Fructose
Fructose, or fruit sugar, is a common monosaccharide, i.e. a simple sugar. It is classified as a reducing hexose, more specifically a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. In terms of structure, it is a C-4 epimer of glucose. A white, water-soluble solid, it is one of the three dietary monosaccharides, along with glucose and galactose.
Fructose is found in honey, tree and vine fruits, flowers, berries, and most root vegetables.
History
Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847. The name "fructose" was coined in 1857 by the English chemist William Allen Miller. Pure, dry fructose is a sweet, white, odorless, crystalline solid, and is the most water-soluble of all the sugars.Etymology
The word "fructose" was coined in 1857 from the Latin for fructus and the generic chemical suffix for sugars, -ose. It is also called fruit sugar and levulose or laevulose, due to its ability to rotate plane polarised light in a laevorotary fashion when a beam is shone through it in solution. Likewise, dextrose is given its name due to its ability to rotate plane polarised light in a dextrorotary fashion.Chemical structure
Fructose adopts both cyclic six- and five-membered structure, The six membered ring can exist as either the β--fructopyranose and α--fructopyranose. The five-membered rings exists as either of two isomers β--fructofuranose and α--fructofuranose. Additionally, an acyclic form exists: keto--fructose. At 70% and 22% respectively, fructopyranose and fructofuranose are the dominant species in aqueous solution.Chemical reactions
Fructose and fermentation
Fructose may be anaerobically fermented by yeast and bacteria. Yeast enzymes convert sugar to ethanol and carbon dioxide. Some of the carbon dioxide produced during fermentation will remain dissolved in water, where it will reach equilibrium with carbonic acid. The dissolved carbon dioxide and carbonic acid produce the carbonation in some fermented beverages, such as champagne.Fructose and Maillard reaction
Fructose undergoes the Maillard reaction, non-enzymatic browning, with amino acids. Because fructose exists to a greater extent in the open-chain form than does glucose, the initial stages of the Maillard reaction occur more rapidly than with glucose. Therefore, fructose has potential to contribute to changes in food palatability, as well as other nutritional effects, such as excessive browning, volume and tenderness reduction during cake preparation, and formation of mutagenic compounds.Dehydration
Fructose can be dehydrated to give hydroxymethylfurfural, which can be processed into liquid dimethylfuran.This conversion has long been proposed, not implemented, as a route to green fuels.
Physical and functional properties
Sweetness of fructose
The primary reason that fructose is used commercially in foods and beverages, besides its low cost, is its high relative sweetness. It is the sweetest of all naturally occurring carbohydrates. The relative sweetness of fructose has been reported in the range of 1.2–1.8 times that of sucrose. However, it is the 6-membered ring form of fructose that is sweeter; the 5-membered ring form tastes about the same as usual table sugar. Warming fructose leads to formation of the 5-membered ring form. Therefore, the relative sweetness decreases with increasing temperature. However, it has been observed that the absolute sweetness of fructose is identical at 5 °C as 50 °C and thus the relative sweetness to sucrose is not due to anomeric distribution but a decrease in the absolute sweetness of sucrose at higher temperatures.The sweetness of fructose is perceived earlier than that of sucrose or glucose, and the taste sensation reaches a peak, and diminishes more quickly than that of sucrose. Fructose can also enhance other flavors in the system.
Fructose exhibits a sweetness synergy effect when used in combination with other sweeteners. The relative sweetness of fructose blended with sucrose, aspartame, or saccharin is perceived to be greater than the sweetness calculated from individual components.
Fructose solubility and crystallization
Fructose has higher water solubility than other sugars, as well as other sugar alcohols. Fructose is, therefore, difficult to crystallize from an aqueous solution. Sugar mixes containing fructose, such as candies, are softer than those containing other sugars because of the greater solubility of fructose.Fructose hygroscopicity and humectancy
Fructose is quicker to absorb moisture and slower to release it to the environment than sucrose, glucose, or other nutritive sweeteners. Fructose is an excellent humectant and retains moisture for a long period of time even at low relative humidity. Therefore, fructose can contribute a more palatable texture, and longer shelf life to the food products in which it is used.Freezing point
Fructose has a greater effect on freezing point depression than disaccharides or oligosaccharides, which may protect the integrity of cell walls of fruit by reducing ice crystal formation. However, this characteristic may be undesirable in soft-serve or hard-frozen dairy desserts.Fructose and starch functionality in food systems
Fructose increases starch viscosity more rapidly and achieves a higher final viscosity than sucrose because fructose lowers the temperature required during gelatinizing of starch, causing a greater final viscosity.Although some artificial sweeteners are not suitable for home baking, many traditional recipes use fructose.
Commercial production
Fructose is produced on an industrial scale from three precursors: starch, sucrose, and inulin.Sucrose is an organic compound with one molecule of glucose covalently linked to one molecule of fructose. All forms of fructose, including those found in fruits and juices, are commonly added to foods and drinks for palatability and taste enhancement, and for browning of some foods, such as baked goods.
Fructose is found in honey, tree and vine fruits, flowers, berries, and most root vegetables.
Starch is hydrolyzed to glucose, which is converted to fructose by the enzyme glucose isomerase. This mixture is high-fructose corn syrup. At 60 °C, the conversion gives a 1:1 mixture of glucose and fructose. Sucrose is hydrolyzed to give its monomeric precursors glucose and fructose. Inulin is also converted to fructose on a commercial scale., about 240,000 tonnes of crystalline fructose were being produced annually.Commercially, maize is a major source of starch. Sugar cane and sugar beets are sources of sucrose is a compound with one molecule of glucose covalently linked to one molecule of fructose. Inulin is found in chicory,
Natural sources
Natural sources of fructose include fruits, vegetables, and honey. Fructose is often further concentrated from these sources. The highest dietary sources of fructose, besides pure crystalline fructose, are foods containing white sugar, high-fructose corn syrup, agave nectar, honey, molasses, maple syrup, fruit and fruit juices, as these have the highest percentages of fructose per serving compared to other common foods and ingredients. Fructose exists in foods either as a free monosaccharide or bound to glucose as sucrose, a disaccharide. Fructose, glucose, and sucrose may all be present in food; however, different foods will have varying levels of each of these three sugars.The sugar contents of common fruits and vegetables are presented in Table 1. In general, in foods that contain free fructose, the ratio of fructose to glucose is approximately 1:1; that is, foods with fructose usually contain about an equal amount of free glucose. A value that is above 1 indicates a higher proportion of fructose to glucose and below 1 a lower proportion. Some fruits have larger proportions of fructose to glucose compared to others. For example, apples and pears contain more than twice as much free fructose as glucose, while for apricots the proportion is less than half as much fructose as glucose.
Apple and pear juices are of particular interest to pediatricians because the high concentrations of free fructose in these juices can cause diarrhea in children. The cells that line children's small intestines have less affinity for fructose absorption than for glucose and sucrose. Unabsorbed fructose creates higher osmolarity in the small intestine, which draws water into the gastrointestinal tract, resulting in osmotic diarrhea. This phenomenon is discussed in greater detail in the [|Health Effects] section.
Table 1 also shows the amount of sucrose found in common fruits and vegetables. Sugarcane and sugar beet have a high concentration of sucrose, and are used for commercial preparation of pure sucrose. Extracted cane or beet juice is clarified, removing impurities; and concentrated by removing excess water. The end product is 99.9%-pure sucrose. Sucrose-containing sugars include common white sugar and powdered sugar, as well as brown sugar.
All data with a unit of g are based on 100 g of a food item.
The fructose/glucose ratio is calculated by dividing the sum of free fructose plus half sucrose by the sum of free glucose plus half sucrose.
Fructose is also found in the manufactured sweetener, high-fructose corn syrup, which is produced by treating corn syrup with enzymes, converting glucose into fructose. The common designations for fructose content, HFCS-42 and HFCS-55, indicate the percentage of fructose present in HFCS. HFCS-55 is commonly used as a sweetener for soft drinks, whereas HFCS-42 is used to sweeten processed foods, breakfast cereals, bakery foods, and some soft drinks.
Carbohydrate content of commercial sweeteners (percent on dry basis)
for HFCS, and USDA for fruits and vegetables and the other refined sugars.Cane and beet sugars have been used as the major sweetener in food manufacturing for centuries. However, with the development of HFCS, a significant shift occurred in the type of sweetener consumption in certain countries, particularly the United States. Contrary to the popular belief, however, with the increase of HFCS consumption, the total fructose intake relative to the total glucose intake has not dramatically changed. Granulated sugar is 99.9%-pure sucrose, which means that it has equal ratio of fructose to glucose. The most commonly used forms of HFCS, HFCS-42, and HFCS-55, have a roughly equal ratio of fructose to glucose, with minor differences. HFCS has simply replaced sucrose as a sweetener. Therefore, despite the changes in the sweetener consumption, the ratio of glucose to fructose intake has remained relatively constant.