A single fuzzy spot on bread is enough for USDA guidance to say the entire loaf should be discarded, not trimmed.
The reason is hidden beneath the surface.
Bread is porous, so mold can send microscopic rootlike threads deep into the food before colored growth becomes obvious.
Those threads, called hyphae, form much of the mold’s body while the visible patch contains spores and surface structures.
USDA specifically lists bread and baked goods among foods to discard because contamination can extend below the surface.
Moldy foods may also harbor invisible bacteria, meaning clean-looking slices cannot be assumed safe.
Some molds can produce mycotoxins, toxic compounds that the FDA monitors in grains and other foods.
USDA also advises against sniffing moldy food because spores can cause respiratory problems.
With bread, cutting away what you see does not remove what may already be hidden.