Agriculture Reference
In-Depth Information
long been major materials used for packaging fresh fruits,
but their permeability to respiratory gases is still too low for
use with highly respiring fruits. Furthermore, PS and PET
films with lower permeability have been employed for fruits
with very low respiration rates. These facts clearly point out
the need to develop films with higher gas permeability; yet
a suitable permeability ratio of CO 2 :O 2 and water vapor
permeability should also be taken into account.
Developing packaging for prolonging the shelf life of
fresh produce has been a challenging task (Rooney, 1995a).
Effective packaging should create an EMA inside the pack-
age to retard undesirable changes due to respiration, tran-
spiration, and ripening, while at the same time anaerobic
respiration and fermentation must also be prevented. In
general, a high gas-permeable package is favorable. The
film material should have gas permeability properties well-
matched with the respiration rate of the packaged produce.
Each commodity has its own unique MA requirement and
different tolerance limits to O 2 and CO 2 , thus suitable pack-
aging materials should ideally be engineered for each item.
Available plastic films with suitably high oxygen trans-
mission for fresh tropical produce are rather limited (Yam
and Lee, 1995). Challenges in this area lie in the develop-
ment of films with greater permeability and a wider range
of permeability ratios (PCO 2 :PO 2 or
ity of the plastic phase and change the gas transmission
rates; and (3) films having pores/micropores or microper-
forated films (Rooney, 1995b; Zagory, 1997).
Several new resins offering high O 2 transmission have
been developed such as metallocene-catalyzed linear low-
density PE (mPE), or polyolefin plastomers and ultra-
low-density ethylene octene copolymers (Dow Chemical,
USA). Blending common LDPE or LLDPE with other
high-permeation polymers such as EVA or mPE is also
an alternative approach in enhancing film permeability
(Chinsirikul et al., 2002). The resulting blended films,
30 μ m thick, possessed a high OTR of greater than
10,000 cm 3
300 cm 3 mm m 2 day 1
atm 1 , exceeding that of conventional commercial PE
films). The processing parameters in film fabrication have
critical effects on crystallinity and orientation of polymers
thereby influencing gas permeation. LDPE films subjected
to chilled air cooling during film formation showed low
degree of crystallinity, and higher gas permeation of
m 2
day 1
(PO 2 >
30%
over that of LDPE films produced and cooled by normal
ambient air (Table 7.1). Another study by Ward et al. (2007)
also revealed that certain blends of octene-LLDPE and
LDPE blown films produced under high draw rate showed
enhanced O 2 permeation even with high degrees of molec-
ular orientation.
From a practical viewpoint, packaging films for fresh
fruits should ideally possess not only high gas transmission
but also an optimal balance of other key properties, that is,
mechanical integrity; clarity (haze of < 10%, or the lowest
possible) no fog or mist during use at chilled conditions;
and easy-opening film (minimal film blocking). The out-
standing properties of mPE film are high O 2 transmission
rate, low water vapor transmission rate (WVTR), low seal-
ing temperature, high clarity, and good toughness (Lipsitt,
1998). Blending with rigid polymers improves the modulus
of mPE films, while preventing film blocking and optimiz-
ing its clarity (Chinsirikul et al., 2008a). By controlling
the morphologies of the films' surfaces via proper mixing
and processing, it was feasible to regulate surface rough-
ness (on a nanoscale level) to balance low film blocking
and clarity. Another attempt indicated by the authors was
to explore an alternative practical approach in preventing
film blocking in PE- or PP-based films without the use of
inorganic antiblocking particulates.
Coextruding films with proper design composition and
layer arrangement appeared to be another practical route
in optimizing all property requirements for fresh produce
films. Among coextruded three-layer films, the layer ar-
rangement of HDPE/mPE blends, either as core or inner
layers, played a significant role in their final mechanical
) than existing com-
mercial films. For fresh produce packaging, critical gas
permeations to be considered are oxygen, carbon dioxide,
ethylene, and water vapor. Most PE and PP films commonly
used in MAP of fresh produce possess oxygen transmis-
sion rates (OTR) ranging from 1,500-7,500 cm 3 m 2 d 1
(PO 2 40-220 cm 3 mm m 2 day 1 atm 1 ) with β of
2-3 (Table 7.1). These films can be effectively used only
for some low- to medium-respiring commodities. For trop-
ical and subtropical fruits exhibiting very high respiration
rates, such as mango, durian, banana, and so on, when pack-
aging these commodities using conventional films which
have too low gas permeation, in-package O 2 levels can fall
to very low concentrations, with the subsequent probabil-
ity of anaerobic respiration and fermentation. A minimum
level of 2-3% O 2 is thus recommended to ensure that poten-
tially hazardous conditions are not created (Bernard, 1987).
For this reason, there has been a great deal of commercial
interest in developing films with high gas transmission rates
(Lange, 2000).
β
High gas-permeable films
High gas-permeable films can be produced or designed
using three basic approaches: (1) films having high gas
permeability by virtue of their polymeric nature; (2) films
containing additives or fillers, which interrupt the continu-
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