precise reasons for this are not known. It is thought that nitrite acts as an
antioxidant to inhibit lipid degradation in the meat although this may
only be part of the story.
4.5.3 Sulfur Dioxide
Sulfur dioxide (SO 2 ) has long enjoyed a reputation for its disinfecting
properties and its earliest use in the food industry was when sulfur
candles were burnt to disinfect the vessels used to produce and store
wine. Nowadays, it is also used as an antioxidant to inhibit enzymic and
non-enzymic browning reactions in some products.
Sulfur dioxide is a colourless gas that readily dissolves in water to
establish a pH-dependent equilibrium similar to CO 2 (see Section 3.3.3).
SO 2 ðgasÞÐSO 2 þH 2 OÐH 2 SO 3 ÐHþþHSO 3 Ð2HþþSO^23 ð 4 : 15 ÞSulfurous acid (H 2 SO 3 ) is a dibasic acid with pKavalues of 1.86 and 6.91.
The unionized forms of SO 2 which can readily penetrate the cell have
the greatest antimicrobial activity. It has been reported that they are
between 100 and 1000 times more active than the bisulfite anion. Since
the unionized forms predominate at low pH values, it follows that SO 2 is
used to best effect in acidic foods. At neutral pH, SO 2 is present as a
mixture of the relatively inactive bisulfite (HSO 3 ) and sulfite (SO^23 ) ions,
although salts of these anions prove the most convenient way of handling
the preservative in the food industry.
SO 2 is a reactive molecule and can disrupt microbial metabolism in a
number of ways. As a reducing agent, it can break disulfide linkages in
proteins and interfere with redox processes. It can also form addition
compounds with pyrimidine bases in nucleic acids, sugars and a host of
key metabolic intermediates. One disadvantageous consequence of this
reactivity is its ability to destroy the vitamin thiamine in foods and the
once widespread practice of using it in meat and meat products has now
been prohibited, with the exception of British fresh sausage.
Sulfur dioxide is active against bacteria, yeasts and moulds, although
some yeasts and moulds are more resistant. Gram-negative bacteria are
most susceptible and in British fresh sausage where sulfite is permitted up
to a level of 450 mg kg^1 , the Gram-negative spoilage flora normally
associated with chilled meats is replaced by one dominated by Gram-
positive bacteria and yeasts. In winemaking the tolerance of the wine
yeastSaccharomyces cerevisiaeto SO 2 levels around 100 mg l^1 is ex-
ploited to control the growth of wild yeasts and acetifying bacteria.
Seasonal surpluses of soft fruits are also preserved by the addition of
high levels of SO 2 to permit jam production throughout the year.
106 The Microbiology of Food Preservation