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Nov 3, 2013 09:47 AM

Help! My Overnight Steelcut Oats have green slime...

I started using the Cooks Illustrated Overnight Steel-cut Oats recipe last winter and loved it. Basically, the recipe is to oil water, add steel-cut oats, leftist it on your stovetop overnight, and finish the cooking in the morning. I made it a few times, and then one morning, I noticed that there was a strange green film on the surface when I went to add the milk and continue cooking. I assumed there was some problem with the pot or the oats, but I've now had this happen every time I've attempted the recipe since. I've tried different brands of oats, boiling the water for longer, putting the pot in the fridge, but to no avail. I don't know what the green stuff is - the pots it's out for about 8 hours, insufficient form old growth, it would seem - but it's unappetizing, and I can't bring myself to eat it.

Anyone had this problem? Have any suggestions or possible reasons for this? I have added salt to the water...could the cause this?

Any help would be appreciated!

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  1. If you do a Google search of "mold on oatmeal" you'll see lots of sites that talk about instances of it including some talking about the overnight method.

    That pic is such a good example of why leaving food out overnight is a bad idea.

    Here's a simplified answer I found on one site:

    Oatmeal can grow mold. Oats are full of starch. Starch turns to sugar under certain conditions, and sugar leads to mold. Oatmeal usually doesn't produce mold until it is exposed to heat or moisture, but extremely high humidity, being stored incorrectly, and other mold-favorable conditions will produce oatmeal mold. - See more at: http://www.chacha.com/question/can-oa...

    3 Replies
    1. re: weezieduzzit

      dry goods like that may already be moldy when they get to your house and just need moisture to bloom.

      1. re: weezieduzzit

        I guess I'm just surprised that a supposedly tried-and-true recipe could go so wrong. And that keeping it in the fridge doesn't remedy it...

        1. re: RosemaryHoney

          it's not the recipe -- it's the ingredient. it was moldy when you bought it, but not visible to the naked eye.

          why would refrigeration kill mold?

      2. Hmmmm… Doesn't seem like something could go bad and generate anything really nasty overnight. Are you smelling anything troublesome?

        I make steel-cut oatmeal all the time and keep it in the fridge for 5-7 days. There's a natural thick colloidal liquid that separates out. But it's not green. That I just pour off or stir back in.

        Is there something else in your oats like raisins or apple that could add a color to that normal liquid?

        If overnight isn't working for you you could try the method I use. I stir my oats in some hot oil (I like walnut oil) risotto style. Then I pour on half of the liquid as hot water and let it come to a boil. I reduce the heat to a simmer and add a pinch of salt and one quarter of the liquid as buttermilk followed by the last quarter of the liquid as hot water (that just gets all the flavor of the buttermilk in the pot). I stir that for a few minutes then put on a lid, turn off the heat and go take a shower. When I'm ready for breakfast the oatmeal is ready. It just needs to be stirred and served.

        I also put chopped nuts and raisins in mine and have it topped with oven roasted applesauce. Yum!

        I do 1 cup of s-c oats and 4 cups of liquid at a time and refrigerate what I don't eat to microwave through the week.

        I hope that might help.

        1. I do a slightly different overnight oats, using regular oats or the quick cooking steel cut- mix one serving oats with liquid the night before in a jar or microwave safe dish with a lid, refridgerate overnight. Add a bit more liquid and reheat either stovetop or microwave for hot oats.
          Since the oatmeal is only heated just before eating there should not be this problem-i make these very often and have never had an issue

          1. Not an expert but it was so intriguing that I looked into it and this is what I found (bottom line- it's probably just fine).

            I am guessing that since you used different brands and batches of oats that the problem is your water. Alkaline water or water with some metal ions in it (well water especially) apparently react with oats to make a green to bluish foam. It is allegedly safe to eat. I suspect it's like blue garlic (when garlic turns blue during cooking). A little disconcerting but not harmful.

            Try making the oats with bottled water or leave your water out overnight before cooking with it and see if that fixes it. If it's not the water, it may be your pan. Are you using a new pan that is leaching metal ions into your food? That could cause the same reaction.


            More technical details:
            Doehlert DC, Simsek S, Wise ML. 2009. The green oat story: possible mechanisms of green color formation in oat products during cooking. J Food Sci. 74(6):S226-31

            Hope this helps! What a fascinating phenomenon.

            5 Replies
            1. re: greymalkin

              Thank you so much for that response. I think that's a more likely possibility, since we have special water filters (for home brewing) and we've used several (high-end) brands and batches of oats, so having multiple varieties come home from the store moldy would be scary! I'll try using different water and report back.

              1. re: greymalkin

                Thank you for posting this! I too have had an incident with green slime in overnight oats. To be honest, when it happened to me, I scooped the green stuff off the top and ate the rest, and I fed the oatmeal to my kids too. It tasted normal and nobody got sick. But I felt funny about it, and since then I rarely make overnight oatmeal anymore, which basically means we no longer eat much oatmeal. Glad to know I didn't actually feed my kids a pot of moldy oats!

                1. re: Westminstress

                  Success!! I made the same overnight steel-cut oatmeal recipe, using filtered water and putting it in a glass bowl (rather than the metal pot I used to use) and they were perfect. No green at all! And this was using the same canister of oats as the photo shown above, so it seems mold wasn't the cause. Thanks for all the help! Mystery solved.

                  1. re: RosemaryHoney

                    Hooray! I'm so glad to hear that you were able to stop the green slime. Yay for tasty warm oats in your future!

                    1. re: RosemaryHoney

                      Such a helpful, validating reply. I make steel cut oats in a crock pot about 8 cups at a time and then portion the batch out. Using filtered water helped me too.

                2. I microwaved 3 cups of steelcut Honeyville bulk oats in a glass bowl and got mint green foam. No overnight, no metal pot but it created a pretty minty green color. Same batch of oats previously had no problem, although we use the bowl for cooking and some previous recipe might have left a small residue in the bowl which changed its Ph this time.