Agriculture Reference
In-Depth Information
6.3 Natural antimicrobials derived from animals
6.3.1 Lactoperoxidase
Lactoperoxidase is an enzyme naturally found in milk, tears and saliva (Reiter and
Harnulv 1984; Pruitt and Tenova 1985; Naidu 2000a). Lactoperoxidase is the
most abundant enzyme in bovine milk, constituting about 1% of the whey proteins
or 10-30 μg/ml of the milk (Reiter 1985). Its fi rst antimicrobial properties
were fi rst identifi ed by Hanssen (1924) and further demonstrated by Wright and
Tramer (1958). Involvement of hydrogen peroxide (Jago and Morrison 1962) and
thiocyanate (Reiter 1978) in the inhibitory phenomena was identifi ed later.
Hydrogen peroxide is a substrate for lactoperoxidase in oxidising thiocyanate
(SCN
−
) to hypothiocyanate (OSCN), and it is the OSCN
−
that exerts an
antimicrobial effect. Collectively the use of the three components as a preservation
method is known as the lactoperoxidase system (LPS). The antimicrobial effect of
LPS can be either static or cidal depending on the target organism. The site of
action is probably the cytoplasmic membrane (Reiter 1978). Paul
et al.
(1980)
describe the isolation and purifi cation of lactoperoxidase by casein precipitation
and adsorption of whey proteins by ion exchange.
Most interest in LPS is in the preservation of raw milk, especially in developing
countries where refrigeration is not available or unreliable, and the source of milk
is a considerable distance from the processing dairy. The method of activating
LPS in milk is to add about 10 mg/l of thiocyanate to the raw milk to increase the
level to 15 mg/l (5 mg/l is naturally present). The solution is mixed for 30 seconds
and then an equimolar amount of hydrogen peroxide (8.5 mg/l) is added. The
activation of the LPS has a bacteriostatic effect on the raw milk and effectively
extends the shelf life of the raw milk for 7-8 hours at ambient temperatures of
30°C or longer at lower temperatures. This allows more time for the milk to be
transported from the collection point to a processing centre without refrigeration
(Anon. 2005). Numerous developing countries use LPS in such a manner. The
benefi ts and potential risks of the LPS system is the subject of a joint FAO/WHO
report (Anon. 2005).
The LPS system has also been shown to increase the heat sensitivity of both
L. monocytogenes
and
S. aureus
(Kamau
et al.
1990). Its use in a calf milk replacer
and in dental hygiene has also been realised. Toothpaste (Biotene™) containing
LPS is commercially available (Naidu 2000a).
6.3.2 Lactoferrin
Lactoferrin is an iron-binding glycoprotein present in milk. Apart from its ability
to bind iron it can also bind other metal ions and, although bovine milk is the
main source, it is also found in various human and animal secretions and fl uids
(Naidu 2000b). Apart from its antimicrobial properties it also has benefi ts as
a neutraceutical. Lactoferrin is a single polypeptide chain with a molecular
weight of 75 000 to 80 000 Daltons. Dry weight determinations, together with
measurement of iron-binding capacities, show combining weights per iron atom
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