Using Microorganisms to Nourish the World

The world's steadily increasing population mandates the efficient use of finite natural resources and the development of new protein supplies to nourish that growing populace. One potential solution that addresses both of these needs is to cultivate microorganisms as a protein source, employ-

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ing industrial by-products currently considered wastes as the growth medium.

The term single-cell protein or SCP was coined in the 1960s to describe the use of unicellular organisms such as yeast and bacteria as a protein source. Today, the term generally encompasses the use of multicellular microorganisms as well, and might more accurately be called microbial biomass. While whole cells can be consumed, a more palatable alternative is to extract proteins from those cells and use them to form textured protein products.

Yeasts are considered the most promising large-scale source of single-cell protein. They multiply rapidly, are larger than bacteria, and are more readily acceptable as a potential food. The most suitable type ofyeast depends on the growth medium employed. The various genera of yeasts utilize differing carbohydrate sources and conditions for growth. For example Kluyveromyces marxianus can be grown on whey, a by-product ofcheese-making; Saccharomyces cere-visiae can grow on molasses, a by-product of the sugar industry; and Candida utilis can multiply on cellulose-containing by-products ofthe pulp and paper industry. Most ofthese wastes must be supplemented with a nitrogen source, as well as various vitamins and minerals, in order to support the growth of yeasts or other microorganisms.

To a lesser extent, the use of bacteria as source of SCP is also being explored. The cyanobacterium Spirulina maxima can be cultivated in alkaline lakes and then harvested and dried for use as a food source. Unfortunately, this requires adequate sunlight and warmth, making large-scale, year-round production impractical in many parts of the world.

One of the chief concerns regarding the consumption of microorganisms as a protein source is their high concentration ofnucleic acid, primarily RNA. Yeast and bacteria contain approximately three to eight times as much RNA per gram as does meat. High levels of nucleic acid in the diet cause an increase in uric acid in the blood, which can lead to gout and kidney stones. Chemical and enzymatic methods are being developed to decrease the nucleic acid in SCP without altering the nutritional value of the protein.

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