Food Protection Trends - September/October 2021 - 469

for E. coli O157:H7 and Salmonella Typhimurium, respectively,
compared with CA (1.10 and 1.40 log CFU/g) and LA
(1.40 and 1.70 log CFU/g). Previous studies demonstrated
that treatments with LA reduced E. coli O157:H7 (1.74 log
CFU/g) and Salmonella Typhimurium (1.73 log CFU/g) on
fresh organic lettuce (51), suggesting also that low pH, the
ratio of undissociated ions, cell physiology, and metabolism
influence the efficacy of organic acids against foodborne
pathogens (52). Overall, organic acid solutions applied by
spray showed a significantly greater reduction for both E. coli
O157:H7 and Salmonella Typhimurium than immersion applications
(P ≤ 0.05). Also, the use of chemical sanitizers (C,
SC, and ASC) showed a significant reduction (P ≤ 0.05) on
samples inoculated with both E. coli O157:H7 and Salmonella
Typhimurium in the order ASC > SC > C. The antimicrobial
capacity of ASC was higher than that of SC and C at 2.6 and
2.8 log CFU/g for E. coli O157:H7 and Salmonella Typhimurium,
respectively. Previous studies reported the ability
of ASC to reduce microbial population on strawberries and
cherry tomatoes by 2 to 3 log CFU/g (2, 63).
EO vapor phase and reduction of foodborne pathogens
in vitro and on strawberries
As shown in Table 2, the antimicrobial activity of thyme
and oregano oil vapor phase at different levels (10, 50, 100,
250, and 500 μL) against E. coli O157:H7 and Salmonella
Typhimurium was evaluated. Both EOs in vapor phase were
active against the tested foodborne pathogens. The diameters
of inhibition zone gradually increased when the level of EOs
increased. Thyme and oregano oil vapor phase produced a
greater inhibition halo against Salmonella Typhimurium than
E. coli O157:H7. Conversely, thyme oil was more effective
than oregano oil overall. The mode of action of the EOs
against bacteria is due to cell membrane damage, increased
permeability, and phosphate ion leakage (10, 38).
The antimicrobial activity of EOs vapor phase against Salmonella
Typhimurium and E. coli O157:H7 on refrigerated
strawberries is reported in Figures 3 and 4. Bacterial populations
remained constant during the 18 days of refrigeration
in the control sample. Conversely, samples exposed to EOs
vapor phase showed a 1-log reduction in Salmonella Typhimurium
populations after 3 day, and a significant reduction
was observed for E. coli O157:H7 compared with the control
sample. During storage, EOs vapor phase showed reduction
of the bacterial populations (3 to 4 log CFU/g) on strawberries
up to 18 days. Previous studies have found that EOs
vapor phase has a greater antimicrobial impact than liquid
EOs in direct contact (31, 49, 60). Other researchers (47)
demonstrated instead that lipophilic molecules in the aqueous
solutions (micelles) restrain the activity of EOs against
microorganisms.
Pullulan coating incorporation in GS-NPs and PP-NPs
for improving quality and safety of strawberries
Antimicrobial activity of NPs by using plate overlay assays
The influence of NPs from GS and PP were evaluated and
reported in Table 3. Both GS-NPs and PP-NPs were active at
TABLE 2. Antibacterial activity of thyme and oregano oil vapor phase against E. coli and
Salmonella Typhimurium in vitro reported as inhibition halos
Inhibition halo (cm)a
Treatment (µL), n = 9
Oil vol (µL)
10
50
Thyme oil vapor phase
100
250
500
10
50
Oregano oil vapor phase
100
250
500
a
E. coli O157:H7
0.5 ± 0.05 eB
1 ± 0.08 dB
1.35 ± 0.13 cB
2.15 ± 0.15 bB
2.45 ± 0.2 aB
0.4 ± 0.05 dB
0.8 ± 0.08 cB
1 ± 0.1 cB
1.7 ± 0.1 bB
2.05 ± 0.15 bB
Salmonella Typhimurium
0.8 ± 0.05 eA
1.2 ± 0.1 dA
1.6 ± 0.15 cA
2.35 ± 0.17 bA
2.65 ± 0.2 aA
0.7 ± 0.05 fA
1.08 ± 0.05 eA
1.51 ± 0.05 dA
2 ± 0.1 cA
2.25 ± 0.1 bA
There are no significant differences between any two means (±SD) in the same column that have the same lowercase letter (P ≥ 0.05).
There are no significant differences between any two means (±SD) in the same row that have the same uppercase letter (P ≥ 0.05).
September/October Food Protection Trends 469

Food Protection Trends - September/October 2021

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