Food Biochemistry and Food Processing (2 edition)

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.