Monitoring Specific Gravity

[Chalkyt]

We occasionally have posts searching for the "holy grail" i.e. slightly sweet carbonated cider. Stovetop pasteurising seems to be the answer, but potential bottle bombs are always in the background.

I had plans to try this by bottling at say, 1.010 and heat pasteurising at 1.007 or thereabouts. However the "show stopper" was monitoring the SG without wasting too much of the cider. It seemed logical to me to keep a hydrometer floating in a test jar for a few days in order to monitor the SG once I was near the desired range, but... it didn't work all that well.

For some reason, after a day or two the hydrometer in the test jar showed quite a difference from freshly tested cider straight out of the fermenter.

So does anyone have any idea why this might be the case.

I had thought of getting a tilt hydrometer for this but their accuracy is only 0.002 and I am mucking around with a 0.003 differential. My finishing hydrometer has 0.001 graduations that are 1/8" apart so reading to 0.0005 or even finer is reasonably easy, but of course "wastes" cider if several readings are needed, especially when trying to chart the SG as suggested by CJ.

My concern is that if the pasteurisation fails and fermentation continues then with beer bottles, bombs are a possibility. Of course I could use champagne bottles.

Any opinions are welcome.

 

[gregbathurst]

You don't say if the test jar was higher or lower sg? I would expect the main ferment to produce more heat and go faster than a test jar. I don't have any problem putting a hydrometer sample back in the ferment, especially an active ferment. If you have some margin for error your pasteurisation is unlikely to fail, it's a pretty reliable process.

 

[Chalkyt]

You are right, the main ferment did go faster. Although I didn't keep a record of the numbers, the test jar was higher, and by the time I realised it the main ferment had gone too low to continue with my plan. I had used Nottingham which fermented quite quickly.

I find that my ferments go from 1.050 to 1.020 in something like two weeks at around 15C (60F), then in secondary will go below 1.010 in another two weeks or so. This seems very fast compared with CJ's Table 14.1 where this change takes 100 days or so and was the basis for the monitoring idea.

With the short time frame it is possible to miss the target SG of 1.010 if you don't monitor almost constantly. What sort of time frames do others achieve without quite low temperatures (fairly difficult to achieve here in Oz during late Spring even though my cool store has a small air conditioner).

 

[RPh_Guy]

Whether it's higher it lower, a separate sample will pretty much always be different than the main batch. A lot of variables affect fermentation speed. Also, TILT hydrometers and similar are often wildly inaccurate, way more than 2 points.

Monitor gravity readings to chart progress. A few readings should provide a reasonably accurate window when the fermentation will hit the target gravity.

I don't have any problem putting a hydrometer sample back in the ferment, especially an active ferment. If you have some margin for error your pasteurisation is unlikely to fail, it's a pretty reliable process.

I agree 100%.
Pour the hydrometer sample back into the fermenter if you're concerned about "wasting" it.

Pasteurization is a pretty fail-safe way to stop fermentation if your thermometer is even in the right ballpark.

With the short time frame it is possible to miss the target SG of 1.010 if you don't monitor almost constantly. What sort of time frames do others achieve without quite low temperatures (fairly difficult to achieve here in Oz during late Spring even though my cool store has a small air conditioner).

A lot of us use equipment to control the temperature rather than relying on ambient. I use a refrigerator + Inkbird 308, and a Fermwrap if needed. So, it's not difficult at all, it just requires the right gear.

 

[gregbathurst]

Nitrogen levels affect fermentation speed a lot. When my trees were young I fertilised them regularly and my ferments would be finished in 5-6 days at ambient of about 21c. Now my trees are plenty big and I don't fertilise, the ferment takes about 15 days and bottle conditioning takes a couple of months. If you can get low nitrogen fruit you will find fermentation takes a lot longer.

 

[bucketnative]

I am trying to determine where you are looking to make these measurements. It seems other people are answering different questions...

1) Are you looking for a means of measuring the target 1.010 sg in the fermenter without losing a lot volume via hydrometer samples?

In this case, I would have two answers: A) Ferment in a clear vessel with a hydrometer flowing in the ferment. This may get murky with foaming or other particulate floating of the surface or stuck to your hydrometer. B) Measure specific gravity via mass. If you have a volumetric pipette and an accurate scale, you can determine sg with a relatively small sample size. Same could be said for a refractometer (corrected for alcohol).

or

2) Are you looking for a means of measuring the drop from 1.010 to 1.007 in the bottle without losing a lot of bottles?

In this case, perhaps it is best to measure another value in the bottled cider, like pressure: https://www.homebrewtalk.com/forum/threads/monitor-your-bottle-pressure.591360/#post-7713665 Pasteurize when you hit the expected vol CO2, which should roughly correlate with sugar consumed.

 

[Maylar]

I find that my ferments go from 1.050 to 1.020 in something like two weeks at around 15C (60F), then in secondary will go below 1.010 in another two weeks or so. This seems very fast compared with CJ's Table 14.1 where this change takes 100 days or so and was the basis for the monitoring idea.

Claude uses low nitrogen apples and ferments at 50°F. At 60 you'll be much faster.

 

[bmd2k1]

I've been thinking about a Tilt to help me out & compliment my hydrometer readings. Looking at other threads there's a bunch of folks that have had very good success with 'em. Seems the less krausen ya have the better. For my D47 ferments...I get Very little.

Cheers....good luck & keep us posted [emoji111]

 

[Chalkyt]

Thanks for the replies, I have been away for a few days... lots of good stuff there, particularly using a refractometer, which I have. I thought that they lost their accuracy once alcohol was involved and wasn't aware that you can make corrections for this.

So plan B or C or whatever might be to take small samples by removing the stopper and dipping a chop stick in to get a drop for the refractometer. Hopefully this can be done without introducing much oxidising air (O2) to the CO2 as this should happen over a time frame of days rather than weeks.

Making up a pressure gauge on a test bottle also sounds good so a few "experiments" using store bought juice sounds worthwhile.

 

[RPh_Guy]

Refractometers cannot approach the reliable accuracy of a hydrometer, even with the correction calculations.