The technique i will explain is called "cryoextraction". This is a technique use all over the world to produce concentrated icewine without having to buy or have real naturally frozen grapes and press at minus 7 Celcius and lower to be legal. In Canada, legally, an icewine can only be labelled icewine if they are over 32 Brix and press at minus 7 celcius and lower. But, as amateur winemakers we can do whatever we whant
You start with a 23 L pail of fresh juice or about 80 lbs of fresh grapes. This will lead to about 8 L of finished high quality icewine. If you start with fresh grapes, you will have to crushed and destem the grapes and add 50ppm free SO2 and let the must in a cold area overnight. After that period, you will rack the wine in two clean 20 L plastic pail. If you are starting with juice, you will have to rack the juice in two 20 L plastic pail too but with no SO2 addition needed. Now, there is no further difference between grapes and juice. You immediately take your two pail of juice and you put them for two weeks in the freezer. The juice consist of approximately 70% of water so the water will freeze but you will still have near 30% highly concentrated, sirupy juice in the pail. After the two week period, take out the pail of the freezer. You will now have to extract those concentrated juice from the pail. There is two way that i know. You can place the pail on a support ( can be made of wood 2X4 ) and drill few hole in the buttom of the pail so the juice can flow and drop in a primary pail that should be place under the support table. Or, you can simply made a V cut on the lid and place the pail head down over an empty primary pail to extract the juice. The pail will hold by himself. This is the technique i use and it works pretty good. The sirup will begin to flow but it will take some time. As the ice begin to melt, you will get more and more juice but the product will soon become to be diluted. At this moment you will have to monitor the must carefully. You will need either a high range refractometer ( more precice than an hydrometer ) or an high range hydrometer ( not all hydrometer have higher range scale ) to take brix or SG reading. If you use the hydrometer, a 50% dilution of the must with water will lead to more accurate result. You should get between 1,138 to 1,167 if the sample is pure or 1,019 to 1,085 if the sample is dilute. If you use a refractometer you should get between 32 to 42 Brix if the sample is pure and between 16 and 21 Brix if the sample is also dilute. Depending on your local temperature, this could take some time to achieve but it is important to test regularly to avoid missing all the work. It could take more than 8 hours for all the process till you got your starting target sugar content. This is a personnal preference but i prefer a icewine on the lower side, ie; 32 brix to 36. My last year Moscato Icewine is 35 Brix and is the best i've made. Now, you will have to measure the total titrable acidity of the wine ( TA ) with an acid test kit. You need around 11gr/L target TA ( between 10 and 12gr/L, to your taste ) but it can sligtly vary to fit your personnal starting SG and target finished SG. But, what is sure is that you will need to add some. To add acid to your must, use tartaric acid at a rate of 1gr/L to get an increase of your TA of 1gr/L. Now if the juice had not been sulphite by the supplier, you should add 25ppm free SO2 as an initial dose since the wine is now exposed to oxydation. Put the lid closely on and begin to start thinking about your yeast, nutrients and rehydratation period.
Fermenting the must ;
Take a temperature reading of the must. It will be very low like 5 Celcius. Wait until the must get above 10 Celcius before preparing the yeast. When the temperature is over 10 Celcius, you will have to rehydrate your yeast carefully with special nutrient. Suspend some Go-Ferm nutrient at the rate of 30gr/hl ( hl = hectolitre = 100L ) in 20 times its weight of clean 43 Celcius water. When the temperature drop to 40 Celcius, add your selected yeast at the usual rate of 5gr/5Gal. even if you only 8-10 Liter of must. Adding even two package of yeast will only help you achieve an healthy fermentation. Up to 80% of the yeast die during the first hours of fermentation due to the osmotic shock in this difficult environnement ( high sugar content ). Stir gently to break any clumps. Let suspension stand for 15-30 min. max and then stir gently again. Now you have two way of fermenting your juice. 1) As the usual way, like any other juice or 2) In little addition of must each day until all the must has been added ( keep the extra must in the fridge ). If you select the second option you have less chance ( if we can call it a chance ) of a stuck fermentation. With proper care you should have no problem by taking the first option ( except if you use a less tolerant yeast strain ). So now, slowly combine a small amount of the must to be fermented to the yeas suspension. This will avoid any cold shock cause by a rapid temperature drop. Wait few minute and continu to add little more must until you fee that the temperature is closer to your must in the primary pail ( use your thermometer ). Add the yeast to the primary pail and stir hard to give enough oxygen to the yeast. Keep the temperature in the proper range ( be safe, don't play near the limit ) of your selected yeast. Now you should add yeast nutrients as they will need them. I recommend, an other time, Lalvin Fermaid K wich is a very good nutrient. The addition rate is 25gr/hl for difficult must ( normally it's 20gr/hl ). Divide in half the quantity you need. Add the first half right after the yeast addition and the second part after 1/3 sugar depletion ( when the Sg is around 1,125 for a starting gravity of 1,150 and finish gravity of 1,075 as an example ). This should be dilute in some must and then stir in the primary. Monitor carefully the ongoing fermentation and take daily reading of the SG and temperature. Stir the must each day during the first week. The fermentation could take one week to reach your target final SG or as much as two months depending on the yeast strain and temperature of the must.
Stopping the active fermentation ;
An icewine is not ferment till dryness or you will get more than 20% alc. with little sugar remaining. So you have to keep some ( i should say big amount ) of sugar by stopping the fermentation. You should target a final gravity of about 1,075 to 1,090 depending on your personnal taste. Experience will tell you over the years what you prefer. When you decide you final target SG, you will have to stop the fermentation a little before this point as an active fermentation does not stop that easy. If you target at 1,080 like i personnaly did with my Moscato this year, you should stop the fermentation at 1,085. How to stop that ? Add between 50 to 100ppm free SO2 to the wine and rack it fast to a cold storage condition between minus 10 and 0 Celcius for a two weeks period. The fermentation will not halt spontaneously, therefore, you will get a further reduction of sugar and will probably hit the 1,080 target. Now the best thing is to coarse filter the wine with number 1 pads. And let the wine in cold storage for a minimum of two more weeks. Then, measure the free So2 and keep it at 50ppm. Let the wine aged in cold storage as much as you can. Add sorbate potassium ( at 0,18gr/L ) to avoid the risk of re-fermentation since you've got lot's of residual sugar and some yeast cell present that could still live. You can let your wine age for few more months before an other filtration with no. 2 or 3 pads and bottling. Your wine will kept for 2 years in this condition. It is however recommended that you sterile filtering the wine for proper aged of your wine for many years but it will skip some good component and draw off some color off the wine. The free So2 level at bottling should be 50ppm. You should get around 8 L of finished ice wine.
Hope this help,
Edited by Seb, 25 February 2006 - 08:04 PM.