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Novel Protein-Based Biopolymer
as a Coating in Food Packaging
Scientific Corner
Scientific Corner
Plastic packaging materials have been under attack due
to both the consumption of crude oil and the persistence
of non-biodegradable polymer resins. So called
biopolymers have been considered as a solution of these
problems depending onwhether being based on renewable
resources or biodegradable. Of course, biopolymers that
are both bio-based and biodegradable address both
accusations at the same time and have raised significant
interest in both academia and industry. A broad range of
biopolymers have been investigated for food packaging
such as polysaccharide-based, protein-based, lipid-based,
and combinations of these components. Formulating
natural antioxidants in food packaging systems has
prompted novel preservation techniques to provide safety
and security of food products. As it is well known, the
oxidation of lipids has been found to be associated with
coronary heart disease, atherosclerosis, cancer and aging
processes. Therefore, packaging that contributes to such
protection will meet consumer interest in healthier food
products.
In the current study, we investigated compounds based
on soy protein and modified lignin; we expected step-
change improvements in oxygen and water vapor barrier,
alongside anti-oxidative and UV-protecting properties
due to the chemical structure of the two blend
components. The soy protein was modified through
enzymatic and physical treatments under film forming
conditions
to
optimize
the
property
profile.
Transglutaminase has been used to catalyze the reaction
between lysine and glutamine aminoacids in the protein
structure to form peptide bonds, and by this, crosslinks
between neighboring protein chains (Figure 1).
Transglutaminase -the enzyme- is found in animal tissue,
body fluids, plants and microorganisms, and has been in
use in the food industry for years, e.g. in Japan for the
setting of ground fish paste (Surimi) since 1920’s. This
enzymatic treatment improved film properties such as
tensile strength, elongation at break, and water absorption
by 50% to 100%, and oxygen barrier by 75% compared to
the control film without the enzyme treatment.
Modified lignin, which is a byproduct of wood processing
industry, was added as a second blend component to
improve not only physicochemical, but also functional
properties of the film. These improvements are due to its
functional groups such as aromatic rings, methoxy
groups, phenol and alcohol groups, which can be used for
chemical reactions. The phenol functionality has been of
special interest in this study since it provides antioxidant
ability. Antioxidants of this kind are known to
prevent oxidation of organic polymers and have
been found to act as a preserving agent for the
packed food stuff. With such approach, Dr. Gómez-
Guillén and his research team at the Institute of
Science and Technology of Food and Nutrition,
Spain, have been able to store salmon muscle in
fish gelatin-lignin film preventing oxidation during
storage.
For the current research, two types of modified
lignin polymers were selected: alkali lignin (AL) and
lignosulphonate (LSS). Films containing alkali
lignin were opaque and the opacity increased as a
function of lignin concentration; on the other hand,
lignosulphonate (LSS) provided films with a high
level of transparency (Figure 2). The addition of
lignin improved tensile strength and thermal
stability of films, but
reduced elongation at
break as a function of
lignin concentration.
The solubility of both
the
enzymatically
modified soy protein
film and the modified
lignin in water-based
solution motivated us
to apply this biopolymer
blend as a coating on
polylactic acid (PLA)
film to test for barrier,
antioxidative and UV
blocking
properties.
Coating PLA film, treated by corona discharge, with the
current biopolymer blend reduced the oxygen and water
vapor transmission rates by about 5x and 8x, respectively,
compared to the neat PLA film (Table 1).
The coated PLA film was applied as the inner layer of a
model package for a sample of soy oil and tested for its
anti-oxidative properties. For this experiment, the soy oil
container had been placed in an oven at 40 °C for one
month to monitor the evolution of lipid oxidation according
to the method described by American Oil Chemists Society
(AOCS 1996). In addition, the generation of pentanal as a
tracer for the rancid smell of degrading soy oil has been
monitored. Results revealed that the soy protein / lignin
containing model package decreased the oxidation rate by
around 75% and reduced the occurrence of off-flavor tracer
by about 40% of the control.
In another experiment, soy oil package was covered by the
coated PLA film and placed under florescent light to
measure UV induced oil oxidation. It was observed that the
combined UV-blocking and anti-oxidative ability of PLA-
coated films containing alkali lignin caused a reduction in
oil oxidation rate (AOCS method) by again more than 75%
compared with the reference system.
Based on the observations above, the enzymatic treatment
of protein-based polymer can be a useful way to improve
the physical properties of protein-based biopolymer film.
The anti-oxidative and UV protecting properties of the bio-
based blend of modified soy protein and lignin add
considerably to the range of properties of biopolymers of
interest for the packaging industry. Both the protein
polymer alone and the lignin blend can be applied to a wide
spectrum of applications such as packaging of food
products, pharmaceutical, and agricultural industries.
Figure 2. Soy protein based film modified with
different concentrations of lignosulphonate (right)
and alkali lignin (left)
Heating/Cooling/
Enzyme
incubation
Lignin
MeO
OH
Lignin
C
H
3
CH
3
OH
C
H
3
SO
3
Na
Lignin
MeO
OH
Lignin
C
H
3
CH
3
OH
C
H
3
SO
3
Na
Lignin
MeO
OH
Lignin
C
H
3
CH
3
OH
C
H
3
SO
3
Na
Lignin
MeO
OH
Lignin
C
H
3
CH
3
OH
C
H
3
SO
3
Na
Lignin
MeO
OH
Lignin
C
H
3
CH
3
OH
C
H
3
SO
3
Na
Lignin
MeO
OH
Lignin
C
H
3
CH
3
OH
C
H
3
SO
3
Na
Lignin
MeO
OH
Lignin
C
H
3
CH
3
OH
C
H
3
SO
3
Na
Lignin
MeO
OH
Lignin
C
H
3
CH
3
OH
C
H
3
SO
3
Na
Lignin
Enzyme crosslinking
Lignin
MeO
OH
Lignin
C
H
3
CH
3
OH
C
H
3
SO
3
Na
Lignin
MeO
OH
Lignin
C
H
3
CH
3
OH
C
H
3
SO
3
Na
Lignin
MeO
OH
Lignin
C
H
3
CH
3
OH
C
H
3
SO
3
Na
Lignin
MeO
OH
Lignin
C
H
3
CH
3
OH
C
H
3
SO
3
Na
Table 1.
Mechanical and barrier properties of different plastics comparing to the PLA-coated film
Tensile Modulus
(MPa)
Elongation at Break
(%)
O
2
TR (cm
3
/.day)
65% RH
WVTR
(g/m
2
, day)
90% RH
PLA coated SPI
modified film
4250
27
15-30
5
EVOH*
96.7
238
1.08
126
PLA**
3834
4
149-165
40-55
BoPP***
2415
200
550
9
HDPE***
1000
600
600
5-10
*source: Material property data website
**source: Handbook of biodegradable polymers
***source:
Abbreviations: O
2
TR: oxygen transmission rate; WVTR: water vapor transmission rate; SPI: soy protein isolate; EVOH: ethylene vinyl alcohol; PLA: polylactic acid; BoPP: Biaxial oriented
poly propylene; HDPE: high density polyethylene
Figure 1.
Proposed mechanism of enzymatic crosslinking and lignin in structure of soy protein chain
1,2-3,4-5,6-7,8-9,10-11 14-15,16-17,18-19,20
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