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Nov 13, 2010 08:13 PM

Potatoes au gratin - Sauce split??

I'm wondering if anyone has any idea what i did to cause my potatoes au gratin to split?

I didn't exactly follow a recipe, just made them like my grandmother always did: bechamel with chopped shallot, 2 tbsp melted butter, 2 tbsp flour, 2 cups milk (skim, lactose free). I prefer a slightly thinner sauce for potatoes au gratin. The bechamel was prepared on the stove with hot milk, I took it off the heat to add my extra old cheddar (1 cup) and seasoned with S&P, mustard, paprika, etc. I combined with thin sliced potatoes, ham and some thawed and pressed spinach and baked for about an hour at 375.

I took it out of the oven and the sauce had split, it looked terrible! I stirred it up and it still tasted ok but I would love to find out why this could happen for the next time I make them. I live alone but it's a dish that I would love to make for company in the future. Thank you. :)

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  1. Could the fact that you didn't use real milk have something to do with it--an emulsion problem?

    1. The milk is the problem. You need either evaporated milk or some form of cream.

      8 Replies
        1. re: greygarious

          I guess I might have to break the lactaid pills out of the cabinet when I want to indulge. I can't get lactose free cream and i think evaporated milk is probably a bad idea. It just seems odd to me because I have a similar recipe for LF mac & cheese and it's never split on me. :(

          Serves me right for trying to make a dairy-loaded food lactose free! On the other hand, it tasted delicious and I wouldn't mind experimenting with it. Maybe whole LF milk, a bit more starch to keep it together or gentler baking? I would be willing to use a bain marie for a good dairy fix...

          1. re: elysabeth

            Yes, it's the sauce, the heat and length of baking time; it's a common problem with potatoes au gratin, ask anyone who makes the dish on a regular basis. Aside from switching up the dairy (I use heavy cream and don't make a bechamel anymore) you could lower the heat by 25-50°, but even that might not totally solve the problem, as it will just up the amount of time it takes to finish cooking, although gentler heat might be the ticket. Precooking the potatoes will help cut down on baking time. Slicing them paper thin may work for you. Try blanching the potatoes first, if you're slicing your potatoes by hand, not with a mandoline. And no evaporated milk for you, if you're lactose intolerant.

            Mac 'n cheese is baked for less than half the time than au gratin potatoes, about a half hour for the cooked macaroni as opposed to 1 hour and more for the raw potatoes to finish cooking through, depending on quantity, and enough time for the proteins to curdle; that's why mac 'n cheese doesn't usually break or not as obviously as au gratin.

            Here's a fairly lengthy thread discussing the perils of au gratin potatoes, and what some posters do to avoid breakage and gain success. As you see, you are not alone:


            1. re: bushwickgirl

              Thanks, that's a great thread. If the problem is extended high heat on casein I'm really starting to wonder if a bain marie might be the way to go. The bechamel did not split until it had boiled again. I sliced the potatoes by hand to about 1/8" but a mandoline would probably be even better.

              Maybe I could bake the potatoes for the first 30 minutes in a small amount of stock & butter before I add the sauce. I really love the flavour of potatoes that have had a chance to brown in the oven but I think it might be necessary to precook them for tender potatoes and smooth silky sauce. :)

          2. re: greygarious

            I use Lactaid skim all the time and it doesn't usually break. And you cannot tell the difference, once it's thickened up.

            1. re: Phurstluv

              lactaid milk tastes the same as cream?

              1. re: alkapal

                Most certainly not. But once I use it in a white sauce, it thickens up just fine. I know that it won't have the same mouthfeel to it as if I had used cream, but the au gratin cheddar sauce I make for my potatoes tastes very similar, even if I use skim.

                1. re: Phurstluv

                  Do you have a recipe for your gratin that doesn't split using Lactaid skim milk?

          3. I'm thinking what happened here is that the potatoes, ham and spinach gave up their water in the baking process and leached into the bechamel, and the low-fat subs. may have been part of the problem too. If you want perfect ones every last time: Slice your requisite amt. of spuds thinly and rinse and dry. Place in large saucepan and cover with milk. 2% is fine, just to cover though. Bring to a boil over medium heat, and let the potatoes cook until BARELY tender and milk starts to look a bit "sludgy." Tip into gratin or shallow baking dish and smooth out: pile your shredded cheese on top, and bake at 350 for about 45 minutes. You will never look back. S,P, and garlic can be added to the poaching milk. It will take forever to soak your saucepan, but for foolproof scalloped? Well worth it.

            5 Replies
            1. re: mamachef

              i cook mine using blanched and well-drained (patted dry, too) potatoes.

              i too wondered about the spinach.

              i don't use skim milk; i don't think it is suitable for cooking anything, is it?

              1. re: alkapal

                In general, skim milk is unobtainable. At least in my kitchen. Low as I can go and not feel totally cheated is 2%, but I almost always run with full-fat. For this recipe, because the potatoes give their starch off into the milk as it heats, 2% works ok though. This is my go-to, along with Edna Lewis' amazing potato casserole, one of the best things you'll ever eat.

              2. re: mamachef

                I have followed this proces ever since I saw Jacques Pepin do it on a cooking show and it has never failed me yet. I also use full fat milk, so I would think the amount of fat in the milk must enter into the equation.

                1. re: NE_Elaine

                  Well, it may. I'm just lucky that this has worked out, but maybe the fact that I also dot with copious amounts of butter (to make up for what I didn't use with the milk ;-) saved the day. Chemicalkinetics should feel free to chime in anytime, though. A thorough explanation will undoubtedly be given. My very BEST guess is that the fats/proteins in the milk combine with the starch in the potatoes to create the "thickening synergy", but what do I know? I'm a cook, not a physicist. Or even very smart, for that matter. ;-)

                2. re: mamachef

                  Thanks, this sounds easy and good, I'll definitely try it.

                3. The lower in fat the dairy product, the more prone it is to being problematic in sauces unless it's already processed by evaporation/condensation.

                  1. I've tried many recipes with varying results, but I used a Cook's Illustrated recipe for our Christmas meal, and it's now my favorite. There were only three of us, so I halved the recipe and it made plenty for dinner plus leftovers. I made a couple slight changes to the recipe. I used Yukon Golds, rather than Russets, and dried thyme instead of fresh. And rather than put all the cheese on top, I put half of the potatoes in the baking dish, topped with half the cheese, and then another layer of potatoes and cheese. They came out perfectly done, rich and delicious.