Can anyone offer advice on below:
In terms of general growth, does the brix content of material have a direct correlation to micro suppression?
For our product micro risk assessment some have taken a brix 65 – 75+ to be a micro control similar to pH or aW.
This does not seem sensible to me but I cant find any literature to prove it.
Products can include, Juices, Vanilla, Honey, Syrups.
Some, maybe, sometimes, in some cases
At least for the juice products - I've had less cause to look into honey/syrup, where things may be slightly different.
High Brix will certainly limit the viability of growth for some organisms, but in terms of general spoilage you have to get really very high before it alone is a sufficient barrier, IMEX.
I don't need literature to prove this, as I've seen enough concentrates at 65-70 Brix (and generally also with pH<4) quite merrily fermenting to be pretty sure that things can grow in them
I too have seen and heard the claim you describe, indeed even from people only a few months after seeing it go wrong...
In reality I see this as taking Brix as being almost analogous to, or at least relatively well inversely correlated with, water activity. At these sort of Brix levels you're into a range where certainly some yeasts, moulds, and bacteria will be inhibited, but not all. I'll see if I can dig out some references, as I know I've got some somewhere, but since I don't use them for any micro justification/risk assessment for our products they're not easily to hand right now!
I have a figure of around 0.80-0.85 in mind for the aw of 65-70 Brix juice concentrates which, as expected based on my experience, means that there are some yeasts and moulds that could still grow. The rate of growth may be suppressed as it might not be an ideal environment, and indeed the presence of growth in effect becomes a probabilistic gamble - in many batches one may simply not encounter the right/wrong sort of yeast, so it would be possible that the Brix alone is sufficient for a proportion of the time. Would I rely on it alone, based on my experience to date? Absolutely not...
It does become further complicated by storage considerations. Looking at these types of factors suggests that, in some cases at least, they are perhaps being relied upon in place of e.g. frozen storage. Again this can work some of the time (or even most of the time, if you're lucky ) for some products, but does also present further challenges - in ambient conditions one can find that evaporation of the water content will condense inside the packaging, and drip back onto the product forming a more dilute area, in which it is then much easier for growth to get started.
The other potential pitfall using Brix rather than aw as a measure/threshold control is that it is a somewhat imperfect measure - what is it really telling you about the product? Easily and commonly equated to %w/w sugar content but that's only valid for a pure sucrose solution, and juices, honey etc definitely aren't that. For syrups and honeys the correlation is potentially going to be close enough (although again I haven't looked in sufficient detail recently to be able to actually substantiate that), but for juices it is certainly more variable - two different juice products at 65 Brix (refractometric) could have actual sugar contents anywhere from potentially 35ish to 65g/100g, and that's quite a difference. The other soluble solids will probably have some impact on micro stability, but characterising and understanding exactly what this will be is potentially not a trivial undertaking.
Having said all that, there are approaches where it can work for defined periods of time, but it requires quite careful control and so I don't think it's entirely reasonable to characterise the Brix alone as the primary factor at play - pasteurisation and filling controls are arguably equally relevant, as it's about not getting things in their to start with that may then grow, whilst also having a product matrix that is going to at least slow the growth of most things if a very low level does get in. Personally I feel much more comfortable with either just storing things frozen (which has additional organoleptic benefits for most juices) or packing aseptically and not having to worry about micro, again subject to getting it right at the start