Formaldehyde in milk? (historic)

[neutron_star]

From Upton Sinclair’s The Jungle, the 1906 exposé of the meat-packing industry in Chicago, outcry over which led Congress to pass the Pure Food and Drug Act later that same year:

@Upton Sinclair

There were many dangers, in which the odds were all against them. The children were not as well as they had been at home; but how could they know that there was no sewer to their house, and that the drainage of fifteen years was in a cesspool underneath it? How could they know that the pale-blue milk that they bought around the corner was watered, and doctored with formaldehyde, besides?

An unscrupulous shopkeeper watering down milk I can understand, but what’s up with the formaldehyde?

[Terminus_Est]

Probably a preservative.