Jung Han. Innovations in Food Packaging
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Quality of packaged foods 37
Processing
Cooling
Sttrlitoiton'
>\ Filling
Sterile environment
Seating
Storage
and
distribution
Figure 3.3 Schematic flow diagram of aseptic packaging system.
a packaging unit for
packaging.
Aseptic packaging offers advantages to the consumer
as well as distribution channels (for example, lower distribution and storage costs, an
extended shelf
life,
relief of pressure on chilled cabinets, cost effectiveness, and free-
dom from additives).
Sterilization of packaging material is a critical step in the aseptic packaging sys-
tem, and the method used should:
1.
Have rapid microbicidal activity
2.
Be compatible
with the
surfaces
treated,
especially packaging material and equipment
3.
Leave minimal residue - i.e. residue should be easily removable from surfaces
4.
Present no health hazard to the consumer
5.
Have no adverse effects on product quality in cases of unavoidable residue or erro-
neous high concentration
6. Present no health hazard to operations personnel around the packaging equipment
7.
Be compatible with the environment
8. Be non-corrosive to the surface treated
9. Be reliable and economical.
Various methods for the sterilization of packaging materials are currently used in
aseptic packaging
systems.
Hydrogen peroxide sterilization followed
by
hot air appears
to have the most potential for use as in-line sterilization of packaging materials.
Sterilization of packaging materials followed by ultraviolet irradiation has also been
accepted for industrial application. Ultraviolet irradiation promotes the breakdown of
peroxides into hydroxyl radicals, and the overall lethal effect is greater than the sum
of the effects of peroxide and irradiation alone.
Ethylene oxide has been used to pre-sterilize paperboard cartons (Hedrick, 1973)
and plastic packaging materials (Alguire, 1973). However, no commercial system
38 Innovations in Food Packaging
currently uses ethylene oxide as a sterilizing agent in the aseptic packaging of low-
acid foods. An important consideration in bulk sterilization of preformed packaging
materials is ethylene oxide's ability to permeate and contact all the surfaces that need
to be sterilized. Extensive aeration of the sterilized material is necessary to remove
residual ethylene oxide because of its toxic nature, and the length of time necessary
precludes its use in in-line applications. Other chemicals, such as peracetic acid, beta
propiolactone, alcohol, chlorine and its oxide, and ozone, have potential for use in
sterilizing aseptic packaging
materials.
However, peracetic acid produces off-flavor in
food if any residual deposit is enclosed in the container, and beta propiolactone lacks
the penetrating power of ethylene
oxide
- although it
is
considerably
more
active against
micro-organisms. Only 2-5
mg/1
of beta propiolactone is required for sterilization,
compared with 400-800
mg/1
of
ethylene
oxide. Ethanol is effective only against veg-
etative cells, and not against conidia or bacterial spores; therefore its use in packaging
of foods is limited to the extension of the shelf life of packaged foods which are nor-
mally stored under refrigeration.
Dry heat, saturated steam, and superheated steam can
be
effective sterilants, but the
degree of heat damages many packaging materials and so they have limited applica-
tion. Sterilization of metal cans and lids by saturated steam under pressure was used
as early as 1920 in the USA (Renter, 1988), and this method is today employed
to
ster-
ilize thermostable plastic cups. Saturated steam under pressure is used to sterilize
moulded polystyrene cups and foil lids at 165°C and 6 bars immediately after deep
drawing. At the same time, the external surface of the cup is cooled to limit the heat-
ing effect to the internal surface of the cups. Moist heat could cause blistering or
delamination of paper-based packaging materials and impair the heat-sealing charac-
teristics of
plastics.
Saturated steam is a preferred treatment method for sterilizing
metal-food contact surfaces downstream from the hold tube, including sterile hold
tank,
filters,
and
the
aseptic packaging
zone.
Superheated steam
was
the method used in
the 1950s for sterilization of
tin-plate
and aluminum cans and lids in the Martin-Dole
aseptic
canning
process.
This process
continuously
sterilizes tin-plate and
aluminum cans
by passing them through 220-256°C superheated steam at normal pressure for
45
sec-
onds (Larousse and Brown, 1997). Hot air is preferred as a sterilant over superheated
steam to
sterilize cardboard laminates
(i.e.
aluminum and
paper)
at 145°C for
3
minutes
(Renter, 1988). However, the method has only been found suitable for products that
have a pH < 4.6. An aseptic packaging system utilizing the heat co-extrusion process
for sterilization is the form-fill-seal packaging system, which relies on temperatures
reached by thermoplastic resin during the co-extrusion process used to produce multi-
layer packaging material to produce a sterile product contact surface. The high tem-
perature (180°C-230°C for up to 3 minutes) produced by extrusion produces sterile
surfaces. Reports as to the efficacy of this heating method vary considerably (Renter,
1988).
Aseptic
filling
into extruded containers appears suitable only for acidic products
with a pH less than 4.6. For products with a pH greater than 4.6, extruded containers
should be post-sterilized with hydrogen peroxide or with mixtures of peracetic acid.
Ultraviolet radiation at a wavelength of 253.7nm is an effective germicide against
yeast, moulds, bacteria, viruses and
algae.
Maunder (1977) suggested the use of high-
intensity ultraviolet light for aseptic packaging material sterilization. However, dust
Quality of packaged foods 39
particles present on the surfaces reduce the effectiveness of ultraviolet irradiation for
sterilization of
aseptic
packaging materials. Doyen (1973) described an aseptic pack-
aging machine that used both an alcohol bath and high-intensity ultraviolet radiation for
the sterilization of
flexible
pouches. Infrared
rays
(0.8-15 X10"^ waveband) have been
used
to
treat
the
interior of aluminum lids with
a
plastic coating on
the
exterior surface,
but
a
temperature
rise in the
packaging material
due to
infrared application results in soft-
ening of the plastics. Gamma irradiation
(25
kGy) has been used to sterilize plastic
bags for the bulk packaging of acid foods using an aseptic bag in box system (Nelson,
1984).
Ionizing rays are not acceptable, as they have harmfiil effects on personnel.
The technology of utilizing short pulses of light is attractive for sterilizing packag-
ing materials and processing equipment in aseptic packaging. The spectrum of light
used for sterilization purposes includes wavelengths from the ultraviolet to the near-
infrared region. The packaging material is exposed to at least one pulse of light hav-
ing an energy density in the range of about
0.01-50
Jcm"^ for
1
|JLS
to 0.2
s
at the
surface (Anonymous, 1994). It is desirable for certain aseptic processes that packag-
ing material be treated with pulses having a relatively high ultraviolet content to min-
imize the total fluence (Jcm~^) necessary to achieve the desired reduction in
microbial population (Dunn et
ah,
1991). Light pulses have not been adequately stud-
ied, so far as their effect on food material is concerned.
The versatility of aseptic technology has given rise to the use of a variety of plastic
and polyolefin materials for packaging, and examples commonly used in aseptic
packaging are presented in Table 3.3. There are several basic requirements that these
packaging materials must meet for successfiil application:
1.
The packaging material must be acceptable for use in contact with the intended
product, and must comply with applicable material migration requirements
Table 3.3 Functional attributes of some aseptic packaging materials
Material
Barrier property Durability
Oxygen Moisture Light Seal quality Stiffness Tear
and adhesion
Puncture
Linear-low density x
polyethylene
Low-density polyethylene x
Polypropylene x
Polystyrene
Polyvinyl id ine chloride x x
Ethylenevinyl alcohol x
Nylon
Ethyieneacrylic acid
Paperboard
Aluminum foil x x
Metallized film x
Source:
Encyclopedia
of
Packaging
Technolo^ (1997).
40 Innovations in Food Packaging
2.
Physical integrity of the package is necessary to assure contamination of the prod-
uct and maintenance of steriUty
3.
The package material must be able to be sterilized and be compatible with the
method of sterilization used (heat, chemical, or radiation)
4.
The package must provide the barrier protection necessary to maintain product
quality until it is used.
The structure and composition of aseptic packaging is more complex, and varies
depending on product application, package size, and package type. Factors such as
strength and integrity, package shape, stiffness, durability, and barrier properties deter-
mine the choice and/or combination of materials required. In most applications aseptic
packages incorporate more than one material in the structure, and these are assembled
by lamination or co-extrusion processes.
Conclusions
Food packaging is an important element in the development of a successful consumer
product. An effective package must prevent spoilage, and have oxygen, temperature,
light, and water-vapor barrier properties adequate to retard quality deterioration of
packaged foods. The packaging must withstand processing conditions and distribu-
tion abuse. Further research is still necessary to achieve microbiologically-safe pack-
aged foods, and maintain their nutritional value and original sensory quality. The shelf
life attained by packaged foods must be enhanced to allow distribution and marketing.
Studies on the influence of different packaging on the shelf life of processed foods
need to be carried out. In addition, modeling of the package atmosphere composition,
respiration rates, and internal atmospheres in the package throughout storage are of
major importance in designing appropriate packaging and predicting the shelf life of
packaged foods of different types.
References
Alguire, D. E. (1973). Ethylene dioxide gas sterilization of packaging materials. Food
TechnoL
27, 64.
Alves,
R. M. V, Van Dender, A. G. R, Jaime, S. B. M. and Moreno, I. (2002). Stability of
"Requeijao cremoso" in different packages exposed to light. Poster, Congrilait, 26th
International Dairy Conference, Paris, France, 24-27 September.
Anon (1974). Shelf life of
foods.
J. Food
ScL
39, 861.
Anonymous (1994). The Pure Bright
Process.
Pure Pulse Proprietary Information.
Beaudry, R. (1999). Effect of
O2
and CO2 partial pressure on selected phenomena affect-
ing fruit and vegetable quality. Postharvest
Biol.
TechnoL
15, 293-303.
Beaudry, R. (2000). Responses of horticultural commodities to low oxygen: limits to the
expended use of modified atmosphere packaging. Hort. TechnoL 10,491-500.
Quality of packaged foods 41
Beaudry, R. M., Cameron, A. C, Shiraji, A. and Dostal-Lange, D. L. (1992). Modified
atmosphere packaging of blueberry fruit: effect of temperature on package oxygen
and carbon
dioxidQ.
J.
Am.
Soc.
Hort.
Sci. 117,
436-441.
Bekbolet, M. (1990). Light effects on food. J.
FoodProt.
53,430^40.
Bosset, J. O., Gallmann, P. U. and Sieber, R. (1994). Influence of light transmittance of
packaging materials on the shelf life of milk and dairy products - a review. In: Food
Packaging and Preservation (M. Mathlouthi, ed.), pp. 222-268. Blackie Academic
and Professional, Glasgow, UK.
Bourne, M. C. (1987). Water Activity: Theory and Application to
Food,
Basic Symposion
Series, pp. 75-99. Marcel-Dekker, New
York,
NY.
Bremner,
A.
H., Olley, J. and
Vail,
A.
M. A. (1987). Estimating time-temperature effects by
a rapid systematic sensory method. In: Seafood Quality Determination
(D.
E. Kramer
and
J.
Liston, eds). Proceedings of an International Symposium (University of Alaska
Sea Grant College Program, Anchorage, Alaska, USA),
pp.
413-435. Elsevier Science
Publishers B.V, Amsterdam, The Netherlands.
Brody, A. L. (2003). Predicting packaged food shelf
life.
Food
Tech.
57, 100-102.
Dalgaard, P., Mejlholm, O. and Huss, H. H. (1997). Application of an iterative approach
for the development of a microbial model predicting the shelf-life of packed fish. Intl
J. Food
Microbiol.
38,169-179.
Deger, D. and Ashoor, S. H. (1987). Light-induced changes in taste, appearance, odor, and
riboflavin content of
cheese.
J. Dairy Sci. 70,1371-1376.
Devlieghere, E, Van Belle, B. and Debevere, J. (1999). Shelf life of modified
atmosphere packed cooked meat
products:
a predictive model.
IntlJ.
Food
Microbiol.
46,
57-70.
Doyen, L. (1973). Aseptic system sterilizes pouches with alcohol and UV Food
Technol.
27,49.
Dunn, J. E., Clark, R.
W.,
Asmus, J. F. et al. (1991). Methods and Apparatus for preserva-
tion of foodstuffs, US Patent no. 5,034,235.
Exama, A., Ami, J., Lencki, R. W., Lee, L. Z. and Toupin, C. (1993). Suitability of plastic
films for modified atmosphere packaging of fruits and vegetables. J. Food Sci.
58,1365-1370.
Farber, J. M.,
Cai,
Y.
and
Ross,
W.
H. (1996). Predictive modeling of the growth of Listeria
monocytogenes in CO2 environments. Ind
J.
Food
Microbiol.
32, 133-144.
Fernandez, P S., George, S. M., Sills, C. C. and Peck, M. W. (1997). Predictive model of
the effect of CO2, pH, temperature and NaCl on the growth of Listeria monocyto-
genes.
IntlJ.
Food
Microbiol.
32, 133-144.
Garcia-Gimeno, R., Castillejo-Rodriquez, A., Barco-Alcala, E. and Zurera-Cosano, G.
(1998).
Determination of packaged green asparagus shelf-life. Food
Microbiol.
15,
191-198.
Giacin, J. R. (1995). Factors affecting permeation, sorption and migration processes in
package-product systems. In: Foods and Packaging Materials. Chemical Interactions
(P.
Ackerman, M. Jagerstad and Y. Ohlsson, eds), p. 231. The Royal Society of
Chemistry, Cambridge, UK.
Gibson, D. M. (1985). Predicting the shelf life of packed
fish
fi-om
conductance measure-
ments.
J.
Appl.
Bacteriol.
58, 465-470.
42 Innovations in Food Packaging
Girard, B. and Nakai, S. (1994). Grade classification of canned pink salmon with static
headspace volatile patterns. J. Food
Sci.
59, 507-512.
Gvozdenovi, J., Popov-Ralji, J. and Curakovi, M. (2000). Investigation of characteristic
colour stability of powdered orange. Food Chem. 70,
291-301.
Hansen, E. (1996). Light-induced oxidative changes in dairy spreads. Project report.
Copenhagen, Department of Dairy and Food Science, The Royal Veterinary and
Agricultural University.
Hedric,
T.
L. (1973). Aseptic packaging in paperboard containers. Food
Technol.
11^
64.
Hong, C. M.,
Wendorfif,
W. L. and Bradley Jr, R. L. (1995a). Factors affecting light-
induced pink discoloration of annatto-colored cheese. J. Food
Sci.
60, 94-97.
Hong, C. M.,
Wendorff,
W. L. and Bradley Jr, R. L. (1995b). Effects of packaging and
lighting on pink discoloration and lipid oxidation of annatto-colored cheeses.
J. Dairy
Sci.
78, 1896-1902.
Hotchner, S. J. (1981). Container shelf Hfe methodology. In: Shelf Life: A Key to
Sharpening Your Competitive Edge Proceedings, p. 29. Food Processors Institute,
Washington, DC.
Jacobsson, A., Nielsen, T., Sjoholm, I. and Wendin, K. (2003). Influence of packaging
material and storage condition on the sensory quality of broccoli. Food Quality
Preference 15(4), 301-304.
Juric,
M., Bertelsen, G., Mortensen, G. and Petersen, M. A. (2003). Light-induced colour
and aroma changes in sliced modified atmosphere packaged semi-hard cheeses. Intl
Dairy
J.
13,239-249.
Kader,
A. A.
(1987). Respiration and gas exchange of vegetables.
In:
Post Harvest Physiology
of
Vegetables
(J.
Weichmann, ed.), pp. 25^3. Marcel Dekker, New York, NY.
Kader, A. A., Zagory, D. and Kerbel, E. L. (1989). Modified atmosphere packaging of
fiiaits and vegetables. CRC
Crit.
Rev. Food
Sci.
Nutr. 28, 1-30.
Kang, J., Park, W. and Lee, D. (2000). Quality of enoki mushrooms as affected by pack-
aging conditions. J. Sci. FoodAgric. 81, 109-114.
Katz, E. E. and Labuza,
T.
P. (1981). Effect of water activity on the sensory crispness and
mechanical deformation of snack food products. J. Food Sci. 46, 403-^09.
Kim, G.-Y, Lee, J.-H. and Min, D. B. (2003). Study on Hght-induced volatile compounds
in goat's milk cheese.
J.
Agric. Food Chem. 51, 1405-1409.
Kristensen, D., Orhen, V, Mortensen G.,
Brockhoff,
P and Skibsted, L. H. (2000). Light
induced oxidation in sliced Havarti cheese packaged in modified atmosphere. Intl
Dairy J. l{S,95-\{)3.
Labuza,
T.
P. and Contreras-Medellin, R. (1981). Prediction of moisture requirements for
foods.
Cereal
Foods World
26, 335-343.
Labuza, T. P. and Hyman, C. R. (1998). Moisture migration and control in multi-domain
foods.
Trends in Food
Sci.
Technol.
9,47-55.
Larousse, J. and Brown, B. E. (1997). Food Canning Technology. John Wiley & Sons,
New
York,
NY.
Leneovich, L. M. (1987). In:
Water
Activity: Theory and Application to
Food,
Basic
Symposion Series 6, pp. 119-136. Marcel Dekker, New
York,
NY.
Lennersten, M. and Lingert, H. (1998). Influence of different packaging materials on lipid
oxidation in potato crisps exposed to fluorescent light. Lebensm.
Wiss.
u.
Technol.
31,
162-168.
Quality of packaged foods 43
Lennersten, M. and Lingert, H. (2000). Influence of wavelength and packaging material
on lipid oxidation and color changes in low-fat mayonnaise. Lebensm. Wiss. u.
TechnoL
33, 253-260.
Leung, H. K. (1987). In: Water Activity: Theory and Application to
Food,
Basic
Symposion Series 6, pp. 27-54. Marcel Dekker, New
York,
NY.
Marsh, R., Kajda, P. and Ryley, J. (1994). The effect of hght on the vitamin B2 and the
vitamin A content in cheese. Nahrung 5, 527-532.
Maunder, D. T. (1977). Possible use of ultraviolet sterilization of containers for aseptic
packaging. Food
TechnoL
31, 36-37.
Michael, S. M. (1997). Aseptic packaging. In: The Wiley Encyclopedia of Packaging
Technology, 2nd edn. (A. Brody and K. S. Marsh, eds), pp. 41-45. John Wiley and
Sons,
New
York,
NY.
Morillon, V, Debeaufort, E, Blond, G. and Voilley, A. (2000). Temperature influence on
moisture transfer through synthetic films. J. Membrane Sci. 168,
223-231.
Mortensen, G., Sorensen, J. and Stapelfeldt, H. (2002a). Light-induced oxidation in semi-
hard cheeses. Evaluation of methods used to determine levels of
oxidation.
1 Agric.
Food Chem. 50,4364-4370.
Mortensen, G., Sorensen, J. and Stapelfeldt, H. (2002b). Effect of light and oxygen trans-
mission characteristics of packaging materials on photooxidative quality changes in
Havarti cheeses.
Packag.
TechnoL
Sci. 15, 121-127.
Mortensen, G., Sorensen, J. and Stapelfeldt, H. (2003a). Effect of modified atmosphere
packaging and storage conditions on photooxidation of sliced Havarti cheese. Eur
Food
Res.
TechnoL
216, 57-62.
Mortensen, G., Sorensen, J. and Stapelfeldt, H. (2003b). Response surface models for pre-
diction of photooxidative quality changes in Havarti cheese.
Eur.
Food
Res.
TechnoL
216,
93-98.
Mortensen, G., Sorensen, J., Danielsen, B. and Stapelfeldt, H. (2003c). Effect of specific
wavelengths on light-induced quality changes in Havarti cheese. J. Dairy Res. 70,
413^21.
(accepted for publication).
Mortensen, G., Bertelsen, G., Mortensen, B. K. and Stapelfeldt, H. (2004). Light-induced
changes in packaged cheeses - a review. Intl Dairy J. 14, 85-102.
Nagy, S. (1980). Vitamin C contents of citrus fruit and their products: a review.
J.
Agric.
Food Chem. 2S,S-l^.
Nelson,
P.
R. (1983). Stability prediction using the Arrhenius model. Computer
Programs
Biomed.
16, 55.
Nelson, P. E. (1984). Outlook for aseptic bag-in-box packaging of products for remanu-
facture. Food
TechnoL
38,
72-73.
Nelson, K. H. and Cathcart,
W.
M. (1984). Transmission of Hght through pigmented poly-
ethylene milk bottles. J. FoodProt. 47, 346-348.
Olley, J. and Ratkowsky, D. A. (1973). Temperature fimction integration and its impor-
tance in the storage and distribution of flesh foods above the freezing point. Food
TechnoL
25,
66-73.
Renter, H. (1988). Aseptic Packaging of
Food.
Technomic, Lancaster, PA.
Risbo, J. (2003). The dynamics of moisture migration on packaged multi-component food
systems I: shelf life predictions for a cereal-raisin system.
J.
FoodEng. 58,239-246.
Robertson, G. (1993). Food
Packaging.
Marcel Dekker, New
York,
NY.
44 Innovations in Food Packaging
Silva, K, Chau, K. and Sargent, S. (1999). Modified atmosphere packaging for mixed loads
of horticultural commodities exposed to two postharvest temperatures. Postharvest
Biol Technol 17, 1-9.
Susana, C. K, Fernanda, A. R. O. and Jeffi-ey, K. B. (2002). Modelling respiration rate of
fresh fruits and vegetables for modified atmosphere packages: a review. J.
FoodEng.
52,99-119.
Sutherland, J.
P.,
Aheme, A. and Beaumont, A. C. (1996). Preparation and validation of
a
growth model for Bacillus
cereus:
the effect of temperature, pH, sodium chloride and
carbon dioxide.
IntlJ.
Food
Microbiol.
30, 359-372.
Sutherland, J.
P.,
Bayliss,
A.
J., Braxton,
D.
S. and Beaumont,
A.C.
(1997). Predictive mod-
elling of Escherichia coli 0157:H7: inclusion of carbon dioxide as a fourth factor in
a pre-existing model.
IndJ.
Food
Microbiol.
37, 113-120.
Troller, J. A. and Christian, J. H. B. (1978). In:
Water
Activity and
Food,
pp. 13^7.
Academic Press, New
York,
NY.
Van Boekel, M. A. J. S. (1996). Statistical aspects of kinetic modelling for food science
problems. J. FoodSci. 61,477.
Watkins, C. (2000). Responses of horticultural commodities to high carbon dioxide as
related to modified atmosphere packaging.
Hort.
Technol.
10, 501-506.
Whiting, R. C. and Buchanan, R. L. (1994). Microbial modeling. Food
Technol.
48,113-120.
Willocx, F. (1995). Evolution of microbial and visual quality of minimally processed
foods;
a case study on the product life cycle of cut endive. PhD Thesis, Katholic
University of Leuven, Faculty of Agriculture and Applied Sciences, Leuven, Belgium,
pp.
228.
Wold, J. P., J0rgensen, K. and Lundby, F. (2002). Non-destructive measurement of light-
induced oxidation in dairy products by fluorescence spectroscopy and imaging.
J. Dairy Sci. 85, 1693-1704.
Surface chemistry of
food,
packaging and
biopolymer materials
Jung H. Han,
Yachuan
Zhang,
and
Roberto
Buffo
Introduction 45
Principles of contact angle and surface energy 46
Techniques for measuring the contact angle 50
Applied research 52
Future trends 57
References 57
Introduction
Surface properties of food packaging polymers, such as wettability, scalability, print-
ability, dye uptake, resistance to glazing, and adhesion to food surfaces or other poly-
mers,
are of central importance to food packaging designers and engineers with
respect to product shelf-life, appearance, and quality control. The most commonly
used food packaging polymers are low-density polyethylene (LPDE), high-density
polyethylene
(HDPE),
polypropylene
(pp),
polytetrafluoroethylene (PTFE), and nylon.
Surface properties of these polymers have been extensively studied (Rucka et ai,
1990;
Michalski et
al.,
1998; Ozdemir et
aL,
1999).
In recent years, environmental concerns have increased the interest in preparing
biodegradable packaging materials (Sanchez
et
a/.,
1998).
Proteins
and
polysaccharides
are the biopolymers of prime interest, since they can be used effectively to make
edible and biodegradable films to replace short shelf-life plastics (Guilbert et al,
1996).
These environmentally friendly films must meet a number of specific func-
tional requirements: moisture barrier, solute and/or gas barrier, color and appearance,
mechanical and rheological features, and non-toxicity (Guilbert, 2000). The type of
film-forming materials and additives will decisively determine whether
a
biodegradable
film can achieve these functional
properties.
Studies of biopolymer films have focused
Innovations in Food Packaging
ISBN:
0-12-311632-5
Copyright © 2005 Elsevier Ltd
All rights of reproduction in any form reserved
46 Innovations in Food Packaging
on mechanical properties, water vapor transfer
rates,
and gas exchange control. Rela-
tively little attention has been paid to surface properties, which may
be
essential to the
behavior of
the
film in relation to water and organic solvents. In addition, they con-
tribute substantially to the structure of the film (Bialopiotrowicz, 2003).
Biopolymer
films
primarily composed of polysaccharides (e.g. gums, cellulose and
derivatives, starch and derivatives) or proteins (e.g., gelatin, zein, gluten) have suit-
able overall mechanical and optical properties (Guilbert et a/., 1996). With regard to
gas permeability, biopolymer films present impressive gas barrier properties in dry
conditions, especially against oxygen (O2). For instance, oxygen permeability of
wheat gluten films has been reported to be 800 times lower than that of polyamide-6,
a well-known high 02-barrier polymer (Guilbert, 2000). On the other hand, because
of their hydrophilic nature biopolymer films are indeed wettable by water. Although
they provide efficient barriers against oils and lipids, their moisture barrier properties
are poor. As water activity (a^) increases, film moisture content increases as well
due to water absorption, following non-linear absorption isotherms. This induces a
decrease in water vapor barrier properties (Guilbert et ai, 1997).
Surface properties of biopolymers provide a supplementary understanding of film
behavior, leading to an enhanced design of packaging materials for specific applications
(McGuire and Yang, 1991). This chapter discusses the physicochemical principles of
surface phenomena, and provides an overview of the research regarding surface prop-
erties of biopolymers used for the manufacturing of biodegradable films.
Principles of contact angle and surface energy
Liquid and solid surface thermodynamics are based on the fimdamental concept of
surface tension, defined as half the free energy change due to cohesion of the material
in a vacuum. Surface tension depends on a number of relatively independent forces,
such as dispersion, dipolar, induction, hydrogen-bonding, and metallic interactions
(Fowkes,
1963).
According
to
the traditional theory, the surface tension of colloid-size
particles is determined by an apolar component, Lifshitz-van der
Waals
(LW) forces;
a polar component, Lewis acid-base (AB) forces; and an electrostatic (EL) compo-
nent. The apolar component depends on the unevenness of the electron cloud sur-
rounding the molecules, the polar component is related to the formation of coordinate
covalent bonds between Lewis acids (electron pair acceptors) and Lewis bases
(electron pair donors), and the electrostatic component is described through the
zeta-potential - a measurement of electrical surface charge (van Oss, 1994; Meiners
etaL,
1995; Rijnaarts etai, 1995; Besseling, 1997).
Contact angle
Wetting properties are essential surface features of packaging
materials.
The wettabil-
ity of
a
solid surface can be determined relatively simply by measuring the so-called
contact angle (Kiery and Olson, 2000). This concept, as applied to a liquid drop on a
material surface, is the result of
the
equilibrium between three surface energies: the