A change over the year is that butcher shops and supermarkets once ground their own hamburger meat, so it was freshly ground with no additives. Most hamburger meat is now ground and packaged at the slaughter house, with a variety of additives. Pink slime as 4meAndyou mentioned, preservatives, and who knows what else.
Also .... I grew up in the grocery business and my dad's store had a butcher shop. Referencing the 1950s, I don't recall fresh meat or poultry products ever being frozen, either to facilitate delivery or in-store. Carcasses were delivered from the packing houses to the store in refrigerated trucks, wrapped in cheese-cloth and the sides or quarters were stored in walk-in refrigerators and further butchered as needed. Without preservatives or sealed shrink-wrap, once butchered it only had a day or two shelf life before turning blackish.
