306 Modern Food Microbiology
least 1813; however, it is not permitted in meats or other foods recognizable as sources of thiamine.
Although SO 2 possesses antimicrobial activity, it is also used in certain foods as an antioxidant.
The predominant ionic species of sulfurous acid depends on pH of milieu, with SO 2 being favored
by pH <3.0, HSO
−
3 by pH between 3.0 and 5.0, and SO
2−
3 by pH ≥6.0.
144 SO 2 has pKs of 1.76 and
7.2. The sulfites react with various food constituents including nucleotides, sugars, disulfide bonds,
and others.
With regard to its effect on microorganisms, SO 2 is bacteriostatic against Acetobacter spp. and the
lactic acid bacteria at low pH, with concentrations of 100–200 ppm being effective in fruit juices and
beverages. It is bactericidal at higher concentrations. When added to temperature-abused comminuted
pork, 100 ppm of SO 2 or higher were required to effect significant inhibition of spores of C. botulinum
at target levels of 100 spores per gram.
198 The source of SO 2 was sodium metabisulfite. Employing
the same salt to achieve an SO 2 concentration of 600 ppm, Banks and Board
9 found that growth of
salmonellae and other Enterobacteriaceae were inhibited in British fresh sausage. The most sensitive
bacteria were eight salmonellae serovars, which were inhibited by 15–109 ppm at a pH of 7.0; Serratia
liquefaciens, S. marcescens, and Hafnia alvei were the most resistant, requiring 185–270 ppm free
SO 2 in broth.
Yeasts are intermediate to acetic and lactic acid bacteria and molds in their sensitivity to SO 2 , and
the more strongly aerobic species are generally more sensitive than the more fermentative species.
Sulfurous acid at levels of 0.2–20 ppm was effective against some yeasts, including Saccharomyces,
Pichia, and Candida, whereas Zygosaccharomyces bailii required up to 230 ppm for inhibition in
certain fruit drinks at pH 3.1.
119 Yeasts can actually form SO 2 during juice fermentation; some
“S. carlsbergensis” and S. bayanus strains produce up to 1,000 and 500 ppm, respectively.
144 Molds
such as Botrytis can be controlled on grapes by periodic gassing with SO 2 , and bisulfite can be used
to destroy aflatoxins.
48 Both aflatoxins B 1 and B 2 can be reduced in corn.
76 Sodium bisulfite was
found to be comparable to propionic acid in its antimicrobial activity in corn containing up to 40%
moisture.
76 (Aflatoxin degradation is discussed further in Chapter 30.)
Although the actual mechanism of action of SO 2 is not known, several possibilities have been
suggested, each supported by some experimental evidence. One suggestion is that the undissociated
sulfurous acid or molecular SO 2 is responsible for the antimicrobial activity. Its greater effectiveness
at low pH tends to support this. Vas and Ingram
202 suggested the lowering of pH of certain foods by
addition of acid as a means of obtaining greater preservation with SO 2 . It has been suggested that
the antimicrobial action is due to the strong reducing power that allows these compounds to reduce
oxygen tension to a point below which aerobic organisms can grow or by direct action on some
enzyme system. SO 2 is also thought to be an enzyme poison, inhibiting growth of microorganisms by
inhibiting essential enzymes. Its use in the drying of foods to inhibit enzymatic browning is based on
this assumption. Because the sulfites are known to act on disulfide bonds, it may be presumed that
certain essential enzymes are affected and that inhibition ensues. The sulfites do not inhibit cellular
transport. It may be noted from Figure 13–1 that metabisulfite acts on germinating endospores during
the outgrowth of vegetative cells.
NITRITES AND NITRATES
Sodium nitrate (NaNO 3 ) and sodium nitrite (NaNO 2 ) are used in curing formulas for meats because
they stabilize red meat color, inhibit some spoilage and food poisoning organisms, and contribute
to flavor development. The role of NO 2 in cured meat flavor has been reviewed.
29,74 NO 2 has been
shown to disappear on both heating and storage. It should be recalled that many bacteria are capable of
least 1813; however, it is not permitted in meats or other foods recognizable as sources of thiamine.
Although SO 2 possesses antimicrobial activity, it is also used in certain foods as an antioxidant.
The predominant ionic species of sulfurous acid depends on pH of milieu, with SO 2 being favored
by pH <3.0, HSO
−
3 by pH between 3.0 and 5.0, and SO
2−
3 by pH ≥6.0.
144 SO 2 has pKs of 1.76 and
7.2. The sulfites react with various food constituents including nucleotides, sugars, disulfide bonds,
and others.
With regard to its effect on microorganisms, SO 2 is bacteriostatic against Acetobacter spp. and the
lactic acid bacteria at low pH, with concentrations of 100–200 ppm being effective in fruit juices and
beverages. It is bactericidal at higher concentrations. When added to temperature-abused comminuted
pork, 100 ppm of SO 2 or higher were required to effect significant inhibition of spores of C. botulinum
at target levels of 100 spores per gram.
198 The source of SO 2 was sodium metabisulfite. Employing
the same salt to achieve an SO 2 concentration of 600 ppm, Banks and Board
9 found that growth of
salmonellae and other Enterobacteriaceae were inhibited in British fresh sausage. The most sensitive
bacteria were eight salmonellae serovars, which were inhibited by 15–109 ppm at a pH of 7.0; Serratia
liquefaciens, S. marcescens, and Hafnia alvei were the most resistant, requiring 185–270 ppm free
SO 2 in broth.
Yeasts are intermediate to acetic and lactic acid bacteria and molds in their sensitivity to SO 2 , and
the more strongly aerobic species are generally more sensitive than the more fermentative species.
Sulfurous acid at levels of 0.2–20 ppm was effective against some yeasts, including Saccharomyces,
Pichia, and Candida, whereas Zygosaccharomyces bailii required up to 230 ppm for inhibition in
certain fruit drinks at pH 3.1.
119 Yeasts can actually form SO 2 during juice fermentation; some
“S. carlsbergensis” and S. bayanus strains produce up to 1,000 and 500 ppm, respectively.
144 Molds
such as Botrytis can be controlled on grapes by periodic gassing with SO 2 , and bisulfite can be used
to destroy aflatoxins.
48 Both aflatoxins B 1 and B 2 can be reduced in corn.
76 Sodium bisulfite was
found to be comparable to propionic acid in its antimicrobial activity in corn containing up to 40%
moisture.
76 (Aflatoxin degradation is discussed further in Chapter 30.)
Although the actual mechanism of action of SO 2 is not known, several possibilities have been
suggested, each supported by some experimental evidence. One suggestion is that the undissociated
sulfurous acid or molecular SO 2 is responsible for the antimicrobial activity. Its greater effectiveness
at low pH tends to support this. Vas and Ingram
202 suggested the lowering of pH of certain foods by
addition of acid as a means of obtaining greater preservation with SO 2 . It has been suggested that
the antimicrobial action is due to the strong reducing power that allows these compounds to reduce
oxygen tension to a point below which aerobic organisms can grow or by direct action on some
enzyme system. SO 2 is also thought to be an enzyme poison, inhibiting growth of microorganisms by
inhibiting essential enzymes. Its use in the drying of foods to inhibit enzymatic browning is based on
this assumption. Because the sulfites are known to act on disulfide bonds, it may be presumed that
certain essential enzymes are affected and that inhibition ensues. The sulfites do not inhibit cellular
transport. It may be noted from Figure 13–1 that metabisulfite acts on germinating endospores during
the outgrowth of vegetative cells.
NITRITES AND NITRATES
Sodium nitrate (NaNO 3 ) and sodium nitrite (NaNO 2 ) are used in curing formulas for meats because
they stabilize red meat color, inhibit some spoilage and food poisoning organisms, and contribute
to flavor development. The role of NO 2 in cured meat flavor has been reviewed.
29,74 NO 2 has been
shown to disappear on both heating and storage. It should be recalled that many bacteria are capable of