Fungi in Food: More Than Just Mushrooms!

I have taught the lab portion of a university course on mycology (the study of fungi) twice, and both times, the most entertaining lab to teach was the "food lab." Students' jaws dropped when they learned how many of their favorite foods contained fungi or required fungi for manufacture. Fungi are an incredibly diverse group of organisms with a wide array of metabolisms and biochemistries, and humans have cultivated many of them as foods, or in order to process foods. Most of these fungi do not produce mushrooms, and in fact very few of them are closely related those that do. Were I to ask you to list off the foods in your kitchen that could not be made without fungi, would you think to include your bottle of Sprite, blue cheese, or-dare I say it-chocolate?

Fungi are useful for processing foods because they are heterotrophs (organisms that get their nutrition from other organisms) that feed by absorption-that is, they secrete powerful enzymes into their environment that break down complex organic molecules into smaller molecules, which they then absorb. Thusly, we can alter the chemical makeup (and therefore the nutrition, taste, and texture) of foods by allowing fungi to grow into them. Fungi also produce chemicals-which are waste products as far as the fungi are concerned-that can impart desirable flavors and aromas to foods.

Many cheeses are ripened through the action of fungi. Milk, sometimes with flavoring or coloring added, is curdled to form cheese curd using bacterial enzymes, and after this stage many types of cheeses (like mozzarella and cheddar) simply allowed to age. Blue cheese, however, is made by introducing (or "inoculating") spores of the fungus Penicillium roquefortii into the curd. (Incidentally, P. roquefortii is closely related to Penicillium chrysogeum, the original source of the antibiotic Penicillin, but I'm sorry to say that eating blue cheese will not clear up any bacterial infections.) The spores germinate and the fungus grows through the curd, altering the chemistry of the cheese and producing the distinctive taste and smell of blue cheese. The powdery blue veins in blue cheese are actually masses of spores produced by the P. roquefortii. Camembert and Brie are produced with methods similar to those used to make blue cheese, but they are ripened with the use of the species Penicillium camembertii instead. The white, fuzzy rind of these two cheeses is made up of the fungus, and is perfectly safe to eat.

Cheeses aren't the only food whose flavors are improved by fungi, however. Many chocolate fans might be surprised to learn that their favorite dessert owes its appeal to a fungus. Chocolate is made from the beans of the cacao tree, which are too bitter to be palatable. The seeds are fermented with two fungi, Candida krusei and a species of Geotrichum. The fungi partially decompose the seeds, producing acetic acid, killing the embryo inside the seed, destroying the bitter compounds, and creating the aromas we associate with chocolate. After fermentation, the hulls of the seeds are removed, and the beans are processed into what we recognize as chocolate.

While foods can be ripened or fermented with fungi to change their flavor, aroma, and texture, fungi can also be used to change the nutrition of a food. For example, fungi are higher in protein than plant materials, so high-protein vegetarian foods can be made by allowing fungi to grow on and in substances derived from plants. Tempeh, a traditional Indonesian food, is made by soaking and cooking soybeans, then inoculating the beans with the fungus Rhizopus (closely related to common bread molds) and allowing the mixture to ferment. The Rhizopus grows throughout the beans, binding them together and converting many of the sugars to protein; tempeh also has more vitamins and dietary fiber than soy beans. A more high tech version of the process is used to make the "mycoprotein" meat-substitute brand-named Quorn; the fungus Fusarium venenatum is grown in vats while it is fed a nutrient mixture, then is harvested and mixed with bindings and flavorings.

Some fungi are used to ferment foods and beverages because they produce what are essentially metabolic waste products that happen to have desireable smells or flavors, or are somehow useful in and of themselves. For example, Aspergillus niger is used to ferment sugars into citric acid, which is used in many soft drinks. Another example is perhaps the best known fungal fermentation used by man: the fermentation of grains or grapes into beer or wine. In both, yeasts such as Saccharomyces cerevisiae are grown in a source of sugars (the grain or grapes) in the absence of oxygen gas; the lack of oxygen forces yeasts to switch to fermentation, a metabolism which can be sustained in the absence of oxygen, and they convert the sugars to ethanol.

These are just a few examples of foods that cannot be made without fungi. Which many people may only begrudgingly tolerate mushrooms and think that all other fungi are just weird or gross, hopefully you can now spread the word-we need fungi to make many of our favorite foods. We should be grateful to them!

References:

For more information about fungi & chocolate, see Tom Volk's webisite: http://botit.botany.wisc.edu/toms_fungi/feb2006.html

Wikipedia has articles about many fermented foods, including this one about tempeh: http://en.wikipedia.org/wiki/Tempeh