Agriculture Reference
In-Depth Information
properties, haze, and film blocking. Interestingly, a low
haze of 7% could be achieved when a blended HDPE/mPE
layer was placed at the core layer (Chinsirikul et al., 2008b).
The above examples demonstrated the benefits of
polymer blending and processing methods in producing
films with controlled gas permeation and other required
properties. The resulting films exhibited high OTR of
>
10,000 cm
3
m
−
2
day
−
1
(PO
2
>
300 cm
3
mm m
−
2
day
−
1
atm
−
1
with a permeability ratio (PCO
2
:PO
2
,
β
)of
∼
3-4.
Moreover, gas transmission of plastic films can also be in-
creased by addition of inorganic additives (e.g., CaCO
3
)
into the matrix, as well as creation of microperforation
and/or micropores in the structure.
A mechanism in controlling gas permeation through per-
forated films is mass flow through holes. It has been re-
ported that microperforated films have permeabilities many
orders of magnitude higher than those of regular poly-
meric films, with permeability ratios of CO
2
:O
2
between
0.8-1.0 (Anderson, 1989). To date, microperforated film
technologies and porous film products are available, but
these technologies are mainly owned by the manufacturers
and are therefore considered proprietary. These include, for
instance, P-Plus, FreshHold, and Fresh Pak (Zagory, 1997),
with a microperforation diameters of 40-200
μ
m. Devel-
opment of microperforated biaxially oriented polypropy-
lene (BOPP) films with varying numbers of micropores
(
∼
50
μ
m pore diameter) could result in a wide range of
OTR and WVTR, as shown in Table 7.1. As reviewed
by Sandhya (2010), microperforated films could be suc-
cessfully used to extend shelf life of several high-respiring
fruits. For fresh tropical/subtropical fruits, early study ex-
hibited clear benefits of high gas-permeable film in retain-
ing freshness quality and prolonging shelf life of 'Kluai
Khai' banana for 30 days (Chaiwong et al., 2005). After
removing bananas from the package, they developed a nor-
mal ripening process with no off-odor and no off-flavor.
Microperforated BOPP films were also effective in extend-
ing shelf life of rambutan to 14 days, as compared to 3 and
8 days of normal BOPP film and BOPP film with macro-
holes, respectively (Winotapun et al., 2010a). Low weight
loss of
permeability of microperforated films were carried out by
Kerddonfag et al. (2008) and Winotapun et al. (2010b);
their method used an OTR instrument containing a coulo-
metric sensor and a developed ultra-high gas-permeable
membrane. The ultra-high permeable membrane was used
as a backup substrate of the sample to help control pres-
sure drop and prevent short-circuits (or undesirable leakage
flow) of the carrier gas through the microholes.
The gas transport of polymers could be greatly enhanced
by creating an interconnecting network of microporous
structures upon stretching mineral-filled polymers (e.g.,
PE/CaCO
3
) or polymers containing specific crystal struc-
ture, for example, beta form of PP crystal (Hale et al.,
2001; Chu and Kimura, 1996). Ultra-high gas-permeable
films/membranes with an OTR of 40,000-10,000,000 cm
3
m
−
2
day
−
1
(Table 7.1) have been developed by stretching
PP containing specifically controlled beta crystal structure
(Kerddonfag et al., 2007, 2010). Most polymer films offer
high permeability ratio of carbon dioxide-to-oxygen (e.g.,
β
ratio
is useful for CO
2
-sensitive fruits where the package allows
CO
2
to escape faster than O
2
can enter. Conversely, films
with low PCO
2
:PO
2
ratios are more applicable to fruits re-
quiring a high in-pack carbon dioxide level as to inhibit
microorganisms. It should be stressed that in addition to
the ratio of PCO
2
to PO
2
, the final headspace gas composi-
tion is also affected by the film's permeability for each gas.
Experiments by Kerddonfag et al. (2007) and Chinsirikul
et al. (2009) utilized various sizes of the ultra-high OTR
membrane (OTR in a range of 300,000-10,000,000 cm
3
m
−
2
day
−
1
with low PCO
2
/PO
2
) as a breathable window, at-
tached to the low-permeable package (i.e., a normal BOPP
bag or tray with BOPP lidding film) (Fig. 7.6). The re-
sults showed that in-package O
2
and CO
2
could be practi-
cally controlled by changing the property and/or area of the
breathable membrane and desirable optimum atmosphere
(EMA) could be achieved.
of 2-4 for PE- and PP-based films); such a high
β
Ethylene-absorbing and ethylene-permeable films
Ethylene is a plant hormone and considered a ripening-
promoting substance for fresh fruits and vegetables. Typi-
cally in fresh fruit packaging, if ethylene accumulates in the
package at even a trace amount (at ppm level), it accelerates
the respiration rate and promotes ripening which can lead to
maturity, senescence, and reduced shelf life (Zagory, 1995).
It has been suggested that ethylene production can be re-
duced by half at an oxygen level of
<
2% and delayed color change of the skin were
observed.
In designing suitable MAP based on microperforated
films, information on film permeability is essential. To date,
no general or standard measuring methods exist to deter-
mine gas permeation through films containing microperfo-
rations. Simple determination of O
2
and CO
2
permeability
under real conditions of packaging respiring foods was
reported by Ozdemir et al. (2005). Other investigations
to develop a practical method for measuring O
2
and CO
2
2.5% (Sandhya, 2010).
The benefits of MAP and cold storage have been found
in reducing ethylene production and activation (Beaudry,
1999), but MAP alone is not adequate to control ethylene
∼
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