My wife bought some beef mince (ground beef) from a shop. It looked red.
When she fried it, it give off an aroma of vinegar. Is it spoiled?
The smell indicates a safety problem. There is no reason to add vinegar to meat (unless you bought already marinated meat, but 1) marinating mince is so unusual, the chances of finding premarinated mince are against zero, and 2) this would have been declared on the label.
The likely source for the smell is bacterial fermentation. Many bacteria produce acetic acid as a waste product (this is how vinegars gets to smell like vinegar). While the vinegar producing bacteria usually colonize other types of food, and meat tends to support bacteria producing other odors (the ones known as rotting meat), strange things can happen, and this smell is a big red flag. Especially with mince meat, which supports much more bacterial growth than a slab of whole meat (bacteria live on the surface only, more surface=more bacteria).
it looked red
This doesn't matter. The color of meat is not a reliable indicator for food safety. First, meat exposed to air quickly gets an unappetizing shade of green or grey. Second, because customers are known to turn up their noses at greenish meat, butchers can just package it under a special atmosphere, or just bathe it in chemicals which prevent discoloration, so it always stays red. Third, while meat does indeed change its look when it rots, this happens rather late in the process. Gas production (smelly or not) will come much earlier, and you should discard it at that point.
Probably. Beef should have a faintly sweet meaty smell. An acidic or vinegary smell is never good. I'd chuck it and/or take it back and get a refund, providing it was still supposed to be in date.
With any question of food safety, you have to make the trade off between your willingness to contract food poisoning and the cost of the item involved. Minced beef would cost, perhaps, £5 tops? Which would you rather, lose £5 (assuming you can't get a refund) or spend a few days in bed/in the bathroom being violently ill?
If the meat is not spoiled, the reason might be that the animal was stressed before killing: The smell can be from lactic acid or its decomposition products.
Other reason, yet not very probable, is that the animal was underfed and suffered from catabolic condition - this could be the case with a milk cow.
Or perhaps the meat was just stored in a wrong temperature: Minced meat gets very fast spoiled when the cold chain breaks.