Food Chemistry

142 2 Enzymes

Table 2.18.Examples for application of enzyme im-
munoassay 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

equipment contrary to use of radio immunolog-
ical methods (RIA). Furthermore, for radio im-
munoassays free antigens have always to be sep-
arated from the ones bound to antibodies (het-
erogeneous immunoassay) while an enzyme im-
munoassay 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 ELISAtest(enzyme linked
immunosorbent assay) is the most important im-
munochemical 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 concentra-
tion in the sample which can be calculated based
on measured standards and a calibration curve.

Fig. 2.44.Principle of non-competitive ELISA (sand-

wich ELISA)

Immobilized antibody,

antigen,

enzyme-marked antibody

2.6.4 PolymeraseChainReaction...............................

With the polymerase chain reaction (PCR), a few

molecules of any DNA sequence can be multi-

plied 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 con-

tent

Wheat Herbicide tolerance

Sugar beet Herbicide tolerance

aThe crop is approved in at least one country.