This has been one of my favourite threads on Homebrewtalk, and at long last I've finally gotten around to brewing my own GaP beer!
I used kumara (New Zealand sweet potato) for converting my starches. As MacBruver notes above, it does supposedly contain beta amylase. I've found some sources that say various varieties of sweet potato have a diastatic power of 150-300 lintner. I also found some sources that suggested that ginger root contains alpha amylase. These seemed a lot less certain, and give no indication of how MUCH, but I figured it was worth a try, and added some ginger to the mash too.
Since I might well have ended up with only beta amylase I did my best to include adjuncts that have relatively high quantities of unfermentables.
Here's the recipe:
9.5L batch
MASH INGREDIENTS
1.0kg Orange Kumara (chopped, frozen, thawed, then blended)
0.9kg Raw soft wheat
0.1kg Toasted raw wheat (toasted for 40 mins @ 185C in the oven. Came out looking a bit lighter than pale chocolate malt.)
0.02kg Ginger root
Mash for:
60 mins at 63C
40 mins slow ramp up to 69C
20 mins at 69C
BOIL INGREDIENTS
0.5kg Treacle @ 60 mins
50g orange peel @ 60 mins
20g orange peel @ 20 mins
5g cinnamon stick @ 5 mins
FERMENTING
Tasti Bread yeast @ 16-18C
OG=1.045
I mashed using a BIAB type method with only the solid ingredients included, leaving the treacle for addition at the start of the boil. The mash used 5L of water, then I did two "dunk" sparges with 3.5L, and one with 2L, giving about 12L into the kettle. Unsurprisingly, with a nothing but kumara and wheat in the mash, lautering wasn't easy.
The first runnings were nice and sweet, and a nice dark gold colour. Later runnings were an ugly green-brown colour. I took 1L of the first runnings and boiled it down to 500ml, hoping to caramelize some of the sugars.
The wort had lower gravity than I'd expected.
Part of this was because I mis-calculated the extract potential of kumara at first.
Part of it was because my efficiency was quite low (about 60%.) This may have been because I didn't let the mash rest long enough to fully convert, though I forgot to pick up some iodine before brewing, so I guess I'll never know.
That said, the kumara and/or ginger definitely DID convert starch to sugar. If I've (finally) got the math right, then the sugars in the ingredients alone should have given an OG of 1.012 or so. Since my OG was 1.045, a good chunk of the sugar in the wort must have been converted from starch.
The wort was pretty tasty. Nice dark brown colour. Quite sweet, with minimal bitterness. Soft, but clearly noticeable ginger and cinnamon flavours.
It's been fermenting away happily for a few days now, and I'm looking forward to trying this when it's ready