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 97

Figure 5.12.The first hydration sphere of most cations M(H 2 O) 6 +, and anions X(H 2 O) 6 −^1. Small water molecules are below the plane

containing the ions, and large water molecules are above the plane.

depends on the substance, and for this reason, some substances

are more soluble than others. Natural waters in oceans, streams,

rivers, and lakes are in contact with minerals and salts. The

concentrations of various ions depend on the solubility of salts

(Moeller and O’Connor 1972) and the contact time.

All salts dissolved in water are completely ionized, even those

formed in the reaction between weak acids and weak bases.

For example, the common food preservative sodium benzoate

(NaC 6 H 5 COO) is a salt formed between a strong base NaOH

and weak benzoic acid (Ka=6.5× 10 −^5 ). The benzoate ions,

C 6 H 5 COO−, in the solution react with water to produce OH−

ions giving a slightly basic solution:

C 6 H 5 COO−+H 2 O↔C 6 H 5 COOH+OH−,

K= 1. 6 × 10 −^10 ,at 298 K.

Ammonium bicarbonate, NH 4 HCO 3 , was a leavening agent be-

fore modern baking powder was popular. It is still called for in

some recipes. This can be considered a salt formed between the

weak base NH 4 OH and the weak acid H 2 CO 3. When NH 4 HCO 3

dissolves in water, the ammonium and bicarbonate ions react

with water:

NH+ 4 +H 2 O↔NH 3 (aq)+H 3 O+,K= 5. 7 × 10 −^10 ,

at 298 K

HCO− 3 +H 2 O↔H 2 CO 3 +OH−,K= 2. 3 × 10 −^8 ,at 298 K

H 2 CO 3 =H 2 O+CO 2 (g).

Upon heating, NH 3 and carbon dioxide (CO 2 ) become gases for

the leavening action. Thus, during the baking or frying process,

NH 3 is very pungent and unpleasant. When sodium bicarbonate

is used, only CO 2 causes the dough to rise. Phosphoric acid,

instead of NH 4 +, provides the acid in baking powder.

Buffer Solutions

A solution containing a weak acid and its salt or a weak base

and its salt is a buffer solution, since its pH changes little when a

small amount acid or base is added. For example, nicotinic acid

(HC 6 H 4 NO 2 , niacin, a food component) is a weak acid with

Ka=1.7× 10 −^5 (pKa=−log 10 Ka=4.76):

HC 6 H 4 NO 2 =H++C 6 H 4 NO− 2 ,

Ka=

[H+][C 6 H 4 NO− 2 ]

[HC 6 H 4 NO 2 ]

pH=pKa+log 10

{

[C 6 H 4 NO− 2 ]

[HC 6 H 4 NO 2 ]

}

(Henderson-Hesselbalch equation).

In a solution containing niacin and its salt, [C 6 H 4 NO 2 −]is the

concentration of the salt, and [HC 6 H 4 NO 2 ] is the concentration

of niacin. The pair, HC 6 H 4 NO 2 and C 6 H 4 NO 2 −, are calledcon-

jugate acid and base, according to the Bronsted-Lowry defini-

tion for acids and bases. So, for a general acid and its conjugate

base, the pH can be evaluated using the Henderson-Hesselbalch

equation:

pH=pKa=log 10

{

[base]

[acid]

}

(Henderson-Hesselbalch Equation).

Adding H+converts the base into its conjugate acid, and adding

OH−converts the acid into its conjugate base. Adding acid and

base changes the ratio [base]/[acid], causing a small change in

the pH if the initial ratio is close to 1.0. Following this equation,

the most effective buffer solution for a desirable pH is to use

an acid with a pKavalue similar to the desired pH value and to

adjust the concentration of the salt and acid to obtain the ratio

that gives the desired pH. For example, the pKafor H 2 PO 4 −

is 7.21, and mixing KH 2 PO 4 and K 2 HPO 4 in the appropriate

ratio will give a buffer solution with pH 7. However, more is

involved in the art and science of making and standardizing

buffer solutions. For example, the ionic strength must be taken

into account.

The pH of blood from healthy persons is 7.4. The phospho-

ric acid and bicarbonate ions in blood and many other soluble