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Yeasts: Fermenters and Tormenters
| Article
# : |
15207 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
4 / 1989 |
2,780 Words |
| Author
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Herman Phaff
Herman Phaff is professor emeritus of food science and
technology and bacteriology at the University of California-
Davis. He has studied yeasts for over 40 years had has
published a book and over 250 scientific articles on yeast. |
The metabolic activities of yeasts were utilized by mankind for thousands of years before they were understood. Exquisitely preserved models of a brewery and a bakery found in a 4,000-year-old tomb at Thebes on the Nile show that the art of brewing and baking was well established in these early days of civilization. Similarly, there is evidence that other ancient civilizations had learned to ferment various sugar-containing liquids to make alcoholic beverages.
The first observation of the causative agents of fermentation was made by Antoine van Leeuwenhoek in 1680. With a primitive microscope, he observed tiny oval or spherical "animalcules" in droplets of various materials, including fermenting beer. However, the connection between the fermentation process and the bodies observed by van Leeuwenhoek was not clarified until nearly 200 years later, when in 1876 Louis Pasteur showed beyond doubt that alcoholic fermentation does not arise spontaneously; rather, it is a vital, energy-yielding process for yeast living in the absence of oxygen. However, while most yeasts used for alcoholic fermentation grow in anaerobic conditions, they can utilize a respiratory type of metabolism when oxygen is available. Those species that lack the fermentative ability altogether can grow only in the presence of oxygen.
Yeasts are generally regarded as unicellular fungi, but they do not constitute a precise, well-defined group in a botanical sense. Like higher fungi or "molds," yeasts lack chlorophyll, and because they lack the photosynthetic ability found in higher plants and algae they are also dependent on organic substrates for their nutrition. They have thick, rather rigid cell walls and a membrane-bound nucleus that contains more than one
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