Aussiedistiller, Home Distilling, Moonshine, Home Brew

Just an observation on residual sugars in dunder and how they affect your starting gravity

I thought it was time to do another rum run, so I dug out 10L of dunder I had saved from my last rum adventures.... nearly 2 years ago. This was 100% boiler stillage from a stripping run of Macs Rum using stock feed molasses and sugar and stored in a big green shed 10L plastic jerry can. At the time I put the boiling dunder in the sterilised jerry and sealed it, similar to the way I would hot cube a beer wort so it would remain sterile (I hadn't heard of 'live' dunder at that time).

Anyway, I opened it up the other day, it smelt file, just like the day a stored it, no growths or other yuckies so I bunged 2 litres of it in with 5kg of Bundaberg Molasses, the usual nutrients and made up to a 20L wash. I expected the SG to be about 1.070ish but the measured gravity was way up at 1.085 :o

I checked the gravity of the dunder and found it to be 1.055. I'm sure the wash it was recovered from finished about 1.010. I can only think the unfermentable sugars from that wash were concentrated when boiled down in the stripping run. All these extra unfermentables will impact the finishing gravity of my current wash, though I checked it this morning and its down to 1.025 and still going strong.

I can see now why, with so much unfermentable sugar available bacterial infection thrive in 'live' dunder.

last month i had 3 rums with the same ingredients, maybe they had dunder from different cubes, but the same amount of dunder.
OG's of 1.090, 1.078 & 1.075
it must be the unfermentables in the dunder?
last year, when using too much dunder, i had an OG of 1.105 :?
it had a PH crash and had to be nursed through to the end (oyster shells are used in every wash now)
looking at my notes, it would seem that the difference between OG & FG is usually the same, whatever it starts at.
ie a 1.090-1.012, 1.070-0.990

I hadn't paid much attention to my starting gravities when I was using stock feed molasses as I had no idea of the quality and/or sugar content. This Bundaberg food grade molasses seemed to be much better quality, no apparent suspended solids when mixed into a wash, so I'm pretty confident the hydrometer readings are fairly accurate.

Checked the SG again today at 1.020, and the ferment has really slowed down, possibly close to or has reached FG. Not surprising given the additional gravity points from the dunder. I'll give it a few more days to see if the SG is stable.

howard wrote:last year, when using too much dunder, i had an OG of 1.105 :?
it had a PH crash and had to be nursed through to the end

The measured pH of this dunder was 4.08 - the starting pH of the wash was 5 and when checked 24 hours into the ferment had dropped to 4.55

i did this side by side comparison, some might say that the eventual result (FG) was the similar, but i'd like to think that during the ferment, the yeasties were not stressed in a higher PH and didn't produce as many off flavours. :?

howard wrote:i did this side by side comparison, some might say that the eventual result (FG) was the similar, but i'd like to think that during the ferment, the yeasties were not stressed in a higher PH and didn't produce as many off flavours. :?

rum wash with oyster figures.xlsx

Thats a great comparison, and an impressive way of displaying it (it plays to my inner geek :ugeek: )
Surprising how the pH really ramps up after day 2 with the shell grit, and that the pH of the non-grit also rose about the same time, though to a much lesser degree.

Was there a noticeable difference in the final products?

I wouldn't really call it "residual sugar" as such in the dunder.

If you had a healthy ferment all the usable sugar should have been converted to alcohol, however with mollasses there will always be unfermentable compounds, weather these be complex sugars or other carbohydrates etc.

Once you have done the distillation and you have concentrated what is left in the boiler then the specific gravity is going to go up as you have extracted the alcohol and some water depending what/how you have run.

As with any rum all that matters is the split between your starting and final gravity to determine your finish ABV of your wash.