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| Hydrometer suspended in grape juice |
The wine must has to ferment for about 2 weeks in the first stage. When fermentation stops, you have to test the specific gravity. When making wine, the Start Gravity should be 1.070 (normal finished ABV will be 10.5%) to 1.090 (normal finished ABV will be 13%). Finish Gravity should be 0.990 (for dry wines) to 1.005 (for sweet wines).
I've seen many recommend that wine should be fermented down to dryness. If you like a sweeter wine, you can then add sugar or grape juice at the end to reach your preferred sweetness. It is a dangerous business stopping the fermentation early.
So the hydrometer is probably the most essential piece of equipment to make wine with success. I've ordered a spare as I can imagine that it's pretty easy to break. I've also read about how to read the hydrometer properly. You have to make sure your hydrometer is floating freely and not stuck against the cylinder. You read at the bottom of the meniscus not the top (I remember that from chem class). It measures the Specific Gravity (SG) of the liquid being fermented. This will then in turn give you a guide to the Alcohol by Volume (ABV) you will be able to produce. The hydrometer is used throughout the fermentation to ensure sugar is being converted into alcohol. As more sugar is converted to alcohol, the SG will fall.
The Alcohol by Volume (ABV) is calculated by subtracting the start gravity from the finish gravity and dividing this figure by 7.362. For example, the starting point for our wine is 1.100 and if this ferments down to 0.990, the drop will be 110 points. This divided by 7.362 is 14.94% ABV. That should kick a punch.
You have to make certain the fermentation is complete and stopped at the right time. Sometimes fermentation slows down or stops too early. A stuck fermentation is one that falls short of reaching the expected final gravity, and as with many things brewing, the term is relative. A wine that stops at 1.000 probably suffers more from poor instrument calibration than it does from stuck yeast. A 1.100 wine that stops at 1.045, though, still has a way to go and needs some help.
There are tricks that one can use to restart the fermentation process. One is temperature. If the hydrometer is placed in water at 20°C, it will read 1.000. Most people only use the hydrometer as a guide but if you want to be really accurate then this should be done with a liquid temperature of 20°C. If the liquid is 5°C higher then add 0.001 and similarly if its 5°C lower then delete 0.001.
Warming up the must is probably the most reliable way to restart a stalled fermentation. Some yeast strains are more temperature-sensitive than others and may require some warmth to complete the job. Some strain are famous for refusing to budge until they are warmed as high as 95°F (35°C). Some British yeasts are stubbornly flocculent; it’s worth giving the must a good swirl a couple of times a day just to keep the yeast cells in suspension until they’re done.