), higher DP malt (Pilsner, 2-Row, rice, corn, dextrose etc), longer mash rest time (~60 min), later or no denaturing of enzymes (<~170ºF)
Does this help? I'll do my best to answer them.
Yes it does. Thank you for your help!