Lemon juice is an essential cooking ingredient, but many of us can't forage for lemons. If you'd like a foraged substitute for lemon juice, use the juice of unripe grapes to make verjuice.
Verjuice (sometimes spelled verjus) is from the French for green juice. Not green in color, but green as in unripe. It can be made from immature crab apples and currants as well as any kind of grape. It's important that whatever fruit you use be super sour, so be sure to taste what you're harvesting as you go along.
The result is an acidic liquid that can be used in place of either lemon juice or vinegar.
Unripe grapes are smaller and less juicy than mature grapes, so you need lots of fruit to make verjuice. Start with five pounds of grapes. You'll also need a food mill, rubber or latex gloves, a mesh sieve, and some coffee filters. Citric acid is optional.
It's important to work quickly during these next few steps to minimize the oxidation of the verjuice. Exposure to the air turns the juice from yellow to brown. It will taste fine, but it won't be as pretty.
Load your food mill with its coarsest plate and start grinding the fruit. It's going to take some hand strength since these grapes are hard, being underripe. You'll end up with a rough mix of juice and pulp. The food mill should eliminate most of the seeds, but don't worry if a few make it into the bowl beneath.
Next, put on your gloves. Why? Because these unripe grapes are full of acid (that's what makes them so sour) and this can sting your hands after prolonged exposure.
With gloved hands, squeeze handfuls of grape pulp over the sieve to release maximum juice. Then, pour the resulting juice through a moistened coffee filter. If sediment builds up in the coffee filter, use a new filter. Wet each coffee filter before using so it absorbs as little juice as possible.
Pour the filtered juice into a mason jar, add 1/2 teaspoon of citric acid, and shake vigorously to combine.
The citric acid is a preservative that will let you hold your verjuice in the refrigerator for up to six months. Refrigerate the verjuice for 24 - 48 hours.
There should now be a layer of sediment on the bottom of the jar. Gently pour the liquid off the sediment into a clean jar and there you have it: verjuice! The flavor is tart and not noticeably grape in nature. It's a versatile substitute for vinegar or lemon juice in salad dressings, sauces, or marinades.
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