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Sweet potatoes messing up au gratin potatoes


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My au gratin potatoes are very reproducible.

 

Except when I add sweet potatoes to yukon gold slices. Then they are really soft and lack the "set" that potato starch usually gives after resting.

 

I've made 50/50 sweet potato and yukon all the way down to 1 in 5 sweet potato. Same result ...mushy au gratin.

 

Is there a sweet potato enzyme working here? Pectinase or something?

 

 

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22 minutes ago, pastrygirl said:

I think they simply lack starch.

 

There is certainly that, but the potatoes are mushy when cooked with SPs.

 

More experimentation is needed

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I think that it is because the sweet potatoes cook in about half the time of the other spuds.

When they cook in liquid, they also seem to become watery with even small overcooking. Pumpkin seems to do the same thing.

Not sure whether they start to absorb water or the water starts to break down the cells.

When you cook them in the microwave (by themselves in their skins or a plastic bad/container) they tend not to become watery.

I roast them in their skins in the oven till the skins are quite dry and the flesh remains intact though soft.

Perhaps try par boiling the spuds for 5 minutes. But it might mess up the flavors from the other ingredients or the thickening provided by the starch.

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What about using a different sweet potato - the Korean and Japanese ones I can buy tend to be a bit drier that the orange/yellow fleshed ones.

 

Quote

Chestnut Sweet Potatoes (Bam-Goguma 밤고구마): These sweet potatoes have a yellow interior and taste similar to chestnuts. These sweet potatoes are denser and drier in texture, similar to chestnuts!

 

Quote

In addition to their color, what sets Japanese sweet potatoes apart from their orange-fleshed relatives is their texture. Because they’re a little drier and starchier than other varieties, Japanese sweet potatoes have a distinct creaminess and a fluffier, lighter texture, a little more like a regular baking potato than a standard orange sweet potato

 

https://www.thespruceeats.com/what-are-japanese-sweet-potatoes-5213007

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Good suggestions, thanks!

 

Initially I thought it was water from the SPs, so I greatly decreased the number of layers and the thickness of the SP slices and the mushiness of the Yukons was unchanged.

 

At this point its maybe 15% SP in the gratin. The total amount of SP water added might be 3/4 Tbsp in a quart pan. I can't imagine that the potatoes that the SP replace don't contribute even more.  And the recipe includes a good bit liquid in the form of cream.

 

I could try preboiling the SP to inactivate any enzymes...which ought to work if the enzyme is not heat stable. (recall from another thread that papain in corned beef results in mush if sous vided but not if boiled).

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I suggest you boil then cool (in the fridge) the sweet potatoes, boiling will deactivate the enzymes, cooling will firm up the starch.

 

As an anecdote, sunchokes work well in a graten with potatoes, no special treatment needed.

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~ Shai N.

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We have made a version of pommes Anna using half sweet potatoes (the usual orange ones) and Yukon golds, and have not noticed that the potatoes get mushy, so maybe it is the liquid. I don't know what the answer would be in a gratin, though.

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