320 Modern Food Microbiology
Gaminara, Infantis, Anatum, Cubana, and Stanley). After a 10-minute exposure, the reductions ranged
between 2 and 3.2 logs for the following sanitizers: 20,000 ppm Ca(OCl) 2 ; 5% Na 3 PO 4 ; 8% H 2 O 2 ;
1% Ca(OH) 2 ; 1% calcinated calcium, 5% lactic acid, and 5% citric acid.
207
The effect of isothiocyanate vapors against antibiotic-resistant E. coli 0157:H7 and S. Montevideo;
and L. monocytogenes inoculated onto lettuce was examined.
118 Iceberg lettuce was inoculated with
up to 10
7 –10
8 cfu/g and held at 4
◦ C for up to 4 days. A reduction of up to 8 logs of E. coli 0157:H7
(in 2 days) and S. Montevideo (in 4 days) was achieved with vapor generated from 400 µl of allyl
isothiocyanate. Overall, allyl isothiocyanate was more effective than methyl isothiocyanate against the
two Gram-negative pathogens but the methyl form was more effective against L. monocytogenes.
118
The control of L. monocytogenes on turkey frankfurters was achieved by the use of any one of four
GRAS chemicals employed at a level of 0.3%/frank. With an inoculum of a five-strain mixture of
10
6 cfu/ml, the franks were dipped for 1 minute each and held at 4, 13, and 22
◦ C.
89 An immediate
decrease of 1–2 logs was achieved by all agents tested alone, and after 14 days at 4
◦ C, the decrease was
3–4 logs while untreated franks spoiled within 7 days at 22
◦ C. When chopped parsley was inoculated
with ca. 10
3 or 10
6 cfu/g of Shigella sonnei and held at 21
◦ C for up to 14 days, the pathogen increased
about 3 logs after 1 day while that held at 4
◦ C decreased by 2.5–3.0 log 10 cfu/g over the 14-day
period.
213 With vinegar (5.2% acetic acid) or 200 ppm free Cl 2 for 5 minutes at 21
◦ C, a >6-log
reduction was achieved, and with 7.6% acetic acid or 250 ppm free Cl 2 , a reduction of 7 to 7.3 logs
was achieved.
213 The effect of hypochlorite and ClO 2 on bacterial spores has been studied, and in one
study using Bacillus subtilis spores, neither hypochlorite nor chlorine dioxide caused the release of
the spore core’s depot of dipicolinic acid (DPA) but spores so treated more readily released DPA upon
a subsequent normally sub-lethal heat treatment than did untreated spores.
219 )
NaCl AND SUGARS
These compounds are grouped together because of the similarity in their modes of action in preserving foods. NaCl has been employed as a food preservative since ancient times. The early food uses of
salt were for the purpose of preserving meats. This use is based on the fact that at high concentrations,
salt exerts a drying effect on both food and microorganisms. Salt (saline) in water at concentrations
of 0.85–0.90% produces an isotonic condition for nonmarine microorganisms. Because the amounts
of NaCl and water are equal on both sides of the cell membrane, water moves across the cell membranes equally in both directions. When microbial cells are suspended in, say, a 5% saline solution, the
concentration of water is greater inside the cells than outside (concentration of H 2 O is highest where
solute concentration is lowest). In diffusion, water moves from its area of high concentration to its area
of low concentration. In this case, water passes out of the cells at a greater rate than it enters. The result
to the cell is plasmolysis, which results in growth inhibition and possibly death. This is essentially
what is achieved when high concentrations of salt are added to fresh meats for the purpose of preservation. Both the microbial cells and those of the meat undergo plasmolysis (shrinkage), resulting in
the drying of the meat, as well as inhibition or death of microbial cells. Enough salt must be used to
effect hypertonic conditions. The higher the concentration, the greater are the preservative and drying
effects. In the absence of refrigeration, fish and other meats may be effectively preserved by salting.
The inhibitory effects of salt are not dependent on pH, as are some other chemical preservatives. Most
nonmarine bacteria can be inhibited by 20% or less NaCl, whereas some molds generally tolerate
higher levels. Organisms that can grow in the presence of and require high concentrations of salt are
referred to as halophiles; those that can withstand but not grow in high concentrations are referred to
as halodurics. (The interaction of salt with nitrite and other agents in the inhibition of C. botulinum
has been discussed earlier under Nitrites and Nitrates.
Gaminara, Infantis, Anatum, Cubana, and Stanley). After a 10-minute exposure, the reductions ranged
between 2 and 3.2 logs for the following sanitizers: 20,000 ppm Ca(OCl) 2 ; 5% Na 3 PO 4 ; 8% H 2 O 2 ;
1% Ca(OH) 2 ; 1% calcinated calcium, 5% lactic acid, and 5% citric acid.
207
The effect of isothiocyanate vapors against antibiotic-resistant E. coli 0157:H7 and S. Montevideo;
and L. monocytogenes inoculated onto lettuce was examined.
118 Iceberg lettuce was inoculated with
up to 10
7 –10
8 cfu/g and held at 4
◦ C for up to 4 days. A reduction of up to 8 logs of E. coli 0157:H7
(in 2 days) and S. Montevideo (in 4 days) was achieved with vapor generated from 400 µl of allyl
isothiocyanate. Overall, allyl isothiocyanate was more effective than methyl isothiocyanate against the
two Gram-negative pathogens but the methyl form was more effective against L. monocytogenes.
118
The control of L. monocytogenes on turkey frankfurters was achieved by the use of any one of four
GRAS chemicals employed at a level of 0.3%/frank. With an inoculum of a five-strain mixture of
10
6 cfu/ml, the franks were dipped for 1 minute each and held at 4, 13, and 22
◦ C.
89 An immediate
decrease of 1–2 logs was achieved by all agents tested alone, and after 14 days at 4
◦ C, the decrease was
3–4 logs while untreated franks spoiled within 7 days at 22
◦ C. When chopped parsley was inoculated
with ca. 10
3 or 10
6 cfu/g of Shigella sonnei and held at 21
◦ C for up to 14 days, the pathogen increased
about 3 logs after 1 day while that held at 4
◦ C decreased by 2.5–3.0 log 10 cfu/g over the 14-day
period.
213 With vinegar (5.2% acetic acid) or 200 ppm free Cl 2 for 5 minutes at 21
◦ C, a >6-log
reduction was achieved, and with 7.6% acetic acid or 250 ppm free Cl 2 , a reduction of 7 to 7.3 logs
was achieved.
213 The effect of hypochlorite and ClO 2 on bacterial spores has been studied, and in one
study using Bacillus subtilis spores, neither hypochlorite nor chlorine dioxide caused the release of
the spore core’s depot of dipicolinic acid (DPA) but spores so treated more readily released DPA upon
a subsequent normally sub-lethal heat treatment than did untreated spores.
219 )
NaCl AND SUGARS
These compounds are grouped together because of the similarity in their modes of action in preserving foods. NaCl has been employed as a food preservative since ancient times. The early food uses of
salt were for the purpose of preserving meats. This use is based on the fact that at high concentrations,
salt exerts a drying effect on both food and microorganisms. Salt (saline) in water at concentrations
of 0.85–0.90% produces an isotonic condition for nonmarine microorganisms. Because the amounts
of NaCl and water are equal on both sides of the cell membrane, water moves across the cell membranes equally in both directions. When microbial cells are suspended in, say, a 5% saline solution, the
concentration of water is greater inside the cells than outside (concentration of H 2 O is highest where
solute concentration is lowest). In diffusion, water moves from its area of high concentration to its area
of low concentration. In this case, water passes out of the cells at a greater rate than it enters. The result
to the cell is plasmolysis, which results in growth inhibition and possibly death. This is essentially
what is achieved when high concentrations of salt are added to fresh meats for the purpose of preservation. Both the microbial cells and those of the meat undergo plasmolysis (shrinkage), resulting in
the drying of the meat, as well as inhibition or death of microbial cells. Enough salt must be used to
effect hypertonic conditions. The higher the concentration, the greater are the preservative and drying
effects. In the absence of refrigeration, fish and other meats may be effectively preserved by salting.
The inhibitory effects of salt are not dependent on pH, as are some other chemical preservatives. Most
nonmarine bacteria can be inhibited by 20% or less NaCl, whereas some molds generally tolerate
higher levels. Organisms that can grow in the presence of and require high concentrations of salt are
referred to as halophiles; those that can withstand but not grow in high concentrations are referred to
as halodurics. (The interaction of salt with nitrite and other agents in the inhibition of C. botulinum
has been discussed earlier under Nitrites and Nitrates.