I left a pack of frozen vegetables (brocolli, carrot, cauliflower, green beans) in the fridge so it thawed. It went sour and slimy after a few days, (maybe 5) -- why?
If I cut the same fresh vegetables and store it in a plastic bag or container, it will be fine in a week or more.
What's the science behind that? Why do bacteria grow more quickly in frozen veggies being thawed than in fresh cut ones?
Very simply; freezing breaks the cell walls.
As ice takes up more space than water, as the vegetables freeze all the cells rupture.
Once defrosted, each cell is then little more than a slowly-leaking bag of nutrient, exposed to the open air. Whether or not any bacteria gets in & starts to breed is almost irrelevant by this time, as your 'leaky bag' is going to dissolve into a pool of its own juices pretty quickly.
This is why frozen veg is cook-from-frozen not cook-from-thawed. An hour might be OK, a day is starting to get a bit much. By 5 days, I wouldn't do anything other than move it to the recycle using tongs.
In additions to Tetsujin's answer, know that freezing, at least of home temps, does not completely stop degrading of the vegetable. One of the main reason most vegetables are blanched before freezing is to stop, or at least slow down enzyme actions that decay and degrade the color, texture and flavor. This helps some, but of course also can contribute to cell breakage and add some water which compounds this, so even this attempt to help stall degrading may aid in faster decay after thawing. That is, the item may be better quality when first thawed that it would have been without the blanching, but the post thaw breakdown might be even faster.
