Yeast, the Basics & Background

This is a portion of an image of Dry Lager yeast placed in a micro titer plate, with 20% malt extract. The cells are at 10X magnification and are about 5 microns in diameter.


and after 10 hours…


Yeast is a single cell fungi used in many different fields, from cooking to cellular research to beer and wine making. Since I make beer and we generally look at cells under a microscope here at Kairos Instruments, it seemed like a good area for some further study. Turns out there are a lot of different types of fungi – 1.5 million species, and there are 1,500 species of yeast alone.

Fungi are a different classification from plants, animals and bacteria. The main difference is the cell wall contains chitin whereas cell walls of plants contain cellulose, and animals actually lack a rigid cell wall. Chitin is a long chain polymer, a derivative of glucose. This polymer is also the exoskeleton of many crustaceans like crabs and shrimp. Yeast is a small cell, 3 to 7 microns in diameter, 7 microns = .00027 inches. Some yeast cells reach 40 microns in diameter, .0016 inches. Many of these cells derive their energy from fermentation, a chemical conversion of carbohydrates to ethanol and carbon dioxide.

The process of using yeast as an industrial organism, that is to produce alcohol, is over 4,000 years old. Yeast was first observed under a microscope by Anthony van Leevwenhoek (1632-1723), “layu-wen-hook”.  Anthony was the first to see blood cells, and he built more than 500 single lens microscopes. His microscopes were very primitive single lenses, where compound microscopes had been around since 1595 by Robert Hook. But what made Anthony’s single lens microscopes so interesting was his ability to grind a very precise lens. His microscopes could magnify to 200x where compound microscopes of the day could only magnify to 20 to 30x.

Louis Pasteur (1822-1895) had an interest in yeast as well. He determined that alcoholic fermentation is conducted by living cells and not a chemical catalyst. Pasteur also noted that bubbling oxygen into a yeast broth increased cell growth while fermentation was inhibited, apparently there are 2 metabolic pathways. When the oxygen is low, ethanol and carbon dioxide are produced. The production efficiency of ATP however is low. ATP (Adenosine triphosphate) was discovered in 1929 by Karl Lohmann. As the oxygen concentration grows, the efficiency of ATP production increases 20x. This is known as the Pasteur effect.

The word ‘yeast’ comes from the old English gist, gist meaning boil, foam or bubble. They reproduce by budding. The bud or daughter cell continues to grow, the nucleus splits then migrates to the bud and finally separates from the parent. Yeast cells reproduce every two to three hours, 30 billion cells make up 1 gram of cells. Each cell undergoes budding 12 to 15x before it is no longer capable of reproduction. That’s 15 x 3 = 45 hours. One yeast cell can ferment approximately it’s own weight of glucose per hour!

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