Saprophytes (Greek, Sapros, decay, Phytos, plant) are fungi, which play a vital role in recycling organic nutrients in environment. Microscopic fungi are decomposers that grow on dead or decaying organic matter such as animal dung and dead wood logs. These act as predators to trap soil worms and commercially used in industrial biotechnology for brewing, reducing paint or oil.

Fungus mycelium (Latin, Myco, vegetative fungus) grows as branching thread-like hyphae, absorbs water and nutrient from decaying organic matter to derive energy and built-up a fruiting body like mushroom. Fungal fruiting bodies are diverse in shape and color; and, release reproductive spores, which decompose by growing on animal body, feces or plant matter. Fungi are found in all moist habitats. These decompose fruit, bread, yogurt, milk, leather, wood, garden refuse, feces, animal body and skin.

Mushrooms are edible form of fungus made from decomposing matter in soil. It is cultivated commercially to produce cheese, beer, antibiotics. Ringworm is a disease caused by fungal manifestation of skin. In biology, fungus is a general term used to encompass the diverse morphologic forms such as yeasts and molds, which are primitive plants with the ability to utilize external organic compounds as source of carbon but lack chlorophyll. Fungi can breakdown complex organic substances of almost every type like sugar, oligosaccharides, lactose, cellulose, starch, and play an essential role in recycling of carbon and its derivatives in biosphere. Yeasts particularly are industrially important in fermenting foods, baking, cooking, brewing wine and alcohol or development medicinal compounds, antibiotics, and other essential biological products.

Yeast is a general term denoting fungi – Saccharomycetaceae (Greek, Saccharos, sugar, Latin, Mycos, vegetative fungus, -aceae, microbial family), which is widely distributed in substrates that contain sugars in addition to soil, vegetative parts of plants. Yeasts possess remarkable ability to ferment carbohydrates thus important to baking and brewing industry. Some commercially important types of yeasts include:
(1)    Brewer’s yeast: It is yeast produced from Saccharomyces cerevisiae, which is a by-product of brewing beer.
(2)    Compressed yeast: It is moist living cells of Saccharomyces cerevisiae in combination with starchy or absorbent base.
(3)    Cultivated yeast: This form is yeast cultured or cultivated for making bread or ferment foods (pickling) that contain carbohydrates.
(4)    Dried yeast: This may be brewer’s dried yeast, debittered brewer’s dried yeast, or primary dried yeast. Dried yeast is also a dietary supplement that contains about 45% proteins
and some Vitamin B complexes.
(5)    Wild yeast: It is uncultivated form of yeast obtained from a natural source. Yeast is naturally distributed in vegetative matter, natural sugars, soil and nitrogenous matter. Brewer's yeast, cultivated yeast and dried yeast are made from wild yeast and commercially important in fermentation industry – bread, brewing, dietary supplements.

Chronological history of yeast
Since prehistoric times, yeasts were known as group of micro-organisms that grow on organic sugars, which mainly found use in brewing wine and beer. Van Leeuwenhoek (Dutch microbiologist) in 1680 investigated this remarkable property of yeast in Grape cultivation and discovered that yeast is also responsible for leavening of dough (Latin, Levare, Levure, leaven, to raise) as it is capable of releasing CO2 during the baking process.

Fermentation of sugar malt by yeast in 1837 was given the name Saccharomyces cerevisiae (yeast strain observed in malt). The English word – Yeast, like Dutch Guist, German Hefe (haf-jon), Greek Zymi are used simultaneously for yeast and dough and form roots in words related to brewing, brooding or fermentation. In fact, modern expression of biological catalysts or 'enzymes' (en + zymi = occurring in yeast) came from yeast in the year 1877 through another Dutch chemist (Kuhne) through discovery from compounds derived from yeast capable of fermenting different sugars. Prior to work of Kuhne, French microbiologist Louis Pasteur in 1857 invented the process of pasteurization for development of vaccines to anthrax, rabies and chicken cholera during his time at Strasbourg University (Louis Pasteur – Études sur la levure de bière). Louis Pasteur correlated fermentation with yeast metabolism in 1876 for unique properties of yeast in about 700 yeast species including S. cerevisiae (a subgroup from 700,000 different fungi) as a preferred organism for research. Moreover, S. cerevisiae and other yeasts yielded a vast majority of industrial and medical applications beneficial to human life like production of different alcohols, cell-free extracts (intermediate metabolites), glycerol and therapeutic proteins (enzymes).

Yeast was introduced as experimental organism in the 20th century and ever since received increasing attention. Lindegren in 1949 gave first genetic map of yeast. Kluyver, Hinnen, Hicks and Fink anticipated basic biological structures and processes conserved throughout eukaryotic life. Yeast genetics and the ease of manipulation gave the technical breakthrough in genetic engineering techniques and recombinant DNA technology.

Yeast has substantially contributed to the enormous growth of molecular biology through now known basic experimental system. In 1980, Hepatitis B vaccine was the first commercial pharmaceutical product obtained from recombinant yeast. Ease of genetic manipulation in yeast has opened the possibility to dissect functional gene products from other eukaryotes (bacteria, plant and animal cells) using the yeast system. It is a well-established fact that yeast is an ideal system in which cell architecture and fundamental cellular mechanisms can be successfully investigated.

Among all eukaryotic model organisms, S. cerevisiae combines several advantages. It is a unicellular organism, which can be grown on defined media giving the investigator complete control over environmental (experimental) parameters. The wealth of sequence information obtained in the yeast genome project is extremely useful reference against sequences of human, animal or plant genes, and those of multitude of unicellular organisms now under study.

Some common therapeutic products derived from yeasts include Prokaryotic products (Tetanus toxin fragment C, Streptokinase), Surface antigens of viruses (Hepatitis B, HIV, Foot and mouse disease, Influenza, Polio), Polyoma, Epstein-Barr strains, Oncogenic retroviruses, Malaria antigens, Animals products (Hirudin, porcine interferon, interleukin, trypsin inhibitor), Human hormones (Insulin, parathyroid hormone, growth hormone, chorionic gonadotropin), Human growth factors (IGF1, INF-alpha and INF-beta1, NGF, EGF, tissue factor, CSF, GM-CSF, TNF), Human blood proteins (Hemoglobin, factors VIII and XIII, aplha-1-antitrypsin), Antithrombin III, Serum albumin, Enzymes, CFTR estrogen receptor.

Yeast fermentation
Fermentation (Latin, Fevere, to boil) is a biochemical (metabolic) process that describes action of yeast on extracts of fruit or malted grains. In biochemical terms, fermentation relates to generation of energy by the catabolism of organic compounds (mostly carbohydrates). There are 5 major groups in commercially important fermentation processes: (a) Microbial cells (biomass production), (b) Microbial enzymes (Biocatalysts), (c) Metabolites (Organic intermediates), (d) Recombinant products (therapeutic rDNA, rRNA and proteins), and (e) Modulator substances (Transformation of polymers). Cultivation of microbial biomass involves yeast production for baking, brewing and organic applications such as production of microbial cells for human and animal food supplements (single cell proteins). Baker’s yeast has been produced on large scale since early 1900s for production of human food supplements.

Components of fermentation process include the following steps:
1.    Formulation of culture media using processing organism (typically, yeast).
2.    Development of inoculums for production process.
3.    Sterilization of medium and production equipments.
4.    Production of active pure culture under optimal conditions with additional nutrient supplements.
5.    Extraction and purification of product after completion of fermentation process.

Sugar metabolism (Glycosis) is an oxidative process that breakdown glucose into pyruvate to enters citric acid cycle (TCA cycle) and cellular respiration for production of energy. Glycosis and Gluconeogenesis cycles in plants and animals recycle (reduced) pyridine nucleotides (NAD+) and (oxidized) pyridine nucleotides NADH. Yeast is a facultative anaerobe i.e. capable of both aerobic oxidation (involving oxygen) and anaerobic oxidation or fermentation (lack of oxygen supply) on carbohydrates. Energy for catalysis, transport and synthesis by yeast is derived by breakdown of sugars and involves recycling of NAD+ and NADH via different regulatory mechanisms depending upon the availability of oxygen. The process of sugar metabolism (cellular respiration or fermentation) involves production of CO2 that give boiling appearance (bubbles) to starch while undergoing transformation. Under aerobic conditions reoxidation of NAD+ occurs via electron transfer through cytochrome – cellular respiratory mechanism. Under anaerobic conditions, NAD+ couples to reduction of organic compound, which is subsequent product of the catabolic pathway.

Pyruvate (from glycolysis) à Acetaldehyde + CO2 à Acetaldehyde à Ethanol
Action of yeast on fruit or grain malts regenerates NADH from NAD+ by coupling reduction of pyruvate to ethanol. Thus, fermentation is an energy-generation process where organic compounds act as both terminal electron donors and electron acceptors.