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


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.
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