Sweet Potato in Secondary

[hbear]

So here's the deal: I've got an English brown sweet potato ale in the fermentor, and I'm trying to decide whether or not to add more sweet potato once I rack to secondary. I'd like to roast it until it's slightly burnt and caramely-sweet and let the yeast go to work. I'm hoping that would help some of the aroma shine through. My concern is that it's going to mess up my clarity. I used Whirlfloc in the boil, but will throwing sweet potato in at this point cloud up the batch?

 

[seabass07]

AFAIK, you need to mash (enzyme mash, not mashed potato mash) the sweet potato before the yeast will be able to ferment it. Sweet potato is mostly starch. You need to cook it to gelatinize the starches and then let amylase go to town on it before it will do what you want. At least this is how I understand it.

 

[hbear]

I already added sweet potato to the mash; I'm just hoping to push more aroma into the batch. Sweet potato is mostly starch, but the process of roasting it converts much of the starch to sugar, which will almost surely ferment. Regardless, I'm mostly concerned with infusing the ale with more sweet potato flavor, not with secondary fermentation. But the question remains: will the addition of sweet potato in the fermentation vessel cloud the beer?

 

[seabass07]

pretty sure adding tons of starch will give you starch haze.

 

[mcl]

but the process of roasting it converts much of the starch to sugar

I don't believe it does.

 

[seabass07]

Nope. It gelatinizes the starches, making them available to amylase. It is still mostly starch after cooking.

 

[Seedly]

The enzymes that are in the sweet potato that would convert the starch to sugar denature (i.e. die) at 160°F, so roasting them will just gelatinize the starch, not convert it.

 

[Obliviousbrew]

I like my sweet potatoes "mashed"

 

[william_shakes_beer]

sooo If I wanted to extract fermentable sugars from sweet potate I would
1. roast the potatoe to gelatinize the starch
2. mash with some basemalt to convert the starch to sugar
3. drain the liquor and ferment as usual

Sorry for the threadjack, I understand that's not the question you asked.

 

[hbear]

Ah, I see. I think I was confused about what gelatinized starch is.

 

[Seedly]

sooo If I wanted to extract fermentable sugars from sweet potate I would
1. roast the potatoe to gelatinize the starch
2. mash with some basemalt to convert the starch to sugar
3. drain the liquor and ferment as usual

Sorry for the threadjack, I understand that's not the question you asked.

In a nutshell, yes. The skin of the sweet potato is rich in alpha alamayse, so they can self-convert if you leave the skin on. Theres a paper ive seen floating around in different HBT threads about an experiment done in the 1920s where they processed sweet potatoes (iirc, grated, kilned until dry, and ground into flower) and then mashed the results successfully and made a syrup.

The problem you may run into is that not all starch in potatoes will liquify at mash temps. You may have some starch settle out of the mash and not convert, leading to a bad starch haze and off flavor.


Finally, while sweet potatoes are rich in sugars and starches, 1lb of fresh sweet potato != 1lb of grain. Much of the weight of the potato is water. There is a thread (i think in the gluten free subforum) where someone went through and calced the potential gravity from sweet potato, and it was underwhelming to say the least (again iirc, about 1.018 PPG for fresh taters, compared to 1.038 for 2-row). Thats likely due mostly to the water-weight of the potato.

So how do you correct for all this? Good question. I would go with:
1) Roast the potato to make sure all the starch is gelatinized
2) Use 6-row for the extra diastatic power since the potato wont contribute anything in that regard after being roasted
3) Account for the extra water you're adding with the potato when figuring your strike-water volume and temp (as you are essentially adding several cups of very hot water with the roasted potato. No idea exactly how much you would be adding)
4) Mash a little longer than you normally would and make sure you stir well when doing your starch tests to kick up anything that settles out of the mash

 

[seabass07]

Interesting. If the enzymes are in the skin, you could peal it, roast it on parchment without the skin, and then mash it with the skin so it can self convert. Not sure if there's a point to doing it that way vs using some 6-row, but interesting that it would work.