Levan is a naturally occurring

fructan A fructan is a polymer of fructose molecules. Fructans with a short chain length are known as fructooligosaccharides. Fructans can be found in over 12% of the angiosperms including both monocots and dicotyledon, dicots such as agave, artichokes, a ...

present in many plants and microorganisms. This polymer is made up of

fructose Fructose (), or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and gal ...

, a monosaccharide sugar, connected by 2,6 beta glycosidic linkages. Levan can have both branched and linear structures of relatively low molecular weight. Branched levan forms a very small, sphere-like structure with basal chains 9 units long. The 2,1 branching allows methyl ethers to form and create a spherical shape. The ends of levan also tend to contain a glucosyl residue. Branched levan tends to be more stable than linear polysaccharides. However, the amount of branching and length of polymerization tends to vary among different species. The shortest levan is 6- kestose, a chain of two fructose molecules and a terminal glucose molecule.

Discovery

Levan was first discovered through research on

nattō is a traditional Japanese cuisine, Japanese food made from whole soybeans that have been Fermentation in food processing, fermented with Bacillus subtilis, ''Bacillus subtilis'' var. ''natto''. It is often served as a breakfast food with rice. ...

, a traditional Japanese dish. Natto was known as a " superfood" which promoted health and longevity in Japan during the late 1800s. In 1881, Edmund Oscar von Lippmann first discovered "lävulan" (levan) as the remaining gum from

molasses Molasses () is a viscous byproduct, principally obtained from the refining of sugarcane or sugar beet juice into sugar. Molasses varies in the amount of sugar, the method of extraction, and the age of the plant. Sugarcane molasses is usuall ...

in sugar beet production. Later in 1901, Greig-Smith coined the name “levan” based on the

levorotatory Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...

properties of this substance in

polarized light , or , is a property of transverse waves which specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. One example of a polarize ...

Production

Levan is synthesized in

archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...

, fungi, bacteria, and a limited number of plant species. Fructans such as levan are synthesized from

sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...

, a

disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose, ...

containing glucose and fructose. In plants, the

vacuole A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...

is where fructan production occurs. Sucrose:sucrose/fructan 6-fructosyltransferase is the fructosyltransferase in the vacuole which creates the beta 2,6 linkages to form the linear form of levan. Bacteria also use a fructosyltransferase known as levansucrase to form levan. These enzymes in bacteria form the 2,1 linkages in the linear basal chains of levan to allow for branching points to occur. Many bacteria produce levan in the cell exterior. This production can be sensitive to temperature, oxygen concentration, pH, and other factors. Levan production in bacteria is typically a sign of growth in population. There are also possible ways to produce by fracturing

soybean The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean. Soy is a staple crop, the world's most grown legume, and an important animal feed. Soy is a key source o ...

mucilage Mucilage is a thick gluey substance produced by nearly all plants and some microorganisms. These microorganisms include protists which use it for their locomotion, with the direction of their movement always opposite to that of the secretion of ...

. Levans are produced by microbes during colonization of a food substrate. '' Erwinia amylovora'' exudes levan and amylovoran as part of its

biofilm A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...

. Together they contribute to its pathogenicity. In 2016, Ua-Arak et al. developed a

sourdough Sourdough is a type of bread that uses the fermentation by naturally occurring yeast and lactobacillus bacteria to raise the dough. In addition to leavening the bread, the fermentation process produces lactic acid, which gives the bread its dis ...

method with high levan output (among other

exopolysaccharide Extracellular polymeric substances (EPS) are natural polymers of high molecular weight secreted by microorganisms into their environment. EPS establish the functional and structural integrity of biofilms, and are considered the fundamental compo ...

s).

Properties

The beta 2,6 linkages of levan allow for it to be soluble in both water and oil; however, the water temperature varies the degree of

solubility In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...

. Levan also is insoluble in many organic solvents such as methanol, ethanol, and isopropanol. The branching of levan allows it to have a high tensile and cohesive strength, while the hydroxyl groups contribute to adhesion with other molecules. The intrinsic viscosity n, a measure of the substance effect on viscosity of a solution, tends to be very low for levan. This allows levan to be utilized in a pharmaceutical setting.

Real world implications

Many industries such as

food Food is any substance consumed by an organism for Nutrient, nutritional support. Food is usually of plant, animal, or Fungus, fungal origin and contains essential nutrients such as carbohydrates, fats, protein (nutrient), proteins, vitamins, ...

, beverages, cosmetics, and even medicine utilize levan in their products. One of the reasons levan is able to be used in such a versatile way is that it fulfills all safety guidelines. Levan does not cause any form of skin or eye irritation, has not shown any allergenic effects, and poses no threat of

cytotoxicity Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are toxic metals, toxic chemicals, microbe neurotoxins, radiation particles and even specific neurotransmitters when the system is out of balance. Also some types of d ...

Food

In the food industry, levan is incorporated due to its prebiotic effects, cholesterol lowering ability, and adhesive properties. It also occurs naturally in low amounts in food for human consumption. Levan is also included in many dairy products as fiber or sweetener. Commercial, non-alcoholic beverages use levan as well in ultra-high-fructose-syrups. Interestingly, levan causes useful bacterial growth and proliferation which can be especially important in the gut because it causes a decrease in population of the pathogenic bacteria.

Cosmetics

Levan can be used for

hair care Hair care or haircare is an overall term for hygiene and cosmetology involving the hair which grows from the human scalp, and to a lesser extent Facial hair, facial, Pubic hair, pubic and other body hair. Hair care routines differ according to a ...

and

skin whitening Skin whitening, also known as skin lightening and skin bleaching, is the practice of using chemical substances in an attempt to lighten the skin or provide an even skin color by reducing the melanin concentration in the skin. Several chemicals h ...

. In hair care products, levan acts to form a film which creates a hair holding effect utilized in various gels and mousses. Levan is used as a skin whitener as well because it has been tested to show inhibition on melanin production by decreasing the activity of the enzyme

tyrosinase Tyrosinase is an oxidase that is the rate-limiting enzyme for controlling the production of melanin. The enzyme is mainly involved in two distinct reactions of melanin synthesis otherwise known as the Raper–Mason pathway. Firstly, the hydroxy ...

which is responsible for melanogenesis.

Medicine

Levan has shown uses for burned tissue, anti-inflammation, and

aquaculture Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. Nelu ...

. By combining levan into a thin film, it is able to activate an enzyme known as metalloproteinase which increases the recovery and healing process. In the case of inflammation, levan interacts with the aggregating cells and affects their adhesion to the blood vessel causing reduced accumulation. In aquaculture, results have shown that levan incorporated diets could possibly cause an increase in aggregation of viruses allowing for easier

phagocytic Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...

removal. Levan produced by ''Pantoea agglomerans'' ZMR7 was reported to decrease the viability of rhabdomyosarcoma (RD) and breast cancer (MDA) cells compared with untreated cancer cells. In addition, it has high antiparasitic activity against the promastigote of ''Leishmania tropica''

References

{{Carbohydrates Polysaccharides Fructosides