2009_Book_FoodChemistry

18.5 References 859
Table 18.44. Isotope discrimination in primary photosynthetic CO 2 binding
Plant group CO 2 Acceptor δ( 13 C) value Foods
‰
C 3 -Plant D-Ribulose-1,5-bis-phosphate −32 to −24 Wheat, rice, oats, rye, potatoes, barley, batata,
carboxylase (RuPB-C)
soybean, orange, sugar beet, grapes
C 4 -Plant Phosphoenolpyruvate −16 to −10 Corn, millet, sugar cane
carboxylase (PEP-C)
CAM-Plant a RuBP-C/REP-C −30 to −12 Pineapples, vanilla, cactaceae, agave
a CAM: Crassulacean acid metabolism.
plant (Table 18.44). The discrimination in C 3 -
plants is the greatest and is caused by the kinetic
isotope effect in the reaction catalyzed by
ribulose-1,5-biphosphate carboxylase. It is considerably
less in C 4 -plants. CAM plants occupy
an intermediate position (Table 18.44) because
the C 3 -ortheC 4 -path is taken depending on the
growth conditions.
The large differences in the masses of
1 H 2 O, 2 H 1 HO, and 2 H 2 O result in considerable
thermodynamic isotope effects on phase
transitions. On evaporation, deuterium ( 2 H)
correspondingly decreases in the volatile phase,
so that surface-, ground-, and rain-water contains
less 2 H than the oceans. The 2 H enrichment
in the oceans is greatest at the equator and
decreases with increasing latitude because the
amount of water evaporating depends on the
temperature.
The hydrogen of plant foods comes from precipitation
and from the ground-water in that particular
location. Therefore, plants of the same type of
photosynthesis, which are cultivated at different
places, differ in their δ( 2 H) values. Kinetic isotope
effects in plant metabolism, which due to the
mass difference 2 H/ 1 H are much higher than in
the case of 13 C/ 12 C, also have an effect on the
δ( 2 H) values.
For isotope analysis, the sample is subjected to
catalytic combustion to give CO 2 and H 2 O. After
drying, the 13 C/ 12 C ratio in CO 2 is determined
by mass spectrometry. The 2 H/ 1 H ratio is determined
in hydrogen, which is formed by reducing
the water obtained from catalytic combustion.
The 2 H/ 1 H ratio can change by 2 H/ 1 H exchange,
e. g., as undergone by OH groups. Therefore, such
groups are eliminated before combustion. For example,
only the δ( 2 H) values of the CH-skeleton
in carbohydrates are determined after conversion
to the nitrate ester.
Table 18.45. δ ( 13 C) and δ ( 2 H) values for orange juice
and sugar of different origins
Food δ ( 13 C)(‰) δ ( 2 H)(‰)
Orange juice, freeze-dried −25.6 ± 0.8 n.a.
Sucrose isolated from −25.5 ± 2.5 −22 ± 10
orange juice
Beet sugar −25.6 ± 1.0 −135 ± 25
Cane sugar −11.5 ± 0.5 −50 ± 20
Glucose-fructose syrup −10.8 ± 0.9 −31
(corn)
n.a.: not analyzed.
Sweetening orange juice with cane sugar or
glucose-fructose syrup from corn starch lowers
the δ( 13 C) value of sugar, which is −25.5‰ in
the native juice (Table 18.45). On the other hand,
the addition of beet sugar (C 3 -plant) can be recognized
only via the δ( 2 H) value. The addition of
synthetic products from petrochemicals (δ( 13 C):
−27 ± 5‰) to foods from C 3 -plants cannot be
detected via the δ( 13 C) value, but via the δ( 2 H)
value in many cases.
Apart from the global 13 Cand 2 H contents of
food constituents, the intramolecular distributions
of these isotopes are typical of the origin and,
therefore, of great analytical importance. They
can be measured after chemical decomposition
of the substance or with 13 Cor 2 H NMR spectroscopy
(example in 5.5.1.5).
18.5 References
Bell, E.A., Charlwood, B.Y. (Eds.): Secondary plant
products. Springer-Verlag: Berlin. 1980
Berger, R.G., Shaw, P.E., Latrasse, A., Winterhalter,
P: Fruits I–IV. In: Volatile compounds in foods and