soluble solids density, and we use this to closely approximate soluble solids content of fruit juice. Because almost all the soluble solids in wine grapes are sugar, we use hydrometers to determine sugar content. I’m not telling you anything you don’t already know, every book on wine making I have says something like this, but did you maybe tuck the “in wine grapes” qualifier into the back of your head and just remember the “… almost all the soluble solids are sugar” part? I know I did, and it was while researching raspberries that I discovered just how much hydrometers can overstate sugar content.
Great data on raspberries upends an old rule of thumb
I was checking university extension offices, googling, and looking anywhere I had found information on fruit composition before when I came across this great paper on raspberries. It’s got excellent data on sugar and acid content, and I highly recommend it if you’re interested in making raspberry wine. I want to zero in on the Brix and Total Sugar measurements:
|Varietal||Brix||Total Sugar||Sugar Brix Ratio|
|Meeker site 1||11||2.83||25.7|
|Meeker site 2||8.6||1.01||11.7|
|Meeker site 3||9.2||2.38||25.9|
|Meeker site 4||9.8||3.28||33.5|
Total Sugar is reported as grams per 100 grams, and if the soluble solids were 100% sugar, then Brix would equal Total Sugar. The last column, Sugar Brix Ratio, is my own calculation and expresses sugar as a percentage of soluble solids. I also reported average values in the last row.
We go from soluble solids being “almost all” sugar in wine grapes to less than 30% (on average) for raspberries! I asked Michael Qian, one of the authors, about this. He said that the data were good but some fruit, like raspberries and blackberries, just have a lot of pectin and other non-sugar soluble solids. I really appreciate him taking the time to help me out; I’m sure he’s a busy guy and this was a pretty basic question from someone outside his target audience (so, if you’re reading this – thank you!).
We know more than before – it just doesn’t feel like it
It’s exciting to learn something new, even if it does make things more complicated. When can we rely on our hydrometers? Wine grapes are probably a safe bet. Raspberries and blackberries are not. We need more information about other fruits, and I’ll be looking into that. If you know something about sugar content and soluble solids of other fruits, please say so in the comments.
Alright, what do we do when we know our hydrometers will read high? I don’t have a good solution yet. For raspberries, we might just adjust the reported Brix by 30% – still not accurate, but closer than the hydrometer reading. If you have an idea, I’d love to hear about it.
Jack Keller once said that a hydrometer is like a compass. Maybe it’s like a magnetic compass. It works well enough much of the time, but the question is, how do we find true north when we really need to?