I made a soup recipe the other night (mainly potato, spinach, and veggie broth) that called for buttermilk. I substituted soymilk and lemon juice, reading online that that was a good substitution for buttermilk.

Now, about 48h of refrigeration later, the leftovers are bitter and a little bit bubbly - it's as if they are 'rising' like a baked good would (I imagine my buttermilk substitute was intended for baking).

What can I put in to stop this chemistry and, for bonus points, nullify the bitterness? It's to the point where it's physically difficult to eat - I'm having trouble keeping the few bites I forced myself through down - but I abhor the idea of wasting food.

(I tagged this baking thinking that people who frequent that tag may have a better than average understanding of the chemistry behind this)

  • That sounds like an acid-base reaction, between the lemon juice and some carbonate-containing base... but I'm at a loss for why it would occur with soymilk. To google!
    – BobMcGee
    Jun 25, 2012 at 22:49
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    Sounds more like curdling to me. I don't know about bitterness, but soy milk will coagulate in anything even mildly acidic, like coffee.
    – Aaronut
    Jun 25, 2012 at 23:37
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    @Aaronut: I can believe that, but how do you account for the foaming?
    – BobMcGee
    Jun 25, 2012 at 23:44
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    @aaronut- FYI. soy milk actually coagulates more readily in the presence of calcium or magnesium ions. Coffee has a fair amount of magnesium in it that I would blame for the coagulation before the acidity. Soy proteins coagulate at a pH of 4.5. Jun 26, 2012 at 4:10
  • @BobMcGee - any thoughts on the foaming? Jun 26, 2012 at 14:58

1 Answer 1


Soy milk is bitter. Enzymes in the beans (lipoxygenase) combine with fats in the presence of water to produce what is usually described as a "beany flavor"; bitter and grassy.

The solution to this problem, although not done in many traditional soy milk preparations, is to cook the soy milk long enough to destroy the enzyme. Many, but not all, soy milk manufacturers will cook the milk as well as add a lot of flavorings to mask the bean flavor.

When enough lemon juice is added to soy milk it will coagulate but in my experience it does not produce flavors more bitter than the milk itself. I have, however, found that, regardless of coagulant, soy milk declines in quality very quickly and, even when I thoroughly boil it to deactivate enzymes and "beany" compounds, it will go from sweet to bitter in just a few days. I have also had homemade soy milk start to turn after a relatively short amount of time developing rancid flavors or activity that implied fermentation such as independently souring or clabbering.

I don't have any proof but based on my own experience I suspect that the lemon is not the culprit. If your milk has become so bitter as to be inedible then I think your soy milk may have spoiled.

As for the foaming- in your comment below you mentioned that the soymilk in question was Silk. Silk is one of the most modified soymilks- they definitely err on the side of flavor rather than simplicity and have a lot of additives for flavor and texture.

One of the additives is Calcium Carbonate. Calcium Carbonate gently coagulates soymilk and is sometimes used for making silken tofu. In Silk it is no doubt used to make the product more creamy.

The existence of Calcium Carbonate in your milk could explain your foaming as it can form CO2 in the presence of acids.


  • Soy milk does not coagulate well with lemon juice. It requires a lot of acid to coagulate and so is overly sour and it abruptly produces a very fragile curd. I agree that the advice that you read must have been for baking. If you can't use real buttermilk then in such a dish I would recommend silken tofu for the creaminess and don't worry about the acidity.

  • In general the way to temper bitterness is to add salt.

  • As a rule of thumb food that is bitter is often poisonous. I don't imply that that is the case here but you should trust your tongue better and not feel forced to eat questionable food.

  • The soymilk was Silk, which I imagine is fairly well cooked/preserved given that I can't pronounce some of the ingredients BUT this does explain why a couple other things that I've thrown soymilk into in the past have gone bitter quickly as well, regardless of whether or not there was lemon present. Do you have thoughts on other non-dairy milks one could use instead, that would hold up better a few days into being a leftover? Also, I appreciate you mentioning that it could be poisonous - I may have forced myself to stomach it, otherwise. Thanks for the thorough answer! Jun 26, 2012 at 14:27
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    @humanstory- to be clear, soymilk goes bitter easily enough that I don't think the bitterness is an indicator of poisonousness. My advice was to in general trust your tongue. As for a buttermilk substitute, again, try adding silken tofu to the soup when it is served just for the creaminess. Don't add the tofu to cook with the soup because it could accidentally further coagulate in the high heat. Jun 26, 2012 at 14:52
  • Hmm, I think that perhaps it was an order of bitter beyond simply bitter soymilk? Generally I can close my eyes and force things down, but the few bites I managed of this kept me feeling a little queasy for almost an hour. Perhaps @bobmcgee above was right that there was more to it than the soymilk. Jun 26, 2012 at 14:57
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    All of the chemistry here makes perfect sense, and the logic here seems sound. Spoilage (or enzyme reactions) give rise to the extra bitterness, possibly in combination with additives. The calcium carbonate + acid gives the foaming, as I suspected. Excellent job analyzing and researching this one, @Sobachatina!
    – BobMcGee
    Jun 26, 2012 at 15:43

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