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

96 Part 1: Principles/Food Analysis

Figure 5.11.Titration curve of a 0.10 mol/L (or M) weak acid HA

(Ka= 1 × 10 −^5 ) using a 0.10 mol/L strong base NaOH solution.

As the H+ions react with OH−,moreH+ions are produced due

to the equilibrium

HA=H++A−,Ka(ionization constant of the acid).

Before any NaOH solution is added, HA is the dominant species;

when half of HA is consumed, [HA]=[A−], which is called

thehalf equivalence point. At this point, the pH varies the least

when a little H+or OH−is added, and the solution at this point

is themost effective buffer solution,as we shall see later. The

pH at this point is the same as the pKaof the weak acid. When

an equivalent amount of OH−has been added, the A−species

dominates, and the solution is equivalent to a salt solution of

NaA. Of course, the salt is completely ionized.

Polyprotic acids such as ascorbic acid H 2 -(H 6 C 6 O 6 ) (Vitamin

C;Ka1=7.9× 10 −^5 ,Ka2=1.6× 10 −^12 ) and phosphoric acid

H 3 PO 4 (Ka1=6.94× 10 −^3 ,Ka2=6.2× 10 −^8 ,Ka32.1×

10 −^12 ) have more than one mole of H+per mole of acid. A

titration curve of these acids will have two and three end points

for ascorbic and phosphoric acids, respectively, partly due to

the large differences in their dissociation constants (Ka1,Ka2,

etc.). In practice, the third end point is difficult to observe in the

titration of H 3 PO 4. Vitamin C and phosphoric acids are often

used as food additives.

Many food components (e.g., amino acids, proteins, alkaloids,

organic and inorganic stuff, vitamins, fatty acids, oxidized car-

bohydrates, and compounds giving smell and flavor) are weak

acids and bases. The pH affects their forms, stability, and reac-

tions. When pH decreases by 1, the concentration of H+,[H+],

increases 10-fold, accompanied by a 10-fold decrease in [OH−].

The H+and OH−are very active reagents for the esterification

and hydrolysis reactions of proteins, carbohydrates, and lipids,

as we shall see later. Thus, the acidity, or pH, not only affects the

taste of food, it is an important parameter in food processing.

Solutions of Amino Acids

Amino acids have an amino group (NH 3 +), a carboxyl group

(COO−), a H, and a side chain (R) attached to the asymmetric

alpha carbon. They are the building blocks of proteins, polymers

of amino acids. At a pH called theisoelectric point, which

depends on the amino acid in question, the dominant species

is azwitterion, RHC(NH 3 +)(COO−), which has a positive and

a negative site, but no net charge. For example, the isoelectric

point for glycine is pH=6.00, and its dominant species is

H 2 C(NH 3 +)COO−. An amino acid exists in at least three forms

due to the following ionization or equilibria:

RHC(NH+ 3 )(COOH)=RHC(NH+ 3 )(COO−)+H+,Ka1

RHC(NH+ 3 )(COO−)=RHC(NH 2 )(COO−)+H+,Ka2.

Most amino acids behave like a diprotic acid with two dissoci-

ation constants,Ka1andKa2. A few amino acids have a third

ionizable group in their side chains.

Among the 20 common amino acids, the side chains of eight

are nonpolar, and those of seven are polar, containing OH,

>C O, or SH groups. Aspartic and glutamic acid contain

acidic COOH groups in their side chains, whereas arginine,

histidine, and lysine contain basic NH or NH 2 groups. These

have four forms due to adding or losing protons at different pH

values of the solution, and they behave as triprotic acids. For

example, aspartic acid [Asp=(COOH)CH 2 C(NH 3 +)(COO−)]

has these forms:

AspH+=Asp+H+

Asp=Asp−+H+

Asp−=Asp^2 −+H+

Proteins, amino acid polymers, can accept or provide several

protons as the pH changes. At itsisoelectric point(a specific

pH), the protein has no net charge and is least soluble because

electrostatic repulsion between its molecules is lowest, and the

molecules coalesce or precipitate, forming a solid or gel.

Solutions of Salts

Salts consist of positive and negative ions, and these ions are

hydrated in their solutions. Positive, hydrated ions such as

Na(H 2 O) 6 +, Ca(H 2 O) 82 +, and Al(H 2 O) 63 +have six to eight wa-

ter molecules around them. Figure 5.12 is a sketch of the inter-

actions of water molecules with ions. The water molecules point

the negative ends of their dipoles toward positive ions, and their

positive ends toward negative ions. Molecules in the hydration

sphere constantly and dynamically exchange with those around

them. The number and lifetimes of hydrated water molecules

have been studied by various methods. These studies reveal that

the hydration sphere is one layer deep, and the lifetimes of these

hydrated water molecules are in the order of picoseconds (10−^12

seconds). The larger negative ions also interact with the polar

water molecules, but not as strongly as do cations. The presence

of ions in the solution changes the ordering of water molecules

even if they are not in the first hydration sphere.

The hydration of ions releases energy, but breaking up ions

from a solid requires energy. The amount of energy needed