Technology
For Heaters:
UV Lights in the heater are used to kill germs, viruses, mold spores, bacteria and fungi as they pass through the heater.
Ultraviolet (UV) light disinfection is getting a lot of attention during the coronavirus pandemic. The main benefit: its ability to kill pathogens like viruses and bacteria.
While UV-C is an extremely effective option for disinfecting, it does come with a safety warning. Many UV-C products use 254 nm, which can penetrate the skin and eyes. Exposure to UV-C can cause burns.
Most products should only be used in empty rooms, which can be challenging for specific industries where there is little downtime.
Right now, there is a threshold limit for the dose of UV in an occupied space over an eight hour period. That may limit the amount of disinfection provided to surfaces. Near UV can help reduce bacterial infection rates in medical facilities and senior care centers.
Killing bacteria and viruses with UV light is particularly effective because it kills germs regardless of drug resistance and without toxic chemicals. It is also effective against all germs, even newly-emerging pathogen strains.
These wavelengths are very close to the visible light spectrum and are believed to be safe for humans.The researchers concluded that the number of bacteria and virus reduction depended on how the device was used.
While it performed well overall (killing between 94.98% and 99.99% of germs), "education on the risks of incorrect application should be included in the travel medical consultation."
The UV light heater will protect you from germs, bacteria, viruses, and other destructive bio-aerosols.
For vegetable washers:
Lettuce is a popular and important leafy vegetable in the human
diet because of its delicious and nutritious characteristics. Accord
ing to a U.S. Dept. of Agriculture report, about 2 million tons
of lettuce were produced in the U.S. in 2014, and the value of
production was over 1 billion dollars (Natl. Agricultural Statistic
Service 2014). Fresh lettuce can be contaminated with foodborne
pathogens at farm by manure and irrigation water. Lettuce, as a
minimally processed and ready-to-eat (RTE) food product, can act
as a vehicle for transmitting foodborne disease. The Centers for
Disease Control and Prevention (CDC) reported a multistate Es
cherichia coli O157:H7 outbreak in 2011 which was directly linked
to romaine lettuce. According to CDC’s investigation, pathogenic
microorganisms such as E. coli O157:H7, Listeria monocytogenes,
Salmonella, and Cyclospora might attach to fresh vegetables during
farming and postharvest storages (CDC 2012). Therefore, an ef
fective washing method during postharvest handling is necessary to
enhance the safety of fresh produce, thereby reducing foodborne
outbreaks.
Electrolyzed (EO) water is a strong and versatile antimicrobial
agent with a wide range of applications in medicine, agriculture,
and food industry (Hricova and others 2008). It is generated from
anodic electrolysis of dilute saline solutions (NaCl) in an elec
trolytic cell (Park and others 2004). During electrolysis, NaCl
in the saline solution is disassociated. The negative ions, such as
MS 20160122 Submitted 1/21/2016, Accepted 5/12/2016. Authors are with
the Dept. of Food Science and Technology, The Univ. of Georgia, 1109 Experiment
Street, Griffifin, GA 30223-1797, U.S.A. Direct inquiries to author Hung (E-mail:
yhung@uga.edu).
chlorine (Cl¯) and hydroxide (OH¯), move to the anode, and
give up electrons. These ions form the components of EO wa
ter including chlorine gas (Cl2), hypochlorite ion (OCl¯), and
hypochlorous acid (HOCl) that contributed to its antimicrobial
activity. EO water is considered an environmental friendly san
itizer, because it reverts to normal water after use. In addition,
many studies have shown that EO water does not lead to negative
effects on the organoleptic properties such as color, aroma, flflavor,
or texture, in various treated food products (Hricova and others
2008; Waters and Hung 2010; Eda and others 2015).
Ozone is a powerful antimicrobial agent which can inactive
bacteria, virus, fungi, and spores at relatively low concentrations
(Alexopoulos and others 2013). The strong oxidation capability of
ozone could rapidly oxidize DNA, lipid, and protein of microor
ganisms, causing their death in a short time (Khadre and others
2001; Patil and others 2014). In 2000, the U.S. Food and Drug
Administration (FDA) approved both gas and aqueous ozone as
a direct food additive for treatment, storage, and processing of
foods. One of ozone’s special properties is that ozone is not sta
ble and can be easily transformed to oxygen in a few minutes,
making it an excellent sanitizer because it leaves no residues in
food (Khadre and others 2001). Several studies on ozone appli
cation have demonstrated that ozone can effectively inhibit the
growth of microorganisms, thus enhancing the safety and quality
of fresh produce (Alexopoulos and others 2013; Patil and others
2014).
Ultraviolet-C (UV-254nm) light has been applied with ozone
to enhance the inactivation of E. coli O157:H7. UV-C has strong
microbicidal properties and it does not leave any residues on
food. The FDA has approved UV-C for inactivation of microor
ganisms on food product surfaces (US-FDA 2011; Syamaladevi
C
2016 Institute of Food Technologists
R
doi: 10.1111/1750-3841.13364
Vol. 00, Nr. 0, 2016 Journal of Food Science M1
Further reproduction without permission is prohibited
M: Food microbiology
& safetyProduce washing using UV-ozonated water . . .
and others 2013). UV-C exposure causes irreversible damage to
bacterial DNA. It may have a synergistic effects when combined
with ozone for microbial inactivation (Otto and others 2011).
However, the UV-C radiation could be absorbed by ozone and
dissociate ozone molecules to form free oxygen atoms and oxy
gen molecules, which neutralize ozone (Vig 1985). Therefore,
one of the objectives of this study was to investigate if UV-C
can work with aqueous ozone and achieve a synergistic antimi
crobial effect on E. coli O157:H7. This study was also conducted
to evaluate the effificacy of using ozonated and slightly acidic EO
water (SAEW) as spray water to reduce E. coli O157:H7 on ro
maine and iceberg lettuces. The synergistic antimicrobial effect
of combined applications of UV and ozonated water was also
investigated.
Materials and Methods
Bacterial culture
Three nalidixic acid-resistant strains E. coli O157:H7, includ
ing 932 (human isolate), 1 (beef isolate), and E009 (beef isolate),
were used as inocula. Each strain was propagated separately in
10 mL tryptic soy broth (TSB; Difco/Becton Dickinson, Sparks,
Md., U.S.A.) with 50 mg/L of nalidixic acid (Sigma-Aldrich, St.
Louis, Mo., U.S.A.) at 37 °C for 24 h. The use of nalidixic acid
resistant strains of E. coli O157:H7 was to differentiate from the
microflflora naturally present on lettuce samples in the presence
of nalidixic acid during sampling, thus facilitating the detection
and recovery of the inoculated pathogens. Following incubation,
each strain was individually transferred into a sterile centrifuge
tube and sedimented by centrifugation at 3900 × g at 5 °C for
15 min (Centra-CL3, Intl. Equipment Co., Needham Height,
Mass., U.S.A.). The supernatant was discarded and cell pellets
were resuspended in 10 mL of sterile 0.1 M phosphate buffered
saline (PBS, pH7). The optical density of resuspended bacterial
suspensions was measured using a spectrophotometer to insure all
3 strains had the same initial optical density. Equal volumes (3 mL)
of each strain suspension were combined in a sterile centrifuge
tube to form a 9 mL inoculum.
SAEW produce washer
SAEW at pH 6.0 were freshly generated by electrolyzing a di
lute NaCl solution (ca. 0.03%) using an EAU EO water generator
(Model #P30HST44T, EAU, Ga., U.S.A.) for each experiment.
The pH of SAEW was measured using an ACCUMET pH meter
(AR50, Fisher Scientifific, Pittsburgh, Pa., U.S.A.), and its free chlo
rine concentration was determined using the DPD-FEAS method
(Hach Co., Loveland, Colo., U.S.A.). Appropriate dilutions were
made to achieve 50 mg/L free chlorine concentration in SAEW.
About 100 L SAEW was prepared and stored in a water tank for the
spray experiments in a custom built spray cabinet as described in
Jadeja and Hung (2014). Instead of a handheld sprayer, the same
kind of nozzle (Full cone nozzle #460.526, Lechler Inc., West
Chicago, Ill., U.S.A.) used in the UV-ozonated water treatment
was used. The distance between the nozzle and lettuce samples
was set at 20 cm. SAEW was sprayed at 1.5 L per min.
UV and ozonated water produce washer
A UV-chilled ozonated water washer (UV-ozone washer,
Sanichill Prototype Operating System; Sanist Technologies LLC,
Pittsburgh, Pa., U.S.A.) was used in this study. The refrigeration
system, ozone gas generator, and 2 UV-C (254 nm) lamps (UV Air
Lamp Assembly ASIH1003 17 T3; Light Spectrum Enterprises
Inc., Feasterville-Trevose, Pa., U.S.A.) were 3 major components
of this washer (Figure 1). The refrigeration system maintained
the temperature of wash water at 4 °C. The UV-C intensity at
1.19 mW/cm2 measured by a UV radiometer (Model UVX-25;
UVP LLC, Upland, Calif., U.S.A.) was applied in this study. The
distance between the UV lamps and the lettuce samples was 20
- There were 6 nozzles (Full cone nozzle #460.526) in the
spray chamber. To reduce water consumption, 2 water tanks were
set up in this system to recycle water. The ozonation tank (1st
tank) was used to continuously generate ozonated water by inject
ing ozone gas from the ozone generator, and the recycling tank
(2nd tank) was used to collect spray water from the spray chamber.
The recycled water was fifiltered and regenerated in the ozonation
tank. The concentration of O3 in the ozonated water (0.5 mg/L)
was determined using the Indigo method with high-range ozone
Accu Vac Ampuls (Hach Co., Loveland, Colo., U.S.A.) using a
Figure 1–The UV-chilled ozonated water washer
diagram.
M2 Journal of Food Science Vol. 00, Nr. 0, 2016
M: Food microbiology
& safetyProduce washing using UV-ozonated water . . .
Table 1–Antimicrobial effect of SAEW on romaine and iceberg lettuce with different holding time after inoculation.
Holding timea
2 h 26 h
SAEW treatment Recovery Reduction Recovery Reduction
Lettuce type Treatment time (min) (log CFU/g) (log CFU/g) (log CFU/g) (log CFU/g)
Romaine lettuce Control A6.17
±
0.08a A6.25
±
0.16a
5 A2.70
±
0.21a 3.5 A3.20
±
0.69a 3.0
10 A1.37
±
1.06a 4.8 A2.76
±
0.72a 3.5
15 A1.59
±
0.48a 4.6 A2.70
±
0.41b 3.6
Iceberg lettuce A6.12
±
0.08a A6.14
±
0.32a
5 B4.34
±
0.13a 1.8 B4.04
±
0.66a 2.1
10 B3.70
±
0.59a 2.4 B3.99
±
0.90a 2.2
15 B3.86
±
0.51a 2.3 B3.23
±
0.18a 2.9
a Mean values with the same capital letter in each column with the same treatment time indicates no signifificant difference between romance and iceberg lettuce (P > 0.05). Mean
values with the same lowercase letter in the same row are not signifificantly different (P > 0.05).
Table 2–Antimicrobial effects of UV, O3 water, and their combination on romaine lettuce.a
Outside Inside
Treatment time Recovery Reduction Recovery Reduction
Treatment method (min) (log CFU/g) (log CFU/g) (log CFU/g) (log CFU/g)
UV Control A6.11
±
0.07a A5.81
±
0.36a
5 A4.18
±
0.23a 1.9 A3.91
±
0.84a 1.9
10 A4.17
±
0.43a 2.0 A3.45
±
0.42a 2.4
15 A3.97
±
0.31a 2.1 A3.04
±
1.06b 2.8
Ozonated water A6.16
±
0.16a A6.27
±
0.00a
5 A4.12
±
0.36a 2.0 AB3.26
±
0.01a 3.0
10 A3.46
±
0.32a 2.7 A3.89
±
0.31a 2.4
15 A3.43
±
0.28a 2.7 A3.47
±
0.21a 2.8
UV + ozonated water A5.97
±
0.16a A5.97
±
0.06a
5 A3.77
±
0.67a 2.2 B2.31
±
1.17a 3.7
10 B1.45
±
1.35a 4.5 B2.02
±
0.69a 4.0
15 B1.14
±
0.79a 4.8 B1.10
±
0.60a 4.9
a Mean values in each column at the same treatment time with the same capital letter indicates no signifificant difference between treatment methods (P > 0.05). Mean values for the
same treatment time and method that are followed by the same lowercase letter in the same row indicates no signifificant difference between inoculation locations (P > 0.05).
colorimeter (model DR/890; Hach Co.). The washer was turned
on for 10 min before every treatment to allow the ozone in the
water to reach a steady level.
Preparation and inoculation of romaine and iceberg
lettuces
Romaine (Lactuca sativa L. var. longifolia) and iceberg (Lactuca
sativa L.) lettuces were purchased from a local grocery store, stored
at 4 °C, and used within 48 h. The outer 2 or 3 damaged lettuce
leaves were discarded, and the next few intact leaves were collected
and trimmed to 20 ± 1 g/leaf. Sample leaves were placed on a
sanitized tray with either outer (abaxial) side or inside (adaxial)
side surface facing up and spot inoculated with 100 µL of the
- coli O157:H7 cocktail by placing 15 to 20 inoculum drops on
the surface of each leaf. To study the effect of drying time (2 vs. -
26 h) after inoculation on microbial inactivation, both romaine
and iceberg lettuces were inoculated on the outer surface and
then air-dried in a laminar flflow hood for 2 h at room temperature
(22 ± 2 °C). Half of the lettuce leaf samples was immediately
treated with SAEW, whereas the remaining half of the samples
was covered with plastic wrap and held at 4 °C for additional
24 h before SAEW treatment. The inoculated romaine lettuce
leaves were also used to study the effects of inoculum location
(outer vs. inside surface) on bacterial inactivation after 2 h drying
at room temperature (22 ± 2 °C). Furthermore, the effificacy of
different washing treatments (for example, UV, ozonated water,
SAEW, and a combination of UV and ozonated water) was also
studied.
Procedures for treating lettuce
Romaine and iceberg lettuces inoculated with E. coli O157:H7
only on the outer surface and dried for either 2 or 26 h in a lam
inar flflow hood were treated with SAEW for 5, 10, and 15 min.
Treated leaves were placed into a 1.5 L of Whirl-Pak bag contain
ing (Nasco, Fort Atkinson, Wis., U.S.A.) 80 mL of Dey-Engley
(DE) neutralyzing broth (Difco/Becton Dickinson) to stop the
SAEW reaction before subjecting to microbiological analysis.
Romaine lettuce leaves, inoculated with E. coli O157:H7 on
both outside and inside surfaces, were treated with UV, ozonated
water, and their combination. Two lettuce leaves were placed side
by-side directly under the UV light and ozonated water spray
nozzles for each treatment. Each treatment had 3 treatment times,
5, 10, and 15 min. Treated leaves were immediately placed into a
1.5 L Whirl-Pak bag (Nasco, Fort Atkinson, Wis., U.S.A.) with
80 mL of DE broth before subjecting to microbiological analysis.
Both romaine and iceberg lettuce inoculated with E. coli
O157:H7 on the outside surface only were treated with either
SAEW or UV-ozonated water combination for 5, 10, and 15 min.
After treatments, the treated leaves were placed in a 1.5 L of Whirl
Pak bag with 80 mL of DE broth before subjecting to microbio
logical analysis.
Vol. 00, Nr. 0, 2016 Journal of Food Science M3
M: Food microbiology
& safetyProduce washing using UV-ozonated water . . .
Microbiological analysis
After washing treatment, all lettuce samples in DE broth were
pummeled in a stomacher (Stomacher 400 Circulator, Seward,
London, U.K.) for 3 min at the speed of 230 rpm. The samples
were serially diluted with PBS and plated onto Sorbital Mac
Conkey agar (Hardy Diagnostics, Santa Maria, Calif., U.S.A.)
containing 50 µg/mL nalidixic acid (SMACN) to determine the
surviving population of E. coli O157:H7. Undiluted DE/lettuce
homogenates (250 µL) were spread-plated in quadruplicate onto
SMACN, to enumerate the low surviving populations of E. coli
O157:H7 from lettuce samples.
Data analysis
Experiments were replicated at least 3 times and each repli
cate consisted of 2 samples for each treatment. Means of bacterial
counts were calculated as log CFU/g using Microsoft Excel
R
One way analysis of variance was used for data analysis followed
by Duncan’s multiple range test to compare means using SAS soft
ware 9.4 (SAS Inst. Inc., Cary, N.C., U.S.A.) with the proc anova
procedure. A P 0.05 was considered to be statistically signifificant.
Results and Discussion
Antimicrobial effect of SAEW
The pH, oxidation reduction potential, and free chlorine con
centration of SAEW are 6.08 ± 0.19, 949 ± 21 mV, and 51
±
0.6 mg/L, respectively. Table 1 shows the surviving popula
tion of
- coli O157:H7 on romaine and iceberg lettuce after
SAEW treatments. With a 2 h drying time after inoculation, a
reduction of 3.5 and 1.8 log CFU/g in E. coli O157:H7 was ob
served in romaine and iceberg lettuce, respectively, after a 5 min
treatment with SAEW. Longer treatment (10 and 15 min) led
to an additional 1 and 0.5 log CFU/g reductions in the bacterial
counts in romaine and iceberg lettuce samples, respectively. With a
longer drying time (26 h) after inoculation, a reduction of 3.0 and
2.1 log CFU/g in the bacterial counts were observed in romaine
and iceberg lettuce after a 5 min treatment with SAEW. Longer
treatment times (10 and 15 min) led to an additional 0.5 log
CFU/g in the reduction of bacterial counts in romaine lettuce.
For iceberg lettuce, a 10 min treatment did not cause additional
inactivation of the bacteria, although a 15 min treatment led to
an additional reduction of 0.8 log CFU/g of E. coli O157:H7.
A similar result was reported by Afari and other’s (2015), which
stated that the reduction of E. coli O157:H7 on romaine lettuce
was higher than on iceberg lettuce by 1 log when both samples
were treated with SAEW.
Shown in Table 1, no signifificant difference was observed on the
reduction of E. coli O157:H7 for both romaine and iceberg lettuce
regardless of holding time after inoculation (2 h vs. 26 h) except
romaine lettuce treated by SAEW for 15 min. Therefore, holding
time (2 h vs. 26 h) had little impact on the viability or attachment
of E. coli O157:H7 on lettuce. In other studies, similar observation
was also reported (Farber and others 1998; Lang and others 2004).
Hence, the rest of the experiments in this report were conducted
using only samples with 2 h holding time after inoculation.
Antimicrobial effect of UV, ozonated water, and their
combination
Table 2 shows the surviving population of E. coli O157:H7 on
romaine lettuce after the treatments with UV, ozonated water, and
their combination. For romaine lettuce, similar E. coli O157:H7
reductions were achieved for the same treatment regardless of the
inoculum location (inside or outside surface). For all types of treat
ments (UV, ozonated water, and their combination), there was no
signifificant difference in the reduction of E. coli O157:H7 between
the samples with inside and with outside surface inoculation, ex
cept for the UV-C treatment for 15 min. Therefore, for the rest
of the study only lettuce leaves inoculated on the outside surface
were used.
UV-C treatment for 5 min resulted in about 1.9 log reduc
tions for both inside and outside surface inoculated romaine let
tuce. However, no further signifificant increase on reduction was
observed after the samples were treated for longer time (10 or
15 min). This may be due to the limitation of penetration depth
of UV-C and bacteria hiding in debris, shaded areas due to the
topology of leaves, or internalized in stomata and cut edges.
A 5-min treatment by ozonated water was able to reduce the
population of E. coli O157:H7 by 2 logs for the leaves inoculated
on the outside surface, whereas a 3 log reductions was observed for
the leaves inoculated on the inner surface. Extending the treatment
to 10 min resulted in an additional 0.7 log reductions for leaves
Figure 2–The comparison of SAEW and
UV-ozonated water combination antimicrobial
effects on romaine and iceberg lettuce. Error
bars represent standard deviations. Capital
letters not followed by the same letter in the
same treatment time indicate signifificant
difference (P 0.05).
M4 Journal of Food Science Vol. 00, Nr. 0, 2016
M: Food microbiology
& safetyProduce washing using UV-ozonated water . . .
inoculated on outside; however. no further reduction was observed
beyond 10 min treatment time. This indicates that ozonated water
are more effective than UV-C treatment at longer treatment time
(10 min). This may be due to ozonated water can wash away
bacteria and organic matter residing on the surface of leaves that
offers protection to bacteria from UV-C lights. Ozonated water
may also able to reach to the shaded areas of leaves that could
not be reached by UV-C light due to the fifixed projection angle.
In addition, ozone gas might vaporize from the ozonated water
and penetrate lettuce surfaces. However, the concentration of the
ozonated water (0.5 mg/L) used in this study may be too low and
hence no further signifificant reduction was observed for longer
treatment time.
According to Table 2, the combination of UV and ozonated
water treatment had signifificant higher reduction than either UV
or ozonated water treatment alone. Although no signifificant differ
ence was observed in the surviving population of E. coli O157:H7
on romaine lettuce among 3 treatments at 5 min, the combi
nation of UV and ozonated water achieved an additional 1.8 to
2.5 log CFU/g reductions at 10 min treatment and 2.1 to 2.7
log CFU/g additional reductions at 15 min of treatment when
compared to either UV or ozonated water treatment alone for the
same treatment time. The synergistic effect of UV and ozonated
water combination treatment may be due to ozonated water plus
ozone gas released from ozonated water can get to the folded
leaves where UV-C light was not able to reach. Therefore, the
combination treatment of UV and ozonated water could deliver a
close to 5 log reductions of E. coli O157:H7 on romaine lettuce
compared to 2 to 3 log reductions using UV or ozonated water
treatment alone.
Comparison between UV-ozonated water combination
and SAEW
The antimicrobial effect of UV-ozonated water combination
and SAEW on both romaine and iceberg lettuces are presented in
Figure 2. For romaine lettuce, both treatments were able to deliver
close to 5 log reductions on E. coli O157:H7 at 10 and 15 min
treatments. However, for iceberg lettuce, a signifificant difference in
microbial inactivation was observed between UV-ozonated water
combination treatment and SAEW. UV-ozonated water combina
tion could achieve similar antimicrobial effect on iceberg lettuce
as compared to the romaine lettuce, whereas SAEW can only
achieve up to 2.5 log reductions on iceberg lettuce regardless
of treatment time. This difference may be due to the different
behavior of bacteria on romaine lettuce compared with iceberg
lettuce (Kroupitski and others 2009; Kroupitski and others 2011).
Kroupitski and others (2009) inoculated Salmonella Typhimurium
onto the surface of both romaine and iceberg lettuce leaves and
observed the distribution of bacteria using both scanning electron
microscopy and confocal microscopy. They found that bacteria
tend to aggregate near and into stomata of iceberg lettuce leaves,
whereas no such attraction of bacteria to stomata was found on
romaine lettuce leaves. In this study, lower antimicrobial effificacy
on iceberg lettuce than romaine lettuce for SAEW may be due to
SAEW cannot reach/penetrate into the stomata on iceberg lettuce
where the bacterium was internalized. However, for UV-ozonated
water combination, UV can stimulate the opening of the stom
ata as suggested by Eisinger and others (2000) and hence allowed
ozonated water and/or ozone gas to inactivate bacteria that may
be internalized in the stomata. Hence, a 5 log reductions of E. coli
O157:H7 using UV-ozonated water combination was observed
for both iceberg lettuce and romaine lettuce. Similar results were
reported by Afari and others (2015). They used neutral pH EO
water (NEO, 155 mg/L free chlorine, pH 7.5) in an automated
produce washer at 65 rpm for 30 min and achieved 4.2 and 3.2 log
CFU/g reductions for romaine and iceberg lettuce, respectively.
They demonstrated that when washing romaine lettuce with NEO
achieved additional one log reduction than iceberg lettuce.
Conclusions
This study demonstrated both UV-ozonated water combina
tion and SAEW washing treatments has strong antimicrobial effect
and achieved approximately 5 log reductions of E. coli O157:H7
on romaine lettuce. UV-ozonated water combination treatment
achieved similar reductions on iceberg lettuce whereas less than
2.5 log reductions on E. coli O157:H7 were achieved by SAEW.
This difference may be due to bacteria aggregation near and within
stomata for iceberg lettuce but not romaine lettuce. The UV light
will stimulate the opening of the stomata for the UV-ozonated wa
ter treatment and hence achieve better bacter