Annex A: Food components A.1 Structure of food macromolecules This annex contains a summary of the chemical structures of the main macromole- cules in foods, the properties, functions and sources of vitamins and minerals, and examples of functional components of foods. More detailed information may be found in food chemistry texts (e.g. Belitz et al., 2009; Damodaran et al., 2007) and nutrition texts (e.g. Webster-Gandy et al., 2011; Gibney et al., 2009). A.1.1 Carbohydrates A.1.1.1 Sugars Sugars may be single molecules, known as ‘monosaccharides’, which can be classi- fied by the number of carbon atoms they contain: diose (2), triose (3), tetrose (4), pentose (5), hexose (6), heptose (7). Two or three monosaccharides joined together are termed ‘disaccharides’ and ‘trisaccharides’, respectively. ‘Oligosaccharides’ have up to 20 monosaccharides, joined by glycosidic bonds, and ‘polysaccharides’ are carbohydrates that contain more than 20 monosaccharides, joined by different types of glycosidic bonds. Monosaccharides contain both hydroxyl groups and carbonyl groups and are classified as ‘aldoses’ if the carbonyl group is an aldehyde (e.g. pentose (five carbon atoms), or hexose (six carbon atoms)) or ‘ketoses’ if it is a ketone (e.g. the corresponding five- and six-carbon molecules are pentulose, hexu- lose). When the position of the aldehyde or ketone group on the molecule allows it to react with oxidants (i.e. act as a reducing agent as, for example in Maillard reac- tions), the sugar is known as a ‘reducing sugar’. Examples include glucose, fructose and arabinose, and the disaccharides lactose and maltose, but not sucrose, which is a nonreducing sugar. Reaction between the carbonyl and hydroxyl groups in the molecule forms monosaccharides into a ring structure. Common types have five- membered (furanose) rings or six-membered (pyranose) rings (Fig. A1). In solution, sugars have an equilibrium mixture of open-chain and closed-ring (or cyclic) structures. In the open-chain form, the carbon atom that has the C 5 O bond is the carbonyl atom, whereas in the cyclic structure the carbonyl atom is attached to the O of the ring and an OH group. Monosaccharides also contain ‘chi- ral’ carbon atoms (atoms that can exist in two different spatial configurations that are the mirror image of each other). Glucose is the most abundant aldose and the two forms are D- and L-glucose, with the D-form occurring naturally in foods. Other common naturally occurring monosaccharides are D-mannose, D-galactose and D-
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Annex A: Food components
A.1 Structure of food macromolecules
This annex contains a summary of the chemical structures of the main macromole-
cules in foods, the properties, functions and sources of vitamins and minerals, and
examples of functional components of foods. More detailed information may be
found in food chemistry texts (e.g. Belitz et al., 2009; Damodaran et al., 2007) and
nutrition texts (e.g. Webster-Gandy et al., 2011; Gibney et al., 2009).
A.1.1 Carbohydrates
A.1.1.1 Sugars
Sugars may be single molecules, known as ‘monosaccharides’, which can be classi-
fied by the number of carbon atoms they contain: diose (2), triose (3), tetrose (4),
pentose (5), hexose (6), heptose (7). Two or three monosaccharides joined together
are termed ‘disaccharides’ and ‘trisaccharides’, respectively. ‘Oligosaccharides’
have up to 20 monosaccharides, joined by glycosidic bonds, and ‘polysaccharides’
are carbohydrates that contain more than 20 monosaccharides, joined by different
types of glycosidic bonds. Monosaccharides contain both hydroxyl groups and
carbonyl groups and are classified as ‘aldoses’ if the carbonyl group is an aldehyde
(e.g. pentose (five carbon atoms), or hexose (six carbon atoms)) or ‘ketoses’ if it is
a ketone (e.g. the corresponding five- and six-carbon molecules are pentulose, hexu-
lose). When the position of the aldehyde or ketone group on the molecule allows it
to react with oxidants (i.e. act as a reducing agent as, for example in Maillard reac-
tions), the sugar is known as a ‘reducing sugar’. Examples include glucose, fructose
and arabinose, and the disaccharides lactose and maltose, but not sucrose, which is
a nonreducing sugar. Reaction between the carbonyl and hydroxyl groups in the
molecule forms monosaccharides into a ring structure. Common types have five-
membered (furanose) rings or six-membered (pyranose) rings (Fig. A1).
In solution, sugars have an equilibrium mixture of open-chain and closed-ring
(or cyclic) structures. In the open-chain form, the carbon atom that has the C5O
bond is the carbonyl atom, whereas in the cyclic structure the carbonyl atom is
attached to the O of the ring and an OH group. Monosaccharides also contain ‘chi-
ral’ carbon atoms (atoms that can exist in two different spatial configurations that
are the mirror image of each other). Glucose is the most abundant aldose and the
two forms are D- and L-glucose, with the D-form occurring naturally in foods. Other
common naturally occurring monosaccharides are D-mannose, D-galactose and D-
xylose. The L-form is less common in nature, but two examples are L-arabinose and
L-galactose, which are units in polymeric carbohydrates. The only ketose found nat-
urally in foods is D-fructose (fruit sugar), which forms � 55% of high-fructose
corn syrup and � 40% of honey. Further details of the structure and orientation of
monosaccharide molecules are given by BeMiller and Whistler (1996) and the
important reactions of monosaccharides are described by Davis and Fairbanks
(2002).
A.1.1.2 Glycogen and starch
Glycogen is a branched glucose polymer that has glucose molecules linked by
α(1�4) glycosidic bonds. Starch occurs in two forms: (1) α-amylose, in which
500�20,000 D-glucose molecules are linked by α(1�4) glycosidic bonds
(Fig. A2A) in linear, helical chains; and (2) amylopectin, in which highly branched
chains of up to 30 glucose molecules are linked through α(1�4) bonds and con-
nected to each other through α(1�6) branch points. Amylopectin is therefore a
much larger molecule than amylose, typically containing 1�2 million residues,
with a mass that is four to five times that of amylose. Starch granules produce low-
viscosity pumpable slurries in cold water, and thicken due to gelatinisation when
heated to � 80�C. Details of starch gelatinisation are given by Maaruf et al. (2001),
Palav and Seetharaman (2006) and Xie et al. (2006).
A.1.1.3 Cellulose
Cellulose is composed of unbranched linear chains of 1000�10,000 D-glucose
molecules, linked together by β(1�4) glycosidic bonds (Fig. A2B). The linear struc-
ture promotes hydrogen bonding that holds together nearby cellulose molecules to
form a three-dimensional structure of microfibres, which in turn interact to form
cellulose fibres. A typical fibre contains � 500,000 cellulose molecules, and the
large number of hydrogen bonds creates a crystalline structure that has a high ten-
sile strength. As a result, cellulose is a stiff material that is used by plants as a
structural molecule to support leaves and stems. In contrast to other more amor-
phous polymeric carbohydrates, the crystalline structure also makes cellulose
CH2OH CH2OH CH2OH
HH
OHH
OH
HH
OH
H
OHOH
H
HO
O
O
OH
H
(A) (B)
Figure A1 Structure of (A) a furanose ring and (B) a pyranose ring.
e2 Annex A: Food components
insoluble and resistant to enzymic breakdown, a property that also makes cellulose
suitable as a packaging film (see Section 24.2.4). Additional details of the proper-
ties of cellulose are given by Chaplin (2014). Carboxymethylcellulose (CMC) is
produced by the reaction of cellulose with chloroacetic acid to substitute polar car-
boxyl groups for hydroxyl groups. This makes the cellulose soluble and more chem-
ically reactive. The functional properties of CMC depend on how many hydroxyl
groups are involved in the substitution reaction and the chain length of the cellulose
backbone.
A.1.1.4 Polysaccharide gums
Hydrocolloids (Table A1) are linear or branched polysaccharides that increase vis-
cosity depending on the molecular weight, shape and flexibility of the hydrated
molecules. Also, the presence of electrically charged groups causes mutual repul-
sion of molecular chains that extends them to create higher viscosities (e.g. algi-
nates, carrageenans and xanthan gums). When they are used to form gels, the
polysaccharide molecules come out of solution to form a three-dimensional network
that is joined together by hydrogen bonding, hydrophobic van der Waals associa-
tions, ionic crosslinking or covalent bonding. Guar and locust bean (or carob) gums
CH2OH
H
HH
OH
H
OH
OH
Hn
O
CH2OH
H
HH
OH
OH
OH
H
O
CH2OH
CH2OH
beta (1→ 4) bondcellulose
H
HH
OH
OH
OH
OH
H
O
(A)
(B)
OCH2OH
O
O
CH2OH
alpha (1→ 4) bondstarch, glycogen
OCH2OH
O
O
Figure A2 Structure of polysaccharides: (A) α(1�4) bonds in starch and glycogen and
(B) β(1�4) glycosidic bonds in cellulose (n is the number of repeating glucose units).
are thickening agents, whose main component is a galactomannan consisting of a
chain of β-D-mannopyranosyl units joined by (1�4) bonds with α-D-galactopyrano-syl branches. Xanthan gum is composed of chains that are identical to cellulose, but
with trisaccharide side-chains of mannopyranosyl and glucuronopyranosyl units.
Carrageenans are a group of .100 types of sulphated galactans that are derived
from seaweed. The basic structure has chains of D-galactopyranosyl units that have
alternating (1�3)-α-D and (1�4)-β-D-glycosidic linkages with sulphate groups
esterified to the hydroxyl groups. The three basic types are named ‘kappa’, ‘iota’
and ‘lambda’, which form gels with potassium or calcium ions. Alginates are the
sodium salt of alginic acid, a polyuronic acid composed of units of β-D-mannopyra-
nosyluronic acid and α-DL-gulopyranosyluronic acid.Pectins are a group of poly-α-D-galactopyranosyluronic acids that have differing
amounts of methyl ester groups along the chains. High-methoxyl pectins have more
than half the carboxyl groups that methyl esters have, whereas low-methoxyl pec-
tins have less that half the carboxyl groups of methyl esters and form gels with cal-
cium ions. Gum arabic has two fractions: highly branched arabinogalactan chains
with side chains of galactopyranosyl units, and another fraction that has proteins as
an integral part of the structure.
A.1.2 Lipids
Lipids are mono-, di- and triesters of glycerol with fatty acids (monocarboxylic
acids). Glycerol is a trihydric alcohol (containing three hydroxyl (�OH) groups that
can combine with up to three fatty acids to form a wide variety of monoacylglycer-
ols, diacylglycerols or triacylglycerols) (Fig. A3). A monoacylglycerol has one fatty
acid unit per molecule of glycerol, which may be attached to carbon atom 1 or 2 on
the glycerol molecule; a diacylglycerol has two fatty acids as either the 1,2 form or
the 1,3 form depending on how they are attached to the glycerol molecule.
Triacylglycerols are the main constituents of vegetable oils and animal fats, and
(A) (B)
(C) (D)
CH3(CH2)7-C=C-(CH2)7-C-OH
CH3(CH2)7CH=CH(CH2)7C(O)O-CH2
CH3(CH2)14C(O)O-CH2
CH3(CH2)14C(O)O-CH2
HO-CH
HO-CH
HO-CH2
||||
|
|
CH3(CH2)7CH=CH(CH2)7C(O)O-CH2
CH3(CH2)14C(O)O-CH2
CH3(CH2)7CH=CH(CH2)7C(O)O-CH|
|
OH H
Figure A3 Components of lipids: (A) oleic acid; (B) 1-monoacylglyceride;
(C) 1,3-diacylglyceride and (D) triacylglyceride.
e5Annex A: Food components
their structure involves a chain of carbon atoms with a carboxyl group (�COOH) at
one end. The triacylglyceride structural formula (Fig. A3D showing olive oil) con-
sists of two radicals of oleic acid and one of palmitic acid attached to the glycerol
molecule (the vertical chain of carbon atoms). Saturated fatty acids (SFAs) have no
double bonds between the carbon atoms, monounsaturated fatty acids (MUFAs)
have only one double bond, and polyunsaturated fatty acids (PUFAs) have more
than one double bond. Nawar (1996) has reviewed different forms of nomenclature
of lipids and in this book the common names for fats and fatty acids are normally
used (Table A2). The numbers at the beginning of the scientific names indicate the
locations of the double bonds. By convention, the carbon atom of the carboxyl
group is number one and Greek numbers such as ‘di, tri, tetra, penta, and hexa’ are
used to describe the length of carbon chains. For example, linoleic acid is 9,12-
octadecadienoic acid, which indicates that there is an 18-carbon atom chain (octa
deca) with two double bonds (di en) located at carbon atoms 9 and 12, with carbon
sine and valine. Amino acids can also be grouped into those that have ionisable
side chains (e.g. arginine, aspartate, cysteine, glutamate, histidine, lysine and tyro-
sine). These amino acids contribute to the charge exhibited by peptides and pro-
teins. Both the amino group and the carboxyl group of each amino acid are
ionisable and the acid dissociation constants (pKa values, see Section 1.1.3) of
amino acid side chains are important for the activity of enzymes and the stability of
proteins. This is because ionisation alters their physical properties such as solubility
and lipophilicity. At physiological pH, amino acids exist as ‘zwitterions’ that have
a negatively charged carboxyl group and a positively charged amino group.
Examples of pKa values of amino acid side chains are given in Table A4. The
α-carbon of amino acids is chiral, which produces two optically active forms, desig-
nated D- and L-forms. The L-forms are most common, although some peptides con-
tain both D- and L-amino acids. Proteins are also chiral and consist of only L-amino
acids, which is important in understanding their function (e.g. some enzymes bind
one stereoisomer of a compound with a thousand times greater affinity than the
other). A covalent peptide bond is formed by dehydration between the carbon atom
in the carboxyl group of one amino acid to the nitrogen atom of the amino group of
another amino acid (Fig. A5).
Peptides are formed by linking amino acids via amide bonds. Amides are made
by condensing together a carboxylic acid and an amine. Peptides that contain ,25
amino acids are ‘oligopeptides’ and longer peptides are ‘polypeptides’. Peptides
have a polarity, with a free amino group on the amino-terminal residue and a free
carboxyl group on the carboxyl-terminal residue (Fig. A6), both of which are ioni-
sable groups. There may also be ionisable groups in the side chains of some amino
acids. The overall charge on a peptide (or protein) is the sum of the charges of each
Table A4 Examples of acid dissociation constants of amino acidside chains
Amino acid pKa Functional group
Arginine 12.0 Guanidinium
Aspartate and glutamate 4.4 Carboxyl
Cysteine 8.5 Sulphydryl
Histidine 6.5 Imidazole
Lysine 10.0 Amine
Tyrosine 10.0 Phenol
Source: Adapted from Gorga, F.R., 2007. Introduction to protein structure. Available at: ,http://webhost.bridgew.edu/fgorga/proteins/default.htm. (last accessed February 2016).
e10 Annex A: Food components
ionisable group, and depends on its amino acid content and the pH of the solution.
When the pH of a solution equals the pKa of an ionisable group, the group exists as
an equal mixture of its acidic form and the conjugate base. The pH at which a zwit-
terion is neutral is known as the ‘isoelectric point’. If the pH is less than the pKa,
(A)
(B)
R1 OH
O
Cα C
H
H2N
R1 H
Cα
C
O O
NH NHCH C
R
C N
H O
H2N
R2 OH
O
Cα C
H
R2 OH
O
Cα C
H
peptide bond
H2N+
H2O+
rotationrestricted
rotationpossible
Figure A5 (A) Formation of a peptide bond between two amino acid residues and
(B) rotation of peptide bond.
NH2 – CH – C – NH – CH – C – NH – CH – C – NH – CH – C – OH
O
R1 R2 R3 R4
O O O
amino-terminalor
N-terminalresidue
carboxy-terminalor
C-terminalresidue
Figure A6 A tetrapeptide showing a free amino group on the amino-terminal residue and
free carboxyl group on the carboxyl-terminal residue of an amino acid.
e11Annex A: Food components
the acid form predominates, whereas a pH greater than the pKa enables the base to
predominate. The further the pH is from the pKa the more unbalanced are the acid
and base groups.
A protein has a series of amino acid residues linked by peptide bonds, with a
‘backbone’ made up by the repeated sequence of three atoms of each amino acid
residue (the amide N, the α carbon and the carbonyl carbon). Because the bond
between the carbonyl carbon and the nitrogen is a partial double bond, rotation
around this bond is restricted and the peptide unit has a rigid structure (Fig. A5).
Rotation in the peptide backbone is restricted to bonds involving the α-carbon and
the chain can rotate to create three-dimensional structures in proteins. There are
four levels of structure that can be described as:
1. Primary structure (the sequence of amino acid residues in the polypeptide chain, which is
determined by the gene that encodes it)
2. Secondary structure (formed by hydrogen bonds between backbone atoms in a chain, pro-
ducing two types of stable structures: α-helices and β-sheets)3. Tertiary structure (the arrangement of α-helices, β-sheets and random coils along a poly-
peptide chain. The polypeptide folds so that the side chains of nonpolar amino acids are
within the structure and the side chains of polar residues are exposed on the outer surface.
The tertiary structure of some proteins is stabilised by disulphide bonds between cysteine
residues)
4. Quaternary structure (the spatial organisation of chains if there are more than one poly-
peptide chain in a complex protein). Not all proteins show a quaternary structure and in
many the polypeptides fold independently into a stable tertiary structure with the folded
units associating with each other to form the final structure. In contrast, quaternary struc-
tures are stabilised by noncovalent interactions including hydrogen bonding, van der
Walls interactions and ionic bonding. These intricate three-dimensional structures are
unique to each protein and it is these that allow proteins to function. Most globular pro-
teins consist of a core composed mainly of hydrophobic residues surrounded by a skin of
mainly hydrophilic residues. Disulphide bonds are formed by the oxidation of thiol (�SH)
groups in cysteine residues (Fig. A7) and they can occur within a single polypeptide chain
where they stabilise the tertiary structures, or between two chains, where they stabilise
quaternary structures.
HS
“H2”-
SH
S S
Figure A7 Disulphide bond formation.
e12 Annex A: Food components
The wide variety of protein configurations is due to the large number of different
sequences of amino acid residues, which by convention is written with the amino
terminus on the left and the carboxyl terminus on the right. Amino acid sequences
can be written using either a three-letter code or a one-letter code (see Table 1.8).
For example, the code for a small eight-residue peptide is written as:
Asp�Ile�Glu�Phe�Arg�Val�Leu�His. de Jongh and Broersen (2012) review
the different types of protein modification including phosphorylation (attachment of
phosphate to serine, tyrosine or threonine), methylation (attachment of a methyl
group to arginine or the N-terminus of the protein), glycosylation (attachment of
carbohydrates to lysine or the N-terminus) and acetylation (attachment of acetyl to
an amino group such as lysine or the N-terminus).
A.2 Vitamins and minerals
Tables A5 and A6 give a summary of the properties, functions and sources of vita-
mins and minerals.
Arsenic is essential in trace amounts; its deficiency depresses growth and impairs
reproduction. Boron may affect the metabolism of calcium and magnesium, mem-
brane function and prevents some forms of cardiovascular disease. Its deficiency is
linked to osteoporosis and arthritis and may be related to vitamin D production.
Chlorine is essential in maintaining cellular fluid and electrolyte balances and its
deficiency can cause hair and tooth loss, poor muscular contraction and impaired
digestion. Cobalt is an integral part of vitamin B12 and deficiency may lead to per-
nicious anaemia, retarded growth and nervous disorders. Fluorine is incorporated
into bones and teeth and may increase the deposition of calcium. However, at high
levels, it is toxic and has adverse effects on many enzyme systems. Gallium con-
trols brain chemistry and may have antitumour activity. Lithium controls aggres-
siveness. Silicon is needed for production of connective tissues (tendons, cartilage,
blood vessels, nails, skin and hair) and with calcium to makes bones. Tin supports
hair growth and can enhance reflexes. Deficiency symptoms include baldness,
reduced response to loud noises and reduced haemoglobin synthesis. Vanadium is
required for development of bones, cartilage and teeth and for cellular metabolism.
Deficiency may be linked to reproductive problems and kidney disease.
Lanthanum, praeseodymium, neodymium, thulium, samarium, europium and ytter-
bium are each involved in enhanced cell growth and extended lifespan.
A.3 Functional components of foods
Table A7 shows examples of the sources and potential benefits of selected func-