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Every year, my family makes peanut brittle using an old family recipe (example recipe, slightly different from ours in quantities). A large portion of this recipe is Karo brand light corn syrup. The brittle is very popular with friends and family because of its foamy, crunchy texture. We achieve this texture by adding a bunch of baking soda at peak temperature, watching the brittle foam up, and dumping it on a tray to cool while it's still really foamy.

This year, we've made two batches, one using a bottle of corn syrup left over from last year, and one using a new bottle we just bought. And there's a difference. The one with the old bottle was properly foamy and held its air bubbles while hardening. The one from the new bottle foamed up, and then went flat before hardening, losing all its air and spreading out more than it usually does.

We'd assume that we made a mistake in our cooking process, except that my mother-in-law, using the exact same family recipe, had the exact same experience. Last year's syrup worked; this year's didn't.

This makes me conclude that something changed about Karo Light Corn Syrup. Maybe the moisture content, maybe the sugar chemistry, maybe something else. There's no published news, and Karo has not responded to our requests for information.

Does anyone know what might have changed about the syrup this year, preferably with citations?

If there are no citations, does anyone have a method by which we could determine how the syrup changed that doesn't require a full chemistry lab?

To forstall questions: no, we don't have both bottles. Also, we cook the sugar+corn syrup to dark amber.

Answering one more question: it was the exact same box of baking soda. And we're talking about batches less than a week apart. Further, my mother-in-law has had the same experience, so we've reproduced the change in behavior in two different kitchens. Absolutely every other variable is accounted for other than the change in corn syrup years.

Update: one more A/B test: we found a bottle of 3-year-old Karo, did a batch of peanut brittle, and it turned out fine.

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    Do you still have the bottles from the 2 different years? That may hold a clue
    – GdD
    Dec 13, 2023 at 21:16
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    if you have the new bottle, look for images of the old bottle online (dates are usually discernable in image searches) and compare the nutrition facts. As you likely know, the ingredients could somewhat change in proportion without changing the label so long as they are sorted greatest>least, but if the calories go down/up, there's your sign.
    – dandavis
    Dec 13, 2023 at 22:52
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    Not a very helpful ingredients list. If you try again, check the temp on the solution when you add the bicarb, it might be critical. pH might affect it too; low pH should enhance the reaction. Might be worth checking pH on the old bottle vs new.
    – bob1
    Dec 14, 2023 at 2:43
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    I use light karo to make sesame cookies at Christmas time. They are usually crisp and brittle (like a very thin peanut brittle would be). My cookies were all soft and gummy. I agree that karo has changed something.
    – Catherine
    Dec 31, 2023 at 13:37
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    Out of curiosity, are you already aware of the dextrose equivalent/manufacturing processes and the differences they can lead to? I got lucky in that I still had a stash of last year's syrup, but I wonder if the new batch is higher in maltose, but not enough to warrant a change in category? In my experience rice syrup (which is high maltose) will lead to a hard, flat, shiny brittle as well. That's just a thought, but might be a lead?
    – kitukwfyer
    Jan 13 at 3:17

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I cannot possibly see how Karo could have changed its formulation in a way which caused you this problem.

Corn syrup is a pretty straightforward substance to manufacture and there’s no significant decisions to be made. I could see a world in which Karo decided to water down its corn syrup a little to save money, but that wouldn’t matter for your recipe because the extra water would’ve boiled off by the time you were done.

Instead, I would wonder about your baking powder. There’s a number of formulations; old baking powder can get “used up”; overall it is way more important to the process than corn syrup, which is basically just there to mess with crystallisation.

I know you had a natural A/B test with the old and new bottle, and I can’t explain your experience there. But I simply cannot see a reasonable way to blame the difference on corn syrup.

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    The only way I can see a problem is during the caramelization of the sugars in the corn syrup, which might come down to exact proportions of glucose, starch and fructose in the syrup. I have to say the baking powder was what sprung to mind for me too. I wonder if both families live in a climate that has been particularly damp this year?
    – bob1
    Dec 14, 2023 at 2:38
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    Blaming the baking powder (soda, actually, per the question) ignores that (presumably the same baking powder/soda, unless there was a new can/box of that for the second bottle of syrup) it worked for the "old" syrup and not for the new syrup.
    – Ecnerwal
    Dec 15, 2023 at 2:23
  • @Ecnerwal yes, I’m aware. See my note about the “natural A/B test”.
    – Sneftel
    Dec 15, 2023 at 7:49
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    What @bob1 says. Notably, the exact proportions of glucose, fructose, sucrose, and maltose in Karo is a trade secret, and one they've gone to court to protect. There's no baking powder, just soda.
    – FuzzyChef
    Dec 15, 2023 at 19:13

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