does anyone have some kind of formula or just general experience for fruit displacement? I have 2.8 gallons of sour blonde and 5lbs of peaches I'm trying to figure out how much blonde I'll have left after putting the sour on peaches in a 3gallon carboy. Thanks
When I did my Apricot Saison I ended up with about 4 gallons in the bottles from a 5 gallon batch... but that included quite a bit of Apricot in the bottles that erupted upon opening. Sorry I don't actually have a formula, but I would assume at least 1 gallon loss? Could you add the peaches to the carboy and see what the volume is? It will settle down some as the yeast beats it up and makes it mushy, but you can pretty much count on losing all the beer incorporated in the peaches.
Fruit is mostly water, so it will be around 8 lbs to the gallon (if anything it will be closer to 7.5 lbs/gallon given the increased density from the sugar).
From my experience, it varies a great deal from fruit to fruit. I think the two main variables are the water content and sugar content of your fruit. Higher water content means more liquid will be introduced to your beer (more beer!), while more sugar will lead to increased yeast propagation and ultimately a thicker trub. As a point of comparison, I added 5 lbs of grapefruit flesh to a 4.5 gallon batch and ended up losing close to another half gallon. Hard to say if peaches will behave similarly to grapefruit.
Peach displacement = the sum of the volume of each peach (sphere) minus the sum of the absorption of each peach
As a follow up should I just transfer to a 5 gallon carbon instead and risk the headspace or deal with leftover sour?
It's always nice to have a little extra if there is any possibility you will barrel age this at some point. Most sours will have some acetic acid...it's just a question of limiting it. A smaller carboy/better bottle and/or periodic CO2 purges will nelp, IMHO.
If you're adding several pounds of fruit to a sour beer and don't leave any headspace, you're going to have a bad day! Head space isn't a big concern in impermeable fermentors. In barrels it is a concern because it allows the wood at the top to dry out and become more permeable. If you are worried, you can purge the carboy after adding fruit and then rack the beer in. Once the airlock is on, the same amount of oxygen will get inregardless of the head space size.
Funny coincidence, I just bottled 5 gallons of lambic style ale that I put onto 7.5 lbs of peaches. Sat for a month and I just bottled 2 cases of 750ml bottles, with 4 12oz "taster" bottles left over. So not a massive loss. I did this in a plastic bucket. Fruit + beer appeared to be about 6 gallons based on the marks on the bucket, but the net into bottles was pretty close to the 5 gallons I measured onto the peaches initially, when I racked the barrel.
