2009_Book_FoodChemistry

142 2 Enzymes
Table 2.18. Examples for application of enzyme immunoassay
in food analysis
Detection and quantification
Type of meat
Soya protein in meat products
Myosin in muscle meat
Cereal proteins as well as papain in beer
Gliadins (absence of gluten in foods)
Veterinary drugs and fattening aids, e. g. penicillin
in milk, natural or synthetic estrogens in meat
Toxins (aflatoxins, enterotoxins, ochratoxins) in
food
Pesticides (atrazine, aldicarb, carbofuran)
Glycoalkaloids in potatoes
Fig. 2.44. Principle of non-competitive ELISA (sandwich
ELISA)
Immobilized antibody,
antigen,
enzyme-marked antibody
2.6.4 Polymerase Chain Reaction
equipment contrary to use of radio immunological
methods (RIA). Furthermore, for radio immunoassays
free antigens have always to be separated
from the ones bound to antibodies (heterogeneous
immunoassay) while an enzyme immunoassay
is suitable for homogeneous tests if
the activity of the indicator enzyme is inhibited by
the formation of an antigen–antibody-complex.
In food analysis, the ELISA test (enzyme linked
immunosorbent assay) is the most important immunochemical
method. In fact, two experimental
procedures are applied: the competitive ELISA,
as shown in Fig. 2.43, and the sandwich ELISA.
While the competitive ELISA is directed at
the detection of low-molecular substances, the
sandwich ELISA is suitable only for analytes
(antigens) larger than a certain minimum size.
The antigen must have at least two antibody
binding sites (epitopes) which are spatially so far
apart that it can bind two different antibodies.
The principle of the sandwich ELISA is shown in
Fig. 2.44. A plastic carrier holds the antibodies,
e. g. against a toxin, by adsorption. When the
sample is added, the toxin (antigen) reacts with
the excess amount of antibodies (I in Fig. 2.44).
The second antibody marked with an enzyme
(e. g. alkaline phosphatase, peroxidase, glucose
oxidase or luciferase) and with specificity for the
antigen forms a sandwich complex (II). Unbound
enzyme-marked antibodies are washed out. The
remaining enzyme activity is determined (III). It
is directly proportional to the antigen concentration
in the sample which can be calculated based
on measured standards and a calibration curve.
With the polymerase chain reaction (PCR), a few
molecules of any DNA sequence can be multiplied
by a factor of 10 6 to 10 8 in a very short time.
The sequence is multiplied in a highly specific
way until it becomes visible electrophoretically.
Based on PCR, analytical techniques have been
developed for species identification in the case of
animal and plant foods and microorganisms. It is
Table 2.19. Examples of approved genetically modified
crops (as of 2003) a
Crop
Property
Cauliflower Herbicide tolerance
Broccoli Herbicide tolerance
Chicory Herbicide tolerance
Cucumber Fungal resistance
Potato Insect and virus resistance
Pumpkin Virus resistance
Corn Herbicide tolerance, insect resistance
Melon Virus resistance, delayed ripening
Papaya Virus resistance
Paprika Virus resistance
Rape seed Higher concentrations of 12:0 und 14:0,
herbicide resistance
Rice Virus resistance
Red Bean Insect resistance
Soybean Altered fatty acid spectrum
(cf. 14.3.2.2.5), herbicide tolerance
Tomato Delayed ripening, increased pectin content
Wheat Herbicide tolerance
Sugar beet Herbicide tolerance
a The crop is approved in at least one country.