Simple Sugar Structure

Monosaccharides are relatively low molecular weight molecules containing five or six carbon atoms with the general empirical formula Cn(H2O)n. They have two enantiomeric forms, D and L, depending on the location of the hydroxyl group at the chiral carbon. Sugars can be present in either 'aldo' or 'keto' forms. On the right fructose, a keto-hexose sugar, is shown using Fisher projections.

These sugars,although present in small quantities in the open chain form remain in a more stable hemiacetal ring conformation. Recall that mutarotation is the transformation of a sugar from one anomer to another. For example the diagram below shows the two anomers of glucose.

Complex carbohydrates are made up of two or more simple sugars linked together in an alpha or beta linkage. Oligosaccharides consist of 2-3 different monosaccharides linked together in a 1-3 or 1-4 linkages. See diagram below. Polysaccharides are the bonding of 10 or more simple sugars also in a or B 1-4 linkages. This can be done is a linear or branched pattern. There is no uniform degree of polymerization but rather a gaussian distribution of molecular weight. Examples of polysaccharides are glycogen (animal), cellulose and starch (plant).

Sugar ReactivityHydrolysis

Hydrolysis is the cleavage of glycoside linkages in acidic or alkaline media. The rate determining step of this reaction is the loss of ROH and the formation of the resonance stabilized carbonium ion. Since carbohydrates are acid sensitive they may be unstable in foods especially if they are subject to high temperatures or high acidity. In the case of polysaccharides, the rate of hydrolysis is proportional to the degree of association between polysaccharide molecules. The following diagram demonstrates this reaction:

Reduction of Monosaccarides

In this reaction the carbonyl group is reduced to an alcohol by a metal catalyzed reaction of hydrogen gas under pressure.

Oxidation Reaction

Like all other aldehydes, aldose sugars are easily oxidized to yield carboxylic acids. There are three chemical reagents used to oxidize sugars: Tollens' reagent (Ag+ in aqueous ammonia), Fehling's reagent (Cu in aqueous sodium tartrate) and Benedict's reagent (Cu in aqueous sodium citrate). These reagents work by oxidizing the sugar and reducing the metallic species. Therefore, all three reagents test for reducing sugars. In addition to aldoses there are some ketoses that are reducing sugars as well. For example, fructose reduces the Tollen's reagent because fructose is readily isomerized to an aldose in basic solution by a series of keto-enol tautomeric shifts. Gycosides however are non-reducing because the acetal group can't open to an aldehyde.

Condensation Reaction/ Glycoside Formation

The sugar in its hemiacetal/ hemiketal form can react with an alcohol (in this case another sugar) with an acid catalyst to form an acetal (or a gylcoside bond). Note that carbohydrate acetals are called glycosides. In this case, the sugars are either dissolved in alcohol or made slightly acidic before the glycoside can be formed. Glycosides, like all acetals, are stable to water. They are not in equilibrium with an open chained form and they do not show mutarotation. They can however, be converted back to free monosaccharides by hydrolysis with and aqueous acid. The following diagram shows the formation of sucrose:

D-glucopyranosyl + D- fructofuranoside ---> sucrose

Starch Structure and GelatinizationStarch is a polymer of glucose in which monosaccharide units are linked together by the 1,4'-a-glycoside bonds. There are two types of starch: amylopectin which is soluble in cold water and amylose which is insoluble in cold water. Amylose accounts for 80% of the starch and ia a linear polymer, where as amylopectin is a branched polymer held together by both the linear 1,4'-a-glycoside bonds and the branched 1,6'-a-glycoside bonds. These branches occur approximately every 35 glucose units.

Starch is is used by most plants as a reserve, therefore it is commonly found in the seeds, roots and tubers. starch is the only polysaccharide to be found in small packets called granules. Below are some pictures of natural starches.

Oat Bean Potato Maize Rye Barley

Gelatinization of Starch

Undamaged starch granules are insoluble in cold water. They will in fact swell slightly but their structure is maintained. When the temperature of the water is increased, the starch molecules begin to vibrate, breaking the intermolecular bonds and allowing water to penetrate and bind. The point where there is a complete loss of crystalinity is the gelatinization point. This occurs over a narrow range of temperatures termed the gelatinization temperature. Since the water becomes increasingly bound, the water acidity drops and the viscosity increases. Starch gelatinization is characteristic of each type of starch granule (corn, wheat, rice, potato, etc.). Gelatinization, viscosity, and characteristics of the remaining solution is dependent not only on temperature, but also on the the other constituents of food already present (e.g. sugars, lipids, proteins, food acids and the amount of water). Recall, that low water activity retards starch gelatinization. This occurs because there may not be enough water available to break open and bind to the starch hydrogen binding sites.

Glass Transition TemperatureMelting is a first-order phase transition where a crystalline material changes from a solid to a liquid. Some substances freeze into an amorphous solid called a glass. In this state, they are solid in the sense that they do not flow readily. Rather, the molecules are randomly distributed as if they were in the liquid state. When a glass is warmed it softens and eventually becomes fluid. This is not a first order transition and therefore occurs over a range of temperatures called the 'glass transition temperature'. Below are four graphs which demonstrate how the coefficient of thermal expansion (a), volume (V), heat capacity (Cp) and enthalpy (H) of a sucrose mixture are effect by temperature. Note that the characteristic of glass transition is the rapid onset of softening.

Figure 1:

Figure 2 depicts the change in the elastic modulus with temperature at the glass phase transition. The elastic modulus is a measure of the stiffness of the material. Note the logarithmic scale of the graph.

Figure 2: Viscosity Curves of amorphous phases of frozen sugar, acid water.

The final graph estimates the viscosity curves of amorphous phases in frozen sucrose-, glucose-, fructose-, and citric acid water systems.

Figure 3:

A mixture of ice crystals and an amorphous matrix is a two phase frozen system. Can you think of any foods which may have this system?