Agriculture Reference
In-Depth Information
and to replenish the antimicrobial agent in a timely manner and maintain a constant,
effective concentration.
Most studies on the effect of organic loading have been conducted with chlorine.
The concept of chlorine demand has been intensively studied in the fi eld of water
sanitation and poultry processing. The chlorine demand is the difference between the
amount of chlorine applied to the washing tank and the amount of free, combined, or
total chlorine residual remaining at the end of the washing. The chlorine demand is
considered an extrinsic property of the water that is affected by pH, temperature, and
dissolved compounds in the water (Tsai and others 1992). When the interaction of
chlorine with microbes is considered, the chlorine concentration is assumed to decay
exponentially, following a fi rst-order kinetic model (Virto and others 2004). Studies
also reported bilinear behavior of chlorine decay, which has been attributed to the
existence of two distinct reaction types in the chlorination of microorganisms (Shang
and Blatchley 2001). In a test using chlorine to wash sliced romaine lettuce, the rate
of depletion of both free chlorine and total chlorine followed a fi rst - order kinetics
model at three produce-to-solution ratios (Zhou and others 2008). The effect of pro-
duce-to-solution ratio on chlorine decay (ppm/min) was also characterized by a semi-
log relation. In a produce washing test, Garg and others (1990) reported that total
aerobic bacteria counts were not reduced on fresh-cut vegetables such as carrot, red
cabbage, and lettuce washed with 300 mg/l chlorine, which was attributed to the rapid
depletion of chlorine by increased levels of organic loading.
Water pH and Chemistry
Because chlorine works most effectively in the pH range 5-7, water supplies that are
buffered or are highly alkaline (higher pH values) often need their pH adjusted for
chlorine to be effective. With acid sanitizers in buffered water, higher concentrations
of acids may be required to reduce the pH to effective levels. Other components
of water, such as organic and inorganic compounds, will react with and consume
oxidizers. Components of hard water such as inert magnesium and calcium ions
generally do not affect most oxidizing chemistries.
Microbial Type, Level, and Attachment
Vegetative cells are less resistant to chlorine than the spore-forming organisms.
E. coli
was found to be generally more resistant to chlorine than other vegetative bacteria.
Although a bacterium is usually readily killed when directly exposed to a sanitizer,
the inactivation becomes much more diffi cult when the microorganism is attached to
produce surfaces. Lund (1983) suggested that contact with host tissue may inactivate
hypochlorite and that the adhesion of bacteria to plant tissues generally increases their
resistance to sanitizers. The population reduction in cell suspensions of
E. coli
O157:H7,
L. monocytogenes
, and
Shigella
treated with acidic electrolyzed water
(AEW) reached over 8.0 log CFU/ml in 1 min (Kim and others 2000). However, less
than 2.7logCFU/g reduction was achieved in a 15-min treatment with AEW when
those bacteria were attached to produce surfaces and air-dried for 1 hr (Park and others
2001) or 7hrs (Stan and Daeschel 2003) after inoculation. Microbial biofi lms also
could develop on produce surfaces when the right growth conditions are satisfi ed. The
removal of biofi lm from produce surfaces remains a major challenge to the food
