Apologies in advance for the exceptionally long post…
In the context that you’re considering, think of Brix and juice content as being completely independent.
It is possible to make a 12% lemon juice product at 12 Brix if you add the right amount of sugar, but it’s also possible to make a 12% lemon juice product at 2 Brix (it’s be a bit flavourless and slightly sour), or at 28 Brix (it’d be disgustingly sweet) by adjusting how much sugar you are / aren’t adding.
For the acid, it’s relatively simple – ignore anything that isn’t contributing acidity, which in this case is everything that isn’t lemon.
Your lemon juice concentrate has an acidity of 18%w/w ACA, taking the midpoint of the spec, so this means that there is 18g of ACA in 100g of concentrate. It’s actually a bit more subtle than this – the figure actually relates to the total titratable acid quoted as anhydrous citric, but you can safely ignore that distinction in this context.
In your recipe for the 10% variant you are adding 25g of concentrate, so this gives you 18*(25/100)=4.5g of citric acid in your blend of 1000g, so this is (4.5/1000)*100=0.45%w/w.
Similarly for the 12% variant you have 18*(30.8/100)=5.544g total in 1000g, so (5.544/1000)*100=0.55%w/w.
(The 0.50% figure that’s quoted for this may be a nominal rounding?)
You can also take a similar approach with the Brix calculations to get yourself an indicative value, although it’s always best to verify this by analysis too
For your 10% variant you’re adding 25g lemon per 1000g, so (25/1000)*100 = 2.5%, and 70.4g sugar i.e. (70.4/1000)*100=7.04%. We’ll take the former as 31 Brix (assuming we’re going to continue working with uncorrected values), and for the latter 100 Brix.
The Brix “contribution” from each is therefore.
31*(2.5/100) = 0.775
And 100*(7.04/100) = 7.04
Summing the contributions gives an expected nominal value of 7.04 + 0.775 = 7.8 Brix for the blend.
For comparison the calculation for the original 12% version (30.8g concentrate per 1000g, so 3.08% in the recipe), the equivalent figures would be:
31*(3.08/100) = 0.955
And 100*(7.4/100) = 7.04
Summing the contributions gives an expected nominal value of 7.04 + 0.955 = 7.995 Brix for the blend.
Considering the latter in terms of soluble solids instead would give:
30.8*(31/100)= 9.54g soluble solids
70.4*(100/100) = 70.4g soluble solids
Total = 70.4 + 9.54 = 79.94g solids per 1000g recipe, so 79.9*(100/1000) = 7.994g soluble solids per 100g, i.e. 7.994 Brix
(Tiny difference between 7.994 and 7.995 inconsequential – no sane person would quote three decimal places for a Brix value for this sort of product!).
The value for the current recipe isn’t unexpected given that the single strength Brix value for from-concentrate lemon juice is 8.0; I’d guess this was originally formulated to behave roughly as a sweeter substitute for pure lemon juice?
You could preserve the perceived sweetness by increasing both the lemon juice content and the sugar content (as mentioned previously, the perceived sweetness is largely down to the ratio of sugars to acids, rather than the absolute values), but higher sugar content will also affect mouthfeel, and lemon is naturally quite a “thin” juice.
As for online courses, I’m afraid I have no idea!
I learned much of this from a mentor early in my career and by teaching myself, and for my staff and others in the business I deliver the training. For general juice familiarity I know that the BSDA does a course on juice processing that covers many relevant things, and it’s delivered by some truly excellent and fantastically knowledgeable tutors. Next one is in a few days: https://www.britishs...nufacture/44184