Not online AFAIK, but "Taste What You're Missing" by Barb Stuckey is a pretty readable book. It's more about how the tasting works on the human side than the actual cooking part, but it goes into some detail about how different tastes work together, as well as why different people taste differently. It was helpful to me to sort of emphasize the high variation in tastes between people.

Modernist Cuisine is probably the most well known reference for actual cooking theory, but it's crazy expensive, and some people have problems with supporting Myhrvold.

It might not be similar but "How To Cook Like Heston" is awesome. It's by Heston Blumenthal who specializes in using science to understand cooking. Give him a shot.

There's an excellent MOOC taught by the likes of McGee, Adria and Dan Barber to name just a few. David Chang has a section on fermentation. https://www.edx.org/course/science-cooking-haute-cuisine-sof...

I know you said preferably online but see if any library you have access to has a copy of the five volume set Modernist Cuisine.

Incidentally the guy behind them has a fascinating, and tech relevant, biography.