Fundamental Food Microbiology, Third Edition - Fuad Fathir

CONTROL BY ANTIMICROBIAL PRESERVATIVES 501
Escherichia, Pseudomonas, and Enterobacter spp. at 200 ppm; Lactobacillus and
Salmonella serovars seem to be resistant to this concentration of NO 2.
The antibacterial effect of NO 2 is enhanced at lower pH (pH 5.0 to 6.0), in the
presence of reducing agents (e.g., ascorbate, erythorbate, and cysteine), and with
sorbate. The current regulatory limit in the U.S. is 156 ppm of NO 2, but this varies
widely in other countries. This amount can also be reduced by supplementing NO 2
with other reducing agents as well as sorbates. The NO 2 effect is also enhanced by
reducing A w and at low Eh. In cured meat products, NO 2 reacts with myoglobin to
form a stable pink color of nitrosyl hemochrome during heating. In bacon, nitrite
can lead to the formation of carcinogenic compounds, nitrosoamines. Because of
this, there is a trend to reduce NO 2 or to use other preservatives to control Clo.
botulinum in low-heat-processed meat products.
B. Sulfur Dioxide (SO 2) and Sulfites (SO 3)
Sulfur dioxide, sodium sulfite (Na 2SO 3), sodium bisulfide (NaHSO 3), and sodium
metabisulfite (Na 2S 2O 5) are used to control microorganisms (and insects) in soft
fruits, fruit juices, lemon juices, beverages, wines, sausages, pickles, and fresh
shrimp.
Currently, these additives are not permitted in the U.S. in meat, as they destroy
vitamin B1. They are more effective against molds and yeasts than bacteria; among
bacteria, the aerobic Gram-negative rods are the most susceptible. The antimicrobial
action is produced by the undissociated sulfurous acid that rapidly enters the cell
and reacts with the thiol groups in structural proteins, enzymes, and cofactors, as
well as with other cellular components. At low pH (£4.5) and low A w, the fungicidal
effect is more pronounced. In bacteria, they are effective at high pH (�5.0), but are
probably bacteriostatic at lower concentrations and bactericidal at higher concentrations.
The concentrations used in foods vary greatly in different countries. In the
U.S., 200 to 300 ppm is generally permitted for antimicrobial uses.
Sulfur dioxide and sulfites are also used as antioxidants in fresh and dried fruits
and vegetables (salads) to prevent browning. However, people with respiratory
problems can be mildly to severely allergic to sulfites. The products need to be
labeled to show the presence of sulfites.
C. H 2O 2
A solution of H 2O 2 (0.05 to 0.1%) is recommended as an antimicrobial agent in raw
milk to be used in cheese processing (to control growth of psychrotrophic Gramnegatives
that produce heat-stable enzymes), liquid egg to facilitate destruction of
Salmonella by low-heat pasteurization, packaging material used in aseptic packaging
of foods, and food processing equipment. In raw milk and liquid egg, catalase is
used before pasteurization to hydrolyze H 2O 2 to water and oxygen. H 2O 2 is a strong
oxidizing agent, and the germicidal action is associated with this property (see
Chapters 16).
Recently, H 2O 2 has been used to produce modified plant fiber flour from straws
for use in low-calorie foods and for bleaching and color improvement of grains,
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