chemical analysis of frozen juice samples

[darlenehayes]

In 2016 I saved various individual pressings with the intension of testing pH, sugar content, titratable acidity, and total tannins. For various reasons, I was not able to do the analysis right away, so I froze the samples. I have recently thawed them and started my analysis. Most of them have significant sediment in the bottom of the test tubes. Using a refractometer to measure Brix I've noticed that I get an impossibly low reading unless I shake up the tube to resuspend the sediment, at which point the reading is in the range of what I would expect. 

Now I'm starting to test total tannin using the Lowenthal permanganate titration. For consistency's sake, I am again resuspending the sediment before taking out my sample. So far the numbers are in the range that I would expect, but I'm noticing that there is sediment at the bottom of my flask, and it appears quite dark, almost as if it's bound to some of the indigo carmine indicator.

Should I be concerned about my numbers? Why would I see a different Brix measurement in an unshaken juice sample?

Darlene

[Andrew Lea]

You get different Brix and acid measurements in thawed unshaken samples due to stratification of solutes during the freeze thaw cycle. This results in almost pure water at the top and dense sugary stuff at the bottom. It is very important to mix thawed juice samples very well before analysis. 

The issue with tannin and sediments is slightly more complicated. Some of the sediment is oxidised tannin which gets polymerised and bound to other juice components. This is what you see. This may mean that you lose a certain amount of tannin irreversibly in frozen thawed samples. However, in practice the losses are likely to be quite trivial for a gross measurement such as Loewenthal. When I was at Long Ashton in the 70’s we froze many hundreds of juice samples for later analysis in the coming months. As long as they were well mixed before analysis, we had no problems. You should filter or decant the thawed samples off the sediment after shaking and resettling. 

Andrew 

Wittenham Hill Cider Portal
www.cider.org.uk

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[darlenehayes]

Thanks, Andrew. Solute stratification. I should have thought of that, especially since in at least one thawed sample the top 2 - 3 mls was as colorless as water.

Darlene

[Dana Glei]

Very enlightening, Andrew.  Your answer to Darlene explains some puzzling results I had recently. 

The best farm juice I can buy locally is sold frozen.  I bought 5 gallons of it and tested the S.G. after thawing the juice, but I didn't take special care to make sure the juice was mixed well.  When my wild yeast fermentation seemed to be slow in starting, I re-tested the SG to confirm whether any fermentation had occurred.  I was surprised and puzzled that the SG measured substantially higher than my initial measurement.  I double- and triple-checked to make sure it wasn't just CO2 that was screwing it up.  I finally decided that I must have screwed up my initial measurement.  Reading your response to Darlene makes me realize why my initial SG measurement was probably distorted.

So, you say that one should filter or decant the thawed samples after shaking and resettling.  I have a very small-batch, low tech operation (as a home cider maker).  What low-tech method would you recommend for decanting:

1)  Rack it with a siphon (as I do during fermentation)? OR 

2)  Pour it carefully off the sediment? (should I run it through a coffee filter as well?)

On Sunday, December 10, 2017 at 2:20:16 AM UTC-8, Andrew Lea wrote:

[Andrew Lea]

I think you are misunderstanding me. When I advised to decant or filter the thawed samples, I was meaning in terms of laboratory analysis. This is to avoid pipettes blocking, suspended solids interfering with optical properties etc etc.

If you are planning to ferment the juice, not to analyse it, then there’s no need to remove the sediment. You will be racking after fermentation anyway.

Andrew

Wittenham Hill Cider Portal
www.cider.org.uk

> On 11 Dec 2017, at 01:43, Dana Glei <danag...@gmail.com> wrote:
>
> Very enlightening, Andrew. Your answer to Darlene explains some puzzling results I had recently.
>
>

[Dana Glei]

Okay.  I understand.  So, for my purposes, I just want to test the SG and the pH prior to fermentation.  In that case, I understand you to be saying that it is sufficient to mix the thawed juice well so I that I get an accurate reading of SG (but there is no need to remove the sediment prior to fermentation).  Thank you for the clarification.