Monday, October 11, 2010

Adding Fruit to Beer Increases Alcohol? (Rant)

Belgian holiday ale with sour cherries.
It really bothers me to hear people talk about the boost in alcohol a beer receives from a fruit addition.  In most cases fruit won't increase the percent alcohol of a beer, and if you're adding fruit to a strong beer it will actually lower the final alcohol content.

The issue is that many people mistakenly assume that they can determine how much gravity the fruit will add by calculating the amount of sugar it contains.  The problem with this method is that in addition to sugar the fruit also contains water (which dilutes the alcohol/sugar already in the beer).  In fact, most fruits have a similar sugar content to a standard gravity wort, between 1.040-1.060.  There are some exceptions, for example in a couple days I'll be getting 5 gallons of Cabernet Sauvingon grapes (to add to sour beers) that have a sugar content of 22 Brix (1.092).  Concentrated and dried fruit are another exception because most of their water content is removed during processing. 

To correctly determine the impact on the alcohol content of adding fruit you need to determine the effective brix/plato of the beer.  To calculate this you need four pieces of information:

1. Weight of the fruit (not including pits/stems/seeds).

2. Brix/plato of the fruit (from a refractometer or packing info).

3. The original brix/plato of the beer (OG reading).

4. The weight of the beer (based on the current volume, but the density of the wort before fermentation.  This can be determined by using the following formula: weight of the beer = original gravity of the beer x volume of the beer x weight of 1 gallon of water)

Effective Brix/Plato = (Weight of beer x Brix/Plato of beer + Weight of fruit x Brix/Plato of fruit) / (Weight of beer + Weight of fruit) 

For example if you have 4.5 gallons of 11 P (1.044) beer it would be 1.044*4.5*8.35 = 39.2 lbs of beer onto 10 lbs of 14 P cherries, the effective OG would be (39.2*11+10*14)/(39.2+10) = 11.61 P.  That is to say the increase in the effective starting gravity was .61 P, enough to boost the alcohol by .3% ABV assuming the same FG (about the same increase in alcohol from the priming sugar). 

Flanders Red with blackberries.On the other hand if you add the same 10 lbs of 14 P cherries into 4.5 gallons of 25 P (1.106) Imperial Stout you would have 1.106*4.5*8.35 = 41.6 lbs the equation would be: (41.6*25+10*14)/(41.6+10) = 22.9 P, a drop of 2.1 P, enough to reduce the alcohol by 1.1% ABV assuming the same FG. 

These two examples demonstrates a key insight, if the gravity of the fruit is lower than the original gravity of beer the addition is going to lower the effective original gravity and thus reduce the ABV (and conversely if the gravity is higher it will raise the effective original gravity and similarly the ABV). 

In general fruits provide sugars that are more completely fermentable than malted/mashed grains, so even with the same OG you may end up with a lower FG and thus marginally more alcohol.  That said, if you determine your effective OG this difference will be included in your ABV calculations when you take the FG reading. 

While you can go through all that work to get a slightly more accurate measure of the alcohol in your beer, the main point of this whole rant is that the change in alcohol due to the addition of fruit is small enough that you can comfortably ignore it.  Although you might want to pay attention if you are adding fruit to a base beer that is exceptionally strong or weak, or using a large amount of a fruit that has a substantially different gravity than the beer it is being added to.

Sorry about all the math, just a bit of the Mad Economist sneaking out...

11 comments:

Taylor-MadeAK said...

This...is probably the best piece of brewing information I've read in a very long time. Now I have an effective way to determine the gravity contribution of fruit to my fermentations (not just beer, this also applies to mead). Thanks, Mike!

While we're on this topic, what would be an accurate way of determining the impact of fruit juice on a beer? I have an all-grain apple brown ale recipe that is 20% apple cider (added to the wort after chilling), and I'm not entirely sure my method of determining its gravity contribution is accurate. Some time back, another homebrewer advised me on how to convert 1.034 OG apple juice to a pppg value (1.005 pppg) for use in ProMash. Using that method, a gallon of apple juice plus 4 gallons of wort works out to 1.071 OG (it's intended to be a high gravity ale). Do you think this is pretty accurate? Or could you suggest a better method?

Seanywonton said...

That's the last time I'm ever adding fruit to a beer!

Seanywonton said...

Taylor-Made, you can calculate it like this:
If apple juice is 1.034 and you add one gallon, plus 4 gallons of wort at 1.0XX, then it's:

(1 x 34 + 4 x XX) divided by 5 = your O.G.

james said...

Right on. I think a lot of people assume that most of the fruit is solid and not liquid and will drop out and no longer account for any liquid volume. It's clearly not the case.

Yes fruit has sugar but also has water. I doubt any of the commonly used software packages have another way of dealing with this other than something like the dilution tool that beersmith has.

The Mad Fermentationist (Mike) said...

Glad you guys enjoyed the post. As Sean said, if you are mixing two liquids the easiest way to figure out the effect is to take a weighted average of the gravities. Giving the liquid a PPPG works, but only if you adjust the total volume of the batch to account for the increased volume.

dwarven_stout said...

Thanks for the rundown. I've thought about this before and come to pretty much the same conclusions, and I like to see you putting it out there on the internet.

Taylor-MadeAK said...

Thanks for the answers, guys!

Seanywonton said...

Promash also had a dilution feature you can use to recalculate your O.G. based on adding water or wort of a specific O.G. You could use that.

Doug said...

Never apologize for "the math"; math is the language of discrete precision, and allows you to convey meaning efficiently. It's part of the reason I read here. The quantification of brewing details is a nice addition to the organoleptic breakdowns so commonly found.

The Mad Fermentationist (Mike) said...

Thanks Doug, glad my nerdish leanings are appreciated!

NT Revisionist said...

You write a great blog. I hope you're doing well by it. I say this as both a homebrewer and a yeast biologist.

Related Posts with Thumbnails