Quoting Alizabeth DeYoung:" I don't feed from the jar. I make his food myself, using a food processor. Put the amount needed for ... [snip!] ... Unless you want to throw away money on jars for just one feeding only to toss it out when your kid only takes a few bites."
It isn't about the jar. You are doing it right. You're supposed to put the jar food in a bowl, so that the spoon doesn't contaminate the whole jar.
But if you make a 4 oz bottle, and the kid only eats 2 oz, you need to throw the rest of that formula out.