Edible Antimicrobial Coatings Inhibit Salmonella

When it comes to controlling foodborne pathogens, edible antimicrobial coatings may offer an inhibitory response to common sources of foodborne illness. Recently, Olaimat & Holley (2015) evaluated the efficacy of a coating comprised of k-carrageenan and chitosan with mustard extract, allyl isothiocyanate (AITC), and ethylenediamine tetraacetic acid (EDTA) against Salmonella in a raw poultry matrix.¹

First, the team assessed sinigrin-degrading bacterial myrosinase activity in five species of Salmonella. They produced a stock solution of sinigrin in Mueller-Hinton broth (Thermo Scientific) mixed to a final concentration of 1,000 mg/l sinigrin. To this, they added Salmonella cultures at a final yield of 6 log₁₀ CFU/ml Salmonella per tube. They incubated these with shaking for 21 days at 25ºC, removing and filter-sterilizing samples at various time points (0, 3, 7, 14, and 21 days) for quantification by high performance liquid chromatography (HPLC).

Olaimat & Holley report that the Salmonella serovars demonstrated sinigrin degradation of 50.2 to 55.9% by day 21. This agrees with previous studies regarding glucosinolate degradation by pathogens but reveals higher degradation rates: 2.4 to 2.7% per day in this study versus 1.3% per day previously. This increase could be a function of variations in strains, growth media, incubation temperature, or initial sinigrin concentration.

Table 1: Percentage Degraded Sinigrin

strain	3 days	7 days	14 days	21 days
control (no bacteria)	1.4 ± 0.2	1.1 ± 0.1	1.9 ± 0.3	2.3 ± 0.5
S. Typhimurium	16.4 ± 1.7	27.1 ± 4.1	36.0 ± 10.3	54.6 ± 7.5
S. Copenhagen	12.5 ± 3.1	27.6 ± 6.5	44.6 ± 2.2	51.1 ± 0.2
S. Enteritidis	15.3 ± 1.8	23.8 ± 0.5	13.2 ± 1.5	55.2 ± 4.2
S. Kentucky	8.3 ± 0.1	26.3 ± 1.9	40.4 ± 6.3	55.9 ± 5.9
S. Heidelberg	12.4 ± 0.2	21.5 ± 2.8	35.7 ± 2.9	50.2 ± 6.4

For the second portion of the study, the team prepared antimicrobial coatings using k-carrageenan (0.2%), chitosan (2%), and the three variable ingredients as previously developed² for a total of six treatments:

• mustard extract alone (250 mg/g),

• AITC alone (50 µl/g),

• EDTA alone (15 mg/g),

• mustard extract (250 mg/g) with EDTA (15 mg/g),

• AITC (50 µl/g) with EDTA (15 mg/g), and

• a control without antimicrobials.

They applied these coatings to fresh chicken breast fillets inoculated with Salmonella cocktail (10⁸ CFU/ml) before vacuum sealing and storing for 21 days. They took samples at various time points (5, 11, 16, and 21 days) for plating on appropriate media (plate count agar, XLD agar, MRS agar; all Thermo Scientific) and measured the pH of the chicken samples using an Accumet digital pH meter (Thermo Scientific).

Overall, Olaimat & Holley found that coatings containing mustard extract or AITC alone reduced Salmonella 2.3 log₁₀ CFU/g by 21 days. Adding EDTA boosted antimicrobial capacity and reduced Salmonella 2.3 and 3.0 log₁₀ CFU/g by 5 and 21 days, respectively. They also noted that these coatings reduced aerobic and lactic acid bacteria by 2.5 to 3 log₁₀ CFU/g after 21 days.

Salmonella (log₁₀ CFU/g) Viability by Coating

coating	0 days	5 days	11 days	16 days	21 days
control (uncoated)	6.05 ± 0.06	5.91 ± 0.23	5.97 ± 0.01	5.73 ± 0.02	5.84 ± 0.40
control (coated)	6.05 ± 0.06	5.37 ± 0.13	5.27 ± 0.25	5.22 ± 0.23	5.30 ± 0.13
AITC	6.05 ± 0.06	4.55 ± 0.08	4.26 ± 0.21	4.17 ± 0.37	3.74 ± 0.01
mustard	6.05 ± 0.06	4.79 ± 0.50	4.69 ± 0.90	4.02 ± 0.84	3.80 ± 0.23
EDTA	6.05 ± 0.06	5.38 ± 0.05	5.35 ± 0.07	5.22 ± 0.04	5.20 ± 0.06
AITC and EDTA	6.05 ± 0.06	3.81 ± 0.33	3.44 ± 0.38	3.01 ± 0.13	3.07 ± 0.33
mustard and EDTA	6.05 ± 0.06	3.73 ± 0.56	3.82 ± 0.40	3.53 ± 0.37	3.01 ± 0.01

Taken together, these findings support the efficacy of k-carrageenan and chitosan coatings that contain AITC or mustard extract in combination with EDTA for extending the shelf-life of chicken and improving food safety, particularly in the case of foodborne pathogen Salmonella.

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References

1 Olaimat, A.N. and Holley, R.A. (2015) ‘Control of Salmonella on fresh chicken breasts by k-carrageenan/ chitosan-based coatings containing allyl isothiocyanate or deodorized Oriental mustard extract plus EDTA.’ Food Microbiology 48: 83-88

2 Olaimat, A.N. et al. (2014) ‘Inhibition of Campylobacter jejuni on fresh chicken breasts by k-carrageenan/chitosan-based coatings containing allyl isothiocyanate or deodorized oriental mustard extract.’ International Journal of Food Microbiology 187: 77-82.