In any case, it seems that the whole thing attracts wild yeast, as all sweet worts will do. If you have no leftover mavi, you can try to ferment your own, lambic style. The cloth cork is just to keep the bigger nasties away and let the carbon dioxide escape.
As you can surmise, open fermentation with wild yeast may also contain wild bacteria, thus the lambic reference. It will ferment on its own. The sun part of the ritual is to add heat and make it confortable to the yeast and friendly bacteria. It also, undoubtly affects taste much as sun affects the taste (In the yummy kind of way) of them Mexican cervezas if you expose them for 5-10 minutes previous to serving. Something that ultraviolet does to the whole concoction.
If you prefer a more beer like drink, add yeast, low attenuation for more sweetness and the classic low alcohol levels. If you prefer dryer and stronger drink, shoot in some champagne yeast in.
I've been researching this for a while cuz I intend to do Belgian style blended beer and one of the layers will be mavi. I'll post my findings later.
By the way, anywhere you find long established colonies of Caribbean people you will find fresh markets and sure as the sun rises, someone imported some bark. I can try to find a supplier locally here in Puerto Rico and ask a friend of mines to post it as a specialty spice on his website. It doesn't hurt to ask.
Here is the website of a comercial version sold in the Dominican republic. They list it as non-alcoholic, so i gues it is just the juice without the fermentation and force carbonation. http://www.grupotaino.com/products.html
Some dude from St. Thomas (they call it mauby there) has this recipe:
... First, I simmered the following for about 10 minutes:
4-5 pieces mauby bark
2 sticks cinnamon (short)
2 bay leaves
2 Tbsp. fresh rosemary
2 tsp. dried marjoram
2 pods star anise
1/4 tsp. grated nutmeg
3 cups water
at the end of which, it had reduced quite a bit. I strained it — saving back the mauby bark — into a 3 gallon carboy, dropping the bark in as well. The mauby bark, incidentally, was $2.50/oz. from a local Caribbean grocery. It’s Bedessee brand. Next, I heated
2 cups brown cane sugar
2 cups white cane sugar
10 cups water
until the sugar dissolved, and allowed it to cool. This was added to the carboy, and shaken well. Finally, I pitched half an expired packet of Lalvin D47 yeast I found in the back of a drawer, figuring it couldn’t hurt, and just might help. Didn’t bother to proof it. To keep the nasties out, and just in case the yeast did decide to kick off an active fermentation, I affixed a waterlock, set it in a warm spot and waited.
It never really developed much of a head… oh, there was a layer of foam on top, and the waterlock was definitely working some, so there was CO2 being produced, but it was nothing like a rolling, active fermentation. I doubt that the D47 had much to do with it. A slight cap persisted for 5 days, at the end of which I decanted it into a pitcher for refrigeration, to halt any further yeast activity.
The verdict: DELICIOUS. Scrumptuously bitter, with lovely herbal and yuletide spice notes. Sweet enough to complement the bitterness — it didn’t even begin to ferment to dryness — without the syrupy heaviness of mauby made from concentrate. It also seems to lack the long, medicinal finish that I noted in the concentrate...