Efficacy of High-Intensity Pulsed Light for the Microbiological Decontamination of Chicken,Associated Packaging,and Contact Surfaces

Abstract

The efficacy of high-intensity light pulse (HILP) technology (3 Hz, maximum of 505 J/pulse, and a pulse duration of 360 μs) for the decontamination of raw chicken and associated packaging and surface materials was investigated. Its ability to reduce microbial counts on raw chicken through plastic films was also examined. Complete inactivation of Campylobacter spp., Escherichia coli, and Salmonella Enteritidis in liquid was achieved after 30 sec HILP treatment. Reductions of 3.56, 4.69, and 4.60 log₁₀ cfu/cm² were observed after 5 sec HILP treatment of Campylobacter jejuni, E. coli, and Salmonella Enteritidis inoculated onto packaging materials and contact surfaces, respectively. The greatest reductions on inoculated chicken skin were 1.22, 1.69, and 1.27 log₁₀ cfu/g for C. jejuni, E. coli, and Salmonella Enteritidis, respectively. Corresponding reductions on inoculated skinless breast meat were 0.96, 1.13, and 1.35 log₁₀ cfu/g. The effectiveness of HILP treatment for reducing microbial levels on chicken increased as the film thickness decreased. HILP treatments of 2 sec did not significantly affect the color of raw chicken although treatments of 30 sec impacted color. This study has shown HILP to be an effective method for the decontamination of packaging and surface materials. Additionally, it has demonstrated the potential of HILP to be used as a decontamination method for packaged chicken.

Introduction

I nfection by Campylobacter jejuni is considered to be the predominant cause of bacterial-mediated diarrheal disease worldwide (Zilbauer et al., 2008). In addition to raw chicken, contaminated packaging material is considered a potential vehicle for the introduction of pathogens, including campylobacters, in retail and domestic kitchens. Studies have demonstrated the presence of Campylobacter as well as Salmonella and Escherichia coli on the external surfaces of materials used to package meats (Harrison et al., 2001; Burgess et al., 2005; FSAI, 2010). These bacteria are recognized as the most frequent causes of bacterial foodborne gastroenteritis worldwide (Wilson et al., 2008; Farzan et al., 2009; Milnes et al., 2009). Contamination of external packaging in particular, poses a potential opportunity for cross-contamination of surfaces, other foods (including ready-to-eat foods) and people in retail premises and consumers homes (Burgess et al., 2005). Risk assessment studies demonstrate that the most effective intervention measures should be aimed at reducing the Campylobacter concentration rather than reducing the prevalence on poultry carcasses and products (Nauta et al., 2009). Sequential reduction in the numbers of Campylobacter coming through the food chain at pertinent stages can result in an overall decrease in the prevalence of foodborne illness associated with that organism.

High-intensity light pulse (HILP) is an emerging technology that has been shown to be highly effective against a wide range of pathogenic microorganisms, including Staphylococcus aureus, E. coli O157:H7, Salmonella enterica, and Cryptosporidium parvum (Bialka and Demirci, 2007; Krishnamurthy et al., 2007; Lee et al., 2008). In a HILP system, broad-spectrum high-intensity light is released as intermittent short pulses. This results in a more effective and rapid treatment for inactivating microorganisms compared with other low-intensity light treatments, such as continuous ultraviolet. Its mechanism of inactivation is through the formation of thymine dimers within a microbe's DNA, which ultimately prevents replication. Photothermal and photophysical effects have also been proposed as inactivation mechanisms (Bialka and Demirci, 2008). Applications of HILP investigated to date include decontamination of milk, juice, fruit, sausages, food powders, and eggs (Fine and Gervais, 2004; Krishnamurthy et al., 2007; Bialka and Demirci, 2008; Keklik, 2009; Sauer and Moraru, 2009; Uesugi and Moraru, 2009). HILP treatment does not result in the development of volatile organic compounds or suspended airborne particulates. It is cost effective and generally does not change the characteristics of food matrices (Luksiene et al., 2007).

The objectives of the current study were (1) to investigate the susceptibility of a collection of Campylobacter isolates from various sources, E. coli (ATCC 25922), and Salmonella Enteritidis (ATCC 13076) to HILP in liquid media, (2) to assess the potential of HILP to decontaminate surfaces of food contact materials, nontransparent and transparent packaging, and (3) to investigate the ability of HILP to penetrate transparent plastic films for the decontamination of packaged raw chicken meat.

Materials and Methods

Microorganisms and culture preparation

A total of 10 Campylobacter strains (seven C. jejuni and three C. coli) were used in the susceptibility studies. All strains were isolated from chicken except for strains NCTC 11168 and 323 BC, which were of human origin. E. coli (ATCC 25922) and Salmonella Enteritidis (ATCC 13076) strains were obtained from the American Type Culture Collection (ATCC). Stocks of cultures of Campylobacter were maintained in defibrinated horse blood, whereas E. coli and Salmonella Enteritidis were stored in Nutrient Broth containing 20% Glycerol at −80°C. Campylobacter strains were resuscitated by inoculating a loopful of the frozen stock into Mueller Hinton broth (Oxoid) containing Campylobacter growth supplement (Oxoid) and incubated microaerobically for 48 h at 42°C. Stocks of E. coli and Salmonella Enteritidis were resuscitated by inoculating a loopful of defrosted stock into Tryptone Soy broth (Oxoid) and incubating for 24 h at 37°C. The enriched Campylobacter cultures were then streaked onto Colombia blood agar (Oxoid), modified charcoal cefoperazone deoxycholate agar medium (mCCDA; Oxoid), and incubated microaerobically for a further 48 h at 42°C. E. coli and Salmonella Enteritidis were streaked onto Tryptone Soy agar (TSA; Oxoid) and incubated for 24 h at 37°C.

Campylobacter cultures were prepared by transferring a single colony of a 48 h blood agar plate to 20 mL of Mueller Hinton broth with Campylobacter growth supplements and incubated for 24 h. The 24 h cultures were centrifuged for 10 min at 30,000 g and the resulting pellet was washed and recentrifuged twice in maximum recovery diluent (MRD; Oxoid) before final resuspension in 20 mL MRD. This resulted in a liquid cell suspension of ∼10⁷ colony forming units per milliliter (cfu/mL). E. coli and Salmonella Enteritidis cultures were prepared by transferring a single colony from a TSA plate to 20 mL of Tryptone Soy broth and incubating at 37°C for 24 h. The E. coli and Salmonella Enteritidis cultures were then centrifuged, washed, and resuspended as described previously. This resulted in cell suspensions containing ∼10⁸ cfu/mL, which were then exposed to various HILP treatments.

HILP unit

The HILP unit was a benchtop SteriPulse-XL system (Xenon). The system comprised a high-energy pulsed ultraviolet/Visible flash lamp (Type C, 190 nm spectral cut-off point) delivering a maximum of 505 J/pulse (1.27 J/cm²). The pulse width produced was 360 μs at a fixed pulse rate of 3 Hz. The pulse energy delivered to the sample varied depending on its distance from the quartz window within the HILP chamber. Table 1 details HILP dose of treatments applied in the current study calculated in accordance with the manufacturer's instructions.

Table 1.

Intensity of Pulsed Light at Selected Distances from the Quartz Window

		HILP dose per treatment (J/cm²)
Distance from quartz window (cm)	HILP dose per pulse (J/cm²/pulse)	1 sec	2 sec	5 sec	30 sec
2.5	1.18	N/A	7.08	N/A	106.2
11.5	0.4	1.2	N/A	6	N/A
14	0.3	0.9	N/A	4.5	N/A

HILP, high-intensity light pulse; N/A, not applicable.

HILP treatment procedures

HILP of bacteria in liquid matrices

All 10 Campylobacter isolates were initially assessed for susceptibility to HILP in a liquid matrix (MRD) and the most resistant isolate was then selected for further study. E. coli and Salmonella Enteritidis were also subjected to equivalent HILP treatments. C. jejuni, C. coli, E. coli, and Salmonella Enteritidis pure cultures (in MRD) were prepared as described previously. Samples (3 mL) were then placed into Petri dishes (50 mm diameter), positioned 2.5 cm from the quartz window. After removal of covers, Petri dishes containing samples were subjected to 2 or 30 sec HILP treatments (corresponding to 7.08 or 106.2 J/cm², respectively). During HILP treatment, samples were placed in an iced bath to minimize heating caused by the infrared portion of the HILP light unit. Sample temperatures were measured pre- and immediately posttreatment using a K-type thermocouple attached to a Grant Data Logger (Squirrel 2040; Grant Instruments) to ensure that the maximum temperature reached was nonlethal to the bacteria under the treatment times investigated (i.e., ≤50°C) as previously reported for Campylobacter, E. coli, and Salmonella (Line et al., 1991; Juneja et al., 2001; Humphrey et al., 2007).

HILP of bacteria on food contact materials, packaging, and films

A selection of food contact materials, nontransparent packaging, and transparent plastic films commonly used in poultry processing were examined in this study (Table 2). Materials were cut into 5 × 5 cm sections, cleaned thoroughly with 70% ethanol, and kept in sterile Petri dishes until required. Before inactivation studies, a range of HILP treatments were investigated on all materials to determine time and distance combinations, which resulted in a final temperature of ≤50°C. The temperature of the top surface of the material was measured immediately after treatment using an infrared thermometer (RS 1327; RS Components Ltd.) and the temperature of the underside of the material was measured using a K-type thermocouple. All materials were subjected to a range of time and distance HILP treatment combinations resulting in the posttreatment temperature not exceeding 50°C. This was to ensure a nonthermal treatment. Subsequently, treatment times of 1 or 5 sec at distances of 11.5 or 14 cm were chosen for subsequent microbiological inactivation studies. Corresponding energy intensities for treatments are presented in Table 1.

Table 2.

Food Contact Surfaces and Transparent and Nontransparent Packaging Materials Evaluated in the Current Study

	Material (abbreviation)	Thickness (μm)	Application
Food contact materials	Stainless steel (SS)	N/A	Common surface in poultry-processing facilities
	Polyethylene cutting board (PCB)	N/A	Food preparation surface
Nontransparent packaging	Black polypropylene tray (BPP)	N/A	Poultry portions
	White polypropylene tray (WPP)	N/A	Poultry portions
	Blue polystyrene tray (PS)	N/A	Whole birds
	Aluminium tray (AL)	N/A	Prepared poultry products
Transparent plastic films^a	Polyolefin (PO)^a	15	Covering of whole birds on trays
	Polyethylene-polypropylene (PET-PP)^a	54	Sealing of polypropylene trays
	Polyvinyl chloride (PVC-25/PVC-16)^a	25, 16^b	Covering of whole birds on trays

Materials used for investigating HILP through plastic films.

PVC of 16 μm thickness was used only for the study of HILP through packaging materials.

Bacterial cultures of C. jejuni (1136 DF), E. coli, and Salmonella Enteritidis were prepared in MRD as described previously and transferred to the upper surfaces of all materials under investigation by applying an aerosolized inoculum using a spray bottle. Before inoculation, the spray bottle was cleaned using 1% Trigene (Medichem International Ltd.) followed by sterile MRD to remove any remains of the disinfectant. The spray bottle was then primed with the bacterial culture before spraying onto packaging, films, and contact surfaces. The spray nozzle was held at a distance of ∼50 cm from the surface of the material, which was held at a 45° angle to the nozzle with a sterile forceps. Each piece received three sprays resulting in an inoculum of ∼3 log₁₀ cfu/cm² for C. jejuni and 4 log₁₀ cfu/cm² for Salmonella Enteritidis and E. coli. After inoculation, the underside of the material was carefully wiped with 70% ethanol solution to remove any inoculum that may have been inadvertently transferred.

HILP of bacteria through plastic films

To investigate whether the HILP treatment could penetrate the various transparent plastic films used in this study, pure cultures of bacteria suspended in MRD (3 mL) were placed in Petri dishes (50 mm diameter), which were covered with plastic films of varying compositions and thicknesses (i.e., polyethylene-polypropylene [PET-PP; 54 μm], polyolefin [15 μm], or polyvinyl chloride [PVC; 16 and 25 μm]). Samples were placed 2.5 cm from the quartz window and exposed to 2 or 30 sec treatments. After treatment of liquid suspensions, the ability of HILP to decontaminate inoculated raw chicken through the transparent plastic films was also investigated. Pieces of raw skinless breast meat and skin (2.5 × 2.5 cm) were dipped in the bacterial suspensions for 20 sec. Inoculated skinless breast meat and skin sections were placed in sterile Petri dishes, and bacteria were left to adhere for 30 min before samples were covered with plastic films and treated by low and high intensity doses of HILP. Skin samples were treated on both sides, which involved treating the upper surface and then aseptically inverting and transferring to a sterile Petri dish for further HILP treatment. Uncovered control samples were also treated in a similar fashion to determine the effect of HILP on samples without film. Untreated control samples were also enumerated to determine bacterial levels before treatment. Sample temperatures were measured posttreatment using an infrared thermometer to ensure the maximum temperature reached was nonlethal to the bacteria under the treatment times investigated (i.e., ≤50°C).

Instrumental color analysis

To determine whether treatment with HILP had any negative effects on visual appearance of skinless breast meat, instrumental color analysis was performed. The Hunter L (lightness), a (redness-greenness), and b (yellowness-blueness) values were measured with a Chroma Meter CR-300 (Konica Minolta Co. Ltd.), which was calibrated using a white tile before use. As the color of untreated raw skinless breast meat was found to differ considerably, the color of each sample was measured before and after treatment. Before treatment, color measurements were taken from three random locations and averaged to get an overall mean measurement for each of three replicate samples. After HILP treatment, triplicate measurements were taken from the most visibly affected regions and analyzed as previously described. The changes in color (ΔL, Δa, and Δb) were calculated by subtracting the values obtained before treatment from the values obtained after treatment.

Microbiological analysis

After treatment of liquid samples, the contents of the Petri dish were transferred to a sterile container. A 10-fold dilution series was prepared in MRD and 0.1 mL of each dilution was spread plated in duplicate onto mCCDA for Campylobacter and TSA for both E. coli and Salmonella Enteritidis. The mCCDA and TSA plates were incubated at 37°C for 48 and 24 h, respectively.

Packaging and surface materials treated by HILP were placed in 20 mL of MRD and gently massaged to remove remaining bacteria. Samples were diluted and spread plated onto appropriate agars and incubated as described previously. After treatment of raw skinless breast meat, 2 g was aseptically removed from the surface and stomached in 18 mL MRD. For skin samples, 2 g of the sample was weighed and stomached in 18 mL MRD. Dilutions, plating, and incubation were carried out as previously described.

Mean counts for each treatment were calculated and converted to log₁₀ cfu values with results for surviving numbers of microorganisms in MRD expressed per mL (cfu/mL). Microbial counts on packaging and surface materials were expressed as cfu/cm², and microbial counts on raw chicken were expressed as cfu/g.

Statistical analysis

Statistical analysis was carried out using a general linear model in SAS version 9.1 (SAS Institute). Each experiment was carried out in triplicate (n = 3).

Results

HILP treatment of bacteria in a liquid matrix

For all microorganisms tested, complete inactivation was achieved after 30 sec HILP treatment. In addition, no viable E. coli or Salmonella Enteritidis were detected after HILP treatment of 2 sec (Table 3). Low levels (<0.85 log₁₀ cfu/mL) of survivors were detected for four C. jejuni and two C. coli strains after the 2 sec HILP treatment. However, statistical analysis demonstrated no differences in resistance between all 10 strains examined (p ≥ 0.05).

Table 3.

Log ₁₀ cfu/mL Campylobacter jejuni, Campylobacter coli, Escherichia coli, and Salmonella Enteritidis in Maximum Recovery Diluent After High-Intensity Pulsed Light Treatment (2 or 30 sec, 2.5 cm from the Quartz Window)

Organism	Strain	0 sec	2 sec	30 sec
C. jejuni	NCTC 11168	6.89 (0.07)^A	0.60 (1.05)^B	ND^B
	1136DF	7.70 (0.14)^A	0.59 (1.03)^B	ND^B
	323 BC	7.01 (0.11)^A	ND^B	ND^B
	1135 DF	6.53 (0.07)^A	ND^B	ND^B
	1147 DF	7.17 (0.08)^A	ND^B	ND^B
	1146 DF	7.30 (0.07)^A	0.25 (0.43)^B	ND^B
	1354 DF	7.06 (0.20)^A	0.49 (0.43)^B	ND^B
C. coli	1140 DF	6.49 (0.06)^A	0.85 (0.91)^B	ND^B
	2124 GF	6.93 (0.28)^A	ND^B	ND^B
	1662 DF	7.23 (0.06)^A	0.34 (0.91)^B	ND^B
E. coli	ATCC 29522	7.84 (0.07)^A	ND^B	ND^B
Salmonella Enteritidis	ATCC 13076	7.55 (0.04)^A	ND^B	ND^B

Means within rows followed by the same letter are not significantly different (p ≥ 0.05). Standard deviation is enclosed in parentheses.

ND, not detected (≤0.22 log₁₀ cfu/mL).

HILP treatment of bacteria on packaging materials and contact surfaces

All HILP treatments resulted in significant reductions of C. jejuni on food contact materials, nontransparent packaging, and transparent plastic films (p < 0.05) (Fig. 1). After the 1 sec HILP treatment times at both distances (11.5 and 14 cm) complete inactivation was observed for all materials with the exception of black polypropylene and polyolefin (PO) with < 1 log₁₀ cfu/cm² being detected (p < 0.05). Increasing HILP treatment time to 5 sec resulted in complete inactivation of C. jejuni on all materials under investigation.

FIG. 1.

Log₁₀ cfu/cm² Campylobacter jejuni on packaging materials and contact surfaces after high-intensity pulsed light treatment (1 or 5 sec, 11.5 or 14 cm from the quartz window). Standard deviations of data within the range of 0–0.79.

All HILP treatments applied to packaging, films, and contact material surfaces resulted in significant reductions for E. coli (p < 0.05) (Fig. 2). HILP treatment for 5 sec at 11.5 cm resulted in complete inactivation of E. coli on AL and PVC-25 and reductions of between 1.50 and 4.17 log₁₀ cfu/cm² were achieved on the other materials (p < 0.05). A similar result was observed for HILP treatment for 5 sec at 14 cm with reductions in the range of 1.57–4.26 log₁₀ cfu/cm² (p < 0.05).

FIG. 2.

Log₁₀ cfu/cm² Escherichia coli on packaging materials and contact surfaces after high-intensity pulsed light treatment (1 or 5 sec, 11.5 or 14 cm from the quartz window). Standard deviations of data within the range of 0–0.60.

Significant reductions of Salmonella Enteritidis were achieved on all materials after each HILP treatment (p < 0.05) (Fig. 3). HILP treatment of 5 sec at 11.5 cm resulted in complete inactivation of Salmonella Enteritidis on PVC-25 and WPP with reductions of between 2.52 and 4.50 log₁₀ cfu/cm² observed after treatment of other materials (p < 0.05).

FIG. 3.

Log₁₀ cfu/cm² Salmonella Enteritidis on packaging materials and contact surfaces after high-intensity pulsed light treatment (1 or 5 sec, 11.5 or 14 cm from the quartz window). Standard deviations of data within the range of 0–0.63.

HILP treatment of bacteria through plastic films

HILP treatment in liquids

The inactivation levels achieved varied depending on the transparent film type and thickness. Both HILP treatments applied to C. jejuni 1136 DF through PO film and PVC-25 and PVC-16 resulted in complete inactivation (Table 4). A significant reduction in C. jejuni levels (0.96 log₁₀ cfu/mL) was achieved by the 30 sec HILP treatment (at 2.5 cm) through the PET-PP film (p < 0.05) although no significant reduction was achieved after the HILP treatment for 2 sec.

Table 4.

Log ₁₀ cfu/mL Campylobacter jejuni, Escherichia coli, and Salmonella Enteritidis in Maximum Recovery Diluent After High-Intensity Pulsed Light Treatment Through Transparent Plastic Films of Various Thicknesses (2 or 30 sec, 2.5 cm from the Quartz Window)

	C. jejuni		E. coli		Salmonella Enteritidis
Plastic film/abbreviation (thickness μm)	2 sec	30 sec	2 sec	30 sec	2 sec	30 sec
No treatment	7.06 (0.13)^A	7.06 (0.13)^A	7.84 (0.07)^A	7.84 (0.07)^A	7.55 (0.04)^A	7.55 (0.04)^A
Uncovered	ND^B	ND^B	ND^D	ND^D	ND^D	ND^E
Polyethylene-polypropylene/PET-PP (54)	6.84 (0.07)^A	6.10 (0.27)^A	7.50 (0.12)^A	7.26 (0.03)^A	7.33 (0.09)^A	7.04 (0.11)^A
Polyvinyl chloride/PVC-25 (25)	ND^B	ND^B	3.74 (0.48)^B	3.59 (0.11)^B	3.77 (0.25)^B	4.51 (0.32)^B
Polyvinyl chloride/PVC-16 (16)	ND^B	ND^B	1.94 (0.26)^C	1.36 (0.10)^C	3.67 (0.12)^B	2.68 (0.33)^C
Polyolefin/PO (15)	ND^B	ND^B	1.15 (0.64)^C	0.77 (0.68)^C	1.72 (0.03)^C	1.54 (0.28)^D

Within columns, means followed by the same superscript capital letter are not significantly different (p ≥ 0.05). Standard deviation is enclosed in parentheses.

ND, not detected (≤0.22 log₁₀ cfu/mL).

For E. coli, the 30 sec HILP treatment through PVC-25, PVC-16, and PO resulted in reductions of E. coli 4.25, 6.48, and 7.07 log₁₀ cfu/mL, respectively (p < 0.05) (Table 4). HILP treatment for 2 sec through the aforementioned films resulted in significant reductions of 4.10, 5.90, and 6.99 log₁₀ cfu/mL, respectively (p < 0.05). No significant reductions were observed after either HILP treatment through the PET-PP film.

Similar trends were observed for HILP treatment of Salmonella Enteritidis. HILP treatment for 30 sec through PVC-25, PVC-16, and PO resulted in reductions of 3.04, 4.87, and 6.01 log₁₀ cfu/mL, respectively (p < 0.05). The corresponding reductions after 2 sec HILP treatment were 3.78, 3.88, and 5.83 log₁₀ cfu/mL, respectively (p < 0.05). Similar to E. coli, reductions after either HILP treatment through PET-PP were not significant.

HILP treatment of raw chicken

Since very similar trends were observed for HILP treatment of both chicken skin and skinless breast meat, only the data for skin are illustrated (Fig. 4). HILP treatment of uncovered chicken skin for 2 or 30 sec (at 2.5 cm) resulted in significant reductions of C. jejuni of 0.46 or 0.91 log₁₀ cfu/g, respectively (p < 0.05). Corresponding reductions for uncovered skinless breast meat were 0.63 or 0.89 log₁₀ cfu/g. Significant reductions of C. jejuni were achieved on skin after HILP treatments through the transparent plastic films examined (p < 0.05) with the exception of the 2 sec treatment in the case of PET-PP, PVC-25, and PVC-16. For skinless breast meat, all C. jejuni reductions were significant (p < 0.05) apart from the 2 sec treatment applied through the PET-PP film. The greatest reductions for C. jejuni on skin and skinless breast meat was 1.22 and 0.96 log₁₀ cfu/g, respectively, both of which were achieved after 30 sec HILP treatment through the PO film.

FIG. 4.

Log cfu/g C. jejuni, E. coli, and Salmonella Enteritidis on chicken skin after high-intensity pulsed light treatments (2 or 30 sec, 2.5 cm from the quartz window) through plastic films of various thicknesses. Standard deviations of data within the range of 0.04–0.74.

HILP treatments of E. coli on uncovered chicken skin for 2 or 30 sec resulted in significant reductions of 1.26 and 1.51 log₁₀ cfu/g, respectively (p < 0.05), whereas a similar effect was observed on uncovered skinless breast meat (1.12 and 1.48 log₁₀ cfu/g, respectively) (p < 0.05). All reductions achieved after HILP treatment of E. coli on both chicken skin and skinless breast meat covered by the various films were significant compared with untreated controls (p < 0.05) with the exception of skinless breast meat treated for 2 or 30 sec through the PET-PP film. Similar to C. jejuni, the 30 sec HILP treatment of skin and skinless breast meat through the PO film resulted in the greatest reductions of E. coli (1.69 and 1.13 log₁₀ cfu/g, respectively).

HILP treatment of uncovered chicken skin inoculated with Salmonella Enteritidis for 2 or 30 sec resulted in significant reductions of ∼1.5 log₁₀ cfu/g (p < 0.05), whereas the corresponding average reduction for uncovered skinless breast meat was 1.2 log₁₀ cfu/g. Significant reductions of Salmonella Enteritidis on both product types were achieved after HILP treatments through all films except for the 2 sec treatment through the PET-PP film. The greatest reductions achieved for Salmonella Enteritidis on skin and skinless breast meat were 1.27 and 1.35 log₁₀ cfu/g after 30 sec HILP treatment through PVC-16 and PVC-25, respectively.

Instrumental color analysis

There were no significant changes in the a values of HILP-treated skinless breast meat for either uncovered or covered product (Table 5). Significant decreases in b values were detected after 30 sec HILP treatment of samples covered by each of the four films under investigation (p < 0.05). With the exception of the 30 sec HILP treatment through PET-PP, PVC-25, and PVC-16, all L values decreased.

Table 5.

Color Change of Skinless Breast Meat After High-Intensity Pulsed Light Treatment Through Transparent Plastic Films of Various Thicknesses (2 or 30 sec, 2.5 cm from the Quartz Window)

Plastic film/abbreviation (thickness μm)	Treatment time (sec)	ΔL ^a	Δa ^a	Δb ^a
Untreated	0	0.00^A	0.00^A	0.00^A
Uncovered	2	−0.57 (0.43)^CD	0.07 (0.08)^A	−0.02 (0.08)^A
	30	−0.12 (0.24)^AB	−0.06 (0.12)^A	−0.72 (0.27)^B
Polyethylene-polypropylene/PET-PP (54)	2	−1.38 (0.17)^E	−0.01 (0.14)^A	−0.50 (0.15)^B
	30	0.37 (0.59)^A	0.08 (0.46)^A	−2.06 (0.48)^CD
Polyvinyl chloride/PVC-25 (25)	2	−2.31 (0.59)^F	0.09 (0.35)^A	−0.33 (0.17)^AB
	30	0.51 (0.23)^G	0.08 (1.42)^A	−2.20 (0.78)^CD
Polyvinyl chloride/PVC-16 (16)	2	−0.87 (0.25)^CD	0.15 (0.08)^A	−0.21 (0.10)^AB
	30	0.30 (0.25)^A	−0.01 (0.22)^A	−1.64 (0.47)^C
Polyolefin/PO (15)	2	−0.43 (0.44)^BC	−0.22 (0.21)^A	0.07 (0.05)^A
	30	−0.10 (0.62)^AB	−0.33 (0.23)^A	−2.28 (0.10)^D

Within columns, means followed by the same superscript letter are not significantly different (p ≥ 0.05). Standard deviation is enclosed in parentheses.

ΔL, Δa, and Δb calculated by subtracting value of untreated from treated.

Discussion

HILP treatment of bacteria in a liquid matrix

All organisms assessed showed complete inactivation after 30 sec HILP treatment, though for the 2 sec treatment, complete inactivation was only observed for E. coli and Salmonella Enteritidis. The detection of Campylobacter survivors after 2 sec HILP treatment suggests that Campylobacter in liquids may be less susceptible than the other organisms tested. Rajkovic et al. (2009) investigated the effect of HILP treatment (20 pulses, 1 pulse/sec, 7 J/pulse, and pulse duration of 30 μs) on C. jejuni and E. coli suspended in Peptone Physiological Salt Solution (PPS) and reported reductions of 2.5 and 1.5 log₁₀ cfu/mL, respectively. Much greater reductions (∼7 log₁₀ cfu/mL) were achieved under the parameters used for the current study (90 pulses, 3 pulses/sec, 505 J/pulse, and pulse duration of 360 μs). Additionally, the sample was 2.5 cm from the light source for the current study compared with 10.5 cm for the study by Rajkovic et al. (2009). Lower reductions observed by Rajkovic et al. compared with our study may have been due to differences in the depths of liquids treated (Demirci and Panico, 2008). This demonstrates that different treatment parameters can significantly affect levels of microbial inactivation in transparent liquids.

HILP treatment of bacteria on packaging and surface materials

HILP was effective for reducing all tested organisms on food contact surfaces, nontransparent packaging materials, and transparent films. Overall, HILP treatments were most effective for C. jejuni on packaging materials and contact surfaces, implying that Campylobacter on surfaces are more sensitive to HILP treatment. In general, HILP was least effective when applied to organisms inoculated onto polystyrene. The more porous nature of polystyrene surfaces compared with other packaging materials may offer some shielding for bacteria from HILP. Although every effort was taken to ensure the more delicate films (PVC and PO) were smoothed out for HILP treatment, bacteria may have been protected by any small crinkles formed in the film. Smooth surfaces such as stainless steel or aluminium would allow for a more uniform HILP treatment, whereas corrugations in black-and-white polypropylene trays could have shielded bacterial populations from HILP treatment.

HILP of bacteria through plastic films

Overall, the results indicate greater microbial reductions with higher doses of HILP and reduced film thickness. A similar study performed by Keklik et al. (2010) using a similar HILP unit achieved significant reductions for Salmonella Typhimurium on unpackaged and vacuum-packaged raw chicken fillet, which is broadly in agreement with the current study for Salmonella Enteritidis. The study involved treating samples for 5 sec at 13 cm, 30 sec at 8 cm, or 60 sec at 5 cm from the quartz window compared with a distance of 2.5 cm and treatment times of 2 and 30 sec used in the current study. HILP treatment of C. jejuni on both chicken skin and skinless breast meat resulted in the lowest reductions out of the three organisms tested. However, reductions of ∼1 log₁₀ cfu/g were achieved for C. jejuni and could be used as part of a Campylobacter control strategy in poultry. For example, quantitative microbial risk assessment models indicate that a 2 log reduction in Campylobacter concentrations on broiler carcasses should substantially reduce the risk of human exposure and associated illness (Rosenquist et al., 2003; Havelaar et al., 2007). Therefore, HILP technology could be applied as part of a sequential risk reduction strategy to achieve a worthwhile effect. The E. coli and Salmonella Enteritidis strains used in the current study are cultivated strains and would be well adapted to the laboratory environment, whereas C. jejuni 1136 DF is a wild-type strain isolated from chicken, which may enhance its ability to survive or increase its tolerance to such an environment.

Color analysis

As the color of untreated raw skinless breast meat samples varied significantly, the color of each individual sample was recorded pre- and posttreatment to obtain an accurate representation of color change. In comparison to untreated skinless chicken breast, neither HILP treatments affected the a (redness) values, whereas a reduction in the L (lightness) and the b (yellowness) values after 30 sec HILP treatments was recorded. Keklik et al. (2010) reported an increase in the a, b, and L values after 60 sec treatment of skinless chicken breast samples placed 5 cm from the quartz window. Our findings suggested an overall increase in the L values for the 30 sec treatment, although not all were significant compared with the untreated control. The samples were visibly affected by the 30 sec HILP treatment, which may resulted from the film being in contact with the sample during treatment as the effect was more pronounced on film-covered samples. Although not reported in the previous study, a calculation of the doses applied in accordance with the manufacturer's technical manual with the HILP unit shows that the dose applied to the vacuum-packaged chicken samples was greater than that used in the current study (i.e., 171 vs. 106.8 J/cm², respectively). This may account for the different trends observed for color between the two studies. Although the technologies are not directly comparable, a study by Liu et al. (2003) on gamma-irradiated skinless chicken breasts also found that as the irradiation dose increased, the b value decreased, the a value increased, whereas the L value was not affected.

Conclusion

The current study shows that HILP is an effective surface decontamination method and is also effective against C. jejuni and C. coli. As there was very little difference between the 2 and 30 sec HILP treatments in terms of microbial reduction, short treatment times could be applied effectively for decontamination of unpackaged or packaged chicken without affecting product quality. Such an approach could potentially be used to reduce the levels of Campylobacter on external packaging surfaces and on raw poultry meat, thereby reducing the risk of exposure to consumers. The authors also conclude that it is important that as much information as possible is provided on the treatment parameters so that accurate comparisons can be made between studies in the future. Overall, these findings demonstrate the potential of HILP as a surface decontamination method for packaging, contact surfaces, and chicken.

Footnotes

Acknowledgments

Funding for this research was provided under the Irish National Development Plan, through the Food Institutional Research Measure administered by the Department of Agriculture Fisheries and Food.

Disclosure Statement

No competing financial interests exist.

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