Hello Hannah. What you are seeing under the lid is almost certainly a harmless chemical reaction rather than mold.
When preserving acidic foods like pickled tomatoes, the combination of vinegar, natural acid, and sulfur compounds in the produce interacts with the metal lid. Metal canning lids are lined with a protective food-grade enamel coating, but minor microscopic imperfections or pinholes in this coating can allow acidic vapors to reach the bare metal underneath. Over time, this causes localized corrosion, which leaves behind a dark gray or black deposit (typically iron sulfide or metal oxide). This phenomenon is well documented in food preservation and gives off that dry, soot-like or graphite-like appearance.
To distinguish between corrosion and mold, look at the physical texture:
1. Mold is an organic growth that typically appears fuzzy, raised, and velvety. It usually grows directly on the food or along the liquid surface where moisture and oxygen meet.
2. Corrosion residue is flat, has no hair or fuzz, and rubs off cleanly like pencil lead or charcoal dust when touched with a napkin or utensil.
Your pickled tomatoes are safe to eat provided that a few basic safety checks pass:
1. The vacuum seal was completely intact before opening (you heard a release of pressure or felt firm resistance when popping the lid).
2. The contents smell normal and pleasant, with no sour, fermented, or foul odors.
3. There is no unexplained fizzing, spurting liquid, or abnormal cloudiness in the brine.
Since the protective coating on that lid has compromised metal underneath, avoid using that specific lid for ongoing storage. You can safely transfer any remaining tomatoes into a glass container with a fresh lid and keep them refrigerated.
