Acid numbers - Fresh pressed juice vs. 12 h with pectinase

[GreginND]

I am really trying to make sense of these numbers. I am getting very different pH results with my just crushed juice vs 12 hours later after pectinase treatment. What is strange is that the TA's are pretty much the same - at least within my error probably. I thought maybe my pH meter was not calibrated well so I made sure to allow it to equilibrate for an hour in pH 4 buffer and then I re-calibrated it this morning. Yes, the pH numbers were different, but I did use the pH meter for my titrations as well. I would have expected those numbers to be different also. I can see that buffering in the must might change as the grapes break down a bit, but these differences are stark especially for the Baltica. And the white with just juice and no skins also changed. Makes me wonder if measuring pH right after crush is misleading.

Here are the numbers from yesterday and today:

ES 5-4-71 crushed and pressed - just juice

10/4 10/5 after pectinase addition
19.4 Brix 19.6 Brix
pH 2.90 pH 3.10
TA 8.1 g/L TA 7.7 g/L

MN 1200 crushed and destemmed - juice sample taken for testing

10/4 10/5 after pectinase addition
26 Brix 25.5 Brix
pH 2.92 pH 3.24
TA 8.1 g/L TA 7.8 g/L

Baltica crushed and destemmed - juice sample taken for testing

10/4 10/5 after pectinase addition
23 Brix 23 Brix
pH 2.64 !! pH 3.10
TA 8.1 g/L TA 8.2 g/L

Thoughts?

 

[GreginND]

I found this link which says the pH can rise .1-.3 24 hrs after crush due to release of potassium. Hmm.

http://www.bcawa.ca/winemaking/acidph.htm

"Whether the grapes are pressed immediately after crushing or let stand on the skins for flavour extraction before pressing, once the juice sample has settled and cleared, the acid and pH readings should be accurate. The same cannot be said for red grapes, however. Most winemakers take their samples immediately after crushing, but the readings are not accurate. Doing a test twenty-four hours later will see an increase in pH of between 0.1 and 0.2 as the direct result of potassium extraction. The TA change will be minimal. A further increase in pH will be observed after pressing due to maceration during fermentation and greater extraction of skin constituents."

 

[Turock]

Yeah Greg---I agree with the answer to yourself! PH readings are not accurate on that first day---best to allow everything to integrate AND as the potassium comes out of grape, you will find the numbers will change. PH readings on the second day are more accurate and even then, the PH changes slightly after the fermentation. But you are always pretty close--in the ballpark--with the second day PH reading.