Vitamin C (ascorbic acid) - properties - synthesis - production

Vitamin C (ascorbic acid)

Ascorbic acid (vitamin C) is an essential organic compound required for umpteen biosynthetic activities in most of the animals and all humans.
Properties of vitamin C
Ascorbic acid, the official name of vitamin C, was first discovered in 1912.
In 1933 it was isolated for the first time in lemons.
It appears as white or light yellow crystals or powder. Its empirical formula is C6H8O6.
It is made up of elements carbon, hydrogen and oxygen.
It is water soluble. In alcohol it dissolves sparingly.
In benzene, ether and chloroform it is insoluble.
It is also known as L-ascorbate. Its chemical name is L-ascorbic acid.
At the temperature of 190-192 degree Celsius it decomposes.

Its bioavailability is rapid and complete. It binds negligibly with proteins.
Its metabolic half life is 30 minutes. It is excreted by the kidneys.
 Biosynthesis of vitamin C in animals and the human inability
It is biosynthesized by reptiles and birds (older orders) in their kidneys. Birds of recent orders and most of the mammals synthesize it in their livers.
In these animals glucose is converted into ascorbic acid with the help of the enzyme L-gulonolactone oxidase.
In most of these animals ascorbic acid is a natural metabolite of liver.
Humans, most of the primates and guinea pigs cannot produce vitamin C in the body and they are required to source it from their food and can develop its deficiency.
It is hypothesized that humans and primates, by a genetic mutation, must have lost their capacity to synthesize L-gulonolactone oxidase long ago due to which they are unable to produce ascorbic acid.
This being a vital nutrient for the biosynthetic activities, is included as 'vitamin C' among essential nutrients.
The pharmacophore (the molecular frame required for the biological activity) of ascorbic acid to function in the biological activities is ascorbate ion.
L-ascorbate, which is related to glucose in structure, occurs naturally attached to hydrogen forming ascorbic acid.
Alternatively it may occur attached to metal ion forming its ascorbate.
This ascorbate ion is highly reactive and scavenging free radicals, functions as antioxidant.
 Absorption, transport and excretion of ascorbic acid
Vitamin C is absorbed into the body by simple diffusion and active transport by SVCTS (Sodium-Ascorbate Co-Transporters) and GLUTS (Hexose transporters).
In normal regular intake absorption is 75-90%.
In heavy doses the absorption rate may fall down to 15%.
There is a renal reabsorption threshold and any concentration above this is excreted through urine.
Though there is a blood threshold for renal reabsorption, many tissues in the body maintain higher concentrations of it.
Adrenal glands, pituitary gland, corpus luteum, thymus and retina accumulate vitamin C as high as 100 times the concentration in blood.
The excess of vitamin C which is not yet excreted is oxidised and broken down by the body by the activity of the enzyme L-ascorbate oxidase.
Commercial production of Ascorbic acid
Earlier vitamin C has been made commercially by extracting it from plants.
Later vitamin C was chemically synthesized. Presently it is being made by two processes.
Reichstein process
In the Reichstein process, which is traditional and earlier, glucose is converted into sorbitol by heat treatment.
Sorbitol is fermented and oxidised (by a microorganism) to form sorbose.
This is made to react with acetone to produce di-acetone sorbose and further oxidised to produce DAKS (di-acetone keto gulonic acid).
Then by dissolving it in a mixture organic solvents the structure of DAKS is rearranged with the help of a catalyst to form vitamin C.
In the next step, by recrystallization vitamin C is purified.
Glucose by heat treatment --> Sorbitol by fermentation --> sorbose by chemical process --> Diacetone sorbose by chemical process --> DAKS by chemical process --> raw vitamin C by recrystallisation --> purified vitamin C
two-step fermentation process
In the two stage fermentation process developed in China, a second stage fermentation of sorbose produces KGA and this like DAKS by the same chemical process is converted into crude vitamin C and then purified.
The later process is cost effective and more realisation of ascorbic acid is achieved.
Much of the world supplies of vitamin C are from China. glucose by heat treatment --> sorbitol by fermentation --> sorbose by fermentation --> KGA by chemical process --> raw vitamin C by recrystallisation --> purified vitamin C
Other uses of ascorbic acid
Calcium, potassium and sodium salts of ascorbic acid are used as food additives for their antioxidant properties.
Ascorbyl stearate or ascorbyl palmitate, which are fat soluble esters of ascorbic acid, are used for protecting fatty food from oxidation.
It is used for enhancing the fermentation process in food preparation. It is also used in chemical processes as reducing agent. Vitamin C (ascorbic acid) is used to fix and keep the red colour of meat products.
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Properties of vitamin C and its production and synthesis.

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