Agriculture Reference
In-Depth Information
Hot-Water Treatment
Vegetables
Lettuce
Several studies have been conducted to investigate the effectiveness of warm-water
treatment either alone or in combination with chlorine to reduce populations of
bacteria in lettuce. Delaquis and others (1999) washed shredded lettuce for 3min
in 100 ppm chlorine at 4 or 47 ° C. They reported a 3 - and 1 - log CFU/gm reduction
in microbial loads of lettuce following washing in chlorine at 47 or 4 °C, respectively,
as compared to the unwashed lettuce. The surviving microbial population, mainly
psychotropic bacteria, was signifi cantly lower on lettuce washed at 47 °C compared
to unwashed lettuce or lettuce washed at 4°C throughout 15 days of storage.
In a follow-up experiment (Delaquis and others 2000), the effect of the two treatments
on the population of inoculated
E. coli
O157 : H7 and
L. monocytogenes
and the
fate of the two pathogens on shredded lettuce during storage at 1 and 10°C were
investigated. Results showed that washing in cold, chlorinated water immediately
after inoculation reduced the
E. coli
O157 : H7 population by 1 log CFU/g. The
antimicrobial effect of chlorinated water was enhanced at 47°C, resulting in a
reduction of the pathogen by more than 2 log CFU/g. The population of
L. monocyto-
genes
was reduced by 1logCFU/g by chlorinated water, either at 4 or 47°C.
Although the cold chlorinated water was detrimental to the survival of
E. coli
O157 : H7
and
L. monocytogenes
at both storage temperatures, washing in warm chlorinated
water favored the growth of both pathogens in lettuce stored at 10°C, suggesting
the importance of appropriate handling and storage temperature during retail display
and distribution. Warm-water washing seemed to improve the quality of shredded
lettuce because the texture and appearance of lettuce washed at the elevated tempera-
ture were better than those washed in warm chlorinated water (Delaquis and others
2000). However, a chlorinaceous off-odor was detected by some panelists, and the
authors speculated that residual chlorine in the lettuce could account for the
off - odor.
Li and others (2001a) reported that dipping fresh-cut iceberg lettuce for 1.5 min in
water at 50 °C or water containing 20 ppm free chlorine at 20 or 50 °C signifi cantly
reduced the initial population of psychrotropic and mesophilic aerobic microfl ora by
1.73-1.96 log CFU/g as compared to nontreated samples. Similar treatments of fresh-
cut lettuce, artifi cially inoculated with
E. coli
O157 : H7, resulted in similar reductions
in
E. coli
populations (Li and others 2001b). Dipping lettuce at 50 °C did not result
in signifi cantly greater reductions in the populations of
E. coli
O157 : H7 compared to
the treatments at 20 °C. The bacterial populations, independent of the temperature and
the chlorine concentration, increased during the subsequent 18-day storage at 15 °C.
However, warm-water treatment (50 °C) enhanced microbial growth at 4 and 7 days.
During subsequent storage at 5°C, the population of
E. coli
O157 : H7 decreased
steadily in all treated samples. It seems that the bacteria grow faster in samples treated
with warm water during storage at harsh temperatures. Two factors may have
contributed to the increase in the population of
E. coli
O157 : H7 during storage.
First, the warm-water treatment may have eliminated some competing background
microfl ora, which favors the growth of
E. coli
O157 : H7. Secondly, the leakage of
