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

5 Water Chemistry and Biochemistry 99

Ionic Strength and Solubility of Foodstuff

Ions are attracted to charged or polar sites of large biomolecules.

Cations strongly interact with large molecules such as proteins.

At low concentrations, they may neutralize charges on large or-

ganic molecules, stabilizing them. At high concentrations, ions

compete with large molecules for water and destabilize them, re-

sulting in decreased solubility. The concentration of electrolytes

affects the solubility of foodstuffs.

One of the criteria for concentration of electrolytes isionic

strength,I, which is half of the sum () of all products of the

concentration (Ci)oftheith ion and the square of its charge

(Zi^2 ):

I=^1 / 2 CiZ^2 i.

However, solubility is not only a function of ionic strength; it

also depends very much on the anions involved.

Thesalting-in phenomenonrefers to increases of protein solu-

bility with increased concentrations of salt at low ionic strength.

The enhancement of broth flavor by adding salt may be due to

an increase of soluble proteins or amino acids in it. At high ionic

strength, however, the solubilities of some proteins decrease; this

is thesalting-out phenomenon. Biochemists often use potassium

sulfate, K 2 SO 4 , and ammonium sulfate, (NH 4 ) 2 SO 4 , for the sep-

aration of amino acids or proteins because the sulfate ion is an

effective salting-out anion. The sulfate ion is astabilizer, because

the precipitated proteins are stable. Table salt is not an effective

salting-out agent. Damodaran (1996) and Voet and Voet (1995)

discuss these phenomena in much more detail.

WATER AS REAGENT AND PRODUCT

Water is the product from the oxidation of hydrogen, and the

standard cell potential (
Eo) for the reaction is 1.229 V:

2H 2 (g)+O 2 (g)=2H 2 O(l),
Eo= 1 .229 V.

Actually, all hydrogen in any substance produces water during

combustion and oxidation. On the other hand, water provides

protons (H+), hydroxide ions (OH−), hydrogen atoms (H), oxy-

gen atoms (O), and radicals (H·,·OH) as reagents. The first

two of these (H+and OH−) also exhibit acid-base properties,

as described earlier. Acids and bases promote hydrolysis and

condensation reactions.

Inesterificationandpeptide synthesis, two molecules are

joined together, or condensed, releasing a water molecule. On

the other hand, water breaks ester, peptide, and glycosidic bonds

in a process calledhydrolysis.

Esterification, Hydrolysis, and Lipids

Organic acids and various alcohols present in food react to yield

esters in aqueous solutions. Esters, also present in food, hy-

drolyze to produce acids and alcohols. Water is a reagent and

a product in these reversible equilibria. Figure 5.13 shows the

Fisher esterification and hydrolysis reactions and the role of wa-

ter in the series of intermediates in these equilibria. In general,

Figure 5.13.Esterification and hydrolysis in aqueous solutions.

esterification is favored in acidic solutions, and hydrolysis is

favored in neutral and basic solutions.

The protonation of the slightly negative carbonyl oxygen

(>C O) of the carboxyl group (C( O)OH) polarizes the C O

bond, making the carbon atom positive, to attract the alco-

hol group R′OH. Water molecules remove protons and rear-

range the bonds in several intermediates for the simple overall

reaction

RC( O)OH+HOR′↔RC( O)OR′+H 2 O

In basic solutions, the OH−ions 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 typical example of hydrolysis.

Triglycerides are hydrophobic, but they can undergo partial hy-

drolysis to become amphiphilic diglycerides or monoglycerides.

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 triglycerides, 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 triglycerides are

partially hydrolyzed in the gastrointestinal tract before absorp-

tion, but most are absorbed with the aid of bile salts, which

emulsify the oil and facilitate its absorption. Many animals

biosynthesize 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.