When you can cut the mold off

There's a specific moment, usually involving cheese, where you hold something up to the light and conduct a negotiation with yourself.

The good news is that this doesn't have to be a judgment call. USDA has an answer, the answer is different for different foods, and the logic behind it is worth understanding because it's not the logic most people assume.

What you're actually looking at

FSIS describes molds as microscopic fungi that live on plant or animal matter. Most are filamentous, meaning threadlike, and they reproduce via spores that travel by air, water, or insects. Nobody knows how many species there are; estimates run from tens of thousands to perhaps 300,000 or more.

The part that matters: unlike bacteria, which are single-celled, molds are made of many cells, and the fuzzy patch you can see is not the whole organism. There's a root structure, and where it goes depends entirely on what it's growing in.

The risks FSIS names are threefold. Some molds cause allergic reactions. Some cause respiratory problems. And a few, under the right conditions, produce mycotoxins, which are poisonous substances that can make people sick.

The rule is about density, not about how bad it looks

This is the reframe. Whether you can rescue something has almost nothing to do with the size of the mold patch and almost everything to do with the moisture and density of the food underneath it.

In a dense, dry food, the root structure can't travel far, so removing a generous margin around the visible growth removes the problem. In a soft or porous food, the roots move through the whole item invisibly, and there is nothing to cut around because the surface patch was never the extent of it.

Which means the small spot of mold on a soft food is a worse situation than the large spot on a hard one, exactly inverting the instinct.

The one-inch rule, and where it applies

For hard and semisoft cheeses, cheddar, parmesan, Swiss, colby and the like, USDA guidance is that you can cut away the moldy section and eat the rest. Cut at least one inch around and below the spot, and keep the knife out of the mold itself so you don't carry it into the rest of the block. Re-wrap in fresh wrap afterward rather than the old packaging.

That single rule is probably the most money-saving item on this blog. Hard cheese is expensive, blocks are large, and a discarded three-quarters of a block of good cheddar is a real loss for what amounts to a one-inch trim.

The same density logic extends to firm vegetables like cabbage and carrots, and to hard salami and dry-cured country hams, where surface mold can be scrubbed off.

What goes in the bin

The discard list is long and it's mostly a list of soft and porous things.

Soft cheeses, meaning cottage cheese, cream cheese, ricotta, chèvre. Yogurt and sour cream. Breads and baked goods. Soft fruits and vegetables. Jams and jellies, where scooping out the visible mold isn't recommended. Nuts, legumes, and peanut butter. Lunch meats, bacon, and hot dogs. Cooked leftover meat and poultry, cooked casseroles, and cooked grains and pasta.

And one that catches people: any cheese that's been shredded, crumbled, or sliced gets discarded, regardless of how hard the original block was. The processing distributed the surfaces, and the cutting tool may have distributed the mold along with them. Firmness was a property of the block, not of the bag.

Mold-ripened cheeses, which are a special case

Blue, Roquefort, Gorgonzola, Stilton, brie, Camembert. The molds used to make these cheeses are safe to eat, which is the entire point of them.

They can still grow mold that isn't part of the plan. The test is whether what you're looking at was there when you bought it. Unexpected growth, in a different color or texture than the cheese's normal veining or rind, means the same rules as any other cheese apply, and for the soft ones that means discarding the piece.

Two small things

Don't sniff it. Inhaling spores to check whether something's still good is the one diagnostic that carries its own risk, particularly for anyone with respiratory sensitivity. Look, don't inhale.

And when in doubt on any of the above, throw it out. That's the standard advice and it's the right advice, because the cost of being wrong is asymmetric and a block of cheese is not worth a coin flip.

The prevention half

Molds need moisture and oxygen, which means the same things that protect everything else in a refrigerator apply here. Wrap cheese properly rather than leaving a cut face exposed. Keep the appliance at temperature, since warm is faster for mold as it is for everything. Don't let things sit uncovered in a drawer for a fortnight, which is easier said than done and is most of what this blog is about.

The takeaway is narrower than a rule of thumb and more useful. Hard and dry, cut an inch and carry on. Soft, porous, or pre-cut, let it go. The visible patch is a report on the surface, and the surface is not the story.