Synergism between high pressure processing and active packaging against Listeria

monocytogenes in ready-to-eat chicken breast.

Alexandros Ch. Stratakosa, Gonzalo Delgado-Pandoa, Mark Lintonb, Margaret F. Pattersonb,

Anastasios Koidisa, *

a Queen’s University Belfast, Institute for Global Food Security, Belfast, Northern Ireland, UK.

b Agri-Food & Biosciences Institute, Belfast, Northern Ireland, UK.

* Corresponding author

Dr Anastasios (Tassos) Koidis

Institute for Global Food Security

Queen's University Belfast

18-30 Malone Road

Belfast, BT9 5BN

Northern Ireland, UK

Tel: +44 28 90975569

email: t.koidis@qub.ac.uk

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Abstract

The effects of high pressure processing (HPP) in conjunction with an essential oil based active

packaging on the surface of ready-to-eat (RTE) chicken breast were investigated as post-processing

listericidal treatment. Three different treatments were used and all samples were vacuum packed: i)

HPP at 500 MPa for 1 min (control), ii) active packaging based on coriander essential oil and iii) active

packaging and HPP. When applied individually, active packaging and pressurisation delayed the

growth of Listeria monocytogenes. The combination of HPP and active packaging resulted in a

synergistic effect reducing the counts of the pathogen below the detection limit throughout 60 days

storage at 4oC. However, when these samples were stored at 8oC growth did occur but again a delay

in growth was observed. The effects on colour and lipid oxidation were also studied during storage

and were not significantly affected by the treatments. Active packaging followed by in-package

pressure treatment could be a useful approach to reduce the risk of L. monocytogenes in cooked

chicken without impairing its quality.

Keywords

High pressure processing, active packaging, coriander essential oil, Listeria monocytogenes, chicken,

ready-to-eat

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1. Introduction

Ready-to-eat (RTE) products, such as cooked meats, are an important segment of the food industry

which is gaining increasing popularity. Although established preservation technologies such as

heating can ensure the safety of these products, cross-contamination might occur during post

processing manipulation from equipment or food handlers. Listeria monocytogenes is a ubiquitous

psychrotrophic organism capable of growing at temperatures as low as 2-4°C which makes its

presence of particular concern in RTE foods with a relatively long shelf-life, such as meat products,.

RTE foods, including cooked meats (e.g. chicken) are considered an important food-borne source of

L. monocytogenes infections in the EU and U.S.A (EFSA 2011; USDA 2012) as they have been

implicated in a number of foodborne outbreaks. Even if the prevalence in contaminated food is low,

the popularity of these RTE products raises a concern for public health. This can be illustrated from

the information provided by EFSA (2014), which states that 1642 cases of listeriosis were reported in

2012 in the E.U., with a mortality rate of 17.8%.

During recent years consumers have been more concerned about the addition of synthetic additives

to food, which has led to increased interest in ingredients from natural sources. It has been

recognised for a long time that some essential oils obtained from plant parts such as flowers, seeds,

leaves, bark, and roots have antimicrobial properties (Burt, 2004; Sacchetti,

Maietti, Muzzoli,, Scaglianti, Manfredini, Radice, & Bruni (2005); Benchaar, Calsamiglia, Chaves,

Fraser, Colombatto, McAllister, & Beauchemin, 2008). Coriander is an herbaceous plant of economic

importance, native to the Mediterranean and Middle East, and is used as a culinary herb and

medicament (Mildner-Szkudlarz, Zawirska-Wojtasiak, Obuchowski, & Gośliński, 2009). It has been

found that coriander essential oil and extracts possess antibacterial, antidiabetic, anticancerous,

antimutagenic, and antioxidant properties (Sreelatha, Padma, & Umadevi, 2009; Zoubiri and

Baaliouamer, 2010). More specifically, coriander essential oil has shown to have an inhibitory effect

against pathogenic microorganisms such as L. monocytogenes and Aeromonas hydrophila as well as

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spoilage microflora in meat products (Burt, 2004). It has been characterised as Generally Recognised

as Safe (GRAS) by The Food and Drug Administration (FDA) and has been included as a food additive

by EU legislation. Essentials oils can be applied in foods directly (Skandamis, & Nychas, 2001;

Solomakos, Govaris, Koidis, & Botsoglou, N. 2008) or indirectly through the use of polymers as

carriers of these oils, which is known as active packaging. Active packaging (AP) refers to the

inclusion of certain active components to the packaging film or within packaging containers in order

to maintain the quality and safety of food products (Kerry et al. 2006).

High pressure processing (HPP) is a non-thermal technology that has become a very useful tool as a

post-packaging decontamination method for RTE products. HPP is currently being applied as a

preservation method in numerous countries to produce a broad range of RTE meat products such as

cooked cured and smoked meats (Garriga & Aymerich 2009).

Pressure levels that are most commonly used for food preservation purposes are between 500 and

600 MPa. HPP is used to extend the shelf life of products and improve safety while preserving the

organoleptic properties of foods. HPP can cause lethal and sublethal injuries to microorganisms and

it has been proved to be effective against several vegetative pathogenic microorganisms such as L.

monocytogenes, Staphylococcus aureus, Escherichia coli and Salmonella Typhimurium

(Rendueles, Omer, Alvseike, Alonso-Calleja, Capita, & Prieto, 2011). However, depending on the

processing conditions, HPP may cause acceleration of the rate of lipid oxidation (Orlien, Hansen, &

Skibsted, 2000) which can increase proportionally with higher pressure levels (Bolumar, Andersen,

& Orlien, V. 2011). Lipid oxidation can negatively affect product quality as well as shelf life.

Depending on the processing conditions HPP can also have an effect on the colour of meat products

(Simonin, Duranton, & de Lamballerie, 2012). Due to the fact that sublethally injured

microorganisms are more susceptible to antimicrobial compounds (Kalchayanand, Sikes, Dunne, &

Ray, 1994), the use of HPP in conjunction with other preservation technologies such as active

packaging appears to be an interesting alternative. The combination the above technologies can

potentially enhance the inactivation of pathogens with the use of lower pressure levels in order to

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better preserve the quality characteristics of the product and thus further augment the commercial

applicability of HPP.

The aim of the study, was to investigate the possible synergistic or additive effect of HPP and active

packaging based on coriander essential oil on L. monocytogenes artificially inoculated on cooked

chicken breast fillet, and also explore their effect on colour and lipid oxidation.

2. Materials and Methods

2.1 Preparation and storage of cooked chicken breast samples

Commercially produced cooked chicken breast fillets were supplied by a local poultry producer and

transferred to the laboratory within 24 hours of production. During transportation to the lab, the

samples were kept in individual packaging under MAP at 4oC. Whole pieces of cooked chicken (10 ±

0.2 g for the challenge study and 100 ± 0.2 g for colour and TBARS measurements) were placed in

polyethylene/polyamide vacuum pouches (Somerville Packaging, Lisburn, Northern Ireland) and

vacuum packed. The oxygen permeability of the pouches was 50 cm3/m2/24h at 1 bar, 23oC and 0%

humidity.

Three different treatments were performed:

(i) Pressure treatment (HPP): vacuum packed samples with a polyethylene/polyamide

vacuum food grade film were pressurised at 500 MPa for 1 min (control).

(ii) Active packaging (AP): samples were vacuum packed using the active film described in

2.2..

(iii) HPP/AP: Samples vacuum packed with the active film were pressurised at 500 MPa for 1

min.

Samples were stored at 4 and 8oC for a period of 60 days (samples were tested during storage at

days 1, 6, 13, 30 and 60).

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2.2. Preparation of the active film

A 10% coriander essential oil solution in ethanol was used to prepare the active films. A food grade

coriander essential oil produced by steam distillation was used (Sigma Aldrich, UK). The active films

were prepared according to Bolumar et al. (2011) with the use of food grade

polyamide/polyethylene film (Culinary Innovations, Leicester, UK) of 12-μm thickness, cut into pieces

of 15 × 15 cm (225 cm2). The produced films were spread and 4 ml of the solution was dispensed

onto each one. Afterwards, it was evenly distributed over the surface with a sterile spreader and left

overnight under aseptic conditions for the ethanol to evaporate. The active film had 0.772 mg

essential oil/cm2. Antimicrobial substances that cannot withstand temperatures used in polymer

processing are usually coated onto the material after forming the polymer (Appendini & Hotchkiss,

2002). Therefore, this method was chosen as the most suitable in terms of avoiding degradation of

the oil as it is thermolabile.

2.3. High pressure processing

Cooked chicken samples were pressure treated in a commercial scale Avure Quintus high pressure

press (Avure Technologies, Sweden), with a pressure vessel 18 cm in diameter and 1.2 m in length,

of 35 L volume. The pressure transmission fluid used was potable water. The pressure come-up time

was approximately 20-25 s per 100 MPa and pressure release time was approximately 10 and 15 s,

depending on the pressure. The initial temperature of the water was 18 oC and the temperature

increase due to adiabatic heating was approximately 2-3oC per 100 MPa. A thermocouple was used

to monitor the temperature of the pressure transmission fluid during processing. The samples were

pressure treated at 500 MPa for 1 min (‘come-up’ and decompression times were not included in the

treatment time).

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2.4. Inoculum preparation and enumeration of Listeria monocytogenes

For each strain used, a loopful of a fresh tryptone soya agar + 0.6% yeast extract (TSAYE) (Oxoid,

Basingstoke, U.K.) slope culture was inoculated into 10 ml of brain heart infusion broth (BHI) (Oxoid,

Basingstoke, U.K.) and incubated at 37 °C for 24 h. Subsequently 100 μl of a 10−4 dilution of this broth

was inoculated into another 10 ml BHI broth and incubated to stationary phase at 37 °C for 48 h. The

cocktail of L. monocytogenes strains that were used are presented in Table 1, including two relatively

pressure resistant strains (Patterson, Mackle, & Linton, 2011).

All cultures were centrifuged at 4000×rpm, for 30 min, washed twice in phosphate buffered saline

(PBS) and re-suspended in a final volume of 10 ml PBS to give approximately 108 -109 CFU/ml. The

suspensions of all 5 strains were combined and mixed well. The combined suspensions were

inoculated (100 μl) onto different chicken samples, to simulate the surface contamination of RTE

products, to give a level of approximately 5 log CFU/g. The ratio of inoculum to weight of sample was

approximately 1:100 thus avoiding any significant change of the aw.

Four samples in total for each of the 3 different treatments (HPP, AP and HPP/AP) were opened

aseptically and the contents were transferred to a sterile stomacher bag with a filter insert

(Interscience, St. Nom La Breteche, France). A 10-1 dilution of the sample was prepared in maximum

recovery diluent (MRD) (Oxoid, CM733, Basingstoke, U.K.) using a variable diluter (Baby Gravimat,

Interscience, St. Nom La Breteche, France). The dilution was homogenised for 1 min in a Seward

stomacher (Lab blender 400, Bury St. Edmunds, UK). When necessary, further 10 fold dilutions were

prepared in 9 ml MRD. An aliquot of 100 μl of each of the 10 fold dilutions was spread plated on

Oxoid chromogenic Listeria agar (OCLA) (Oxoid, CM1084B Basingstoke, UK) supplemented with OCLA

selective supplement (SR0226E) and Brilliance Listeria differential supplement (SR0228E) and left for

incubation at 37oC for 48 h. Each sample was plated in duplicate.

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2.5. Physicochemical characteristics

Chicken samples were characterised in six replicates. The pH of the cooked chicken, mixed with

deionised water (1:1) was measured using a Jenway pH Meter Model 3505. The moisture content

was calculated by drying the sample in an oven at 104oC until a constant weight was reached. Lastly,

water activity (aw) was measured by means of a Hygrolab 3 aw meter (Rotronic instruments, Crawley

UK).

2.6. Colour measurement

Colour measurements were conducted by means of a reflectance colorimeter (Minolta Chroma

Meter CR-400, Konica-Minolta, Basildon, U.K.). The CIELab system L*, a* and b* was followed. The

L* value represents the colour from black (0) to white (100). The a* value represents redness (+) or

greenness (−), and the b* value represents yellowness (+) or blueness (−) (Kaushik, Pal Kaur,

Srinivasa Rao, & Mishra, 2014). The measurements were performed throughout the 60 day storage

on 4 samples per measurement on 2 different points on the surface of the sample, stored at 4oC and

8oC.

2.7. Lipid oxidation analysis

The extent of lipid oxidation was measured conventionally by thiobarbituric acid reactive substances

(TBARS) using the method described by Delgado-Pando, Cofrades, Ruiz-Capillas, Solas, Triki, &

Jiménez-Colmenero, 2011). TBARS were expressed as mg malondialdehyde/kg dry meat. The values

were the means of duplicate measurements of two different samples of the same treatment. The

procedure was as follows: 5 g samples were weighed and 35 ml Trichloroacetic acid (TCA) (7.5%) was

added. Afterwards, the samples were homogenised for 30 s and centrifuged at 3000 × g for 2 min

and filtered. 5 ml of filtrate was transferred to test tubes and 5 ml of 0.02M thiobarbituric acid (TBA)

(1:1, TBA: filtrate) was added. The samples were vortexed and left to stand for 20 h at 20 0C.

Subsequently, samples were vortexed again and the absorbance was measured at 532 nm using a

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plate reader (Teca, Sufire2, Reading, UK) with the use of 98-well plates. The blank was prepared by

adding 5 ml TCA and 5 ml of TBA. The calibration curve was made using 1,1,3,3-

tetramethoxypropane. The measurements were performed at the intervals described previously

during the 60 day storage using 4 samples per measurement and each sample was measured in

duplicate.

2.8. Statistical analysis

The entire experiment was replicated on two different occasions. Data for L. monocytogenes counts

were subjected to analysis of variance (ANOVA) to compare the interactions between treatment and

storage time. For colour and lipid oxidation values two way ANOVA was used, which included

treatment and storage time as the main effects and their interaction. Differences between effects

were assessed by the Tukey test (P < 0.05) unless otherwise indicated. When L. monocytogenes

counts were below the detection limit, (<50 CFU/g) the value of 50 CFU/g was used as the value for

the statistical tests (Patterson et al. 2011).

3. Results and Discussion

3.1. Physicochemical characteristics of RTE chicken

The initial pH, aw and moisture values of cooked chicken were 6.07±0.05, 0.95±0.002 and

66.89±2.91 (%), respectively. The values shown here are similar to the ones obtained in other studies

regarding cooked chicken breast (Naveena, Sen, Kingsly, Singh, & Kondaiah, 2008; Patterson, McKay,

Connolly, & Linton 2010; Sampaio, Saldanha, Soares, & Torres, 2012). As expected, the cooked

chicken represents a good matrix for the growth of L. monocytogenes. Moreover, the colour

parameters of untreated cooked chicken were measured and found to be 81.56±0.90, 2.85±0.32 and

13.53±0.468, for the L*, a* and b* parameters, respectively.

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3.2. Fate of L. monocytogenes during storage at 4oC

High pressure treatment of RTE cooked chicken samples packed in a conventional food-grade film

produced a significant and immediate reduction of L. monocytogenes counts of 1.4 log CFU units (Fig

1a). This survival of the L. monocytogenes was expected, especially as pressure-resistant strains were

included in the inoculum (Patterson et al, 2011). However, during storage the cooked chicken matrix

supported the rapid growth of L. monocytogenes and the pathogen had reached values >

8 log CFU/g at day 30. It has been shown that a pressure level of 600 MPa for 2 min is needed to

reduce the counts of L. monocytogenes by about 3.3 log cfu/g in cooked chicken (Patterson et al.

2011). The observed ability of L. monocytogenes to grow in cooked chicken at refrigeration

temperature, even when HPP treated, showed the need of using additional hurdles in order to

inhibit its growth in the RTE chicken breast during storage. When the active packaging was applied

alone at 40C storage, it resulted in a reduction of L. monocytogenes of approximately 0.8 log CFU/g,

after 1 day of storage (Figure 1a). Although, the reduction was significantly lower (P < 0.05) than

when observed with HPP, the counts remained relatively stable throughout storage, thus exerting a

bacteriostatic effect. This low reduction of the pathogen is consistent with other active packaging

films prepared in different studies. The study of Marcos, Aymerich, Monfort, & Garriga, (2008)

showed that when cooked ham was packaged using alginate films containing 2000 AU/cm2 of

enterocins, a low reduction of L. monocytogenes (≤0.6 log CFU/g) was observed after 1 d of storage

at 1 oC and 6 oC.

In dry cured ham packaged with films containing nisin (200 AU/cm2), no significant immediate

reduction of L. monocytogenes was observed (Hereu, Bover-Cid, Garriga, & Aymerich, 2012).

According to Quintavalla & Vicini, (2002) the efficiency of antimicrobial packaging is heavily

influenced by the direct contact with the food surface, which permits the gradual release of the

antimicrobial substance from the film to the food.

When HPP and antimicrobial packaging (HPP/AP) were applied in conjunction, the counts of the

pathogen were reduced below the detection limit (1.69 log CFU/g) and remained there throughout

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storage. This observation led to the conclusion that there was a synergistic effect between HPP and

AP since the reduction was higher than the sum of the individual reductions observed for these two

technologies. A study by Jofré, Garriga, & Aymerich, (2007) showed that high pressure combined

with interleavers containing bacteriocins resulted in higher reduction of L. monocytogenes (about 4

log CFU/g) compared to when antimicrobial packaging was applied alone, in ham slices during a two

month storage. In the same way, antimicrobial packaging containing enterocins followed by high

pressure was able to postpone the growth of L. monocytogenes when applied individually, however

only the combination of both was able to achieve a reduction of inoculated levels without recovery

during storage of cooked ham (Marcos et al. 2008). Similar results have been observed for other

pathogens. The combination of HPP (400 MPa for 10 min) with interleavers containing nisin led to

the absence of Salmonella spp. in 25 g of sample after 24 h in sliced cooked ham with an initial

inoculum level of 4 log CFU/g, which could not be achieved with individual treatments.

Understanding the mode of action of the different food preservation technologies that potentially

could be applied simultaneously can help in the selection of meaningful combinations. Recent

research has shown that the mode of action of essential oils has to do with the disruption of cell

membranes caused by compounds in the essential oils (Burt, 2004). This mechanism may result in

membrane expansion, increased fluidity and permeability, alteration of transport processes,

disturbance of embedded proteins in the cytoplasmic membrane (Baby & George 2009). Also,

essential oil compounds might be able to cross the membrane and thus interfere with cell

metabolism. The cell membrane of microorganisms can also be damaged by pressure in

microorganisms resulting in leakages and increased uptake of compounds (Patterson, 2005).

Therefore, the synergistic effect observed might be explained by the fact the site of action for both

methods is the cell membrane.

3.3. Fate of L. monocytogenes during storage at 8oC

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In various occasions, domestic as well as commercial fridges may operate at a higher temperature

than the recommended. Therefore, an abuse temperature was included in the experimental design.

The abuse temperature chosen to be used was 8oC, which was found to be the temperature in 75%

of domestic fridges (EFSA, 2007). At the abuse storage temperature a very different trend was

observed (Figure 1b). For HPP alone, after the immediate reduction of the counts, the pathogen

grew exponentially without an apparent lag phase and reached approx. 8.13 log CFU/g at day 13. It is

noteworthy, that the counts for the HPP treatment were always higher (P < 0.05) compared to the

AP and HPP/AP after day 6. When applied alone, the antimicrobial packaging (AP) was able to delay

the growth of the pathogen until day 6 of storage (approx. 4.6 log CFU/g). Moreover, when HPP and

the active packaging were used in combination a delay in the growth of the pathogen was also

observed. In fact, the HPP/AP treatment reduced the counts by a further 0.53 log CFU/g at day 6,

resulting in a statistically significant difference between HPP/AP and AP. The counts of AP and

HPP/AP were not statistically different from day 13 onwards showing that the combination of the

two methods was not able to provide any additional protection against the pathogen after that

point. Although the combination of the two methods provided a greater reduction of L.

monocytogenes, it is obvious from the results that the clear synergism observed between higher

pressure processing and antimicrobial packaging at 40C was not evident at 80C.

The study of Hereu et al. (2012) showed that the combination of HPP and active packaging

containing nisin was able to provide a wider margin of safety against L. monocytogenes in RTE cured

ham compared to individual treatments throughout storage at 8oC. However, the product studied

did not support the growth of L. monocytogenes. A different study by Marcos, Aymerich , Garriga, &

Arnau (2013) found that HPP in conjunction with antimicrobial packaging containing nisin did not

result in any extra protection against L. monocytogenes compared to antimicrobial packaging alone.

The lack of any additional effect was attributed to the low water activity and the presence of lactate

in the food matrix tested (i.e. fermented sausages), that exerted a protective effect against HPP . In

the present study, it seems that the 80C temperature did not allow a synergistic effect to be

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observed by promoting growth and possibly by facilitating recovery of cells from sublethal injuries. It

has been shown that high storage temperature enables L. monocytogenes injured cells to speedily

repair both their cytoplasmic membranes and internal physiologies, leading to a fast and efficient

recovery (Bull et al. 2005). 

3.4. Effect on colour of RTE chicken during storage at 4 and 80C

Colour parameters for samples stored at 40C are presented in Table 2. In general, chicken breast

meat showed a bright colour for all treatments and during storage with L* values ranging from 80.70

to 83.48. For storage at 40C, significant interaction between treatment and storage time was

observed only for b* values. L* and b* values did not show a clear trend during storage for all three

treatments whereas a* values increased at the end of storage. No effect on colour (L* and a*) was

evident between different treatments for all sampling days. The b* values of the HPP treatment

were found to be higher only at the end of storage (day 60) compared to AP and HPP/AP.

For samples stored at 8oC (Table 3), significant interaction between treatment and storage time was

observed for a* and b* values. L* values showed very similar trends during storage between all

treatments with AP and HPP/AP treatments showing significantly higher values compared to HPP

throughout storage. Moreover, an increase of a* values was observed during storage for all

treatments. Again, for this storage temperature, b* values for the HPP treatment were found to be

higher at the end of storage compared to AP and HPP/AP. b* values for AP and HPP/AP showed a

decreasing trend during storage. In general, it was shown that active packaging when applied alone

or in combination with high pressure was able to better preserve lightness (L*) and keep yellowness

(b*) to lower levels at both storage temperatures.

Colour is one of the most important quality characteristics in meat products that heavily influence

consumer acceptance and packaging used should not affect the colour of the meat. Generally, the

application of high pressure affects the colour of raw meat such as chicken beef muscle, cod and

mackerel, as well as meat products (e.g. sausages) (Ohshima, Ushio, & Koizumi, 1993; Crehana,

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Troya, & Buckley, 2000; Serra, Grèbol, Guàrdia, Guerrero, Gou, Masoliver, et al. 2007; Kruk., Yun,

Rutley, Lee, Kim, & Jo, 2011). The discolouration of meat products is a result of myoglobin

denaturation, and/or heme displacement or release (Carlez, Veciana-Nogues, & Cheftel, 1995; Mor-

Mur & Yuste, 2003). However, literature has shown that cooked RTE meat products (e.g. ham) are

less prone to colour changes due to pressurisation (Goutefongea, Rampon, Nicolas, & Dumont,

1995; Hayman, Baxter, O’Riordan, & Stewart 2004; Pietrzak, Fonberg-Broczek, Mucka, & Windyga,

2007) which is consistent with this study since the L*, a* and b* values of non-treated chicken

samples evaluated initially (3.1.) did not show any statistically significant differences compared to

HPP samples (p < 0.05). This can be explained by the fact that in previously cooked meat

products, myoglobin turns into the pigment nitrosyl-haemochromogen, which is not

affected by pressure (Mor-Mur & Yuste 2003). Here, overall the active packaging with

coriander essential oil did not introduce any considerable changes on the colour of the

chicken breast meat.

3.5. Effect on lipid oxidation during storage

The lipid oxidation of the RTE chicken breast in this study was monitored throughout the storage

period using the TBARS assay. Results showed that there was interaction between treatment and

storage time for samples stored at 4 and 8oC. TBARS did not provide any clear or consistent trend of

lipid oxidation during storage at either storage temperatures (Table 4). The drop in the profile of

TBARS values during storage observed here can be explained by the fact that malonaldehyde formed

during storage might undergo intermolecular reactions, including polymerization as well as reactions

with other constituents in the meat, especially amino acids and proteins. Therefore, the rate of

malonaldehyde consumption during storage may surpass the rate of formation through lipid

oxidation, resulting in a decrease in TBARS values (Jamora and Rhee, 2002).

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Significant differences in the TBARS values after one day of storage (Day 1) were observed only at

8oC. At this temperature, AP and HPP/AP samples had higher values than the HPP treated ones. This

could be potentially attributed to the high amounts of unsaturated fatty acids present in coriander

essential oil (from leaves, seed, pericarp and fruit) which promote oxidation (Neffati and Marzouk

2008; Sriti, Talou, Faye, Vilarem, & Marzouk 2011; Sriti, Talou, Wannes, Cerny, & Marzouk 2009).

However, this effect was not observed at 4oC, therefore we cannot state conclusively that the high

amount of unsaturated fatty acids of the essential oil affected lipid oxidation in chicken meat.

Since coriander essential oil has shown to possess both antimicrobial and antioxidant activity,

investigating if the oil coated to the packaging film could delay or inhibit lipid oxidation in

pressurised samples during long term storage was appropriate. The antioxidant activity of coriander

essential oil has been associated with the presence of compounds such as limonene, α-pinene,

camphor and geraniol (Darughe, Barzegar, & Sahari, 2012). No clear inhibitory effect against lipid

oxidation was evident due to the presence of the active film when HPP and HPP/AP were compared.

This might relate with the relatively small quality used for coating of the film and the delivery system

but is something that can be optimised further. However, the study of Camo, Beltran, & Roncales,

(2008) showed that specific active films with oregano extract showed a significant effect against lipid

oxidation during storage in lamb meat prolonging both fresh colour and odour without disclosing

details about the system used.

In general, results showed that the level of lipid oxidation in the samples was low in all treatments at

both storage temperatures compared to other studies (Sampaio et al. 2012). This is most likely

attributed to the fact that unsaturated fat and myoglobin levels in chicken breast are lower

(Bragagnolo, Danielsen, & Skibsted, 2006) in comparison with other meat types. Another cause for

the low oxidation observed was the type of packaging, i.e. the exclusion of oxygen due to vacuum.

This is consistent with the study of Wiggers, Kröger-Ohlsen, & Skibsted (2004), who found that HPP

chicken breast had higher oxidation levels in air than in vacuum, and Martínez, Djenane, Cilla,

Beltrán, & Roncalés (2006), who stated that lipid oxidation is dependent on the oxygen

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concentration for pork sausages. The quality of meat is greatly affected by lipid oxidation and it has

been shown that high pressure processing may trigger lipid oxidation depending on temperature,

pressure level and duration of the process. It has also been established that critical pressure levels

that can induce lipid oxidation in chicken meat, assessed by TBARS, are between 300 and 600 MPa

(Orlien et al. 2000; Bolumar et al. 2011). In this study, the treatment used did not seem to result in

high levels of lipid oxidation.

4. Conclusions

The combination of high pressure processing and an active film based on coriander essential oil led

to a synergistic effect against L. monocytogenes allowing for application of milder processing

conditions (i.e. 500 MPa for 1 min) during storage at 4oC. However, the synergistic effect was not

observed at 8oC showing that control of the storage temperature is still necessary in order to reduce

the risk from the pathogen. No negative effect on the colour of the chicken was observed due to the

active film. Furthermore, it appears that the incorporation of AP in the treatments (AP and HPP/AP)

was able to better preserve the colour of meat during storage. Lipid oxidation levels also remained

at very low levels throughout storage for both storage temperatures. In conclusion, the use of active

film based on coriander essential oil, followed by in package pressurisation can improve the

efficiency of the HPP treatment and could be considered as a good strategy to further enhance the

safety of RTE products during extended storage at refrigeration (≤40C) temperatures.

Acknowledgements

The research leading to these results has received funding from the European Community’s Seventh

Framework Programme FP7, Theme KBBE.2011.2.1-01, research project STARTEC: “Decision Support

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Tools to ensure safe, tasty and nutritious Advanced Ready-to-eat foods for healthy and vulnerable

Consumers”, grant agreement No. 289262.

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