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Unformatted text preview: h pH [NaCl] From the standpoint of pH, the bacteria of most concern in the food industry are
neutralophiles and acid-tolerant neutralophiles. Acid tolerant neutralophiles exhibit
optimal growth rates around pH = 7 but still show significant growth down to a pH
between 4 and 5. These organisms produce acidic waste products of their fermentative
mode of catabolism so have to tolerate low pH.
Several types of canned and bottled vegetables have a pH of less than 5. This includes
pickled foods which also have a high salt content.
Figure 9-9 shows results of a typical experiment measuring the effect of pH on
microbial survival in refrigerated food, in this case, E. coli O157:H7 in Greek eggplant
salad. Note the critical threshold pH required to decrease bacterial counts. At pH 4, about
the pH of lemon juice, the bacteria show a steep exponential death curve, whereas at pH 4.5,
the bacteria remain viable for many days.
To be safe, low pH should only be considered a bacteriostatic treatment; many
types of vegetative bacteria and endospores remain viable and will grow again if the pH is
increased. Further, many exotoxins are acid stable, so if the food was contaminated prior to
bottling/canning, it may not be safe to eat.
No foods are made intentionally alkaline to inhibit the growth or kill bacteria. However
alkaline detergents and soaps are exploited for the disinfection of surfaces in food processing
plants. In addition to their membrane disrupting properties, the alkalinity of detergents and
soaps helps to dissolve organic matter including bacterial tissue. The alkaline pH
characteristic of these products does not, however, harm endospores.
NaCl added to certain foods at high concentrations (meat, fish, pickled foods) to prevent
the growth of spoilage bacteria as well as the food bourne pathogens. It is an effective
treatment because most spoilage bacteria and food bourne pathogens are nonhalophiles (An
exception is halotolerant S. aureus because of its ability to synthesize large amounts of
the amino acid proline as a compatible solute, Chapter 8).
Elevated salt concentration causes plasmolysis of bacterial cells. Depending on its
severity, plasmolysis may be bacteriostatic or bacteriocidal. Either way, bacterial
reproduction and exotoxin production in the food product are halted. High salt does not
harm exotoxins already present in food. While high salt does not harm endospores,
it does inhibit their germination. O2 and toxic
oxygen species A number of antibacterial treatments employ toxic oxygen derivatives. H2O2 H2O2 is used to sterilize piping in food manufacturing plants. Chlorine gas and
chlorine bleach Cl2 is used in the disinfection of drinking water. When this gas is dissolved in water,
nascent (atomic) oxygen is produced.
Cl2 + H2O HOCl
Oxygen + Nascent oxygen can also be created using household bleach, sodium hypochlorite
(NaOCl) which is used for the disinfection of surfaces. HCl FOOD-300 Ozone gas O3 is used in the...
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- Fall '12