Agriculture Reference
In-Depth Information
electrolyte after warm-water treatment may provide an easier access of the nutrients
for the growth of bacteria.
Fan and others (2003) studied the effect of combining warm-water treatment with
irradiation treatment on reducing the adverse affect of irradiation treatment alone on
lettuce quality. Lettuce dipped at 47 °C for 2 min and irradiated at 0.5 and 1 kGy had
better overall visual quality and less tissue browning than the corresponding irradiated
samples dipped at 5 °C. A dipping temperature of 50 °C resulted in injury to lettuce
tissue.
Hydrogen peroxide (H
2
O
2
) and organic acids have been applied at an elevated
temperature to enhance treatment effi cacy. Lin and others (2002) found that the com-
binations of 1.5% lactic acid and 1.5% H
2
O
2
at 40 ° C for 15 min reduced
E. coli
O157 : H7 and
S.
Enteritidis by 4 log CFU/g and
L. monocytogenes
by 3 log CFU/g on
fresh-cut iceberg lettuce. However, the sensory quality of lettuce deteriorated follow-
ing the treatment. However, treating lettuce leaves with 2% H
2
O
2
at 50 ° C (1 or
1.5 min) was effective not only in reducing pathogenic bacteria but also in maintaining
sensory quality for up to 15 days. Up to 4-log reduction in
E. coli
O157 : H7 and
S
.
Enteritidis were achieved with the 2% H
2
O
2
treatment, whereas a 3-log reduction in
L. monocytogenes
was observed. Hydrogen peroxide residues were undetectable (the
minimum level of sensitivity was 2 ppm) on lettuce surfaces after the treated lettuce
was rinsed with cold water and centrifuged with a salad spinner. The authors con-
cluded that the treatment of lettuce with 2% H
2
O
2
at 50 ° C for 1 min is effective in
reducing the populations of foodborne pathogens and maintaining the product quality.
A consumer acceptance study showed that lettuce treated with 2% H
2
O
2
at 50 ° C for
1min compared favorably to untreated lettuce with regard to sensory properties
(McWatters and others 2002).
Wei and others (2005) reported a study in which a fi eld salad vegetable (
Valerianella
locusta
) inoculated with
L. innocua
and
S.
Typhimurium was treated with hot water
(45 - 50 ° C), and H
2
O
2
(1-4%) for treatment times of 1-3.5 min. It was observed that
H
2
O
2
at concentrations of more than 2% caused deterioration of overall visual quality
of the salad. The populations of
L. innocua
and
S.
Typhimurium in the salad were
reduced by 2.5 and 2.6 log CFU/g, respectively, immediately after treatment with 2%
H
2
O
2
at 49 ° C for 1 min). A 3.0 - log reduction in
L. innocua
and 2.5 log in
S
. Typhimurium
was observed after 7 days storage at 4 °C (Wei and others 2005).
Broccoli and Beans
Stringer and others (2007) reported a study in which broccoli fl orets and beans were
washed for 90 s in tap water at either 20 °C or 52 °C and then stored at 7 and 10 °C.
The hot-wash treatment improved the initial appearance of the vegetables and resulted
in a signifi cant reduction in the populations of all groups of endogenous microfl ora,
including bacteria, yeast, and mold counts on the vegetables. However, three patho-
gens (
L. monocytogenes, B. cereus,
and
E. coli
O157 : H7), inoculated after treatment,
grew faster in hot-washed (52 °C, 90 s) broccoli than in the equivalent product washed
at 20°C (Stringer and others 2007). Therefore, even though hot-water washing of
broccoli fl orets and beans can be used to reduce bacteria, yeast, and mold populations,
it can also lead to more rapid and extensive growth by pathogens and spoilage organ-
isms if posttreatment contamination occurs after the heat treatment.
