The Scotch broth mix I use consists of pearl barley, yellow split peas, green split peas, blue peas and red split lentils. Prior to use, it requires soaking for 8-12 hours and then draining and rinsing before cooking.
Due to other commitments, I often cannot return to changing the water for 18-24 hours. This results in the mix starting to give off a distinct undesired aroma, I assume down to fermentation.
Is there a way to slow down this reaction?
Refrigeration is a very standard way to slow down fermentation. Use it.
I've soaked beans in the fridge for more than a week without off-flavors (but I do generally change the water every day or two, as I'm not a fan of preserving extracted oligosaccharides.)
In my personal experience presoaking would only make much difference to the lentils, in your mixture. I came to split peas from other beans, and maintained my prior practice of pre-soaking them, but found there was very little difference if I did not do that. Well, other than making felafel with them, where they obviously need to be pre-soaked.
