Food Chemistry

858 18 Fruits and Fruit Products

Table 18.42.Phenolic compounds as indicator substances for the detection of adulteration of fruit products

Compound Occurrence Detection

Quercetin-3-rutinoside Common, but not in strawberries Elderberry juice in strawberry juice
Quercetin-3-O-(2′′-O-α-L- Red currants Red currants in products from
rhamnosyl-6′′-O-α-L-rhamnosyl)- black currants
β-D-glucoside
Naringin or naringenin Grapefruits Grapefruit juice in orange juice
Apigenin-6-C-β-D- Figs Fig juice in grape juice
glucopyranosyl-8-C-α-L-
arabinopyranoside (schaftoside)

18.4.2 Species-Specific Constituents

The occurrence of species-specific constituents
is also analytically useful. The composition of
the plant phenols of individual fruits can be
analyzed quickly and very accurately by using
HPLC. These data have shown that certain
compounds are suitable indicators of adulteration
(Table 18.42). These indicators must be fixed
with great care. In fact, phloretin-2-glucoside
(phloridzine) and isorhamnetinglucoside have
been proposed as markers for apples and pears.
Improvements in the analyses, however, showed
that phloridzine and isorhamnetinglucoside
widely occur in low concentrations in fruit,
and the last mentioned glucoside also occurs in
apples, among other fruit.
It must be guaranteed that the selected indicator
substance is stable under the production condi-
tions for the particular fruit product. Therefore,
anthocyanins are generally not suitable. For fer-
mented products, O-glycosides are not suitable
because they are degraded by yeast enzymes.
Suitable compounds are C-glycosidically bound

flavonoids which are resistant to enzymatic
hydrolysis and common chemical hydrolysis,
e. g., schaftoside (cf. Table 18.42) can be detected
even in wine and champagne when the must is
adulterated with fig juice.
The analytical importance of amino acid
(cf. 18.1.2.1.2), protein, enzyme (cf. 18.1.2.1.1),
and carotinoid patterns (cf. 18.1.2.3.2) have
already been mentioned.
Adulteration of orange juice by the addition of
an aqueous extract of the pulp, which remains af-
ter pressing of the juice (pulp wash), is detected
by the marker N,N-dimethylproline. The levels of
this amino acid are higher in pulp wash than in
juice.

18.4.3 Abundance Ratios of Isotopes

The content of the isotopes^2 Hand^13 C is a criter-

ion of the origin of the food or of individual con-

stituents, e. g., sugar used to sweeten fruit juice.

The method is based on the fact that isotopomeric

molecules, e. g.,^12 CO 2 and^13 CO 2 , react at differ-

ent rates in biochemical and chemical reactions

(kinetic isotope effect). In general, the molecules

with the heavier isotope react slower, so that this

isotope is enriched in the products.

The resulting change in the abundance ratio is ex-

pressed as theδ-value, based on an international

standard (Table 18.43).

δ=

Rsample−Rstandard

Rstandard

× 1000 [‰] (18.47)

R=

C 1

C 2

(18.48)

c 1 /c 2 : concentrations of heavy/light isotopes.

Theδ(^13 C) value, which is− 8 ±1‰ for atmo-

spheric CO 2 , increases during CO 2 fixation as

a function of the type of photosynthesis of the

Table 18.43.Abundance of important isotopes and in-

ternational standards for their determination

Isotope Rel. mean natural International standard

abundance [atom %]

Name Ra

(^1) H99. 9855 V-SMOWb 0. 00015576
(^2) H0. 0145
(^12) C98. 8920 PDBc 0. 0112372
(^13) C1. 108
aAbundance ratio (Formula 18.48).
bVienna Standard Mean Ocean Water.
cPee Dee Belemnite (CaCO 3 from the Pee Dee forma-
tion in South Carolina).