Naturally carbonating in kegs?

[Xander Cameron]

I am about to set up a kombucha keg system and was wondering about how to go about doing a natural carbonation method in the kegs.  When I bottle I usually add my 20 percent juice to the finished tea and keep a sealed plastic bottle filled so I can feel how carbonated it is getting but obviously I can't do that with a keg.  I will be doing Co2 as well but like the flavor you get from letting it carbonate off of the fruit so I want to explore that option.  

Also when I would bottle my kombucha I would often have the problem of over carbonation depending on what fruit I used and what not.  It seemed to be a pretty fine line like I would check the carbonation one day and it was nearly there and then a day later its fizzing over the top when I crack the bottle.  So my question is if I over carbonate my flavored batch of kombucha in the keg how will it come out the keg tap?  Foamy or do I have a little play there?  And if its over naturally over carbonated in a keg is there any way to fix it?  Thanks

[Jared Englund]

Yeast carbonation is hard to control.  If you use a hydrometer you can tell how much sugar there is which corresponds to the carbonation potential.

But, I can give you a few pointers if you don't have a hydrometer.  First, the sweeter the fruit the more quickly it will carbonate.  Second, solid pieces of fruit or fruit sediment will cause it to carbonate quicker.

In kegs, you can tell how pressurized they are by lifting the pressure release valve.  If tons of gas come blasting out, you know your Kombucha is over-carbonated. 

If you really have your heart set on yeast carbonation, just do it for a couple of days then hook your keg up to 9psi and it will get perfect.

Jared

[rses...@gmail.com]

If you are using a hydrometer, what is ideal for when you are ready to put it in the keg for natural carbonation?

Also, can you connect a pressure gauge to the "out" port on your keg and stop it when it gets to a certain point? I am trying to hold it in my keg until I am ready to fill bottles. I am trying to avoid giving my friends kombucha foam grenades. Any pointers you have for bottling from a keg would be appreciated. Thanks.

[Jared Englund]

I don't have an exact number for you, some trial-and-error is in order.  I do like your idea about a pressure gauge on the keg.  You will still have to refrigerate bottles because any residual sugar will keep fermenting.

[brewer]

Jared gave good advice.  Maybe I can add a few things.

When exposed to any fermentable sugar, yeast will convert it to alcohol.  The by-product is CO2.  

Fermentation will end when the yeast colony has consumed all of the available fermentable sugars.  

Fermentation can nearly-stop when temperature drops considerably below the activity threshold for yeast.  It can re-start if temperature later increases.  

Fermentation will end if temperature exceeds the survival threshold for yeast.  (Pasteurization kills yeast, ending fermentation potential forever.)

Secondary fermentation occurs when we take a liquid that has a viable yeast community in it, and add more sugar.  The yeast organisms are microscopic and floating around in the liquid.  If you strain a fresh beer or bucha through a coffee filter and then add sugar to the liquid and store it at room temperature, a secondary fermentation event will occur because thousands of yeast organisms flew through that coffee filter with ease.    

If you add a relatively small amount of sugar to a certain volume of yeast-containing liquid, and you then seal the container and store it at room temp, the yeast will consume all of the new sugar.  In so doing, they produce a reasonable amount of CO2 before they eat all the sugar.  Because the CO2 has no way to escape, it is forced into the liquid, ie carbonation.  

If you add too much sugar for the same volume of liquid and then seal it and keep it at fermentation temperature, the yeast will convert sugar into alcohol and CO2 until the container explodes.  Bottle bomb.

If you add too much sugar and then refrigerate after 2 days, fermentation will nearly halt, not because the yeast are running out of fuel, but rather because they are sold cold that they go nearly dormant.  If they remain cold, it will take a very long time (months) for over-carbonation to occur.  UNLESS, of courses someone buys your refrigerated product and then puts it in their pantry and forgets it.  In that case, the bottle will explode in their pantry about a week later, and they will call you.  It is possible that a cold bottle of over-primed beverage could explode in a fridge after a few months.  

Kegs!  Corny kegs are great for the OP's plan, because they have a built-in pop-off valve that will release pressure before it exceeds the burst rating of the keg.  If you pull the ring on the pop-off valve, you can release pressure yourself.  

Optimal carbonation levels for low-fizz "good" beers occur at about 7-8 psi of CO2 pressure in a keg.  American pilsners have 9-11psi of CO2 pressure.  

If you want a great bucha, shoot for 7psi.  It's just barely fizzy.  Goes well with the acidic character.  

If you research on beer sites, you will find methods and free calculators for calculating the exact amount of sugar addition necessary to naturally create a certain style of fizz in a certain volume of beer.  Pay attention to the stouts and porters genre.  They have the lower-fizz character that you want.  You can also find threads where the authors discuss equivalent sugar conversion factors for various masses or volumes of fruit or juice.  This will help you estimate an amount of fruit or juice to add to your keg for natural secondary carbonation in the keg.  Different fruits contain different types of available fermentable sugars.  Ripe strawberries are "hot".  A small quantity is all you need.  Raspberries are less so.  Ginger is pretty "cool".  Ripe peaches are "hot".

So what we have learned here is that you won't get enough fizz from natural carbonation if you don't add enough fruit-sugar to the keg for the secondary ferment.  If you add just enough, you'll get perfect carbonation, and if you add too much, you can just vent the keg once per day on day 2, 3, 4, 5, 6 until it starts to seem less angry, at which point the yeast have consumed all the sugar and the CO2 has been balanced properly by your venting procedure.  

However, you will still need a regulator and CO2 setup, because a full keg that is perfectly carbonated will only "self-dispense" a fifth of it contents as perfectly carbonated.  Why?  Because as the "air gap" increases inside the keg, CO2 will break out of the fluid to find a balance, which will lower the carbonation level of the remaining beverage in the keg.  

Which means that you need a bottle and regulator, which then questions the merits of natural carbonation.  However, if that is your marketing plank, then do it anyway.  Use natural carbonation, and then hook up to a CO2 bottle just to retain a consistent dispensing pressure.  

CO2 breaks out of a liquid much faster when the liquid is not cold.  To properly fill bottles or growlers with a carbonated beverage, you need a wand like the ones you see at growler stations.  AND, you need cold containers.  Do not dispense into a room temp container.  that will cause an instant loss of some of the carbonation, regardless of wand technique.  And, you need to reduce your dispensing pressure to 3 psi just before you start filling.  This will slow the transfer speed, which further reduces CO2 breakout.  Clarifying: leave the CO2 pressure at 7psi and keep the keg cold for storage.  If you want to fill a pint glass for a customer, just fill it.  If you want to fill a growler, reduce pressure to 3psi, bleed the pop-off valve for a second or two, fill the growler, then turn the pressure back up.  If you want to fill 39 bottles, reduce pressure, fill them all, then send the keg to the dishwasher.    

Lastly, choose cap systems purposefully.  Most growler caps are designed to leak CO2 above 10 or 12 psi.  Grolsch-style flip caps are also designed to leak CO2 before bottle failure.  These caps will hold 8psi just fine, but leak at higher pressures.  

If you fill a growler for someone who then drives 4 hours away without a cooler, the liquid will come to room temp, most of the CO2 will break out, most of the released CO2 will leak out the cap, and then the customer will have a flat beverage even after they re-refrigerate the growler for a week.  

If you fill a commercial plastic bottle with kombucha that is perfectly carbonated, and it has a leak-proof cap, and the customer gets it warm for 2 days, and then puts it back in the fridge, if the bucha stays in the fridge for at least 4 days, the released-but-trapped CO2 will infuse back into the cold liquid.  

Avoid confusion by clearly labeling the bottles: keep refrigerated.  

As Jared pointed out: bulk fruit decreases viable shelf life considerably compared to filtered fruit juices and extracts.  

have fun!