Two monosaccharide molecules form a disaccharide through a glycosidic bond. Lactose, sucrose and maltose are common disaccharides. They must be broken down into monosaccharides in small intestine before their absorption and utilization.
| Disaccharide | ||
|---|---|---|
| Lactose | Glucose + Galactose | mammal milk |
| Sucrose | Glucose + Fructose | It can store energy in the plant. |
| Maltose | Glucose + Glucose | Plant tissue in which starch hydrolysis occurs |
Physical, Chemical Properties and Structure, bio Function of Sucrose
Sucrose or table sugar is the daily disaccharide. It’s found in almost all naturally sweet or processed foods. The standard reference for relative sweetness is sucrose due to its broad application and moderate sweetness. It has a sweet taste and white crystal appearance with a melting point of 186°C. However, the crystals decompose into glucose and fructose before reaching this temperature. Above 170°C, you will get a brown viscous caramel with a special flavor. The multi-hydroxy structure gives it an excellent water solubility that enhances greatly with temperature. At room temperature, about 200g sucrose can be dissolved in 100 g water. This figure rises to about 323g at 70°C.
When heated in water with acids, sucrose is hydrolyzed to glucose and fructose or invert sugar. Polarized light is rotated to right in a sucrose solution. However, it showed a slight left-handed rotation in hydrolysate because L-fructose exerted a stronger effect than D-glucose.
One pyranose glucose and one furanose fructose are connected by one glycosidic bond in sucrose. It’s different to reducing carbohydrates. Tollens' reagent or Fehling's reagent can’t react with sucrose because hydroxyl groups on the hemiacetal are involved in glycosidic linkage and reformation of free aldehyde or ketone groups is impossible. Most of glucose from photosynthesis is converted to sucrose, one of the most abundant saccharide in plants. It is transported from leaves to fruits or roots for storage or metabolism. Sugarcane and sugar beets are two sources of edible sucrose manufacture.
Sucrose can’t be absorbed directly and has to be broken down enzymatically into monosaccharide components in small intestine. Glucose goes straight into the glycolytic pathway for energy production. Fructose must be phosphorylated for glycolytic pathway. It is highly caloric and provides nutrient for bacterial growth in the oral cavity. Health problems such as obesity, type 2 diabetes, and tooth decay may knock at your door due to excessive intake.
Physical, Chemical Properties and Structure, bio Function of Lactose
Lactose is a carbohydrate found primarily in the milk of mammals. Only about 40% sweetness make it much less sweet than sucrose or table sugar. Its crystal is a white solid at room temperature. The glucose is in alpha or beta form, but galactose is only in the beta form. Thus, there are two types of lactose crystals—α and β-lactose. At room temperature, α-lactose has a solubility of only 7g and is less sweet, whereas β-lactose has a solubility of 50g and is much sweeter. When supersaturated solution is below 93.5°C, the precipitate is α-lactose. Thus, most of the lactose available in market has alpha configuration. When you dissolve α-lactose in water, some will convert to β configuration again. When chemical equilibrium is in balance, about 20 g lactose will be found in 100 g water. Both types of lactose make polarized light undergo right-hand rotation.
Its reducing nature comes as a result of the hemiacetal on glucose that forms an aldehyde group in solution. Tollen’s reagent shows a silver mirror and Fehling's reagent turns red when meet lactose. It is also involved in Maillard reaction to give milk and dairy products a caramelized flavor during cooking.
Lactose must be broken down into glucose and galactose in small intestine before absorption. It’s beneficial for stable blood sugar that the low solubility slows down glucose absorption. Galactose is essential for baby brain development since it forms indispensable glycolipids in nerve tissues. This’s not only an energy source but also helps intake of some vital minerals like calcium and magnesium. Probiotics in the intestine (e.g. Lactobacillus) are boosted by lactose to aid digestion and inhibit harmful bacteria.
Almost all babies can digest lactose, but some adults don’t have enough enzyme to digest lactose present in milk. As they consume milk, the undigested lactose is fermented by bacteria and gas is produced. Abdominal bloating and diarrhea are common symptoms of this fermentation. The intolerance to lactose is relatively widespread among populations of East Asian and African origin, but scarce in Europeans.
Physical, Chemical Properties and Structure, bio Function of Maltose
Maltose or malt sugar is abundant in germinating seeds or fermenting dough. Its sweetness is moderate, about 30-60% that of sucrose. Melting point ranges from 160 to 165°C. The solubility is about 100g at room temperature. The polarized light undergoes right-hand rotation in this solution.
Two glucose units are linked together by an α(1→4) glycosidic bond in maltose. The reactions with Tollens' reagent or Fehling's reagent indicate it's a reducing sugar. Maltose can’t be absorbed by the small intestine directly. It must be broken down into monosaccharides by enzymes before absorption.
Germinated seeds and glutinous rice are used to produce traditional maltose. It mainly consists of maltose, with small amounts of starch and dextrin. Traditional maltose is a viscous syrup that holds a significant place in food industry. The moisture and non-crystallizing properties contribute to the softness of pastries. Additionally, the reduced sweetness of pastries gives you a milder flavor, or it would be too sweet to only use sucrose. It's also served as an adhesive to bind various ingredients together, such as in Sachima or nougat.