SWEET POTATO FLY (KVASS) FERMENTED SODA RECIPE
Background:
Originating in South American Guyana, sweet potato fly is a fermented sweet potato
drink made by grating sweet potatoes, adding spices, eggshells, and citrus, and letting it
ferment for several days. I originally heard of this recipe in Sandor Katz's awesome book
The Art of Fermentation.
You can let your sweet potato fly ferment with the bacteria and yeasts that are already
present in the sweet potato using wild fermentation or you can kick-start the process
using a culture starter. This can include a ginger bug starter, water kefir, whey, and
many other starters that are used to make fermented soda.
This sweet potato kvass can be a refreshing drink over ice, and at the holidays, makes an
awesome stand-in for eggnog and hot toddy cider drinks. Consider it another great
seasonal drink that you’ll come to love.
EQUIPMENT:
1/2 gallon jars or jugs for Primary Fermentation
Fermentation lid <– These silicone lids work well for us
(1) 16 oz. glass flip-top beer bottles <– We love these
(1) 16 oz. heavy duty plastic bottle (such as Perrier or a soda bottle)
INGREDIENTS FOR SWEET POTATO FLY:
1/2 sweet potato, coarsely shredded (~10 oz.)
1 cup sugar (3/4 cup for simple version)
1 quart non-chlorinated water
Procedure:
1. Grate fresh sweet potato and put in a large bowl.
2. Rinse grated sweet potato with non-chlorinated water a couple times or until the liquid
becomes a little clearer. This is to remove a lot of the starch from the potato so it will
have a cleaner taste.
3. Strain off liquid and add sweet potato to the 1/2 gallon jar. Add sugar, spices, and non-
chlorinated water and stir until sugar is dissolved.
4. Add a cover or fermentation lid and ferment for 1-2 days. While fermenting, make sure
to stir occasionally so mold and kahm yeast doesn’t try forming on top. Feel free to taste
to see if you like the flavor as it’s fermenting. Note that you can also add a ginger bug
starter at this step if you’d like to get it started quicker, but we haven’t needed a soda
starter for sweet potato fly.
5. Strain off liquid when done with primary fermentation and pour the sweet potato fly
into bottles to carbonate. If you don’t like carbonation, simply skip this step. Watch out
when carbonating! Sweet potato fly can build up pressure very quickly!
NOTE! – We like one of the bottles to be a plastic #1 soda bottle so we can squeeze the
bottle to get an idea of how much pressure is building up. You might have to “burp” the
bottle to release some of the pressure.
SWEET POTATO FLY – WHAT TO EXPECT DURING FERMENTATION
During primary fermentation you’ll notice that after one day your grated sweet potato
will look pretty even throughout and not much action should be happening except for
small bubbles forming on top. If you added a starter culture like a ginger bug soda
starter you might be seeing quite a bit more bubbling action.
After 48 hours of fermentation, our grated sweet potato began separating from the liquid
and started floating to the top of the jar. This was due to all of the bubbles that were
forming and causing the grated sweet potato to float. After stirring, you’ll see a lot of
bubbles being released.
After 3 days of fermentation the soda is definitely separating more from the sweet
potatoes and the liquid has become a slight orange white color. After tasting, we decide
that it probably tastes best if you only ferment for one to two days but it still tastes
good!
What’s the alcohol content of our homemade sweet potato fly? We’ve had this question
for a while along with the alcohol content of ginger ale so we picked up a hydrometer to
measure the alcohol content.
We measured the gravity of our sweet potato drink before and after the fermentation
process and plugged it into the generally accepted formula:
Alcohol By Volume (ABV) Formula
ABV = (og – fg) * 131.25
Original Gravity (og): 1.05
Final Gravity (fg) (after 4 days): 1.044
ABV = (1.05 – 1.044) * 131.25 = 0.79% ABV
After about four total days, we have a fermented sweet potato fly soda estimated at just
under 1% alcoholic content. Wow!
