So I was looking into using sweet potatoes in a beer and stumbled upon some interesting information. Apparently sweet potatoes have enough diastatic power to convert themselves without using barley. And not only do they have diastatic power, but it's mainly from Beta-amylase.
Here is paper from the 1920's explaining how to go about making a sweet potato syrup, and it's pretty simple. All you do is basically steep them in water at a certain temperature like you would any other beer, then boil them, mash the potatoes, and separate the liquid from the solids.
I'm pretty sure people on this forum have tried this but I don't know how it has turned out, and I don't know if anyone has tried to ferment ONLY sweet potatoes. From what I have read they don't have any free amino nitrogen (FAN) which the yeast need. I don't know if yeast nutrient would take care of this or not, but it's definitely worth doing a one gallon experiment.
People seem to also be of the opinion that the sweet potato flavor doesn't come through very much in the final product.
Well, this sweet potato syrup interested me so I did some searching on the internet and came across an interesting paper.
This is not a very fun read but it's only 3 pages and it's got some interesting information. Some people did a study on the diastatic power of sweet potatoes, particularly in use with sorghum to increase yield for brewing. It turns out that sweet potatoes have a lot of Beta-amylase, which sorghum is lacking. They made the sweet potatoes into a flour, and added it in varying weight percentages to malted sorghum, and saw a good increase in yield, with a 4 fold increase in maltose.
Now this got me thinking. I probably wouldn't go to the trouble to make a flour, but I bet you finely grate sweet potatoes and throw them in with any grain/starch of your choosing and convert it into something fermentable and make beer out of it. And then you could throw in some Alpha-amylase enzyme that's cheap and readily available at homebrew stores. Maybe do a little math and crunch some numbers to figure out the right amount to use and you've got a mash with a similar diastatic activity to barley. With that mix you could have a lot of control over the profile of the finished beer, how fermentable it is, and how much body it will have by choosing your mash temperature.
Or maybe I'm wrong. But I know I'm going to be doing some experiments in the near future.
However, there are a few difficulties I foresee with this. One is the low amount of FAN, but I'm hoping some yeast nutrient, high pitching rate, and good oxygenation will make the yeast happy enough. The other problem has to do with the consistency of the sweet potatoes. To make the syrup, they had whole sweet potatoes in water converting their own starches, then they boiled to make the potatoes soft so they could mash them, rinse, and strain. Doing it this way, though, you can't use the sweet potato enzymes to convert anything else. The other guys made a flour which I'm guessing will allow the enzymes to be soluble in water and make the potatoes cook really quick. I'm wondering how finely I would have to chop the sweet potatoes so that enough of the enzymes would be able to dissolve into the water for the mash. I'm thinking I might just pulverize them in a food processor or blender with some water.