Food Biochemistry and Food Processing

5 Water Chemistry and Biochemistry 123

libria. In general, esterification is favored in acidic
solutions, and hydrolysis is favored in neutral and
basic solutions.
The protonation of the slightly negative carbonyl
oxygen (CO) of the carboxyl group (C(O)OH)
polarizes the CO bond, making the carbon atom
positive, to attract the alcohol group ROH. Water
molecules remove protons and rearrange the bonds in
several intermediates for the simple overall reaction

RC(O)OH
HOR↔RC(uO)OR
H 2 O

In basic solutions, the OHions are attracted to
the slightly positive carbon of the carbonyl group.
The hydrolysis is the reverse of esterification.
Hydrolysis of glycerol esters (glycerides: fat and
oil) in basic solutions during soap making is a typi-
cal example of hydrolysis. Triglycerides are hydro-
phobic, but they can undergo partial hydrolysis to
become amphiphilic diglycerides or monoglyc-

erides. Esterification and hydrolysis are processes in

metabolism.

Lipids, various water-insoluble esters of fatty

acids, include glycerides, phospholipids, glycolipids,

and cholesterol. Oils and fats are mostly triglyc-

erides, which is a glycerol molecule (CH 2 OH–

CHOH–CH 2 OH) esterified with three fatty acids

[CH 3 (–CH 2 )nCOOH, n 8–16]. Some of the tri-

glycerides are partially hydrolyzed in the gastroin-

testinal tract before absorption, but most are ab-

sorbed with the aid of bile salts, which emulsify the

oil and facilitate its absorption. Many animals bio-

synthesize lipids when food is plentiful, as lipids

provide the highest amount of energy per unit mass.

Lipids, stored in fat cells, can be hydrolyzed, and

upon further oxidation, they produce lots of energy

and water. Some animals utilize fat for both energy

and water to overcome the limitation of food and

water supplies during certain periods of their lives.

Figure 5.13.Esterification and hydrolysis in aqueous solutions.