Fructose
Fructose, or fruit sugar, is a simple ketonic monosaccharide 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 galactose, that are absorbed directly into blood during digestion. 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.
Fructose is found in honey, tree and vine fruits, flowers, berries, and most root vegetables.
Commercially, fructose is derived from sugar cane, sugar beets, and maize. High-fructose corn syrup is a mixture of glucose and fructose as monosaccharides. Sucrose is a compound with one molecule of glucose covalently linked to one molecule of fructose. All forms of fructose, including 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. About 240,000 tonnes of crystalline fructose are produced annually.
Excessive consumption of fructose may contribute to insulin resistance, obesity, elevated LDL cholesterol and triglycerides, leading to metabolic syndrome. The European Food Safety Authority stated that fructose may be preferable over sucrose and glucose in sugar-sweetened foods and beverages because of its lower effect on postprandial blood sugar levels, and noted that "high intakes of fructose may lead to metabolic complications such as dyslipidaemia, insulin resistance, and increased visceral adiposity". The UK's Scientific Advisory Committee on Nutrition in 2015 disputed the claims of fructose causing metabolic disorders, stating that "there is insufficient evidence to demonstrate that fructose intake, at levels consumed in the normal UK diet, leads to adverse health outcomes independent of any effects related to its presence as a component of total and free 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.Chemical properties
Fructose is a 6-carbon polyhydroxyketone. Crystalline fructose adopts a cyclic six-membered structure owing to the stability of its hemiketal and internal hydrogen-bonding. This form is formally called -fructopyranose. In water solution, fructose exists as an equilibrium mixture of 70% fructopyranose and about 22% fructofuranose, as well as small amounts of three other forms, including the acyclic structure.Reactions
Fructose and fermentation
Fructose may be anaerobically fermented by yeast or bacteria. Yeast enzymes convert sugar to ethanol and carbon dioxide. The carbon dioxide released during fermentation will remain dissolved in water, where it will reach equilibrium with carbonic acid, unless the fermentation chamber is left open to the air. The dissolved carbon dioxide and carbonic acid produce the carbonation in bottled fermented beverages.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 readily dehydrates to give hydroxymethylfurfural.This process, in the future, may become part of a low-cost, carbon-neutral system to produce replacements for petrol and diesel from plants.
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 lower 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.
Food 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 table 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 a 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 table white granulated sugar and powdered sugar, as well as brown sugar.
| Food Item | Total Carbohydrate including "dietary fiber" | Total Sugars | Free Fructose | Free Glucose | Sucrose | Fructose/ Glucose Ratio | Sucrose as a % of Total Sugars |
| Fruits | |||||||
| Apple | 13.8 | 10.4 | 5.9 | 2.4 | 2.1 | 2.0 | 19.9 |
| Apricot | 11.1 | 9.2 | 0.9 | 2.4 | 5.9 | 0.7 | 63.5 |
| Banana | 22.8 | 12.2 | 4.9 | 5.0 | 2.4 | 1.0 | 20.0 |
| Fig, dried | 63.9 | 47.9 | 22.9 | 24.8 | 0.9 | 0.93 | 0.15 |
| Grapes | 18.1 | 15.5 | 8.1 | 7.2 | 0.2 | 1.1 | 1 |
| Navel orange | 12.5 | 8.5 | 2.25 | 2.0 | 4.3 | 1.1 | 50.4 |
| Peach | 9.5 | 8.4 | 1.5 | 2.0 | 4.8 | 0.9 | 56.7 |
| Pear | 15.5 | 9.8 | 6.2 | 2.8 | 0.8 | 2.1 | 8.0 |
| Pineapple | 13.1 | 9.9 | 2.1 | 1.7 | 6.0 | 1.1 | 60.8 |
| Plum | 11.4 | 9.9 | 3.1 | 5.1 | 1.6 | 0.66 | 16.2 |
| Vegetables | |||||||
| Beet, Red | 9.6 | 6.8 | 0.1 | 0.1 | 6.5 | 1.0 | 96.2 |
| Carrot | 9.6 | 4.7 | 0.6 | 0.6 | 3.6 | 1.0 | 77 |
| Red Pepper, Sweet | 6.0 | 4.2 | 2.3 | 1.9 | 0.0 | 1.2 | 0.0 |
| Onion, Sweet | 7.6 | 5.0 | 2.0 | 2.3 | 0.7 | 0.9 | 14.3 |
| Sweet Potato | 20.1 | 4.2 | 0.7 | 1.0 | 2.5 | 0.9 | 60.3 |
| Yam | 27.9 | 0.5 | tr | tr | tr | na | tr |
| Sugar Cane | 13–18 | 0.2 – 1.0 | 0.2 – 1.0 | 11–16 | 1.0 | high | |
| Sugar Beet | 17–18 | 0.1 – 0.5 | 0.1 – 0.5 | 16–17 | 1.0 | high | |
| Grains | |||||||
| Maize, Sweet | 19.0 | 6.2 | 1.9 | 3.4 | 0.9 | 0.61 | 15.0 |
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)
Data obtained from Kretchmer, N. & Hollenbeck, CB. Sugars and Sweeteners, Boca Raton, FL: CRC Press, Inc. 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.
Nutritional information
Providing 368 kcal per 100 grams of dry powder, fructose has 95% the caloric value of sucrose by weight. Fructose powder is 100% carbohydrates and supplies no other nutrients in significant amount.Fructose digestion and absorption in humans
Fructose exists in foods either as a monosaccharide or as a unit of a disaccharide. Free fructose is absorbed directly by the intestine. When fructose is consumed in the form of sucrose, it is digested and then absorbed as free fructose. As sucrose comes into contact with the membrane of the small intestine, the enzyme sucrase catalyzes the cleavage of sucrose to yield one glucose unit and one fructose unit, which are then each absorbed. After absorption, it enters the hepatic portal vein and is directed toward the liver.The mechanism of fructose absorption in the small intestine is not completely understood. Some evidence suggests active transport, because fructose uptake has been shown to occur against a concentration gradient. However, the majority of research supports the claim that fructose absorption occurs on the mucosal membrane via facilitated transport involving GLUT5 transport proteins. Since the concentration of fructose is higher in the lumen, fructose is able to flow down a concentration gradient into the enterocytes, assisted by transport proteins. Fructose may be transported out of the enterocyte across the basolateral membrane by either GLUT2 or GLUT5, although the GLUT2 transporter has a greater capacity for transporting fructose, and, therefore, the majority of fructose is transported out of the enterocyte through GLUT2.
Capacity and rate of absorption
The absorption capacity for fructose in monosaccharide form ranges from less than 5 g to 50 g and adapts with changes in dietary fructose intake. Studies show the greatest absorption rate occurs when glucose and fructose are administered in equal quantities. When fructose is ingested as part of the disaccharide sucrose, absorption capacity is much higher because fructose exists in a 1:1 ratio with glucose. It appears that the GLUT5 transfer rate may be saturated at low levels, and absorption is increased through joint absorption with glucose. One proposed mechanism for this phenomenon is a glucose-dependent cotransport of fructose.In addition, fructose transfer activity increases with dietary fructose intake. The presence of fructose in the lumen causes increased mRNA transcription of GLUT5, leading to increased transport proteins. High-fructose diets increase transport proteins within three days of intake.