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Keywords:

  • antimicrobial films;
  • carvacrol;
  • cinnamaldehyde;
  • organic leafy greens;
  • Salmonella

Abstract

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgments
  8. References

The objective of this study was to investigate the antimicrobial effects of carvacrol and cinnamaldehyde incorporated into apple, carrot, and hibiscus-based edible films against Salmonella Newport in bagged organic leafy greens. The leafy greens tested included organic Romaine and Iceberg lettuce, and mature and baby spinach. Each leafy green sample was washed, dip inoculated with S. Newport (107 CFU/mL), and dried. Each sample was put into a Ziploc® bag. Edible films pieces were put into the Ziploc bag and mixed well. The bags were sealed and stored at 4 °C. Samples were taken at days 0, 3, and 7 for enumeration of survivors. On all leafy greens, 3% carvacrol films showed the best bactericidal effects against Salmonella. All 3 types of 3% carvacrol films reduced the Salmonella population by 5 log10 CFU/g at day 0 and 1.5% carvacrol films reduced Salmonella by 1 to 4 log10 CFU/g at day 7. The films with 3% cinnamaldehyde showed 0.5 to 3 log reductions on different leafy greens at day 7. The films with 0.5% and 1.5% cinnamaldehyde and 0.5% carvacrol also showed varied reductions on different types of leafy greens. Edible films were the most effective against Salmonella on Iceberg lettuce. This study demonstrates the potential of edible films incorporated with carvacrol and cinnamaldehyde to inactivate S. Newport on organic leafy greens.

Practical Application

Antimicrobial edible films made from apples, carrots, and hibiscus calyces can be used by the food industry to inactivate Salmonella in bagged organic leafy green salads. This study provides a scientific basis for large-scale application of edible fruit- and vegetable-based antimicrobial films on foods to improve microbial food safety.


Introduction

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgments
  8. References

The consumption of organic leafy greens has increased in recent years due to health concerns from consumers (Winter and Davis 2006). Previous studies reported that organically grown produce samples had higher microbial diversity and indicator bacterial population than conventionally grown samples (Oliveira and others 2010; Wetzel and others 2010). The increased demand for organic produce may raise the risk of foodborne illness outbreaks owing to the consumption of contaminated produce. In the United States, organic food production is regulated by the U.S. Dept. of Agriculture's National Organic Program. The chemical antimicrobials allowed in the post-harvest treatment of organic produce are very limited. There is a need for studying the use of natural antimicrobials which are generally recognized as safe (GRAS) ingredients to control foodborne pathogen contamination on organic produce.

A number of studies have investigated the effectiveness of natural plant compounds against foodborne pathogens. For example, citron oil was effective against Salmonella Enteritidis, Escherichia coli, and Listeria monocytogenes in fruit-based salads (Belletti and others 2008). Kim and others (2011) reported that 10% clove extract reduced Salmonella Typhimurium and E. coli O157:H7 populations on lettuce leaves by 4 log10 CFU/g. Oregano oil and its main ingredient, carvacrol, and cinnamon oil and its main ingredient, cinnamaldehyde, were effective against Campylobacter jejuni, E. coli O157:H7, L. monocytogenes, and S. enterica in laboratory media (Friedman and others 2002) and in apple juice (Friedman and others 2004). Among 11 plant essential oils evaluated against foodborne pathogens and spoilage bacteria on ready-to-eat vegetables, oregano and thyme essential oils had the greatest activity against all the tested bacteria (Gutierrez and others 2008). In our previous studies, carvacrol and cinnamaldehyde showed antimicrobial activity against antibiotic-resistant C. jejuni and S. enterica in vitro and on celery and oysters (Ravishankar and others 2008, 2010).

Edible films and coatings can serve as carriers for various food additives, including antimicrobials. The incorporation of antimicrobial compounds into edible films or coatings provides a novel way to control foodborne pathogen contamination (Cagri and others 2004). This suggestion is reinforced by the following published studies. Cinnamon, clove, and lemongrass oils or their active compounds cinnamaldehyde, eugenol, and citral incorporated into alginate films reduced E. coli O157:H7 population by 4 logs on fresh cut Fuji apples (Raybaudi-Massilia and others 2008b). Malic acid and essential oils and their main active compounds incorporated into an alginate-based edible coating significantly reduced S. Enteritidis population in inoculated fresh-cut melon (Raybaudi-Massilia and others 2008a). Lemongrass oil (1.0% to 1.5%) and oregano oil (0.5%) containing apple puree-alginate edible coatings exhibited strong antimicrobial activity against L. innocua, reducing the bacterial population to below the limit of detection within 7 d (Rojas-Graü and others 2007). The use of 1.5% to 2.0% chitosan in the methyl cellulose coating of fresh-cut cantaloupe reduced the growth of mesophilic aerobe, psychrotrophs, lactic acid bacteria, and total coliform by 3 to 4 log CFU/g (Krasaekoopt and Mabumrung 2008).

In our previous studies, the antimicrobial efficacy of carvacrol and cinnamaldehyde in apple-based edible films against S. enterica, E. coli O157:H7, and C. jejuni on chicken breast surfaces as well as against L. monocytogenes on ham and bologna surfaces was evaluated (Ravishankar and others 2009; Mild and others 2011; Ravishankar and others 2012). Carvacrol and cinnamaldehyde are GRAS ingredients for human consumption, and can be used as additives in organic leafy green production. The objective of this study was to investigate the antimicrobial effects of carvacrol and cinnamaldehyde incorporated into apple, carrot, and hibiscus-based edible films against Salmonella Newport in bagged organic leafy greens.

Materials and Methods

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgments
  8. References

Bacterial culture and media

Salmonella enterica serovar Newport was used for this study. Stock culture of the organism was maintained in cryovials (MicrobankTM Austin, Tex., U.S.A.) at –80 °C and activated by transferring 100 μL into tryptic soy broth with 0.6% yeast extract (TSBYE; Difco, Sparks, Md., U.S.A.). The bacterial cultures were maintained in TSBYE at 4 °C with biweekly transfers. For experimental use, an overnight culture of the organism was grown in TSBYE at 37 °C for 18 to 24 h. All dilutions were made in 0.1% peptone water (Difco). Enumerations for S. Newport were done by plating on xylose lysine desoxycholate agar (XLD; Difco).

Food products, antimicrobials, and edible films

Four types of organic leafy green samples were tested in this study: Romaine lettuce, Iceberg lettuce, mature spinach, and baby spinach. They were obtained from local grocery stores in Tucson, Ariz., U.S.A. The tested antimicrobials included carvacrol and cinnamaldehyde incorporated into apple, carrot, and hibiscus films at 0.5%, 1.5%, and 3.0% concentrations. The films were made at the USDA-ARS-WRRC facility in Albany, Calif., U.S.A. as described in our previous paper (Ravishankar and others 2012). Briefly, apple, carrot, or hibiscus purees were mixed with pectin solution and glycerin, then carvacrol or cinnamaldehyde was added into the mixture to obtain final antimicrobial concentrations of 0.5%, 1.5%, and 3.0%. These film solutions were poured on a polyester film, cast with a draw down bar, and cut into 5-cm diameter circles.

Antimicrobial activities of films against S. Newport on organic leafy greens

Produce samples were washed thoroughly under deionized water. After washing, adult spinach, Romaine, and Iceberg lettuce were cut into 1-cm-wide slices. For baby spinach, whole pieces were used. Ten grams of each produce was weighed for each sample. Samples were put under UV light in a biohood for 1 h to reduce spoilage organisms, and then immersed in 107 CFU/mL S. Newport culture solution made in buffered peptone water (BPW, Difco) for 2 min. After inoculation, samples were dried inside a biohood for 1 h. Each sample was put into a Ziploc bag. Eight pieces of edible film (0.3 g total weight) were put into the Ziploc bag with the produce sample and mixed well. The bags were sealed and stored at 4 °C. Samples were taken at day 0, 3, and 7 to enumerate the surviving Salmonella populations. Samples were transferred to stomacher bags, and 90 mL of BPW was added into each bag. The samples were mixed using a stomacher (Lab-blender 400; Seward, London, U.K.) at normal speed for 1 min. Serial dilutions were made and plated on XLD agar. Enumerations were done after 18 to 24 h incubation at 37 °C.

Statistical analysis

Each experiment was repeated 3 times. Mean and standard deviations for the surviving bacterial populations for each sampling time point were calculated. Data were analyzed by one-way analysis of variance (ANOVA) using SAS 9.2 software (SAS Inst. Inc., Cary, N.C., U.S.A.). Tukey's test was used to determine differences between the various types of films and between the various concentrations of antimicrobials within each type of film at the 5% significance level.

Results and Discussion

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgments
  8. References

In our previous studies (Ravishankar and others 2009, 2012), carvacrol-containing films exhibited stronger antimicrobial activity against S. enterica, E. coli O157:H7, and L. monocytogenes on meat products than did cinnamaldehyde-containing films. The results from the present study were consistent with the findings from previous studies. The results are shown in Tables 1 to 4. All 3 types of films (apple, carrot, and hibiscus) containing 3% carvacrol reduced the Salmonella populations to below the detection limit (1 log CFU/g) at day 0 in all 4 types of leafy green samples. In baby spinach samples with apple films containing 3% carvacrol, there were 4.6 and 1.8 log reductions of Salmonella cells at day 3 and 7, respectively. Mature spinach with apple films containing 3% carvacrol also showed 2.0 and 2.1 log reductions of Salmonella populations at day 3 and 7, respectively, and the results were still significantly different (P < 0.05) from the results from the other apple films (0.5% to 1.5% carvacrol and 0.5% to 3% cinnamaldehyde). The efficacy of films containing 1.5% carvacrol varied with the type of film and the type of leafy green. For example, carrot films with 1.5% carvacrol showed 3 log reductions on Iceberg lettuce at day 0, and reduced the Salmonella population to below the detection limit at day 7. On Romaine lettuce, baby spinach and mature spinach, carrot films with 1.5% carvacrol caused 1 to 2.5 and 1 to 2 log reductions at day 0 and day 7, respectively. Hibiscus films with 1.5% carvacrol showed 1.2 log reductions on Iceberg lettuce at day 0 and 4 log reductions on day 7. On Romaine lettuce, baby spinach and mature spinach, hibiscus films with 1.5% carvacrol showed 1 to 2 and 1 to 2.3 log reductions at day 0 and day 7, respectively. For the films containing 0.5% carvacrol, only slight population reduction was observed on the samples irrespective of the type of film, leafy green or storage time.

Table 1. Populations of S. Newport (log CFU/g) in bagged Iceberg lettuce containing edible films
Type of filmDay 0Day 3Day 7
  1. Data are shown as mean ± SD, N = 3. Data with same superscript letters in the same column are not significantly different.

Apple film   
control5.83 ± 0.13AB5.27 ± 0.10AB5.02 ± 0.36AB
3% carvacrol<1.00 ± 0.00E<1.00 ± 0.00F<1.00 ± 0.00F
1.5% carvacrol3.46 ± 0.81D3.85 ± 0.29DE1.13 ± 0.23F
0.5% carvacrol5.49 ± 0.05ABC4.47 ± 0.17BCDE4.24 ± 0.45ABC
3% cinnamaldehyde5.11 ± 0.28BC4.37 ± 0.33BCDE2.33 ± 1.31DEF
1.5% cinnamaldehyde5.51 ± 0.32ABC4.57 ± 0.08ABCD3.82 ± 0.27BCD
0.5% cinnamaldehyde5.69 ± 0.06ABC5.72 ± 0.07A4.90 ± 0.22AB
Carrot film   
control5.92 ± 0.13AB5.16 ± 0.27ABC5.13 ± 0.14AB
3% carvacrol<1.00 ± 0.00E<1.00 ± 0.00F<1.00 ± 0.00F
1.5% carvacrol2.89 ± 0.65D1.66 ± 1.14F<1.00 ± 0.00F
0.5% carvacrol5.32 ± 0.10ABC4.55 ± 0.08ABCD4.40 ± 0.23ABC
3% cinnamaldehyde5.17 ± 0.18BC4.52 ± 0.10ABCD3.21 ± 0.27CDE
1.5% cinnamaldehyde5.45 ± 0.04ABC4.65 ± 0.13ABCD4.46 ± 0.02ABC
0.5% cinnamaldehyde5.61 ± 0.09ABC5.58 ± 0.09AB5.33 ± 0.20AB
Hibiscus film   
control6.11 ± 0.09A5.47 ± 0.24AB5.48 ± 0.23A
3% carvacrol<1.00 ± 0.00E<1.00 ± 0.00F<1.00 ± 0.00F
1.5% carvacrol4.88 ± 0.26C3.27 ± 0.91E1.82 ± 1.43EF
0.5% carvacrol5.72 ± 0.05AB5.33 ± 0.11AB4.95 ± 0.03AB
3% cinnamaldehyde5.69 ± 0.08ABC3.98 ± 0.66CDE<1.00 ± 0.00F
1.5% cinnamaldehyde5.90 ± 0.11AB4.35 ± 0.62BCDE1.13 ± 0.23F
0.5% cinnamaldehyde5.86 ± 0.06AB5.36 ± 0.07AB5.29 ± 0.19AB
Table 2. Populations of S. Newport (log CFU/g) in bagged Romaine lettuce containing edible films
Type of filmDay 0Day 3Day 7
  1. Data are shown as mean ± SD, N = 3. Data with same superscript letters in the same column are not significantly different.

Apple film   
control6.02 ± 0.09A5.39 ± 0.33ABC5.25 ± 0.22A
3% carvacrol<1.00 ± 0.00E<1.00 ± 0.00F<1.00 ± 0.00D
1.5% carvacrol4.94 ± 0.46BC4.65 ± 0.05CDE4.31 ± 0.26ABC
0.5% carvacrol5.67 ± 0.31AB5.14 ± 0.33ABCD4.79 ± 0.20AB
3% cinnamaldehyde5.41 ± 0.43ABC4.76 ± 0.20BCDE4.42 ± 0.35ABC
1.5% cinnamaldehyde5.76 ± 0.37A5.13 ± 0.32ABCD4.51 ± 0.60ABC
0.5% cinnamaldehyde5.87 ± 0.28A5.62 ± 0.08A5.48 ± 0.26A
Carrot film   
control5.98 ± 0.07A5.59 ± 0.04AB5.12 ± 0.05A
3% carvacrol<1.00 ± 0.00E<1.00 ± 0.00F<1.00 ± 0.00D
1.5% carvacrol4.81 ± 0.69C4.63 ± 0.76CDE3.11 ± 1.62BC
0.5% carvacrol5.89 ± 0.07A5.22 ± 0.20ABCD4.46 ± 0.04ABC
3% cinnamaldehyde5.45 ± 0.06ABC4.65 ± 0.24CDE4.00 ± 0.50ABC
1.5% cinnamaldehyde5.87 ± 0.10A5.43 ± 0.18ABC5.01 ± 0.43A
0.5% cinnamaldehyde5.81 ± 0.08A5.67 ± 0.17A5.26 ± 0.41A
Hibiscus film   
control5.93 ± 0.17A5.32 ± 0.15ABCD5.08 ± 0.05A
3% carvacrol<1.00 ± 0.00E<1.00 ± 0.00F<1.00 ± 0.00D
1.5% carvacrol3.73 ± 0.35D4.03 ± 0.54E2.79 ± 0.60C
0.5% carvacrol5.66 ± 0.09AB5.28 ± 0.16ABCD4.65 ± 0.22AB
3% cinnamaldehyde5.55 ± 0.18ABC4.50 ± 0.16DE3.80 ± 0.71ABC
1.5% cinnamaldehyde5.73 ± 0.15AB4.99 ± 0.05ABCD4.40 ± 0.15ABC
0.5% cinnamaldehyde5.75 ± 0.12AB5.54 ± 0.20AB5.21 ± 0.23A
Table 3. Populations of S. Newport (log CFU/g) in bagged baby spinach containing edible films
Type of filmDay 0Day 3Day 7
  1. Data are shown as mean ± SD, N = 3. Data with same superscript letters in the same column are not significantly different.

Apple film   
control6.08 ± 0.07A5.63 ± 0.22AB5.28 ± 0.30AB
3% carvacrol<1.00 ± 0.00E1.99 ± 0.41F3.51 ± 0.61E
1.5% carvacrol5.72 ± 0.02AB4.83 ± 0.13BCD4.56 ± 0.10BCD
0.5% carvacrol5.85 ± 0.19AB5.45 ± 0.20ABC5.09 ± 0.07AB
3% cinnamaldehyde5.60 ± 0.07AB4.80 ± 0.12BCD4.44 ± 0.39BCDE
1.5% cinnamaldehyde5.96 ± 0.12AB5.38 ± 0.14ABC4.43 ± 0.40BCDE
0.5% cinnamaldehyde6.12 ± 0.19A5.88 ± 0.21A5.78 ± 0.36A
Carrot film   
control6.10 ± 0.06A5.37 ± 0.15ABC5.09 ± 0.16AB
3% carvacrol<1.00 ± 0.00E<1.00 ± 0.00G<1.00 ± 0.00F
1.5% carvacrol2.66 ± 0.53D2.95 ± 0.89E3.90 ± 0.39DE
0.5% carvacrol5.74 ± 0.10AB5.17 ± 0.25ABCD4.99 ± 0.24AB
3% cinnamaldehyde5.45 ± 0.08BC4.62 ± 0.17CD4.51 ± 0.05BCD
1.5% cinnamaldehyde5.78 ± 0.05AB5.24 ± 0.25ABCD4.98 ± 0.10ABC
0.5% cinnamaldehyde5.93 ± 0.14AB5.62 ± 0.21AB5.35 ± 0.20AB
Hibiscus film   
control6.00 ± 0.05AB5.37 ± 0.09ABC5.07 ± 0.12AB
3% carvacrol<1.00 ± 0.00E<1.00 ± 0.00G<1.00 ± 0.00F
1.5% carvacrol4.91 ± 0.53C4.37 ± 0.18D4.02 ± 0.51CDE
0.5% carvacrol5.68 ± 0.06AB5.41 ± 0.05ABC5.16 ± 0.07AB
3% cinnamaldehyde5.73 ± 0.10AB4.81 ± 0.35BCD4.47 ± 0.48BCD
1.5% cinnamaldehyde6.06 ± 0.04A5.30 ± 0.45ABC4.94 ± 0.46ABC
0.5% cinnamaldehyde5.92 ± 0.09AB5.64 ± 0.17AB5.28 ± 0.28AB
Table 4. Populations of S. Newport (log CFU/g) in bagged mature spinach containing edible films
Type of filmDay 0Day 3Day 7
  1. Data are shown as mean ± SD, N = 3. Data with same superscript letters in the same column are not significantly different.

Apple film   
control6.12 ± 0.05ABC5.52 ± 0.05ABCD5.14 ± 0.06ABCDE
3% carvacrol<1.00 ± 0.00G3.44 ± 0.86G3.02 ± 0.91G
1.5% carvacrol5.26 ± 0.12E4.72 ± 0.09DEF4.35 ± 0.23F
0.5% carvacrol5.63 ± 0.10BCDE5.40 ± 0.21ABCDE5.36 ± 0.13ABC
3% cinnamaldehyde5.55 ± 0.16DE4.80 ± 0.08CDEF4.49 ± 0.10DEF
1.5% cinnamaldehyde5.75 ± 0.15BCDE5.15 ± 0.05ABCDEF5.20 ± 0.31ABCD
0.5% cinnamaldehyde6.00 ± 0.22ABCD5.69 ± 0.22A5.90 ± 0.06A
Carrot film   
control6.31 ± 0.14A5.56 ± 0.05ABC5.41 ± 0.08ABC
3% carvacrol<1.00 ± 0.00G<1.00 ± 0.00H<1.00 ± 0.00H
1.5% carvacrol3.88 ± 0.51F4.57 ± 0.02F4.44 ± 0.05DEF
0.5% carvacrol5.83 ± 0.12ABCD5.48 ± 0.08ABCD5.32 ± 0.16ABC
3% cinnamaldehyde5.78 ± 0.03ABCDE4.87 ± 0.17BCDEF4.67 ± 0.16CDEF
1.5% cinnamaldehyde5.97 ± 0.09ABCD5.47 ± 0.04ABCDE5.05 ± 0.29BCDEF
0.5% cinnamaldehyde6.18 ± 0.15AB5.83 ± 0.02A5.61 ± 0.12AB
Hibiscus film   
control6.10 ± 0.10ABC5.71 ± 0.21A5.55 ± 0.20AB
3% carvacrol<1.00 ± 0.00G<1.00 ± 0.00H<1.00 ± 0.00H
1.5% carvacrol4.05 ± 0.35F4.49 ± 0.39F4.47 ± 0.05DEF
0.5% carvacrol5.75 ± 0.03BCDE5.58 ± 0.05ABC5.48 ± 0.05AB
3% cinnamaldehyde5.63 ± 0.18CDE4.68 ± 0.09EF4.39 ± 0.10EF
1.5% cinnamaldehyde5.94 ± 0.17ABCD5.46 ± 0.49ABCDE5.15 ± 0.21ABCDE
0.5% cinnamaldehyde5.81 ± 0.03ABCD5.66 ± 0.26AB5.60 ± 0.21AB

Films containing cinnamaldehyde slightly reduced the Salmonella population at day 0 on all types of leafy green samples, while the results varied at day 7 for the different types of films and leafy greens. At day 7, hibiscus films with 3% cinnamaldehyde reduced Salmonella to below the detection limit on Iceberg lettuce, and hibiscus films with 1.5% cinnamaldehyde caused 4.3 log reductions on Iceberg lettuce. On other type of leafy green samples, hibiscus films with 3% and 1.5% cinnamaldehyde reduced the Salmonella population by 0.6 to 1.3 and 0.1 to 0.7 log CFU/g, respectively. Apple and carrot films with 3% cinnamaldehyde showed 0.7 to 2.7 and 0.6 to 1.9 log reductions, respectively, on different leafy green samples at day 7. Apple and carrot films with 1.5% cinnamaldehyde also showed 0 to 1.2 and 0.1 to 0.7 log reductions, respectively, at day 7. Films with 0.5% cinnamaldehyde did not have a bactericidal effect in most cases. These observations with films are also consistent with our previous study on the relative antimicrobial potencies of carvacrol and cinnamaldehyde against Salmonella in laboratory media. Ravishankar and others (2010) reported that carvacrol showed stronger activity than did cinnamaldehyde at low concentrations against S. enterica in PBS buffer.

Edible films with carvacrol and cinnamaldehyde had different bactericidal effects on different bacteria. Our previous study (Mild and others 2011) reported that apple films containing cinnamaldehyde were more effective against C. jejuni on chicken meat than carvacrol films. The variation in antimicrobial potencies of carvacrol and cinnamaldehyde may be related to their different mechanism of action. Studies showed that carvacrol dissolves in the phospholipid bilayer of cells and interacts with the cell membrane. This would cause expansion and destabilization of the membrane and could cause an increase in membrane fluidity and passive permeability (Ultee and others 2000, 2002). Carvacrol causes leakage of phosphate ions from cells of Staphylococcus aureus and Pseudomonas aeruginosa (Lambert and others 2001). Adenosine triphosphate release following disruption of E. coli cell membranes by carvacrol was confirmed with the aid of a different physical technique (Friedman 2006). On the other hand, cinnamaldehyde did not disintegrate the outer cell membrane. Instead, it interfered with the activity of some enzymes. For example, Wendakoon and Sakaguchi (1995) examined the action of cinnamaldehyde against Enterobacter aerogenes and hypothesized that its carbonyl group could bind to proteins, preventing the action of amino acid decarboxylases in cells.

For edible films containing the same antimicrobial concentrations, no significant differences in antimicrobial activities were observed among different types of films (apple, carrot, and hibiscus) on Romaine lettuce, baby spinach, and mature spinach, but some types of films did show significantly different (P < 0.05) antimicrobial activities on Iceberg lettuce. For example, carrot film containing 1.5% carvacrol caused greater reduction in Salmonella population than apple and hibiscus films containing the same concentration of carvacrol, while hibiscus film containing 1.5% to 3% cinnamaldehyde showed better antimicrobial activity than apple and carrot films containing the same concentration of cinnamaldehyde.

The results from the present study demonstrated that edible films have different antimicrobial activities on different types of leafy green samples. For example, the films were most effective on Iceberg lettuce. This may be due to differences in plant tissue morphology and hydrophobicity, since these factors may impact how deep the Salmonella cells and the antimicrobials could penetrate into the tissues, thereby affecting the interactions between Salmonella cells and antimicrobials. Moore-Neibel and others (2012) investigated the antimicrobial activity of lemongrass oil against S. Newport on Iceberg and Romaine lettuce, baby and mature spinach, and similar results were observed. Lemongrass oil showed best antimicrobial activity against S. Newport on Iceberg lettuce.

In our present study, edible film pieces were mixed with leafy green samples in sealed plastic bags. Since carvacrol and cinnamaldehyde molecules are volatile, the advantage of using edible films in salad bags is that the films can release carvacrol or cinnamaldehyde vapors constantly into the surrounding environment, thus maintaining constant activity. Although not all the sample leaves were directly in contact with the films, some types of films (for example, 3% carvacrol films) reduced the Salmonella cells to below the detection limit. This showed that the vapor might have had antimicrobial effects on Salmonella.

The following related previous studies provide a helpful frame of reference or perspective for the observations in the present study:

  • Cinnamon, thyme, and oregano oils exhibited vapor-phase activities against foodborne pathogens (López and others 2007).
  • Carvacrol vapors eliminated S. enterica on raw chicken (Burt and others 2007).
  • Chicken wrapped with carvacrol- and cinnamaldehyde-containing edible films was acceptable to a human taste panel (Du and others 2012).
  • Carvacrol in films was stable to long-term (up to 7 wk) storage at 4 °C (Du and others 2008a, 2008b).
  • Oregano and cinnamon oil containing edible films were active against S. enterica on agar both by direct contact with the bacteria and indirectly by vapors emanating from the films (Du and others 2009a, 2009b).
  • Carvacrol and cinnamaldehyde in vapor phase were effective against Salmonella and E. coli O157:H7 on sliced and whole tomatoes (Obaidat and Frank 2009).
  • Cinnamon oil exerted vapor-phase synergistic antimicrobial activity with chitosan against foodborne pathogens and fungi (Wang and others 2011).

We did not evaluate sensory properties of the organic leafy greens exposed to the antimicrobial vapors released from the edible films inside the salad bag. However, the following observations might be relevant to sensory aspects.

a. Oregano oil, in which carvacrol is the main bioactive component, is widely used as a salad dressing (Skandamis and Nychas 2000; Abdalla and Roozen 2001), and cinnamaldehyde is widely used as a food flavor (Friedman and others 2000).

b. Paired preference tests by a human panel indicated no difference between baked chicken pieces wrapped with apple and tomato films containing carvacrol and cinnamaldehyde compared to chicken wrapped with the same films without the antimicrobials (Du and others 2012).

Conclusion

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgments
  8. References

Edible films incorporated with natural plant antimicrobials can be used as additives into organic bagged salads to control pathogenic contamination. The results of the present study showed that edible films incorporated with natural plant antimicrobials effectively reduced S. Newport in contaminated bagged leafy greens. Films containing carvacrol exhibited better antimicrobial activity against S. Newport than did cinnamaldehyde-containing films. The tested films showed the greatest antimicrobial effects on Iceberg lettuce. These results suggest that the antimicrobial vapors emanating from the films could be retained in the sealed salad bags and maintain the antimicrobial activity. These results could provide the leafy green industry with an option for choosing novel functional food packaging. The sensory evaluation of salad samples exposed to the 3 edible films needs further investigation.

Acknowledgments

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgments
  8. References

This work was supported by the U.S. Dept. of Agriculture–Natl. Inst. of Food and Agriculture–Organic Research and Extension Initiative Competitive Grant nr. 2010-51300-21760.

References

  1. Top of page
  2. AbstractPractical Application
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusion
  7. Acknowledgments
  8. References
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