I had to look this one up. It is when food and agricultural waste is upcycled into skincare and other different products
So sidestream raspberries are not grown specially for skincare. Waste raspberries (maybe the overripe or squashed ones) are recycled (so to speak) into other uses.
It seems you can use it as a noun, adjective and verb. I wonder if it will become more – ha ha – mainstream?