Food Safety Issues/Possible food poisoning
I am a pretty nutritionally educated person so this has me a bit worried. I made a vegetarian sandwich with organic mozarrella, zucchini, sprouts, lettuce, onion and pannini pressed it in ghee. I added pesto at the last minute from a jar in the fridge. I also used honey mustard as a dipping sauce so I did not taste the rancid pesto. Upon cleaning I noticed the expiration date on the pesto, 2 months ago! I smelled it and sure enough, rancid. I tried to vomit as much as I could. So upon research I see this could be a very dangerous bacterial growth and illness. Is there any more I can do? Eat, drink, herbs? How serious is this? The pesto was from cosco, which has me even more worried.
Commercially processed foods packed in oil (garlic in oil, pesto, roasted tomatoes, etc) have all been acidified and are not subject to the growth of harmful bacteria. Home prepared garlic in oil, pesto, etc are capable of supporting the growth of clostridium botulinium which can cause the deadly illness -- botulism. Rest assured your Cosco purchased pesto has been manufactured in a way to prevent the growth of c. botulinium and other harmful bacteria.
As you've probably read, consuming rancid oil is not recommended as it forms harmful free radicals in the body which can cause cellular damage. Since you consumed a very small amount and in turn you vomited what you did consume, I would think your exposure to the rancid oil was greatly minimized.
Since you have already purged your stomach of the sandwich, I don't think there is anything else you need to do. Of course if you develop any symptoms such as fever, nausea, diarrhea, vomiting, etc then I would visit your physician.
FYI -- the "use by" date found on food is a quality date -- not a safety date. The manufacturer guarantee's the highest quality for the product if used by the suggested date. The date does not mean that consuming the product after the date will cause harm, only that the quality will not be as high as a new product.
Here's hoping that by the time you read this responce you are completely fine and that this experience is just a distant memory.