Natural sources of fructose
As the name suggests, fructose, a monosaccharide, is found in fruit (and fruit juices) from which sometimes the name of “fruit sugar“.
Indeed glucose, generally in concentrations similar to those of fructose, and sucrose are usually present in fruit as well, so the previous name is in part improper.
It is also known as levulose.
Its content in fruit oscillates between 1% to 7%, even if some fruit contain considerably higher quantities. When ripe fruits become sweeter it is because contented sucrose is enzymatically cleaved into its component monosaccharides glucose and fructose.
Conversely, the monosaccharide is present in high amount in honey, where it may also represent half the weight.
Finally it may be taken up together with glucose when they form sucrose.
In nature, among sugars, it is the most sweet, though its sweetness depends on the physical state and the temperature in which it is.
In the crystalline form it is about twice (1.73) as sweet as sucrose, while dissolved in water its sweetness depends on temperature. This is due to the fact that levulose dissolved in water is present in four different ring structures:
- two with six carbon atoms, called pyranosic ones, respectively β-D-fructopyranose and α-D-fructopyranose, responsible for the sweetness of fructose;
- two with five carbon atoms, called furanosic ones, respectively β-D-fructofuranose and α-D-fructofuranose, and no one of the two last structures are sweet.
The four structures can interconvert each other with a velocity depending on the temperature, that is, it is as much faster as the higher.
While β-D-fructopyranose (the structure responsible for the sweetness of the monosaccharide) is present for 68% at 25°C (77 °F), its concentration decreases rapidly with the increasing of temperature, and only about 50% of fructose is present as β-D-fructopyranose at 80 °C (176 °F).
In crystalline form only β-D-fructopyranose is present, and this explains its higher sweetness than sucrose.
Note: levulose has an high affinity for water and it is difficult to crystallize. Just for this difficulty, only recently effective and economic methods for its crystallization are discovered, from which the successive sale in the shops.
Fructose and beverages
It is used to sweeten beverages, but also in this case things are not simple as they might seem.
As previously seen, its sweetening power, once dissolved in water depends on liquid temperature: as it increases the sweetening power lowers, which does not occur with sucrose. In fact, if fructose and sucrose have about the same sweetening power at 40 °C (104 °F), fructose, for many people, is about 20% less sweet than sucrose at 60 °C (140 °F).
So, if levulose is used as sweetener for tea, at the beginning this may also be twice as sweet as sucrose use, but as soon as the percentage of the sweet form decreases because of high temperature the sweetness decreases as well.
It is 20% to 50% sweeter than glucose, of which it has virtually the same caloric content (4 kcal/100 g), at body temperature therefore it could be used in its place ,but in lower amount.
Fructose and recipes
Currently, the complete substitution of sucrose with fructose in recipes is not feasible because of table sugar has a structural function as well, the base for the right consistency of the products.
Moreover, being levulose more similar than sucrose for water, it should cause the production of moister products (on the contrary in ice-cream industry it is used for some times together to sucrose, glucose and other sweeteners).
In the body it may have two metabolic fates, according to it reaches the liver or the other tissues.
In the liver, thanks to the presence of an enzyme, the fructokinase, it may be converted to triglycerides and other molecules.
In the other tissues, because of the absence of the above-said enzyme, it enters in the glycolitic pathway and may be used for energy production.
Fructose and gout
It should be noted that the use and abuse of great amount of fruit juices, and so fructose, might promote the deposition of uric acid, and then the development of gout. But, why? It occurs a great production of adenylic nucleotides, then a considerable catabolism of the same ones with production of uric acid.
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