the coatings is based on the leaching
of silver ions over time that are assimilated into the microbial cells where
the ions disrupt proteins (Figure 1). L.
monocytogenes exposed to silver has been
shown to initially decrease in number,
but regrowth occurs due to emergence
of resistant mutants.
16 In addition, silver
gradually leaches out of films, thereby
providing only short-term benefits. The
same disadvantages have been observed
for copper coatings, with additional
cytotoxic concerns being voiced.
17 Antimicrobial coatings have been prepared
using films impregnated with quaternary ammonium salts, chitosan, bacteriophages and bacteriocins. However, the
majority of coatings have been tested
only under laboratory conditions, with
very few being commercialized due to
economic feasibility and technical challenges.
Direct food-contact antimicrobial coatings
The key limitation of current food-
contact surface coatings is that many
have to be applied at the point of man-
ufacture of the equipment. Consequent-
ly, when the antimicrobial agent (e.g.,
silver) is depleted, the units have to be
disassembled, then sent for recoating.
An alternative approach is to apply an-
timicrobial paint. Although more con-
venient to apply compared with other
coatings, it cannot be used on food-
contact surfaces due to the risk of toxic
residues from the coating migrating into
18 In this regard, relatively
few coatings that include binders to ad-
here to substrates such as stainless steel,
plastic or rubber can be directly applied
on food-contact surfaces.
19 The U.S.
Food and Drug Administration regula-
tory approval for food-contact coatings
is described in 21 C.F.R. 177.168, which
essentially provides a list of permitted
polymer and resin materials.
Considering the permitted polymers/
materials for food-contact surfaces, sev-
eral antimicrobial coatings have been
developed, with those modified with
titanium dioxide being of particular
21 Anatase titanium dioxide is
widely used as a photocatalyst and anti-
microbial surface coating, among other
applications (Figure 1).
22 The underlying
mechanism relies on the formation of
free radicals from the breakdown of wa-
ter and oxygen following illumination
with UV light in the range of 254–395
nm (UV-C to UV-A).
Titanium dioxide has been introduced within a range of polymer bases,
which retain the antimicrobial agent
and also provide mechanical robustness
to the coating. For example, polyurethane modified with titanium dioxide
was assessed for inactivating a surface
inoculated with L. monocytogenes,
Salmonella, Pseudomonas aeruginosa and
23 Although the surface
had good durability, the log reductions
of the aforementioned bacteria were
limited to 0.5–1.0 log CFU despite illumination of the modified film with
Polylactide as a Food-Contact
Polylactide (PLA) is a biodegradable polymer produced from lactate
and lactide monomers derived from
the fermentation of raw materials such
as corn, sugar cane or other sources
of sugar. PLA has a generally regarded
as safe status and has been applied in
food-contact applications such as tea
bags, single-serve coffee containers, jugs,
cups, linings and coatings (Figure 2).
PLA can be coated on a range of materials, including paper, ceramic, glass,
rubber and stainless steel to produce
high-adherent coatings with mechanical
As a coating, PLA has a weak negative charge that prevents strong attachment of microbial cells and exhibits
a degree of antimicrobial activity.
Antimicrobial agents such as silver and
titanium dioxide can be incorporated
into PLA by inclusion during film formation or via surface modification.
PLA modified with silver ions has been
reported to support a 5-log CFU reduction of L. monocytogenes, Staphylococcus
aureus, E. coli and Salmonella, although
the leaching rate of silver ions was
32 PLA modified with titanium
Figure 2. Applications for Polylactide