Food Chemistry

392 5 Aroma Compounds

To calculate the average number of aroma
molecules bound to a biopolymer, the specific
binding capacity,r, has to be introduced:

r=

(BA)

(B)+(BA)

(5.40)

The concentration of the complex BA from Equa-
tion 5.39 is substituted in Equation 5.40:

r=

K·cf(B)

(B)+K(B)cf

=

Kcf

1 +Kcf

(5.41)

When a biopolymer binds not only one molecular
species (as A in the above case) but has a num-
ber (n) of binding groups (or sites) equal in bind-
ing ability and independent of each other, thenr
has to be multiplied byn, and Equation 5.41 ac-
quires the form:

r=

n·Kcf

1 +Kcf

(5.42)

r

cf

=K·n−K·r=K′−K·r (5.43)

whereK′=overall binding constant.
The evaluation of data then follows Equa-
tion 5.43 presented in graphic form, i. e.
a diagram ofr/Cf= f(r). Three extreme or
limiting cases should be observed:

a) A straight line (Fig. 5.37, a) indicates that

only one binding region on a polymer, with

Fig. 5.37.Binding of aroma compounds by biopoly-
mers. Graphical determination of binding parameters
according toSolms, 1975

n binding sites (all equivalent and independ-

ent from each other) is involved. The valuesn

and K′are obtained from the intersection of

the straight line with the abscissa and the or-

dinate, respectively.

b) A straight line parallel to the abscissa

(Fig. 5.37, b) is obtained when the single

binding constant,K, is low and the value

of n is very high. In this special case,

Equation 5.48 has the form:

r=K′·cf (5.44)

c) A curve (Fig. 5.37, c) which in approximation

is the merging of two straight lines, as shown

separately (Fig. 5.37, d). This indicates two

binding constants,K 1 ′,andK 2 ′, and their re-

spective binding groups,n 1 andn 2 , which are

equivalent and independent of each other.

By plottingrversuscf,valuesofK′are obtained

from the slope of the curve. An example for

a model system with two binding regions (case c)

is given by aroma binding to starch. It should be

remembered that starch binds the volatiles only

after gelatinization by trapping the volatiles in

its helical structure, and that starch is made up of

two constituents, amylose and amylopectin. The

binding parameters are listed for some aroma

compounds in Table 5.37. Numerous observa-

tions indicate thatK 1 ′and binding regionn 1 are

related to the inner space of the helix, whileK′ 2

and then 2 region are related to the outer surface

of the helix.K 1 ′is larger thanK′ 2 ,whichshows

that, within the helix, the aroma compounds are

bound more efficiently to glucose residues of the

helix. The fraction 1/nis a measure of the size of

the binding region. It decreases, as expected, with

Table 5.37.Binding of aroma compounds by potato

starch

Compounds Binding constant

K′ 1 n 1 K′ 2 n 2

1-Hexanol 5. 45 · 101 0 .10 – –

1-Octanol 2. 19 · 102 0 .05 2. 15 · 101 0. 11

1-Decanol 1. 25 · 102 0 .04 1. 29 · 101 0. 11

Capric acid 3. 30 · 102 0 .07 4. 35 · 101 0. 19

Menthone 1. 84 · 102 0 .012 8. 97 0. 045

Menthol 1. 43 · 102 0 .007 – –

β-Pinene 1. 30 · 101 0 .027 1. 81 0. 089