What Causes Brown Spots on Mushrooms?

It is a common experience to select perfect, fresh mushrooms at the store, only to discover they have developed brown spots after a day or two in the refrigerator. This can be a source of uncertainty, raising questions about quality and safety. Understanding why these spots form is the first step toward knowing how to handle them, as this discoloration is a natural process and not always a sign of spoilage.

What Causes Brown Spots on Mushrooms

The primary reason brown spots appear on mushrooms is physical damage. Fungi are delicate and bruise easily from handling, pressure during transport, or bumping against each other. This bruising ruptures the mushroom’s cells, causing compounds inside to react with oxygen in the air. This oxidation process is chemically similar to how a cut apple darkens.

This discoloration is also a natural part of the mushroom’s life cycle. As mushrooms age, their cells begin to break down, a process called senescence, which leads to browning. Sometimes, these spots are caused by environmental factors, such as being stored in conditions with too much moisture, which can encourage bacterial growth.

One such issue is bacterial blotch, which appears as yellowish or dark brown spots on the mushroom cap. This is caused by Pseudomonas bacteria that thrive in damp conditions. Storing mushrooms in a way that traps moisture creates an ideal environment for these bacteria to multiply.

Are Spotted Mushrooms Safe to Eat

Determining whether spotted mushrooms are safe to eat involves inspecting their texture, smell, and the nature of the spots. If the brown spots are just bruises, the mushroom will still feel firm and dry. These spots are cosmetic and do not affect the safety or flavor; in some cases, older, slightly discolored mushrooms may even have a deeper, earthier flavor.

A fresh mushroom should have a pleasant, earthy smell. If you notice any off-odors, particularly a sour, fishy, or ammonia-like smell, this is a strong indicator of spoilage. Mushrooms with a foul odor have started to decay and should be discarded.

Texture provides another signal. Mushrooms that have gone bad will develop a slimy or sticky film on their surface. This sliminess is a result of bacterial decomposition and is a definitive sign of spoilage. If the mushrooms feel mushy, soft, or shriveled instead of firm, they have lost their freshness and should be composted.

Widespread or very dark discoloration is another warning sign. While a few isolated brown spots from bruising are fine, extensive dark patches suggest a more advanced state of decay. If you also see fuzzy white or dark mold, the mushrooms are certainly spoiled and must be thrown away.

How to Properly Store Mushrooms

Proper storage is key to minimizing browning and extending the freshness of mushrooms. The goal is to manage moisture and allow for air circulation. The original plastic-wrapped containers from the grocery store are not ideal because they trap moisture, which accelerates discoloration and spoilage.

The best method for storing whole, unwashed mushrooms is in a porous paper bag in the refrigerator. A paper bag absorbs excess moisture from the mushrooms while still allowing them to breathe, preventing them from becoming slimy. Placing a dry paper towel inside the bag can further help absorb any condensation, which helps preserve their firm texture.

Sliced mushrooms have a much shorter shelf life and should be stored using the same paper bag method, though they are best used within a couple of days. It is also advisable to wait to clean your mushrooms until just before you are ready to use them. Rinsing them introduces moisture that can hasten their decline, even if dried thoroughly.