Food Biochemistry and Food Processing

122 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking

For water softening by the lime treatment, the
amount of dissolved Ca^2
and Mg^2
is determined
first; then an equal number of moles of lime,
Ca(OH) 2 , is added to remove them, by these reac-
tions:

Mg^2

Ca(OH) 2 (s) ↔Mg(OH) 2 (s)
Ca^2

Ca^2

2 HCO 3 
Ca(OH) 2 (s) ↔2 CaCO 3 (s)

2 H 2 O.

Permanent hard waterscontain sulfate (SO 4 2-),
Ca^2
, and Mg^2
ions. Calcium ions in the sulfate
solution can be removed by adding sodium carbon-
ate due to the reaction:

Ca^2

Na 2 CO 3 →CaCO 3 (s)
2Na
.

Hard waters cause scales or deposits to build up in
boilers, pipes, and faucets—problems for food and
other industries. Ion exchange using resins or zeo-
lites is commonly used to soften hard waters. The cal-
cium and magnesium ions in the waters are taken up
by the resin or zeolite that releases sodium or hydro-
gen ions back to the water. Alternatively, when pres-
sure is applied to a solution, water molecules, but not
ions, diffuse through the semipermeable membranes.
This method, calledreverse osmosis, has been used
to soften hard waters and desalinate seawater.
However, water softening replaces desirable cal-
cium and other ions with sodium ions. Thus, soft
waters are not suitable for drinking. Incidentally,
calcium ions strengthen the gluten proteins in dough
mixing. Some calcium salts are added to the dough
by bakeries to enhance bread quality.

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 organic mole-
cules, stabilizing them. At high concentrations, ions
compete with large molecules for water and destabi-
lize them, resulting in decreased solubility. The con-
centration of electrolytes affects the solubility of
foodstuffs.
One of the criteria for concentration of elec-
trolytes is ionic strength, I, which is half of the sum
(
) of all products of the concentration (Ci) of the
ith ion and the square of its charge (Zi^2 ):

I^1 ⁄ 2
CiZi^2.

However, solubility is not only a function of ionic

strength; it also depends very much on the anions

involved.

The salting-in phenomenonrefers to increases

of protein solubility 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 ion-

ic strength, however, the solubilities of some pro-

teins decrease; this is the salting-out phenomenon.

Biochemists often use potassium sulfate, K 2 SO 4 ,

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

tion of amino acids or proteins because the sulfate

ion is an effective salting-out anion. The sulfate ion

is a stabilizer, 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), 
Eo1.229 V

Actually, all hydrogen in any substance produces

water during combustion and oxidation. On the oth-

er hand, water provides protons (H
), hydroxide

ions (OH), hydrogen atoms (H), oxygen atoms

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

of these (H
and OH) also exhibit acid-base prop-

erties, as described earlier. Acids and bases promote

hydrolysis and condensation reactions.

In esterificationand peptide synthesis, two mol-

ecules are joined together, or condensed, releasing a

water molecule. On the other hand, water breaks

ester, peptide, and glycosidic bonds in a process

called hydrolysis.

ESTERIFICATION, HYDROLYSIS, ANDLIPIDS

Organic acids and various alcohols present in food

react to yield esters in aqueous solutions. Esters,

also present in food, hydrolyze 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 water in the series of intermediates in these equi-