A glycerine and sugar syrup is now frequently used to make 'identical' freezer and shelf life boilies. The base mix is the same, but instead of freezing after drying, the baits are given a run around in the bath and then dried again.
Freezer baits tend to have an attraction 2 or 3 days after thawing as the enzymes, salts and sugars migrate to the surface due to moisture, where I personally think shelf life's have the added instant attraction due to the glycerine. They may both be food baits, the same recipe but the glycerine make them more instant. The two can be used together, or separately.
Shelf life boilies I have found to be harder, and the longer you leave them, the harder they get to the point of drilling.
You can air dry, without freezing the standard bait to rock hard, it will need drilling to go on the hair, but they take on water more quickly and almost explode.
Back in the early 2000's I played around with bait soaks and glugs, with ideas from the original Nutrabaits Bait soaks which were Nutramino and Multimino PPC and added my flavour and essential oil combination; the Peach Nutrafruit was on glycerine/glycerol. The hookbaits after a couple of weeks were rock hard and able to withstand the attentions of small silvers and chub in the lakes and rivers.
I have played with other glugs, often based around Liquid Yeast Extract
I did play with matching of flavour sprays, but they did need watering down to put in an atomiser bottle.
One thing I did find was neat flavours could be a repellant or create a feeding area or attention area actually away from the hookbait. I can't remember which Tim Paisley book it is in, probably Carp, but both him and Rod Hutchinson came to the conclusion that the flavour was acceptable a distance away from the bait, and it is there that the carp boiled, rolled or attempted to feed.
So use flavours at low level, or avoid them altogether and stick with natural attraction.