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"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.