Citrus allergy, need substitute

If you are not allergic to citric acid you can use it as a substitute for lemon juice in canning.

Related Can I use citric acid instead of lemon juice when canning?

I'm allergic to citrus too. For savory dishes, I substitute sumac. For other uses, I mix one part mango balsamic with one part rice vinegar. You can also try cider vinegar in savory dishes. Citric acid is not a problem. In the US, it comes from corn and does not have the protein that causes citrus allergies.

So with an alergy, you will always have to be careful and test things before going all in.

For a lemon flavour Lemongrass is close and is not a citrus fruit. This can be pulped and should add some flavour. You'll still need something to increase the acidity as the other answers have said.

It looks like the primary purpose of the lemon juice in preserves, jam, or jelly is to increase acidity for safety.

So you should then be able to substitute any other acid to serve this purpose. The other common and versatile acid that comes to mind from cooking would be vinegar. It looks like vinegar can be substituted. I've seen instructions to use about half as much white vinegar as lemon juice. That last link also mentions wine as a possible substitute.

Since the other result of changing lemon juice for plain white vinegar would be less flavor, you may want to try to add some flavor back in. You could use a more flavorful vinegar - apple cider vinegar is apparently preferred for this reason, even though I think it'd take more of it. Or you could try adding some other flavor in to sort of balance the jam/jelly. Something with sour notes, or bitter ones, or just something different and contrasting to make the flavors pop. I'd think something like cranberries for the sour, or pine needles (as a tea in various ways or as infused sugar or syrup) for the citrusy aroma - or just some other juice or syrup you might have on hand to give a little something to your recipe.

There are a large number of alternative acids you can use successfully. As I am not an allergy expert, I make no claims that any of these will be safe for you and you should use your own judgment / consult your doctor before trying this.

1. White wine vinegar. The acid in vinegar is acetic acid, and good white wine or champagne vinegars will have complex flavors that can add a lot to a fruit preserve.
2. Malic acid. This is the acid present in green apples (and on warheads candies) and will add a reminiscent flavor. It is quite tart and should be used drop-by-drop in 10:1 solution.
3. Tartaric acid. This acid is the primary acid in wine and has a bracing, drying effect. Like malic acid, it is potent and should be used drop-by-drop in 10:1 solution. I probably wouldn't use this as my only acid in canning as it would alter the flavor too much.
4. Citric acid. If your allergy is not to citric acid specifically, but to some proteins in citrus fruits, then a 4:1 solution of citric acid can be added drop-by-drop.

All of the plain acids can be purchased as refined powders on Amazon, Modernist Pantry, or sometimes in the baking sections of upscale grocers. If you're canning, you should use a pH meter to verify the safety of your jams and jellies (below 4.6).