Thursday, January 24, 2008

Fermentation Tapering Off

This morning the must was reading 2° Brix at 77° F at the punchdown. After the punch I dropped the lower temperature setpoint to 75° F, where it will stay through malolactic fermentation.

I finally received the lab analysis of the juice sample I pulled before fermentation:

Disssolved Solids24.0°Brix
Titratable Acidity4.21g/L
Assimilable Nitrogen124mg/L

The sugar is lower than the WinePod Brix sensor was reporting, by 2° Brix. The pH is not particularly high in my experience, but the T.A is a little low. I will be re-evaluating the taste, pH and T.A. after malolactic and pressing to see if there is need (and room) for another tartaric acid addition. The potassium is a bit low.

This is the main challenge of working with frozen fruit: the freezing precipitates most of the potassium bitartrate out of solution before the ferment. In ordinary circumstances, the tartaric acid equilibrium is not established until well after fermentation, and is affected by the alcohol content of the wine.

My training and experience with acid correction in commercial ferments is not of much help with frozen fruit. Some of the tartrate in the WinePod juice will re-solubilize at the higher temperatures of fermentation – it will be interesting to see what the pH and T.A. are after malolactic. Anyway, I'm glad that I added the half-gram of tartaric before inoculation, and gladder still that I did not add more – a larger addition could have driven the pH uncomfortably low.

The nutrient status of this juice was quite low as well. The combined ammonia/nitrogen of 173 ppm is low enough to starve yeast expected to ferment to over 14% alcohol. It turns out that the nutrient additions I made should have added about 140-150 ppm of available N to the must. If I had known that the N was this low early on I probably would have added another 50 ppm of N as DAP to the juice. At any rate, at somewhere around 320 ppm the total N of the must is what I would describe as "barely adequate" to conduct a clean and complete fermentation.

So it's probably not just coincidence that the ferment smelled a little of sulfide this morning.

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