Food Biochemistry and Food Processing (2 edition)

(Steven Felgate) #1

BLBS102-c05 BLBS102-Simpson March 21, 2012 12:2 Trim: 276mm X 219mm Printer Name: Yet to Come


100 Part 1: Principles/Food Analysis

Figure 5.14.Hydrolysis and peptide-bond formation
(polymerization).

Catalyzed by enzymes, esterification, and hydrolysis in bio-
logical systems proceed at much faster rates than when catalyzed
by acids and bases.

Water in Digestion and Syntheses of Proteins

The digestion and the formation of many biopolymers (such
as proteins and carbohydrates) as well as the formation and
breakdown of lipids and esters involve reactions very similar to
those of esterification and hydrolysis.
The digestion of proteins, polymers of amino acids, starts with
chewing, followed by hydrolysis with the aid of protein-cleaving
enzymes (proteases) throughout the gastrointestinal tract. Then,
the partially hydrolyzed small peptides and hydrolyzed individ-
ual amino acids are absorbed in the intestine. Water is a reagent
in these hydrolyses (Fig. 5.14).
Thedeoxyribonucleic acids(DNAs) store the genetic infor-
mation, and they direct the synthesis ofmessenger ribonucleic
acids(mRNAs), which in turn direct the protein synthesis ma-
chinery to make various proteins for specific body functions and
structures. This is an oversimplified description of the biological
processes that carry out the polymerization of amino acids.
Proteins and amino acids also provide energy when fully oxi-
dized, but carbohydrates are the major energy source in normal
diets.

Water in Digestion and Synthesis
of Carbohydrates

On earth, water is the most abundant inorganic compound,
whereas carbohydrates are the most abundant class of organic
compounds. Carbohydrates require water for their synthesis and
provide most of the energy for all life on earth. They are also part

of the glycoproteins and the genetic molecules of DNA.Car-
bohydratesare compounds with a deceivingly simple general
formula (CH 2 O)n,n≥3, that appears to be made up of carbon
and water, but although their chemistry fills volumes of thick
books and more, there is still much for carbohydrate chemists to
discover.
Energy from the sun captured by plants and organisms con-
verts CO 2 and water to high-energy carbohydrates,

6CO 2 +12H 2 O∗→(CH 2 O) 6 +6H 2 O+6O∗ 2

The stars (∗) indicate the oxygen atoms from water released as
oxygen gas (O∗ 2 ). Still, this is an oversimplified equation for
photosynthesis, but we do not have room to dig any deeper.
The product (CH 2 O) 6 is ahexose, a six-carbon simple sugar,
ormonosaccharide,that can be fructose, glucose, or another
simple sugar. Glucose is the most familiar simple sugar, and
its most common structure is a cyclic structure of the chair
form. In glucose, all the OH groups around the ring are at the
equatorial positions, whereas the small H atoms are at the axial
locations (Fig. 5.15). With so many OH groups per molecule,
glucose molecules are able to form several hydrogen bonds with
water molecules, and thus most monosaccharides are soluble in
water.
Thedisaccharidessucrose, maltose, and lactose have two sim-
ple sugars linked together, whereas starch and fiber are polymers
of many glucose units. A disaccharide is formed when two OH
groups of separate monosaccharides react to form an –O– link,
called aglycosidic bond, after losing a water molecule:

C 6 H 12 O 6 +C 6 H 12 O 6 =C 6 H 11 O 5 –O–C 6 H 11 O 5 +H 2 O.

Disaccharides are soluble in water due to their ability to form
many hydrogen bonds.
Plants and animals store glucose as long-chainpolysaccha-
ridesinstarchandglycogen, respectively, for energy. Starch is
divided into amylose and amylopectin.Amyloseconsists of lin-
ear chains, whereasamylopectinhas branched chains. Due to the
many interchain hydrogen bonds in starch, hydrogen bonding to
water molecules develops slowly. Small starch molecules are
soluble in water. Suspensions of large starch molecules thicken

Figure 5.15.Chair form cyclic structure of glucose C 6 H 12 O 6 .For
glucose, all the OH groups are in the equatorial position, and these
are possible H-donors for hydrogen bonding with water molecules.
They are also possible sites to link to other hexoses.
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