To make dietary staples like cheese, yogurt and yeast breads, and other foods requiring fermentation, microorganisms are essential. But they may also 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 microorganisms 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 substrates.
More recently, scientists have become interested in the massive culturing of microorganisms on a commercial scale.
Many microbes are high in protein, as much as 50% to 70% 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.
Microbe Protein for Commercial Production
Single-cell protein can be produced from a range of microorganisms: 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 revealed 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 MIT, 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 microorganisms. Added to cookies were dried cells of Fusarium graminearium in fine powder form, and a commercial cake mix was used to make muffins containing Paecilomyces variotti.
The project was organized so that 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. After a week's rest, the process was reversed. The so-called 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. Two people who ate the muffins containing the Paecilomyces had mild skin rashes, but such reactions are common in 1% to 4% of the population, even to traditional foods. These symptoms of food sensitivity thus occurred no more frequently than might be expected with many ordinary foods.
A Favorable Balance
Another group of volunteers tested the nutritive value of these products. Foods prepared with single-cell protein scored impressively: Almost 83% were as digestible as casein, the major milk protein. (The lower value may be due to poorly digestible cell walls.) Two other measures of protein quality compared well to values for casein, indicating that the single-cell protein foods contain a favorable balance of essential amino acids. And when the sulfur-containing amino acid methionine is added to the test regimen, the protein quality rises even higher.
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.
Fortunately, such processing is available. Microorganisms can easily be grown in fermentation chambers under a controlled atmosphere, and then treated to reduce their nucleic acids. The dried product yields a powder or granular material that is 55% protein, which may be spun into fibers similar to those prepared from soybeans. From there, it can be used directly or in combination with meat and other foods.
What does this mean for the home consumer? It's not likely that you will shop for "bacteria burgers" in the near future. But single-cell products may someday bring us closer to a richer supply of available protein.
Limiting the Danger of Stomach Bezoars From Persimmon Fruit
Question: You wrote recently about the nutritional value of persimmons. Shortly afterward, I read that persimmons can form indigestible balls in the stomach and cause an obstruction. Is that true?
Answer: Yes, although it hardly appears to be a common problem. Bezoars, as these balls are called, are something of a paradox. Their name comes from a transliteration of Arabic ( badzehr ), Persian ( padzahr ), or Turkish ( panzehir ), all meaning anti-poison or antidote.
From antiquity until the 18th Century, bezoars from the stomachs of goats, gazelles and vicunas were valued for their magical or medicinal properties against such diverse ills as snakebite and plague. A gold-framed bezoar was even included in Queen Elizabeth I's crown-jewel inventory in the 1600s.
But times change, and today we have ceased to view them as an asset.
Persimmons are not the only food that can form bezoars. They have also been associated with excessive consumption of berries, orange and grapefruit pulp, figs, apple skins, green beans, cabbage and potato peel. (Hair and many other non-food items also form these indigestible balls.)
Persimmon bezoars were first recorded in the medical literature about 50 years ago in Japan, where the fruit remains popular and is traditionally served on New Year's Day. In this country, a series of case reports in 1938 described symptoms correlated with bezoar formation, 73% of which were traced to persimmons.
How bezoars come to form is not completely clear. It involves phlobutanin, a substance present in the skin of ripe fruits and the unripe pulp associated with its astringent properties. On contact with stomach acid, phlobutanin apparently coagulates to produce the ball, which may then be modified by the internal environment in the gut.
Although a variety of non-surgical techniques are used to remove bezoars, surgery is sometimes required.
For normal healthy people, the risk of bezoar formation from persimmons is not something to worry about, unless you do what would be unnatural: eat tremendous quantities of the fruit, especially when it is underripe.