After a thunderstorm you open your refrigerator to get a glass of cold milk and notice that it has gone sour. What happened? 

Lactobacilli love thunderstorms

It is an old country saying that thunderstorms make milk go sour. That was true in the old days, when there were no refrigerators. Milk could not be kept fresh over long periods of time and on a hot and humid day, lactobacilli would reproduce at a fairly high rate. That caused the milk to go sour quite fast.

Tarts may also go sour

But even today this can happen in a closed refrigerator. According to analytical chemists this is, however, pure coincidence. Nevertheless, housewives often call up institutions for chemical analyses and report problems with sour milk. Bakers also report frequently that their tarts went sour during thunderstorms.

Are electromagnetic impulses responsible?

Weather conditions such as atmospheric pressure, temperature or humidity are not believed to play a significant role in this effect since they are very changeable even without storms. A member of the Max-Planck-Institute for biochemistry and lay researchers is convinced that the reason for milk going sour during storms can be found in so-called "sferics".

More research required

Sferics are short electromagnetic impulses that can be measured at distances of up to 500 kilometres. How sferics affect organisms, such as lactobacilli, cannot yet be scientifically explained. Experts believe that sferics may also be the reason that many individuals are so sensitive to changes in the weather. No matter what the explanation may be, the old country rule still seems to apply today. Milk sometimes goes sour during thunderstorms, in the 21st century just like 300 years ago.