Molds - General characteristics . Molds grow on most foods and require air and water, although they need less water than bacteria. In appearance, their masses of growth usually are fuzzy and can be nearly any color. However, the mycelium ("roots") of mold can spread invisibly through soft foods.

Some molds produce toxins (mycotoxins) that are carcinogenic. These molds commonly grow on fruits, fruit products, grains, grain products, and some cheeses.

Controlling Growth of Molds - Molds grow best at room temperature, but some can grow (slowly) at refrigerator temperatures. They are relatively tolerant of acid and salt. This is apparent because they grow on pickles, fruits, luncheon meats, and bacon. Molds require oxygen, so wrapping food tightly will reduce the opportunity for mold growth. Molds are easily killed by heat. Most begin to die at about 140 ° F; almost all are dead when food temperatures reach 190°F.

What to Do with Moldy Food? - Discard all moldy foods except hard cheeses. A spot of mold growing on a soft food, such as yogurt or an orange, may have spread its mycelia and possibly its toxins throughout the food.

If mold growth is not extensive on hard cheese, trim off the moldy area, removing at least ½-inch below the mold.

Yeasts - General characteristics . Yeast can grow with or without air and require more water than molds. Their masses in or on food appear as slime, scum, or murkiness. Yeast fermentation (link to glossary) in food is recognized by gas bubbles, froth, or foam, which result from the fermentation activity and the production of carbon dioxide gas. Depending on the specific growth conditions, yeast produce acids (vinegar), alcohol (beer and wines), or carbon dioxide (raised bread) during fermentation. This can be desirable or result in spoiled food.

Controlling Growth of Yeasts - Yeast grow best at room temperatures and are destroyed by heating foods to temperatures of 140°F to 190°F. They grow slowly on refrigerated foods. Yeast grow best on or in acidified foods and fruits. Yeast growth in these foods can raise the pH, changing the conditions to be more favorable for bacterial growth.