The kitchen is the very core of any household. Its function has been rigidly defined in legal texts of great antiquity as: Camera Necessaria pro usus cookare; cum saucepannis, stewpannis, dressero, et stovis inter alia, et omnia pro roastandum, boilandum, fryandum et plum-pudding-mixandum. In such a power-house, naturally, chaos ensues if anything goes wrong - for example, if the milk turns sour.
Until a few decades ago milk was unpasteurised, and therefore prone to sourness anyway. Moreover, the domestic refrigerator was a luxury available only to the few, so many clever ploys were tried to keep the liquid drinkable.
One such was to place the container of milk in a shallow basin of water, into which a muslin cloth covering the jug was allowed to drape.
By this arrangement the water used the muslin like a wick, and its evaporation from the cloth resulted in a drop in temperature. But this method of keeping the milk fresh works only when the air is relatively dry; when the air is moist there is little evaporation, and consequently only a slight drop in temperature and perhaps another consignment of sour milk the following morning.
Another troublesome situation arose when there was thunder in the air. It was observed that milk was more likely to go sour in thundery conditions, and it was widely assumed that this was something to do with electricity in the atmosphere.
This notion, however, was disproved as long ago as 1913 by two scientists called Duffield and Murray. In a series of experiments they drew air through a tube in which were fixed two live electrodes; a high voltage was applied, and the air "electrified" by the resulting discharge was allowed to "bubble" through a flask of milk.
They found that atmospheric electricity, far from turning the milk sour, tended, if anything, to act as a preservative.
The association of thunderstorms with sour milk is linked to the production of lactic acid, in turn caused by the action of bacteria called, appropriately enough, bacilli acidi lactici. These bacteria are comparatively inactive at temperatures below 7C, but their multiplication becomes increasingly rapid at progressively higher temperatures - up to somewhere in the region of 38C.
The tendency to sourness, therefore, is caused not by thunder or lightning, but by the coincidence of the warm humid conditions which usually precede a summer thunderstorm. Rather than electricity in the air, it is the favourable temperature and a high humidity which prevents the muslin from being an effective remedy and turns the milk sour. And this also explains why the problem does not exist in the case of winter thunderstorms.