Milk also has the capacity to generate antimicrobials in the presence
of hydrogen peroxide. The enzyme lactoperoxidase constitutes about
0.5% of whey proteins and catalyses the oxidation of thiocyanate by
hydrogen peroxide. Thiocyanate is naturally present in milk and its level
can be boosted by consumption of brassicas which are rich in thiocyanate
precursors. Hydrogen peroxide can be generated by endogenous enzyme
activity or by the aerobic metabolism of lactic acid bacteria. The reaction
produces short lived oxidation products such as hypothiocyanate which
can kill Gram negative bacteria and inhibit Gram positives, possibly by
damaging the bacterial cytoplasmic membrane (Figure 3.5).
3.2.5 Water Activity
Water is a remarkable compound. Considered as a hydride of oxygen
(H 2 O) it has quite exceptional properties when compared with the
hydrides of neighbouring elements in the periodic table such as ammonia
(NH 3 ), methane (CH 4 ), hydrogen sulfide (H 2 S), and hydrofluoric acid
(HF), see Table 3.9. Life as we know it is totally dependent on the
presence of water in its liquid state. The reactions which take place in the
cytoplasm do so in an aqueous environment and the cytoplasm is
surrounded by a membrane which is generally permeable to water
molecules which may pass freely from the cytoplasm to the environment
and from the environment to the cytoplasm. This dynamic two way flow
Figure 3.5 The lactoperoxidase system
Table 3.9 The boiling points ( 1 C) of hydrides of elements surrounding oxygen in
the periodic table
B 2 H 6 CH 4 NH 3 H 2 OHF
92.5 161.7 33 100 19.4
SiH 4 PH 3 H 2 S HCl
112 87 60 85H 2 Se
4236 Factors Affecting the Growth and Survival of Micro-organisms in Foods