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Fatty Microbes: The Concept of Single Cell Oils
Although not so widespread as single cell proteins, microbial production of oils and fats, termed as Single Cell Oils (SCO), is a concept that is becoming increasingly popular among the scientists and industrialists. These microbial oils can be used for a variety of production lines ranging from edible oils and fats to biodiesel. Whereas the current trends of research in SCO mostly concentrate on the production of biodiesel, this article focuses on the production of edible oils and fats of microbial origin.

Oleaginous Microorganisms

Although all microbes have the ability to produce some amount of lipids for their structural components such as membranes, only a few of them are able to accumulate lipid in amounts which are of commercial importance. Those microorganisms which have the ability to amass lipids more than 20% of their dry cell weight are characterised as oleaginous microorganisms.

Oleaginous microorganisms fall within the groups of bacteria, algae, yeasts and fungi, the properties and the compositions of the oils varying on the species.

Among these, yeasts and fungi are the most efficient producers of oils. They produce a variety of oils that are structurally similar to plant oils. Moreover, they produce larger amounts of oils than other microbes and their oils can be easily extracted. These characteristics are considered important in order permit an industrially feasible production.

Micro algae also produce up to 20-40% (w/w dry weight) lipid content and their lipids include several polyunsaturated fatty acids of dietary significance. However, they require specific growth conditions such as warm temperature, sunshine and clean water making them a not so suitable choice for industrial applications. Furthermore, special recovery techniques are needed for the extraction of algal oils, thus further restricting their use. However, micro algae are an excellent choice to be used in waste or sewage treatment plants thereby permitting the production of oils simultaneously with the bioremediation process. In addition, several researches have been carried out and been successful on growing algae heterotrophically.

Bacteria, however, are not extensively used as oil producers. Though many species of bacteria do produce lipids, only a few of them accumulate high enough amounts of extractable lipids. Some species of bacteria such as Mycobacterium, Corynebacterium and Rhodococcus accumulate up to 30-40% of lipids but these lipids are hard to extract and are associated with toxic or allergic factors, making them undesirable as edible oil producers. However, there are on-going researches on using glycoloipids isolated form these organisms as surfactant. Some bacteria, such as Arthrobacter AK 19 can store up to 80% of lipids as its biomass. But this bacterium is slow growing, making its use in commercial applications somewhat restricted.

Physiology of Microbial Oil Production

Microorganisms produce and accumulate oils as an energy reserve. When the microorganisms are grown in a medium in which carbon is plentiful but another essential nutrient is scarce, the cells rapidly grow and proliferate until the limited nutrient is exhausted. Then the microbes [b]stop dividing but continue to grow. Usually, in industrial fermentations nitrogen is the growth limiting nutrient. When all the nitrogen in the growth medium is used up, the cells cannot create new cells since the protein and nucleic acid synthesis is stopped. But they continue to assimilate carbon and convert it into oils and fats.

The amount and the nature of the accumulated lipids depend on the type of microorganism and are genetically predetermined. ATP citrate lyase[b] is the enzyme that is responsible for lipid accumulation in larger quantities. The presence of this enzyme implies that the microorganism can stock lipids more than 20% their biomass.

[b]Industrial Production of Single Cell Oils

In commercial manufacturing plants, oil rich biomass is produced by fermentation of sugar rich media. These cells are then seperated by centrifugation. Then the oils are extracted by disrupting the cell walls in the presence of a solvent such as hexane and evaporating the solvent by drying under vacuum conditions. Finally, the extracted crude oil is refined through a series of steps including neutralisation, degumming, bleaching and deodorisation.

[Image: Figure12.png]

The Advantages and Disadvantages of Single Cell Oils

The synthesis of lipids by microorganisms has several advantages over the production of plant or animal derived oils. Microbes have shorter life cycles than plants or animals, warranting a rapid production. Unlike other sources, there is no requirement for farm lands and therefore the effect of climatic factors on the production is insignificant. Moreover, microbial fermentation involves less labour and the scaling up the production is easier than in other methods.

On the other hand, the microbial synthesis of lipids is not as economical as the plant oil production due to the high cost of the carbon sources and the requirement of sophisticated extraction techniques. Furthermore, the oil yields which can be obtained by microorganisms are comparatively lower than that with plants or animals.

However, these organisms can be genetically engineered to utilise cheaper substrates such as industrial wastes. Another more economical approach is modifying these organisms into producing more value-added specialty fats and oils or products which can’t be extracted through other sources. This will be more profitable than modifying plant oils into products of higher value such as cocoa butter.

Single Cell Oils in the Market

An infant formula including a blend of single cell oils, namely arachidonic acid (ARA) and docosahexaenoic acid (DHA) is currently available in Europe, Australasia, Far East and USA. In addition, cocoa butter-like products are produced using several species of yeasts such as Cryptococcus curvatus. Furthermore, many polyunsaturated fatty acids (PUFA) which have gained interest as dietary supplements and nutraceuticles, are being synthesised using microorganisms, mostly fungal and algal species.


Biotechnology for the Oils and Fats Industry edited by Colin Ratledge, Peter Stephen Shevyn Dawson, James Rattray

Food Biotechnology (2nd edition) edited by Kalidas Shetty, Gopinadhan Paliyath, Anthony Pometto and Robert E. Levin
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The edible oils extracted from microorganisms that are single-celled entities present at the bottom of the food chain are known as single cell oils. Different types of fungi and yeasts are the best producers with highest oil contents followed by various key algal groups with the capability of producing high levels of PUFA with nutritional significance. The potentiality of algae to produce PUFA has currently galvanized the interest in these fatty acids enriched single cell oils essential for the well being of mankind that are actually not available from plants or animals.
Although potential producers of single cell oils, opportunities for the biotechnological exploitation of the microorganisms are yet limited to the high-value products. Recently an attempt to produce a species of yeast, oil equivalent to cocoa-butter did not materialize due to a reduction in the price of cocoa butter in the world market and not due to inability of development of correct formulation of fatty acids. Production of PUFAs of both the n-3 and n-6 series from bacteria, fungi or algae are gaining better prospects in recent times. It is also observed that certain microbial PUFAs are devoid of other PUFAs, making the purification of desirable fatty acids quite easy than from other sources. Some microorganisms also generate precursors of prostaglandin and also prostaglandins themselves along with cerebroside lipids that are not usually regarded as of microbial origin.
Various microorganisms now have evolved to produce SCO as the commercial source of docosahexaenoic acid (DHA) and arachidonic acid (ARA). SCOs are now extensively used as dietary supplements in infant products. To improve lipid yields a better understanding of genetics and biochemistry of oil accumulation in microorganism is quite essential. Also an understanding of the mechanism involved in the polyunsaturated fatty acids (PUFAs) production is also highly desirable for increasing the oil content. A report on the biosynthetic machinery revealed that oil accumulation in oleaginous species accounts of lipid build up to more than 70% of the cell biomass. Whilst most of the microorganisms use a conventional fatty acid synthase (FAS) pathway for production of polyunsaturated fatty acids (PUFA) followed by a series of elongases and desaturases., it now appears to be via a polyketide synthase (PKS) system in Schizochytrium sp and probably in related thraustochytrid marine protists. It clearly represents a major deviation from the conventional mechanism of fatty acid biosynthesis, possibly as a way to decrease the amount of NADPH needed in the overall production process.
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