Food Protection Trends - July/August 2022 - 281

TABLE 1. Effect of treatment application on the reduction (mean ± standard deviation) of
L. monocytogenes biofilms
1st Treatmenta
Lac
Lac
Lac
15-UV
30-UV
Qua
Qua
Qua
15-UV
30-UV
Poa
Poa
Poa
15-UV
30-UV
a
2nd Treatmenta
15-UV
30-UV
Lac
Lac
15-UV
30-UV
Qua
Qua
15-UV
30-UV
Poa
Poa
Biofilm cell reduction (log CFU/cm2
3.06 ± 0.85a
5.11 ± 0.66c
4.78 ± 1.02c
3.26 ± 0.62a
4.02 ± 0.67b
2.61 ± 0.91a
3.28 ± 1.32ab
4.02 ± 1.19c
3.45 ± 0.93bc
3.85 ± 0.84bc
3.66 ± 0.90a
4.66 ± 0.66c
4.38 ± 0.91bc
4.00 ± 0.81ab
4.51 ± 0.92bc
15-UV, 15-min exposure to UV-C light; 30-UV, 30-min exposure to UV-C light; Lac, 4% lactic acid sanitizer; Qua, 400 ppm
quaternary ammonium-based sanitizer; Poa, 100 ppm peroxy acid-based sanitizer.
bThe population in control biofilms was 6.04 ± 0.49 log CFU/cm2
significantly different (P < 0.05).
the 15- and 30-min UV-C light exposures, with 4.66 ± 0.66 and
4.38 ± 0.91 log CFU/cm2
reductions, respectively. Likewise,
no difference (P > 0.05) was found between 15 and 30 min
of UV-C exposure when UV-C light treatments preceded
the sanitizer (4.00 ± 0.81 and 4.51 ± 0.92 log CFU/cm2
reductions, respectively).
DISCUSSION
In the food industry, L. monocytogenes is widely known for
forming biofilms in difficult-to-clean sites such as floors, waste
water pipes, conveyor belts, and stainless steel surfaces (12). In
the present study, we found that 4-day-old multistrain biofilms
of L. monocytogenes were reduced by 3.06, 2.61, and 3.66 log
CFU/cm2
by lactic acid (4%), quaternary ammonium (400
ppm), and peroxy acid (100 ppm), respectively.
The use of lactic acid to control microbial biofilms has been
previously studied. Yang et al. (38) observed a reduction of
4.21 log CFU/cm2
in a 7-day-old L. monocytogenes biofilm on
polyethylene surfaces treated with 0.18% lactic acid, a greater
log reduction than that found in our study. This discrepancy
in results may be attributed to key factors such as biofilm age,
attachment surface, and conditions under which the biofilms
were grown (e.g., temperature and shear force). Wang et al.
. Within treatment groups, means with different letters are
)b
(35) found that lactic acid can cause a leakage of proteins
through the cell membrane, especially in the first 2 h of
sanitizer exposure, and can inhibit the synthesis of bacterial cell
soluble proteins. These authors found that L. monocytogenes in
the planktonic state was reduced below detectable limits after
exposure to 0.5% lactic acid for 2 h. However, because biofilms
provide bacteria with increased protection from stressors,
a higher concentration of antimicrobial solutions or the
combination with other intervention strategies might offer a
more effective way to control biofilm formation. Ban et al. (4)
achieved a 0.92 log CFU per coupon reduction in L. monocytogenes
biofilms on stainless steel after exposure to 2% lactic
acid for 30 s. However, a 4.5-log reduction was obtained when
the sanitizer was combined with steam for 20 s. In our study,
when 4% lactic acid was combined with UV-C light, a 5.11-log
reduction was obtained.
QAC sanitizers have been evaluated by many researchers
for control of microbial biofilms on various surfaces (5, 21,
26, 27). Concentrations of 200 ppm of QACs used to treat
7-day-old L. monocytogenes biofilms resulted in a 1.35 log CFU
per well reduction on polystyrene (21) and a 3.4 log CFU/
cm2
reduction on stainless steel (26). In our experiment with
4-day-old L. monocytogenes biofilms, a reduction of 2.61 log
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