Agriculture Reference
In-Depth Information
(
)
2
+
−
Ca ClO
+
H O
→+
Ca
H O
+
2
ClO
2
2
2
(
)
→
(
)
Ca ClO
+
2
H O
Ca OH
+
2
HClO
2
2
2
+ −
The term
free available chlorine
consists of chlorine gas (Cl
2
), hypochlorous acid
(HClO), or hypochlorite ions (ClO
−
). Hypochlorous acid is the main active ingredient
formed. The dissociation of HClO depends on the pH and equilibrium between HClO
and ClO
−
, which is maintained even when HClO is constantly consumed through its
antimicrobial activity (Beuchat 1992). The mode of action of hypochlorous acid
destroying microorganisms has not been fully understood. It is thought that HClO
allows oxygen to emerge, which in turn supposedly combines with components of cell
protoplasm, destroying the organism. Thomas (1979) reported that the lethality of
HClO is attributed to its interactions with cell membrane proteins to form nitrogen-
chlorine (N-Cl) derivatives (chloramines or chloramides), which would interfere with
cell metabolism. Beuchat (1992) proposed that because of the low chlorine level
required for bactericidal action, chlorine must inhibit some key enzymatic reactions
in the cell. The inhibition of essential cytoplasmic metabolic reactions should be
largely responsible for the destruction of both bacterial and fungal cells.
The typical concentration of chlorine used in a sanitation treatment is 200 mg/l at
a pH of
Cl
aa
HClO
↔+
H
8.0 and with a contact time of 1-2 min for raw fruits and vegetables process-
ing (Beuchat 1996). Brackett (1987) reported that when the free chlorine concentration
was
<
50 mg/l, there was no observed antimicrobial effect on
L. monocytogenes
(initial
concentration of about 10
8
CFU/g). Exposing the microbes to chlorine at
<
≥
50 mg/l for
a contact time
20 sec, however, resulted in no recoverable cells. Park and Beuchat
(1999) used a chlorine washing solution at concentration of 2000 mg/l and reduced
aerobic bacteria by over 2 logs in 3 min on honeydew melons, but washing with chlo-
rine at 200 mg/l reduced the population by only 1 log. Mazollier (1988) observed that
a further increase of chlorine concentration above 50-200 mg/l did not reduce the
population of the total aerobic count on lettuce more than a concentration of 50 mg/l
of free chlorine treatment. The effectiveness of chlorine on produce is dependent on
the produce type. For instance, leafy produce with surfaces having a lot of folds can
provide better protection to bacteria than relatively smooth-surfaced honeydew melons
(Adams and others 1989). For leafy greens, a moderate increase in chlorine concentra-
tion or treatment time may not be effective in microbial inactivation.
The effi cacy of chlorine wash is affected by the pH of the chlorine solution. The
time required to achieve a 99% reduction in spore populations was shorter at pH 4-5;
the times required to reduce the spore population at higher pH values were longer
(Sykes 1965). Adam and others (1989) observed that an adjustment of pH from 9.0
to 4.5 with inorganic or organic acids introduced an additional 1.5- to 4.0-log reduc-
tion in the number of microbes on lettuce leaves. Sykes (1965) reported an equivalent
activity of a chlorine solution with 50 mg/l at pH 9 to that with free chlorine of
≥
≥
100 mg/l at pH 10, indicating that pH is sometimes more important than the chlorine
concentration. However, a low pH (4-5) treatment is often corrosive to equipment
surface, and a pH of 6.5-7.0 (at which the percentage of HClO is near 97%) is recom-
mended for use in washing fresh produce (Beuchat 1996).
