I don't know much about elderberries specifically, but, generally, fermentation is a good thing. Generally increases the bioavailability of the active ingredients. Also, dark berries, dark fruits, purple, blue. Grapes, blueberries, things like that, generally have the good stuff.
A good, health promoting food/beverage, which I used to drink more often, now, not as much, is a perpetual kefir.
Go to the health food store and buy natural kefir, maybe split the kefir up into 2 mason jars, so that you have more, and then top each of the mason jars off with good stuff you want to ferment. The kefir that you're buying at the store is fermented milk, but you can ferment pretty much anything. If you have elderberries, add the elderberries, you can ferment garlic, ginger, turmeric, etc. Drink some, top it off, drink some more, top it off, perpetual. You might want one that's more of a health one that doesn't taste that good, and that you don't drink as much of, and you might want one that's tastier. The one with garlic, ginger, turmeric, hot peppers, cayenne pepper, etc., that one you don't consume as much. The other one you might consume as much of, that one might have the raw milk and the fruit juices in it mostly.
Wide mouth plastic lids on ball jars are air tight - they don't let air in - but they aren't so sealed that the co2 doesn't escape - they won't explode.