http://durgan.org/2016/September%202016/22%20September%202016%20Concord%20Grapes/HTML/ 22 September 2016 Concord Grapes Eighty five pounds, three bushels, of concord grapes were picked in Fontill Hill ON and made into 42 liters of pressure canned juice. The grapes cost $15.00 per bushel pick your own. Process was wash, remove the grapes from their substrate, cover in cooking pot with water about five liters for 25 pounds, cook until soft about 20 minutes, beat into a slurry with a hand blender. Strain the juice in a food mill with a 2 mm mesh screen. This effectively removes the seeds. The strained juice is then placed in liter jars and pressure canned at 15 PSI for 15 minutes. A few statistics. A bushel of grapes weighed 30 pounds, the removed substrate from 85 pounds of grapes weighed 10 pounds, the seeds removed from the cooked grapes weighed 13 pounds. A liter of pressure canned grape juice contains about 2 pounds of grapes. Pure grape juice is too thick texture to drink, hence about 15 liters of water is added to make 42 liters of juice. Pictures depict the simple process.
It's interesting to see how you pressure can your grape juice. We've just canned 26 litres of juice from home-grown green (Himrod) and red (Canadice) seedless grapes using a similar process. I briefly searched the Web for grape juice pressure canning information and found nothing; all of the usual sites discuss boiling water bath canning only. Information from the USDA recommends a boiling water process time of 5 minutes for quarts; so I used 6 psi for 10 minutes to be on the safe side. Your 15 psi for 15 minutes looks like overkill but I'm sure it doesn't hurt. One interesting thing about the pressure canning is that it changes the colour of the green grape juice from an ugly greenish beige to a more appealing pink. This grape juice is canned at full strength and can be diluted 1 to 1 with water to make a very palatable drink; so the grapes from two vines grown up a south-facing deck are providing over 50 litres of very healthy and tasty juice, a very good return on a small investment of garden space. Because it is not clarified, the juice contains fibre and presumably other beneficial substances that are not found in commercial canned juices. Potassium bitartrate crystals do appear in the jars but mostly remain on the bottom, and I don't find them objectionable. Durgan, does your Concord grape juice produce tartrate (tartaric acid) crystals?
I have 14 liters now pressure canned and seven more in the canner. The 14 are exactly the same color as the unpressurized ones. It is impossible to tell them side by side apart by color. (Even my tomatoes are a cherry red). I now contribute retaining color by boiling enough to remove all air prior to straining whether factual I am only speculating. The grape taste is perfect and texture may be slightly on the thick side but not objectionable. I have never found crystals, but I do consume the grape juice in probably less than 8 months. It is highly popular and desirable. I now never do water bath canning. My view is it is not worth the hassle. My pressure canning method of 15 PSI at 15 minutes method is justified by making everything into an homogeneous slurry, hence even heat. Never has there been one single jar spoiled. The 15 PSI for 15 minutes was chosen, since it is a worst case scenario, picked after much thought and experimentation on myself. I refused to become involved those silly timing charts. There has to be a worst case scenario where one time and pressure fits all, even if it is overkill. I contemplated concentrate but considered it an unnecessary complication to save a little storage space. I have four grape vines and usually get around 80 pounds on a good year, but they are cheap commercially and available locally so usually buy.