When I was 13, my sister and I spent the summer on a dairy farm. We worked in the creamery, where (twice a day) we bottled fresh raw milk, as well as cleaned all the equipment. It was really a fun summer overall.

One evening we had a very bad thunderstorm. It knocked out the power and curdled all the milk in the creamery. My uncle was visibly distressed, but my aunt made it memorable by serving us all (by candlelight) fresh milk curds and brown sugar (something I was reluctant to try but ended up really enjoying). She matter-of-factly explained that sometimes "electrical storms" did that, and it was just an accepted risk. We had all the curds and brown sugar or maple syrup we wanted for a few days, and a lot was given away as well.

The next morning we had to call all the customers to tell them there would be no milk that day. It was the only time that happened, though there were other storms.

I'm a wife, and I'm not young, but this is not an old wives' tale. It wasn't a matter of spoiling. Our milk was all fresh. Milk that came in from the barn was filtered, rapidly cooled in a special apparatus to (very cold - I'm sure I once knew the temperature but don't recall any more), bottled immediately while cold (the bottles were room temperature), and refrigerated. After we met the demand for the next day's milk with a couple of dozen to spare, excess milk was fed to the calves. Any bottle of milk older than 24 hours was marked, and if not sold within one more day, was tossed. But that evening, every bottle of milk curdled.

I just accepted all these years that thunderstorms could curdle milk, until I ran across it today, where it was labeled an old wives' tale. The only support I could find (besides old newspaper articles discussing it) was a blurb on the Naked Scientists website stating

Electrostatic fields within a certain range can break up emulsions by polarising droplets and causing them to coalesce head to tail. During the build-up to a lightning discharge, the field strength will presumably pass through this range and may cause exposed milk to separate into its aqueous and fatty components. Milk in a metal container would be shielded from the field and remain emulsified.

Our refrigerators had metal cases.

I like yoghurt and sour cream, I've had milk animals and made yoghurt and cheese, but I can't tolerate even slightly sour milk. None of my milk ever curdled, and we have plenty of storms where I live (we have at least a dozen storm-related power outages a year, some lasting days). What I had back then wasn't spoiled milk, it was separated right in the bottle into fresh curds and whey (like when making cheese).

I've googled this and can't find a decent paper, so I'm posting here. How can thunderstorms curdle milk?

EDIT – Reference for notability: "Thunderstorm Curdled Milk Puzzle Still Baffles Science", The Telegraph (June 2, 1937)

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    might have more to do with the power outage than the lightning – ratchet freak Jan 24 '15 at 21:47
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    @ratchetfreak - that's my point. Milk doesn't go from fresh to curdled in a few hours. If it was due to a power outage, in a refrigerator packed with milk bottles at 36°, say, it would take a significant number of hours for the temperature to climb, and a significant number of hours for the milk to show hints of turning. Of course, this is the easy answer, but it's not the right one given the facts. Given, the milk was raw (unpasteurized), which would turn sooner, but not in a matter of a few hours. – medica Jan 24 '15 at 21:59
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    @DennisWilliamson - It's been addressed in the media before. I found it online. Does that not meet the site's requirements? (Off to read the help site...) – medica Jan 29 '15 at 0:41
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    Personally, I think the notability is met with the personal observation and reference. It wouldn't be unfair if moderators cleaned up the comments (provided it's clear notability is no longer an issue). Fascinating experiences, @medica - thanks for posting. – Brian M. Hunt Apr 23 '15 at 2:06
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    @Oddthinking - I don't know! I would need to be convinced that I imagined what I experienced, I guess. But that's a hard one; those curds with brown sugar were mighty good. – medica Apr 23 '15 at 4:54
up vote 9 down vote accepted

A correlation between thunderstorms and the souring or curdling of milk has been observed for hundreds of years. According to the 1685 book The Paradoxal Discourses of F. M. Van Helmont: Concerning the Macrocosm and Microcosm, Or the Greater and Lesser World, and Their Union:

Now that the Thunder hath its peculiar working, may be partly perceived from hence, that at the time when it thunders, Beer, Milk, &c. turn fower in the Cellars ... the Thunder doth everywhere introduce corruption and putrefaction

By the 1800s the mainstream explanation was that there is no causal effect of thunderstorms on the souring of milk. Exemplary of the mainstream scientific explanation is the 12 June 1858 article "Lightning and Milk" Scientific American volume 13, issue 40.

although it has long been known to us, and almost every other person we suppose, that milk is liable to become sour during thunder storms. We attribute this influence to the state of the atmosphere, not the thunder concussions, as it is well known that the weather is generally sultry or hot just prior to a thunder storm, and this warm condition of the air is very favorable to the development of lactic acid in the milk.

Nonetheless, there were scientific investigations in the late 1800s and early 1900s concerning alternative theories of a causal relationship, all the while the mainstream explanation being that there is only a correlation.

One alternative explanation was that ozone formed by lighting directly, or by enhancing the growth rate of bacteria, caused increased souring. See for example.

"The Souring of Milk During Thunder-Storms" Science Vol. XVIII, No. 425, pages 178-179, from 1891.

Another alternative explanation put forward in "Thunder and Sour Milk" British Journal of Medicine vol. 2 page 651, from 1890, is:

viewed in relation to the experiments of Professor Lodge, confirmed by myself, on the precipitation of smoke by the static spark, and stated in my book on Static Electricity in Medicine, it is a fair inference that atmospheric dust, together with organic germs, are thus precipitated during a thunderstorm, the precipitation of the latter causing the rapid souring of milk.

(It is now well known that bacteria are present in clouds and fall during storms. See the 2013 NPR story Bird, Plane, Bacteria? Microbes Thrive In Storm Clouds and Surprising Find: Live Bacteria Help Create Rain, Snow & Hail.)

"Influence of Thunderstorms on Milk" The Creamery and Milk Plant Monthly, Volume 11, page 40 from 1922, has data from days with and without thunderstorms, and shows much (factor of 10) greater bacterial levels in milk on days with thunderstorms, relative to other summer days.

It speculates that the falling air pressure associated with thunderstorms causes an increase in bacteria in the cow's udder, and emphasizes that temperature was constant. This article also discusses the influence of ozone, produced by lightning, on the growth rate of bacteria.

None of these alternative explanations gained acceptance. For example According to a University of Nebraska & US Department of Agriculture publication "Milk in Your Meals", (1946)

Another fallacy is that thunderstorms cause milk to sour. In reality the souring was probably due to the warm weather that is associated with thunderstorms.

After pastuerization and refrigeration became widespread, eliminating bacteria growth, interest in this issue almost disappeared.

Still, the prestigious medical journal The Lancet, 18 August 1951, page 278 discusses "The old observation that milk clots more readily in a thunderstorm" and says the work of Trillat is "worth repeating with the more exact methods available today". Trillat had investigated the possibility that not just temperature, but also atmospheric pressure of a storm, can influence the growth rates of bacteria in milk. (See English summary of Trillat's research here) Original French paper is "Étude sur les causes du caillage du lait observé pendant les périodes orageuses" in COMPTES RENDUS DES SÉANCES DE L'ACADÉMIE DES SCIENCES 26 February 1912.

  • Your "The Fermentations of Milk" link doesn't work, and you might want to mention that "Thunder and Sour Milk" dates from 1890, "The Souring of Milk During Thunder-Storms" dates from 1891, and "Influence of Thunderstorms on Milk" dates from 1922. – Mark Apr 25 '15 at 0:49
  • @Mark ok, I fixed the link. That one is from 1889. Yes, it seems that prior to widespread pasteurization there was much more interest in the phenomenon. – DavePhD Apr 25 '15 at 2:18
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    So, the only reference you have less than 90-odd years old is 70 years old, and it says it isn't true? Perhaps the reason why there isn't interest in the phenomena in the past 80 years is because a better understanding of microorganisms and electricity reveals that there is no causation? – Oddthinking Apr 25 '15 at 6:25
  • @Oddthinking none of the references say electricity is the direct cause. One says pressure of the storm influences bacteria growth in the udder. One says ozone increases growth rate of the bacteria. One says lightning causes precipitation of particles in the air including bacteria. "Most explanations are that it is only a correlation, not a causal relationship" as my answer says. – DavePhD Apr 25 '15 at 11:07
  • @Oddthinking In other words, it is true that (at least in the time period when this was studied) there was a correlation between souring and thunderstorms. The mainstream explanation is that it is just a correlation: hot weather increases bacterial growth and souring. The alternative explanations also involve enhanced bacteria growth. – DavePhD Apr 25 '15 at 11:28

This answer is currently a li'l sketchy.

The same forum thread from August 2009 referred to in the OP contains a post that includes a copy of the following article from July 17, 2009:

"Didn't you know, milk goes sour when there are thunderstorms around?" I was surprised to hear this old canard during the recent spell of thundery weather. I can't remember having heard it for ten or twenty years, which illustrates how changing lifestyles have seen off some old myths. In this case we no longer associate sour milk with thunder because most of us have fridges. Let me explain why.

It was a deeply-held notion until at least the 1960s, notwithstanding a widely publicised laboratory experiment which had been conducted way back in 1913 by Professor D.G. Duffield and Mr J.A. Murray. Maintaining an ambient temperature of 20°C, two flasks of milk were connected to a discharge tube through which an electrical current was passed for an hour, simulating repeated lightning discharges, and the air thus affected was bubbled through the milk containers. The artificial lightning was repeated at regular intervals thereafter, for up to nine hours after the beginning of the test. Several slightly differing tests were carried out, with variations in the intensity and frequency of the discharge, and variations in the speed of air passing through the milk. A 'control' flask of milk was kept at an identical temperature in order to provide a sample of the fluid not affected by the electrical discharges.

The results showed that, in all the test variations, the level of acidity - the sourness - rose less quickly in the 'electrified' milk samples compared with the 'control' sample. In other words milk actually sours less quickly in thundery weather, according to the Duffield and Murray experiment.

Milk goes sour due to bacteria - bacilli acidi lactici - which produce lactic acid. These bacteria are fairly inactive at low temperatures, which is why we keep milk in the fridge and it remains drinkable for several days even after opening. Once the temperature climbs above 7°C, however, the bacteria multiply with increasing rapidity until at 50°C conditions become too hot for them to survive. The myths concerning souring in thundery weather probably arose because most thundery activity in Britain occurs during hot and humid summer weather with only a limited cooling off at night. During such episodes the temperature increases sharply inside houses, even in the coolest rooms such as pantries and larders, to levels probably not matched at any other time of the year. Thus in pre-refrigerator days the milk would go off within 12 hours of it being opened.

It should also be borne in mind that during the earlier decades of the twentieth century it could take three or four days for milk to get from the farm to the urban doorstep, adding to the problem. The introduction of pasteurisation in the early 1900s added 12 to 24 hours to the life of milk and resulted in a marked drop in infant mortality rates in the hot summer of 1906 compared with the heatwaves of 1900 and 1901.

The byline for the copy-pasted article credits a Philip Eden who is probably a British weather journalist/historian of some note who has written articles for a number of websites. But I have been unable to locate the above article anywhere else. It does not appear to be available any more.

However, the paper used by Eden as a basis for his article does exist. In 1914, W. G. Duffield and J. A. Murray did publish a paper titled Milk and Electrical Discharges in the journal of the London Röntgen Society. The paper is however stuck behind a paywall with only the following preview available:

The experiments to be described originated from a curiosity to test the popular belief that the electrical conditions during a thunderstorm are responsible for the rapid souring of milk.

Our method was to pass air containing the products of an electric spark discharge through milk, and to observe the effect upon the rate at which the acidity increased; the …

A Google Books copy is only available in snippet view. But there appears to be sufficient circumstantial evidence to suggest that this 101-year-old paper cited in the Eden's article is legitimate. Extending this confidence to the rest of the article, (electrical discharges during) thunderstorms do not promote curdling in milk. If anything, they retard it.

In 1968 NASA, through the assistance of the Israel Program for Scientific Translation, translated from Russian the 1964 book "The Earth in the Universe", with chapters written by top Soviet scientists.

In a chapter written by Prof. A. L. Chizhevskii, who was honorary president of the first International Congress of Biophysics and Biocosmics held in New York, wrote (quoting pages 295-296):

It is known that a minimum amount of electro- magnetic energy may markedly lower the stability of the dispersed phase of certain colloidal fluids. Experiments have shown that radio waves from telegraphic transmitters caused relatively rapid precipitation of the solid phase of certain colloids, yet the field intensity is only of the order of 0.001 to 0.00001 of the intensity of the so-called electrical interference. When placed in sheet-iron chambers the same colloids were not affected by radio transmitters (Wilke and Miiller). It has been known for a long time that milk curdles considerably faster on days with marked atmospheric- electrical disturbances (thunderstorms) than on other days. It was experimentally proved that the curdling is quite independent of bacterial processes. Evidently, under the effect of the factors mentioned above, syneresis and disruption of the protein- colloid system occurs in the milk. Experimental coagulation of milk by treatment with short waves was reported, thus excluding from the process any thermal phenomena (Kerber, Goetinck). Similar observations were made on various gels and emulsions in which the suspended phases precipitated during thunder storms (Wedekind and others).

The statement in the OP attributed to the nakedscientists.com really originates from the 15 February 1997 issue of New Scientist:

Sour cream

Q: One evening in the summer of 1994 I retired to bed with a glass of milk. During the night there was a tremendous thunderstorm with plenty of lighting and the following morning the remainder of the milk had curdled into a solid mass.

My elderly relatives who remembered pre-refrigerator days held it as common knowledge never to leave milk out in a thunderstorm. I had never heard of this. What process had taken place?

A: Electrostatic fields within a certain range can break up emulsions by polarising droplets and causing them to coalesce head to tail. During the build-up to a lightning discharge, the field strength will presumably pass through this range and may cause exposed milk to separate into its aqueous and fatty components. Milk in a metal container would be shielded from the field and remain emulsified.

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    When reading this, be careful not to transfer NASA's imprimatur onto the translation. The work is not endorsed by them, or the Israel Program for Scientific Translation. Note: There doesn't appear to be any attempt to test Chizhevskii's speculations in actual thunderstorms. – Oddthinking Apr 26 '15 at 4:27
  • @Oddthinking The US statement is "Published Pursuant to an Agreement with THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION and THE NATIONAL SCIENCE FOUNDATION, WASHINGTON, D. C.", but I agree it should be considered as the work of Chizhevskii alone. I haven't seen any references by someone as reputable as Chizhevskii, reputable enough to have an article: "In memoriam Alexander Leonidovich Tchijevsky" in International Journal of Biometeorology Volume 9, Issue 1 , p 99. – DavePhD Apr 26 '15 at 12:05
  • Thank you for this answer! Maybe I'm not crazy after all! :) – medica Apr 26 '15 at 17:40

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