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Even slightly spoiled milk has a taste that I don't like, while plain yogurt tastes either great, or at least nowhere near as bad. A dozen sources (all of them) say that the spoiled odor and taste of spoiled milk and also of yogurt is caused by the presence of lactic acid. Does spoiled milk have something else in it besides lactic acid that gives it the taste that I dislike that yogurt (which has plenty of lactic acid it it) almost never has. Or is there more than one kind of lactic acid?

Is the taste of spoiled milk due to lactic acid and one or more amino acids?

https://foodsafety.foodscience.cornell.edu/sites/foodsafety.foodscience.cornell.edu/files/shared/documents/CU-DFScience-Notes-Milk-Flavor-Defects-Sensory-Eval-04-10.pdf has this to say:

"CHARACTERISTICS OF MILK OFF-FLAVORS... Typical Milk: No criticism. Very little distinct odor, pleasantly sweet and clean with no aftertaste... Acid: Basic taste sensation. Sour, tart, may cause tingling sensation on tongue. “Cultured milk” or “sour” odor may be present. Cause: Growth of lactic acid producing organisms such as Lactococcus lactis, due to poor refrigeration, especially when temperatures exceed 70°F (21°C)."

Like everyone, the Cornell author explains the spoiled taste and odor of spoiled milk by referencing only lactic acid.

https://www.nationalgeographic.com/culture/article/when-good-milk-goes-bad is no different when talking about slightly spoiled milk, as opposed to the much later very spoiled moldy milk, which involves yeasts and molds and ...danger.

https://www.reddit.com/r/chemistry/comments/j15maj/what_does_lactic_acid_smell_like/ says pure lactic acid does not smell bad. Aha! On the other hand, it does say whether or not it smells like spoiled milk. What a pity.

I found out recently (with a lot of help from the folks at Seasoned Advice -thanks guys) that what I used to call 'sour milk' a lot of people call 'spoiled milk', and what I call 'fermented milk products' a lot of people call 'soured/sour milk products'. I think this has been a cause of quite a bit of talking at cross purposes involving me, both here and at Seasoned Advice. I have edited the question accordingly.

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Probably. Lactic acid bacteria produce other volatile compounds such as acetaldehyde and diacetyl that could be perceived as off-flavors. Both where fermentation is the objective and in the case of spoilage, the VOC profile can vary with the identity of the responsible bacteria but also of the milk and its treatment.

For instance, in Ref. 1 they discuss the effect of different lactic acid bacteria:

During fermentation, S. thermophilus and L. delbrueckii ssp. bulgaricus generate lactic acid and a variety of volatile organic aroma compounds. These compounds impart particular flavors to fermented milk (Beshkova et al., 1998). The overall flavor of any fermented milk product is formed by a large number of these volatile compounds. Among these compounds, carbonyl compounds and organic acids such as acetaldehyde, diacetyl, and acetic acid play a crucial role and can be used to evaluate the flavor quality of dairy products (Zha et al., 2015). Ott et al. (1997) found that acetaldehyde provided the typical flavor of dairy products such as yogurt and buttermilk. Erkus et al. (2013) also reported that diacetyl contributed the characteristic sour buttery flavor to some fermented dairy products.

The offending chemical could also be vinegar (acetic acid). Again from Ref. 1:

Acetic acid has been found previously as a volatile in yogurt by Panagiotidis et al. (2001). High levels of acetic acid contributes most to the tart flavor of yogurt and is particularly associated with a “vinegary, pungent, acidic” taste (Panagiotidis et al., 2001).

If you search for "chemical profile of spoiled milk gc" you'll find a wealth of references discussing the effect of different milk (due to species, handling or forage), milk treatment, and spoilage bacteria on the VOC profile.

In addition, what makes the milk taste bad may depend on the bacteria and on you. That makes narrowing the answer down to a particular compound impossible.

References

  1. T. Dan, D. Wang, R.L. Jin, H.P. Zhang, T.T. Zhou, T.S. Sun, Characterization of volatile compounds in fermented milk using solid-phase microextraction methods coupled with gas chromatography-mass spectrometry, Journal of Dairy Science, Volume 100, Issue 4, 2017, Pages 2488-2500, ISSN 0022-0302, https://doi.org/10.3168/jds.2016-11528.
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    $\begingroup$ Diacetyl is an interesting compound. In grad school, one fellow in our research group needed to use it and he could not smell it at all. Me and the 3 other grad students were stunk out by the overwhelming movie theater popcorn smell. Unforgettable. $\endgroup$
    – Ed V
    Apr 21, 2021 at 20:28
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    $\begingroup$ I think Table 1 from this paper is also useful $\endgroup$
    – Curt F.
    Apr 21, 2021 at 20:47
  • $\begingroup$ Maybe my experience with spoiled milk is different than most people. In my experience, it definitely smells different than vinegar, lactic acid, or diacetyl. Definitely seems more like short-chain fatty acids that are responsible for the sour rot smell. $\endgroup$
    – Curt F.
    Apr 21, 2021 at 20:49
  • $\begingroup$ @CurtF. Table 1 from that paper marsiliconsulting.com/pdfs/j_ag_food_chem_2000_48-8.pdf and the whole paper in fact is great. I learned so much about milk and how it spoils. Everyone should read it. On the other hand, it doesn't explain why sour milk seems to me to always taste exactly the same, as if the sour milk taste was caused by a single chemical or single type of bacterium. $\endgroup$
    – Matthew Christopher Bartsh
    Apr 21, 2021 at 21:40
  • $\begingroup$ @CurtF. I also smell a rancid odour, like when fat are involved. Must be some kind of aldehyde. Can the acids be freed? They else smell not that good... $\endgroup$
    – Alchimista
    Apr 22, 2021 at 7:59
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Raw milk compared to pasteurized milk when spoiled I think would tell us a lot. Raw milk sours slowly from the day it exits the cow, the lipids separating, with a sour yoghurty kind of smell that gets stronger and stronger until it gels into a sour yoghurt. Pasteurized smells like nothing at all for a week or so and and then suddenly smells like something created by an antichrist from another planet. I mean it screams "What kind of a maniac would even touch this let alone try to drink it" Whatever the processes happening, I would imagine that they are very different bacterial/fungal and chemical processes.

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