2009_Book_FoodChemistry

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-Larabinopyranoside
(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 conditions
for the particular fruit product. Therefore,
anthocyanins are generally not suitable. For fermented
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 after
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 criterion
of the origin of the food or of individual constituents,
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 different
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 expressed
as the δ-value, based on an international
standard (Table 18.43).
δ = R sample − R standard
× 1000[‰]
R standard
(18.47)
R = C 1
C 2
(18.48)
c 1 /c 2 : concentrations of heavy/light isotopes.
The δ( 13 C) value, which is −8 ± 1‰ for atmospheric
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 international
standards for their determination
Isotope Rel. mean natural International standard
abundance [atom %] Name R
a
1 H 99.9855 V-SMOW b 0.00015576
2 H 0.0145
12 C 98.8920 PDB c 0.0112372
13 C 1.108
a Abundance ratio (Formula 18.48).
b Vienna Standard Mean Ocean Water.
c Pee Dee Belemnite (CaCO 3 from the Pee Dee formation
in South Carolina).