I have a bunch of veggie pulp left over from juicing, and it is packed down into little patties. After mushing in some black beans, it conforms to a hamburger shape, but is still very brittle. Is there anything I can add that will make it stickier, so it won't fall apart while cooking or eating?
Buy some powdered wheat gluten at a health food store. Add it to the mix, add some water if necessary, and stir it around in the same direction for 30 to 50 strokes. The gluten will bond and start forming long chains, and bring the whole mixture together. Nothing else really works as well.
Eggs and cheese will also help hold a veggie burger together, but only gluten really gives it cohesiveness and a springy texture.
FWIW, I can't imagine that veggie burgers made from juicer pulp will be even remotely appetizing.
Being mainly veg pulp, you need lots of binders to form it into a patty that won't be crumbly and brittle. I would make it by first frying the onions in some olive oil, then I would add the black beans and off the heat I would sprinkle on 1/3 cup of bread crumbs, an egg yolk and 1/4 cup of grated cheese (gruyere say). Then I would mash it all together and combine it with the veg pulp.
Hopefully this would bind it together and give it a perhaps more meaty texture. Hope this helps.