Re: Why does mold grow differently on different foods?

There are many more kinds of molds than there are kinds of people. They are like us in that they use oxygen and give off carbon dioxide. One way of limiting mold growth is to keep oxygen or air away.

Molds are fungi, along with mushrooms. They have three life stages. It starts with a spore – a single, microscopic, inert cell that floats through the air. Spores land everywhere; we inhale them with nearly every breath. It seems they usually germinate (begin their life processes) only when they land in a favorable environment. What is favorable differs among molds: some grow well in the outdoors, and others seem to grow best in our refrigerators.

In a favorable environment, the spore stretches out into long strings (hyphae) that form a microscopic network like roots in, for example, foods. This network is called the mycelium. When the mycelium has developed to a certain stage, it begins to put out "fruiting bodies" that extend above the surface of the food. The types of fruiting bodies are the principle basis for classification of molds, because they are relatively easy to see with a microscope. They are what make the food surface look fuzzy; their purpose is to produce and scatter more spores. If colors are seen (green, blue, black, etc.), these are the colors of the spores.

Ideal conditions for mold development are generally what people like – warm, not too salty, and not to acid. However, almost everywhere that there are mold spores, there are also bacteria that grow faster than molds under these conditions. For example, bacteria grow in milk and produce lactic acid from the milk sugar (lactose). When too much lactic acid accumulates, the bacteria essentially die. Then the molds take over, even in the refrigerator, and we see their fruiting bodies at the surface. Which molds grow depends on which spores landed, which competing bacteria were present, and the composition of the food. Molds can grow in saltier and more acid conditions than most bacteria; and of course they grow in the refrigerator, but not as fast as they do at room temperature.

Food in cans has been heated enough that any spores that may have been present were killed. Other ways to prevent or delay mold growth are refrigeration (especially freezing), covering food to keep spores from landing on it, and wrapping food tightly to keep air away. Food that is wrapped air-tight must be refrigerated carefully, to prevent other foodborne diseases.

Although molds generally look yucky on food, most of them are relatively harmless. However, some produce poisonous substances (mycotoxins) that can cause serious illness in people and animals, so it is better to discard moldy food. Remember that by the time you can see the fruiting bodies on the surface of the food, there is already a mycelium down inside, so washing, scraping, or peeling the surface won't help much. It is safer to wrap and discard the moldy food so that people and animals are not exposed, and then clean the surfaces that the moldy food touched or was near.

Not all molds are bad, though. Some are used to make special cheeses: blue (French bleu) cheese has veins of blue in it because of the colorful mold spores. Other kinds of cheeses and some salamis depend on white molds for their character. Swiss cheese is made with bacteria that produce propionic acid, which discourages mold growth. Various Asian and other foods also depend on mold growth for their character; but if you aren't sure the mold is "friendly," don't eat the food!

Dean O. Cliver

Reference: http://www.fsis.usda.gov/factsheets/molds_on_food/index.asp