Talking about green natural olives, the brine usually has the suspension of solids and phenols from the olives themselves, which can make the medium cloudy and give an extra dark color to the brine, and secondly to olives.
On the other hand, this mature brine is supposed to have free acidity from the fermentation, so it can be used as a good starter medium for the 1st phase of the fermentation.
Finally, the mature brine has a microorganism content higher, principally because of mold and yeast, responsible of giving flavors to olives.
In Spain, there has been some experience, consisting in filtering the mature brine through carbon filters, but in the canning brine only, and not in the fermentation brine.
- Using this mature brine diluted in the new fresh brine, (20:80), can be useful, but increasing salt content previously until the normal % salt used in the fresh new brine, in this case, 10%, to get after equilibrium, safe salt concentration.
- Talking about black natural olives, the brine has suspension solids and phenols from the olives themselves. This is a good thing for improving the color of the olives. In fact, in Greece and South America, some companies are using mature brine from kalamata olives fermentation, in order to improve the external color of olives during fermentation.
Again, you will have inside it high content in microorganisms, yeast, and mold, which can affect the initial phase of the fermentation.
- Using this mature brine diluted in the new fresh brine, (40:60), can be useful, but increasing salt content previously until the normal % salt used in the fresh new brine, in this case, 10%, to get after equilibrium, safe salt concentration.
I hope my answer can be useful, but as you say, there is no scientific reference concerning this issue.