I don’t use man made flavours either Carpmaster, I prefer to use additives that give off food signals, and investigation triggers.
As you probably know fish don’t smell like us, as smell is attributed to scent in the air.
fish use olfaction, the reason most flavours work is because they are either acidic or alkaline and therefore detectable by the fish and are considered investigation triggers.
i like additives like belachan and fish sauce, so not concerned about them staying fresh as they are pretty rotten to start with as they are partially decomposed already.