When I store basil in the fridge, sometimes it gets brown spots. Is it still usable? Is that valid also for Thai basil? Throwing it all away after 2-3 days seems such a waste. Also, how can I prevent this from happening?
You can store it with the stems in a glass of water, like a bouquet, preferably not in the refrigerator. A few black spots that aren't moldy or slimy doesn't make it totally unusable, but it isn't very appealing to eat.
This is actually precisely the reason why it's not recommended to refrigerate fresh basil leaves.
I would not recommend consuming basil that has turned brown/black, especially if it is "slimy" to the touch. Even though a few brown spots are probably safe, it will be bitter and, well, slimy. Throw it away - and consider using some of the storage methods discussed in the link above.
If you have leftover basil you can also freeze it so it doesn't go to waste. It won't be that nice for salads or drinks, but it will still be perfectly good for putting in pasta sauces or pesto.
If it's only been a few days, there's no sliminess or weird texture, and the smell is still fresh and basil-like with no hints of unpleasant decomposition, then what I've found is it's just a matter of presentation. It won't be very good for garnish, but if all you need out of it is its flavor, then it's fine. Blended into a puree or simmered in a sauce at the last minute, it still tastes fine to me. Even just by themselves, the mottled leaves have tasted fine, just a little rougher texture. So it's not necessarily a complete waste.
I just finished making a pesto with my brownish basil leaves. It tastes good to me,just like "regular" green basil leaves, only a little browner.
As long as they are NOT slimy and well cleaned use them. I blend in food processor with fresh garlic, olive oil and a bit of good Parma just so it sticks. Take a large tray or cookie sheet, cover with waxed paper drop by large spoonfuls (they spread a bit) and freeze. When frozen take off waxed paper and freeze in ZIP lock bags. You have a good beginning pesto for use in soup, stew/goulash pasta or risotto. The sky is the limit. I make this frequently in summer and again winter when I run out!!
My garden harvest just turned half black while leaving it to soak in water. I quickly blanched it ...this will stop the blackening reaction. I’ll make pesto tomorrow. NYT Cooking recommends blanching the bright green basil for pesto, as it will keep the final product very green. It true.