To make dietary staples, such as cheese, yogurt and yeast breads, as well as other foods requiring fermentation, micro-organisms are essential. But they also may contribute to our food chain in a strikingly different and more direct way. Out of a quest for new sources of protein has come the notion of using micro-organisms as food.
The idea of eating microbes is hardly new. In parts of Africa, algae have been cultivated and eaten for centuries. During both world wars, protein was produced in Germany from yeast grown on a variety of substances.
More recently, scientists have become interested in the massive culturing of micro-organisms on a commercial scale.
Many microbes are high in protein, as much as 50 to 70 percent on a dry-weight basis. They grow rapidly, usually 100 to 1,000 times faster than higher plants and animals. And they can be raised on a large scale, independent of weather and soil conditions.
Single-cell protein (SCP) can be produced from a range of micro-organisms: bacteria, yeast, algae and fungi. But most algae, which include seaweeds and other aquatic plants, contain chlorophyll and require carbon dioxide and continuous sunlight. Those without chlorophyll, particularly yeasts and fungi, show more promise for commercial production.
Tests have shown that single-cell proteins are well tolerated by rats, chicken and swine. Until recently, humans have received them less favorably. People did not like the taste of microbial food. And volunteer trials often resulted in disagreeable side effects, such as intestinal cramping or nausea. Not long ago, however, in a study conducted at Massachusetts Institute of Technology, two novel single-cell foods were well-digested by 150 human volunteers. Subjects were divided into two groups and given bakery goods prepared with one of two fungal micro-organisms. Added to cookies were dried cells of Fusarium graminearium, in fine powder form. A commercial cake mix was used to make muffins containing Paecilomyces variotti.
The project was organized so neither the attending physician nor the volunteers knew whether the baked product was the ``control`` food or the fungal preparation. For 30 days, participants ate either a control food or an experimental fungal food. Then, after a week`s rest, the process was reversed. The controls were given the single-cell product for 30 days, while those initially given the test foods became the controls.
Participants found acceptable the foods prepared with the single-cell proteins. And there were no adverse side-effects from cookies baked with the fusarium products.
Another group of volunteers tested the nutritive value of these products. Foods prepared with single-cell protein scored impressively: Almost 83 percent were as digestible as casein, the major milk protein. (The lower value may be due to poorly digestible cell walls.)
As a human food, single-cell protein is imperfect. Microbes are high in nucleic acids, compounds which are not well-metabolized. Increased intake may lift uric-acid levels in blood and urine. Without processing to cut the amount of nucleic acids, single-cell foods may cause gout if consumed in great quantities.
Luckily, such processing is available. Micro-organisms can easily be grown in fermentation chambers under a controlled atmosphere, then treated to reduce their nucleic acids. The dried product yields a powder or granular material that is 55 percent protein, which may be spun into fibers similar to those prepared from soybeans.