bioplasticsMAGAZINE_0902

From Science & Research
Use of Biopolymers
in Antimicrobial
Food Packaging
The demand for safe, minimally processed, ‘fresh’
food products presents major challenges to the
food-packaging industry to develop packaging concepts
for maintaining the safety and quality of packaged
foods. Recent outbreaks of foodborne pathogens such as
Escherichia coli O157:H7, Salmonella spp. and Listeria
monocytogenes continue to push for innovative ways to
inhibit microbial growth in foods while maintaining quality,
freshness and safety. As an additional hurdle to nonthermal
processes, antimicrobial packaging can play an
important role at reducing the risk of pathogen contamination
of minimally processed foods. Antimicrobial packaging
systems incorporate antimicrobials into the packaging
to prevent microbial growth on the surface of solid
foods and to reduce the need for excessive antimicrobials
in liquid foods. Currently, food application of an antimicrobial
packaging system is limited due to the availability
of suitable antimicrobials, new polymer materials, regulatory
concerns and appropriate testing methods.
Polylactic acid (PLA) is a biodegradable and compostable
polymer well known as suitable for different kind of
packaging of foods such as milk, water, bakery, cheese,
and produce. The special characteristics of PLA, such as
GRAS status (i.e. Generally Recognized As Safe (FDA)),
biodegradability and being a bio-resource put PLA in a
unique position for food applications. Pectin is a water
soluble, hygroscopic polymer. Pectin has been used as a
thickening, coating and encapsulating material. It can be
used as a vehicle to carry and deliver a variety of bioactive
substances. Relatively few studies have been reported on
the use of pectin or PLA, alone or in combination, as a
base packaging material for antimicrobial food packaging.
However, neither PLA nor pectin possess antimicrobial
properties; therefore, a natural antimicrobial called nisin
was combined with the polymers. Nisin is nontoxic, heat
stable and does not contribute to off-flavors. Additionally
it is commercially used in a variety of foods including dairy,
eggs, vegetables, meat, fish, beverages and cereal-based
products.
In this study, an extruded composite food packaging
film containing pectin and polylactic acid polymers was
developed. Nisin was loaded into pectin/PLA and PLA
films by a diffusion coating method post extrusion.
Experiments were conducted to evaluate the potential
use of these films in antimicrobial food packaging to
inhibit cells of pathogenic Listeria monocytogenes.
Listeria monocytogenes was used as a model in this
study because of recent implications in several fatal
outbreaks of foodborne illness. The presence of L.
monocytogenes in ready-to-eat foods is a special
concern for at-risk populations. The USDA has set a
zero tolerance level for L. monocytogenes in ready-toeat
food products. Microbial growth medium (pH 6.9),
orange juice (pH 3.8), and liquid egg white (pH 8.7) were
selected for this study because they represented neutral,
high acid and low acid foods, respectively.
PLA from NatureWorks, pectin and nisin (Nisaplin®)
from Danisco were used in this project. Adding pectin
to PLA slightly reduced film strength but increased film
flexibility. The addition of nisin to films had no effect on
the film thickness or other mechanical properties.
To simulate a test for films used to wrap a solid food,
each film sample was placed on a surface-inoculated
microbial growth agar plate, on which 106 CFU (Colony
Forming Units 1 ) per ml of L. monocytogenes were
seeded. The agar plates were incubated at 37 ºC for
24 h. Zones of inhibition were formed after incubation.
The larger the zone of inhibition indicated higher
antimicrobial activity of the film. Figure 1 indicated that
there was a zone of inhibition formed around a film
sample containing pectin/PLA/nisin. In contrast, there
was no zone of inhibition observed around the film with
PLA/nisin, indicating the PLA film lost nisin during the
coating process. Therefore, pectin played an important
roll at embedding nisin into the film.
When used in a liquid medium, nisin was gradually
released from pectin/PLA films causing an inhibition
30 bioplastics MAGAZINE [02/09] Vol. 4