Fructose (also levulose or laevulose) is a simple reducing sugar (monosaccharide) found in many foods and is one of the three important dietary monosaccharides along with glucose and galactose. Honey, tree fruits, berries, melons, and some root vegetables, such as beets, sweet potatoes, parsnips, and onions, contain fructose, usually in combination with glucose in the form of sucrose. Fructose is also derived from the digestion of granulated table sugar (sucrose), a disaccharide consisting of glucose and fructose, and high-fructose corn syrup (HFCS).
Crystalline fructose and high-fructose corn syrup are often mistakenly confused as the same product. The former is simply pure (100%) fructose. The latter is composed of nearly equal amounts of fructose and glucose. Crystalline fructose is held to offer many unique benefits such as improved product texture, taste and stability. Specifically, when combined with other sweeteners and starches, crystalline fructose is said to boost cake height (in baked goods) and mouth-feel of foods and beverages and to produce a pleasing brown surface color and pleasant aroma when baking.[1]
Contents[hide]
1 Chemical Properties
1.1 Classification and Structure
1.2 Chemical Reactions
1.2.1 Fructose and Maillard Reaction
1.2.2 Fructose and Fermentation
2 Physical and Functional Properties
2.1 Relative Sweetness
2.2 Fructose Solubility and Crystallization
2.3 Fructose and Starch Functionality in Food Systems
3 Food Sources
3.1 Table 1 – Sugar Content of Selected Common Plant Foods (g/100g)
3.2 Commercial Sweeteners (% of Carbohydrate)
4 Digestion and Absorption
4.1 Capacity and rate of absorption
4.2 Malabsorption
5 Fructose Metabolism
5.1 Fructolysis
5.2 The Metabolism of Fructose to DHAP and Glyceraldehyde
5.3 Synthesis of glycogen from DHAP and Glyceraldehyde 3 Phosphate
5.4 Synthesis of Triglyceride from DHAP and Glyceraldehyde 3 Phosphate
6 Health effects
7 See also
8 References
9 External links
//
[edit] Chemical Properties
[edit] Classification and Structure
Fructose, also referred to as fruit sugar is a simple monosaccharide with a ketone functional group. Fructose is an isomer of glucose with the same molecular formula (C6H12O6) but with a different structure. Fructose is a 6-carbon polyhydroxyketone. Like glucose, it forms ring structures when dissolved in solution. When fructose forms a 5-member ring, the OH group on the fifth carbon atom attaches to the carbonyl group that is on the second carbon atom (D-Fructofuranose). Alternatively, the OH group on the sixth carbon may attach to the carbonyl carbon to form a 6-member ring (D-Fructopyranose). Fructose may be found at equilibrium containing a mixture of 70% fructopyranose and 30% fructofuranose [2]
Figure 1 Isomeric Forms of Fructose
[edit] Chemical Reactions
[edit] Fructose and Maillard Reaction
Fructose undergoes the Maillard reaction, non-enzymatic browning, with amino acids. Because of fructose exists to a greater extent in the open-chain form than does glucose, the initial stages of the Maillard reaction occurs more rapidly than with glucose. Therefore, fructose potentially may 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. [3]
[edit] Fructose and Fermentation
Fructose may be anaerobically fermented by yeast or bacteria. [4] Yeast enzymes convert sugar (glucose, or fructose) 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 bottle fermented beverages. [5] Colonic bacterial fermentation of fructose and the osmotic retention of additional water in the colin may cause gas, cramps, and diarrhea in people with fructose malabsorption. [6]
[edit] Physical and Functional Properties
[edit] Relative Sweetness
The primary reason that fructose is used commercially in foods and beverages is because of its relative sweetness. It is the sweetest of all naturally occurring carbohydrates. Fructose is 1.73 times sweeter than sucrose[7][8] .
Figure 2 Relative Sweetness of Sugars and Sweeteners
The Sweetness Intensity Profile of Fructose The sweetness of fructose is perceived earlier than that of sucrose or dextrose, and the taste sensation reaches a peak (higher than sucrose) and diminishes more quickly than sucrose. Fructose can also enhance other flavors in the system[7]
Sweetness Synergy 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[9].
[edit] Fructose Solubility and Crystallization
Compared to other sugars and sugar alcohols, fructose has the highest solubility. As a result, fructose is difficult to crystallize from an aqueous solution.[7] Sugar mixes containing fructose, such as candies, are softer than those containing other sugars because of the greater solubility of fructose [10].
Fructose Hygroscopicity and Humectancy Fructose is quicker to absorb moisture and slower to release it to the environment than sucrose, dextrose, or other nutritive sweeteners [9]. Fructose is an excellent humectant and retains moisture for a long period of time even at low relative humidity (RH). Therefore, fructose can contribute to improved quality, better texture, and longer shelf life to the food products in which it is used.[7]
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.[7]
[edit] 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 [11].
[edit] Food Sources
The primary food sources of fructose are fruits, vegetables, and honey [12]. Fructose exists in foods either as a free monosaccharide or bound to glucose as the disaccharide, sucrose. Fructose, glucose, and sucrose can all be present in a food; however, different foods will have varying levels of each of these three sugars.
The sugar content of common fruits and vegetables are presented in Table 1. In general, foods that contain free fructose have equal amount of free glucose. In other words, the ratio of fructose to glucose roughly equals 1:1. A value that is above 1 indicates higher proportion of fructose to glucose and vice versa. Some of the 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 apricots contain less than a half of fructose than glucose.
Apple and pear juices are of particular interest to pediatricians due to the juices’ high concentration of free fructose relative to glucose, which can cause diarrhea in children. The cells of the small intestine, enterocytes, have lower affinity for fructose absorption compared with that for glucose and sucrose [13]. 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 details in Health Effects section.
Table 1 also shows the amount of sucrose found in common fruits and vegetables. Sugar cane and sugar beet have a high concentration of sucrose, and are used for commercial preparation of pure sucrose. Extracted cane or beet juices are clarified from the impurities and concentrated by removing excess of water. The end product is 99.9% pure sucrose. Sucrose containing sugars include common white granulated sugar, powdered sugar, as well as brown sugar [14].
Crystalline fructose and high-fructose corn syrup are often mistakenly confused as the same product. The former is simply pure (100%) fructose. The latter is composed of nearly equal amounts of fructose and glucose. Crystalline fructose is held to offer many unique benefits such as improved product texture, taste and stability. Specifically, when combined with other sweeteners and starches, crystalline fructose is said to boost cake height (in baked goods) and mouth-feel of foods and beverages and to produce a pleasing brown surface color and pleasant aroma when baking.[1]
Contents[hide]
1 Chemical Properties
1.1 Classification and Structure
1.2 Chemical Reactions
1.2.1 Fructose and Maillard Reaction
1.2.2 Fructose and Fermentation
2 Physical and Functional Properties
2.1 Relative Sweetness
2.2 Fructose Solubility and Crystallization
2.3 Fructose and Starch Functionality in Food Systems
3 Food Sources
3.1 Table 1 – Sugar Content of Selected Common Plant Foods (g/100g)
3.2 Commercial Sweeteners (% of Carbohydrate)
4 Digestion and Absorption
4.1 Capacity and rate of absorption
4.2 Malabsorption
5 Fructose Metabolism
5.1 Fructolysis
5.2 The Metabolism of Fructose to DHAP and Glyceraldehyde
5.3 Synthesis of glycogen from DHAP and Glyceraldehyde 3 Phosphate
5.4 Synthesis of Triglyceride from DHAP and Glyceraldehyde 3 Phosphate
6 Health effects
7 See also
8 References
9 External links
//
[edit] Chemical Properties
[edit] Classification and Structure
Fructose, also referred to as fruit sugar is a simple monosaccharide with a ketone functional group. Fructose is an isomer of glucose with the same molecular formula (C6H12O6) but with a different structure. Fructose is a 6-carbon polyhydroxyketone. Like glucose, it forms ring structures when dissolved in solution. When fructose forms a 5-member ring, the OH group on the fifth carbon atom attaches to the carbonyl group that is on the second carbon atom (D-Fructofuranose). Alternatively, the OH group on the sixth carbon may attach to the carbonyl carbon to form a 6-member ring (D-Fructopyranose). Fructose may be found at equilibrium containing a mixture of 70% fructopyranose and 30% fructofuranose [2]
Figure 1 Isomeric Forms of Fructose
[edit] Chemical Reactions
[edit] Fructose and Maillard Reaction
Fructose undergoes the Maillard reaction, non-enzymatic browning, with amino acids. Because of fructose exists to a greater extent in the open-chain form than does glucose, the initial stages of the Maillard reaction occurs more rapidly than with glucose. Therefore, fructose potentially may 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. [3]
[edit] Fructose and Fermentation
Fructose may be anaerobically fermented by yeast or bacteria. [4] Yeast enzymes convert sugar (glucose, or fructose) 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 bottle fermented beverages. [5] Colonic bacterial fermentation of fructose and the osmotic retention of additional water in the colin may cause gas, cramps, and diarrhea in people with fructose malabsorption. [6]
[edit] Physical and Functional Properties
[edit] Relative Sweetness
The primary reason that fructose is used commercially in foods and beverages is because of its relative sweetness. It is the sweetest of all naturally occurring carbohydrates. Fructose is 1.73 times sweeter than sucrose[7][8] .
Figure 2 Relative Sweetness of Sugars and Sweeteners
The Sweetness Intensity Profile of Fructose The sweetness of fructose is perceived earlier than that of sucrose or dextrose, and the taste sensation reaches a peak (higher than sucrose) and diminishes more quickly than sucrose. Fructose can also enhance other flavors in the system[7]
Sweetness Synergy 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[9].
[edit] Fructose Solubility and Crystallization
Compared to other sugars and sugar alcohols, fructose has the highest solubility. As a result, fructose is difficult to crystallize from an aqueous solution.[7] Sugar mixes containing fructose, such as candies, are softer than those containing other sugars because of the greater solubility of fructose [10].
Fructose Hygroscopicity and Humectancy Fructose is quicker to absorb moisture and slower to release it to the environment than sucrose, dextrose, or other nutritive sweeteners [9]. Fructose is an excellent humectant and retains moisture for a long period of time even at low relative humidity (RH). Therefore, fructose can contribute to improved quality, better texture, and longer shelf life to the food products in which it is used.[7]
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.[7]
[edit] 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 [11].
[edit] Food Sources
The primary food sources of fructose are fruits, vegetables, and honey [12]. Fructose exists in foods either as a free monosaccharide or bound to glucose as the disaccharide, sucrose. Fructose, glucose, and sucrose can all be present in a food; however, different foods will have varying levels of each of these three sugars.
The sugar content of common fruits and vegetables are presented in Table 1. In general, foods that contain free fructose have equal amount of free glucose. In other words, the ratio of fructose to glucose roughly equals 1:1. A value that is above 1 indicates higher proportion of fructose to glucose and vice versa. Some of the 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 apricots contain less than a half of fructose than glucose.
Apple and pear juices are of particular interest to pediatricians due to the juices’ high concentration of free fructose relative to glucose, which can cause diarrhea in children. The cells of the small intestine, enterocytes, have lower affinity for fructose absorption compared with that for glucose and sucrose [13]. 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 details in Health Effects section.
Table 1 also shows the amount of sucrose found in common fruits and vegetables. Sugar cane and sugar beet have a high concentration of sucrose, and are used for commercial preparation of pure sucrose. Extracted cane or beet juices are clarified from the impurities and concentrated by removing excess of water. The end product is 99.9% pure sucrose. Sucrose containing sugars include common white granulated sugar, powdered sugar, as well as brown sugar [14].
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