If you grow tomatoes, you end up with green tomatoes at the end of the season. For us, this was not so much because we had a frost (our first frost is generally in December or January), but because we just got tired of having tomatoes come ripe every week and I decided to chop down perfectly fine tomato plants.
How fine? Well, this one was still flowering like mad when I felled it.
At any rate, I ended up with a bucket full of green tomatoes, and decided to try my hand at green tomato jelly. Don't do a web search for this, because oh god the recipes are horrific, largely involving jello. Since I refuse to use jello that way, I will be working out my own recipe and sharing it here. This time: the juice.
My first step was to wash the tomatoes and remove the bad bits. Mostly the stem ends and any russeting that had occurred. Then I put them in a big pot on the stove and filled it with water to not cover but come close to covering the tomatoes.
Then I simmered. I simmered the tomatoes for about two hours, which may or may not be the case for you. This is not a high-attention simmer: you can leave this on the stove and go fold laundry or work on your architectural thesis as needed. But when it's done, the tomatoes should be soft enough to mush them with a wooden spoon, and the water should be fairly thick with tomato ooginess.
Learning from the quince jelly experience, I put the whole mess through the food mill to make a nice mash. This squeezed quite a bit more juice out of the tomatoes than there would have been.
Then it was the usual jelly business: put the mash in a jelly bag and leave it to drip for about 24 hours. Don't squeeze the bag. If you must squeeze the bag, squeeze it into a separate container and make cloudy jelly with that one.
Here you have the first pour-off of juice: I had so much mashed boiled tomato that I poured the juice off once at the beginning then again at the end. The mash then makes a totally fantastic addition to the compost heap or worm bin.
Next time: flavouring and jellying. I'm still working out ideas for adding some spice to the jelly.