So, let's talk about partigyle, second runnings, and the myth of free beer. For anyone who doesn't know, partigyle is the practice of making two (or more) beers out of the same mash by using the second (or more) runnings to brew an additional batch. It is a combination of the two terms "partial" and "gyle", which is an old term for mash runnings. Fuller does it with their London Porter to do their London Pride beer, for example.
The theory is that you can brew a batch of beer, especially a big one where the efficiency tends to drop, and after you've hit your volume and numbers for that batch, you can continue sparging and collect more wort to make a second batch. One mash, two beers, and best yet, since you were going to make the first batch anyway, the second one is kind of a bonus. Free beer!
How does it work? Well, let's say you brewed a big beer, like a 1.080OG Double IPA kinda thing, and you used about 8Kg of grain to do it. Typically, the more grain you mash (above a certain point), your efficiency drops, so whereas on your 1.050 APA you might be getting 75% efficiency, here you'd only get 66%.
Now, for simplicity lets assume you are using strictly 2-Row grist. 2-Row has a potential yield of 37 points per gallon (PPG). This means that, theoretically, if you extracted ALL the sugar from ONE pound of grain, into ONE gallon of water, you will have a gallon of 1.037 wort. From that number you can derive the amount of sugar you can expect in your mash. We'll look at the calculation in metric:
1 kg is 2.2 pounds, so 37*2.2=81.4 Points Per Kilo.
3.74 Liters to the gallon, so 81.4*3.74 = 304.5 Points Per Kilo Per Liter
In our case, we have 8 kilos, so that's 8*304.5 = 2436 total points in our batch
2435 divided by 20L (batch size) = 121.8. Convert to S.g. that's 1.122 wort.
Now of course, that's a maximum potential yield. We are expecting an efficiency of 66% so 121.8 * .66 = 80.388. or 20 Liters of 1.0838 wort. We'll take that :)
Now lets look at what's left behind. We had 2436 total points in the grain to extract. We actually extracted 80.38*20= 1607 pf them. That means that we still have 828 points in there. Divided by 20L we get 40.4, or 20L of 1.040 wort. That's plenty good for another beer. Right?
Well... no. Or at least, not quite.
First of all, you don't ever sparge your grain to depletion, the general recommendation is to stop sparging at 1.010 in order to avoid tannin extraction. Second, that's figure of 828 is maximum potential theoretical yield. Your results WILL vary.
So how much can you get? Well.... We said that your typical efficiency on your system was 75%, and in this big batch you got 66%. Let say that because of all the extra sparging your total efficiency (both beers combined) will be 81%. So you get 15% of the original maximum yield. 2435*,15 = 365
or 20L of 1.018 wort. That's a much more reasonable number. In general I think your runnings after you got your first full batch will be between 12 to 18 percent of your initial maximum theoretical yield. YMMV.
1.018 is not exactly a DIPA strength. In fact, if you consider that even a Mild, probably the lowest alcohol beer on the BJCP guideline, starts at about 1.030, it's pretty useless. So no free beer for you with those numbers.
What CAN you do with it? Several options:
- Make a half batch - Instead of 20L of 1.018 you can have 10L of 1.036. That'll get you in the range of some English beers or maybe even a blond ale. Remember that you still need to boil, hop, cool, pitch yeast and ferment this though. Not sure everyone would think it's worth the work for 10L
- Cap it - Add more grain to the mash and let it mash while you're brewing your first batch. Remember that a kilo of grain will add a theoretical 304 points to your batch. And since these weren't mashed in the first batch, we can calculate about 75% from them, so about 11.5 increase in a 20L batch (304/20*.075). Or in other words: chuck another 2 kilos of grain on top of your mash and you'll get 20L of about 1.041 wort. Respectable, but you need a pretty big mashtun.
- Add extract - Same concept as capping, but you don't need the a bigger mashtun, and you have a known quantity because you know exactly how much extract you're adding and what its yield. Extract is generally more expensive than grain though, and sometimes you can run into freshness issues.
- Add specialty grain - Remember in the beginning when I said you're using 8KG of two row? Well who said you have to stay there? A typical "Crystal" or Roast malt will give you about 20PPG when steeped in water (see http://www.howtobrew.com/section2/chapter12-4-1.html) so steeping about a kilo of specialty grain will give you about an 8.2 point bump in SG for a 20L batch. (20*2.2 = 44 * 3.74 = 164.5/20=8.228) This is not enough to bring your 1.018 wort into respectability, but combined with capping or extract it will allow you to make a completely different beer style. For example, a mild, a brown porter, dry stout, or an American Amber are all fairly small beers with a high percentage of specialty malts.
My favorite approach is to combine methods 3 and 4 and to try to have a specific goal in mind in terms of the second beer. I can then brew my first, see what kind of extra runnings I get, and adjust accordingly with steeped grain and extract.
In this part I've walked you through the basic calculation of parti-gyle, and how to approach the recipe development. In part 2 I'll go into two other types of 2-for-1 brewing: Split batch and high gravity brewing. Hopefully, it wont take another year to write... :)