Food Chemistry summary
By Emma Burgwal, wur
Carbohydrates
Carbohydrates can be classified into monosaccharides, oligosaccharides, polysaccharides, glycosides
and sugar derivates.
Monosaccharides: sugar aldehydes (known as aldoses) or
ketones (known as ketoses). They can be further classified
according to the number of carbon atoms; six atoms is called a
hexose, five atoms a pentose. Each D-sugar
has an L-enantiomer, the mirror image of
the whole D-molecule. This mirroring
happens at the highest numbered chiral C-
atom. A cyclic hemiacetal (a ring structure) can be formed due to the
intramolecular reaction between the hydroxyl and the carbonyl group. A pyranose
is a six-membered ring, a furanose a five-membered ring. In a ring, the anomeric
carbon atom is the atom between the two oxygen atoms. An alfa-anomer has an
OH pointing downwards, a beta-anomer has an OH pointing upwards. A sugar in an open ring form
can reduce cupper ions and is thus a reducing sugar (very reactive at elevated temperature). After
opening the ring, the sugar is converted in another anomeric form, called mutarotation (temperature
dependent). Different anomers differ in optical activity.
Different monosaccharides; glucose: D-glucose is also called dextrose, and is an aldohexose. Glucose
most often occurs in the pyranose form. Fructose: glucose isomerase can convert glucose to fructose.
Fructose has a high sweetening capacity. D-fructose is a ketohexose, and usually occurs as pyranose.
Due to its tautomerism, fructose can difficulty crystallize. Mannose: building block of
polysaccharides. Galactose: building block of oligo- and polysaccharides, including lactose, raffinose
and pectin. Xylose: D-xylose is an aldopentose, and a building block of xylan and other
hemicelluloses. Xylitol is the alditol of xylose. Both sugars can be used as sweetener in food for
diabetics (non-fermentable). Arabinose: building block of hemicellulose, pectin and mucilage, mostly
in furanose form.
Uronic acids are monosaccharides of which the primary hydroxyl group (C6) is oxidized to a
carboxylic group. D-galacturonic acid (of galactose) occurs in pectin. These sugars can have a
negative charge, depending on the pH.
Disaccharides: two monosaccharides linked via a glycosidic bond. A nonreducing disaccharide is
formed when the glycosidic linkage is made between two hemiacetal OH-groups (saccharose), and a
reducing disaccharide is formed when a hemiacetal OH and an alcoholic OH are linked (maltose).
Most oligosaccharides (disaccharides) have a trivial name, saccharose, and a systematic name.
Saccharose: also sucrose, where glucose and fructose are linked
(1,2). There is no reducing capacity ad no mutarotation. Invert
sugar can be obtained via hydrolysis. Lactose: galactose and
glucose are linked (1,4 linked). Lactose has reducing properties.
For the digestion, the enzyme lactase or beta-galactosidase is
used. Maltose: consists of two glucose monomers and is 1,4
linked. It is a reducing sugar.
Oligosaccharides are composed of 2-20 monomeric units. Alfa-
,galacto-oligosaccharides are part of the raffinose family oligosaccharides (RFO), which is a saccharose
molecule (linked 1,6). These sugars have no reducing properties and are non-digestible
oligosaccharides (NDOs). Lactose oligosaccharides are also NDOs. Malto-oligosaccharides can be
formed during the acidic or enzymatic hydrolysis (use of alfa-amylases, beta-amylases, glucoamylases
and pullulanases) of starch to starch syrup. Starch syrups slow down the crystallization of saccharose.
For every glycosidic bond broken down during starch degradation, a reducing end is formed. Degree
of starch degradation (DE) = 100*(number of reducing ends/total number of glycosidic units). In
practice, the number of reducing ends is expressed as a concentration. Fructo-oligosaccharides are
present in natural products and belong to the NDOs.
Glycosides are molecules in which a saccharide is bound to a non-saccharide. N-glycosides, or
glycosylamines, are formed by coupling the carbonyl group to a primary amino group of an amino
acid, peptide or protein. N-glycosides show mutarotation. O-glycosides are formed by substitution of
the C1 hydroxyl group with another hydroxyl group in a second compound (for example methanol).
Polysaccharides are polymers of monosaccharides and the number of monosaccharides (variable) is
indicated by the degree of polymerization. Polysaccharides are a collection of molecules with
different DP’s, with no uniform molecular weight, and are thus polydisperse. Glycan is the scientific
name for polysaccharides. Glycans can be heteroglycan (different monomers) or homoglycan (same
monomers). Homoglycans can be linear or branched. Heteroglycans can have a homoglycan main
chain with a different side-chain, or they can be totally heteroglycan. Classification can also be done
due to their biological function (storage or plant cell wall). The properties of polysaccharides (e.g.
solubility, thickening agent etc) can be determined by: kind of monosaccharide units, conformation
of the monosaccharide units (alfa or beta), nature of the glycosidic linkage and the arrangement of
different linkages, branching pattern, degree of polymerization, presence of charged groups and
substitution. Cellulose (NDO) and amylose (not NDO) are both made of only glucose monomers, but
have different properties due to the factors above.
The most important properties of sugars are: solubility and crystallization, water binding, sweetness,
digestibility, viscosity and gelling and fat-substitution. Due to the many polar groups, sugars are
hydrophilic. If a solution has an exceeded solubility, the solution becomes over-saturated and
crystallization can occur. Solubility often depends on temperature, however with saccharose, the
solubility is hardly affected by the temperature. Food products with high sugar concentrations have a
ERH P
low water activity or equilibrium relative humidity (ERH). =a w = . Sugar solutions have a
100 P0
vapour pressure (P) that is lower than the saturated water vapour pressure (P 0). Water molecules are
bound to sugar molecules, and therefore loose their free motion. The water activity influences the
rate of chemical reactions. Humectants, additives with a high water binding capacity, can lower the
water activity of foods. Monosaccharides and oligosaccharides and their sugar alcohols are sweet.
Sugars differ with respect to the quality and the intensity of the sweetness, which depend on
chemical structure, temperature, pH and the presence of other components. The relative sweetness
of saccharose is set to 1. Sugars can neutralize a sour taste; the ratio of Brix (orange-juice industry)
and amount of acids can be titrated with alkali. A ratio of 10 or higher is well drinkable. Sugar
substitutes are applied in food as sugars, but are converted in the human body without insulin
response, such as sugar alcohols. The caloric value of this substitutes is either 0 or very low.
NDOs are not degraded by the digestive enzymes and can therefore function as prebiotics. These
prebiotics can simulate the growth and activity of health promoting bacteria. Dietary fibre includes
all soluble and insoluble polysaccharides in a food, with the exception of starch (all are NDOs). A
number of polysaccharides can increase the viscosity or form gels. Viscosity depends on the size and
form of molecules and the conformation they have in the solvent. Very flexible polysaccharides occur
, as random coils, however most polysaccharides form
stiff bars. Linear polysaccharides take up a lot of
space and they usually collide, giving viscosity.
Viscosity depends on the degree of polymerization,
the volume, form and flexibility of the dissolved
polymer chain. Polysaccharides with negative
charges, are shaped as
stretched molecules (due
to repulsion) and are thus
linear with a high viscosity. For gelling, the interaction between water and
polysaccharide is important. When a sufficient number of polysaccharides
aggregate (stick together), a 3D-network is formed (a gel). Syneresis is the
shrinking of a gel by pressing out water.
Under processing and storage conditions, chemical
reactions in food occur which can have an impact on
food quality. The ripening of vegetables can result in
change in colour and texture (due to degradation of cell wall polysaccharides).
There are different types of chemical reactions.
Hydrolysis: the glycosidic bond between two sugar monomers is cleaved
enzymatically or chemically by the addition of a water
molecule.
Enolization: occurs when a reducing sugar solution is
heated or when the pH changes below 3 or above 7.
Enolization of glucose leads to a 1,2-enediol and this enediol can react back
to glucose or form mannose or fructose. This isomerization reaction is also
called the Lobry de Bruyn-van Ekenstein isomerization. Bases are more
effective catalysts than acids (slow reactions) for the enolization.
Dehydration: the removal of a water molecule of an enediol, called beta-
elimination. Happens slow at alkaline conditions and fast at acidic
conditions. First, an alfa-dicarbonyl compound is formed (by losing water) and after losing a water
molecule, the ring is closed and hydroxymethylfurfural (HMF) is formed. HMF is an indicator that the
product has been heated.
The 1,2-enediol can react further in different ways under alkaline conditions: oxidation (the double
bond is dissociated and two carboxylic acids are formed), formation of alfa-dicarbonyl (by beta-
elimination of a hydroxyl group) and fragmentation (chain cleavage of the 1,2-enediol, reactive
compounds are formed). These reactions are examples of primary reactions, with very reactive
products which can further react to secondary products by aldolization reactions and intramolecular
reactions. Many of these secondary products contribute to aroma and colour. The different
aldolization reactions will now be described.
Caramelization: reactions starting with sugars leading to a dark brown caramel and many aroma
compounds. It occurs during dry heating of sugars or during heating of concentrated sugar solutions.
Enolization, dehydration, fragmentation and condensation reactions occur, where condensation
leads to brown pigments due to unsaturated polymers with complex, variable structures formation.
Flavouring purposes: concentrated syrups in a neutral buffered medium are used, colouring:
solutions at lower pH are used.
Maillard reaction: reducing sugars react with amino acids, resulting in brown coloured and aromatic
substances. Besides reducing sugars, amino groups can also react with ascorbic acid and degradation
products of fat oxidation. The nitrogen-containing intermediates during the Maillard reaction are
