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Additives are blended into polymers to improve mechan-

ical or chemical properties such as ﬂexibility, cling, and

thermal stability. When the additive is a small molecule

and it is soluble in the polymer, the polymer is likely to be

plasticized. This effect increases the diffusion coefﬁcient for

permeant molecules. The solubility coefﬁcient is unaf-

fected. The effect can be small if the additive is included

at less than about 1 wt%. However, at larger concentrations

the effect can be large. The oxygen permeability of poly

(vinyl chloride) increases by about 10 times when enough

plasticizer is added to make the resulting ﬁlm ﬂexible. The

phenomenon of antiplasticization is under investigation.

The potential exists that, for some small molecule addi-

tives, the diffusion coefﬁcient can be decreased.

If the additive is not soluble in the polymer, the result is

more complicated. For an inorganic ﬁller, the permeability

might increase if the polymer does not wet the ﬁller. The

permeability will decrease if the polymer wets the ﬁller.

However, the effect is likely to be small unless the loading

of ﬁller is greater than about 20 wt%. Loadings this high

are avoided since these composites are typically difﬁcult to

handle.

Filler in the form of platelets can lower the permeabil-

ity more than ﬁller with a more compact shape. If the

platelets tend to lie in the plane of the ﬁlm, permeant

molecules must make wide detours (tortuous path) while

traversing the ﬁlm. This gives a greatly reduced effective

diffusion coefﬁcient.

Crystallinity is an overrated contributor to barrier in

polymers. First, if a polymer has crystallinity, the level of

crystallinity typically exists within a narrow range with

only modest variation allowed from fabrication variables.

Hence, crystallinity is not a strong design parameter.

Second, the same properties that lead to crystallinity

also lead to efﬁcient packing in the amorphous phase.

Efﬁcient packing in the amorphous phase gives a low

diffusion coefﬁcient. Polyoleﬁns are glaring exceptions

because they have considerable crystallinity and high

diffusion coefﬁcients.

Orientation is frequently cited as a contributor to

barrier in polymer ﬁlms. If the polymer molecules truly

are oriented in the plane of the ﬁlm, either uniaxially or

biaxially, the permeability is probably lower than that in

an unoriented ﬁlm. However, sometimes the word ‘‘or-

iented’’ merely means that the ﬁlm has been stretched. If

the polymer molecules relax during stretching, little or-

ientation results and little effect is expected. A few cases

have been noted where stretching has created microﬁs-

sures in the polymer and the permeability has increased.

Table 7 contains permeability data for elongated ﬁlms.

The results vary. A practitioner is wise to test permeabil-

ity before concluding that a fabrication involving elonga-

tion will lower the permeability. When elongation does

lower the permeability, the causes are a combination of

better packing among parallel molecules, difﬁculty in

moving perpendicular to the alignment of the polymer

molecules, and, when crystallinity is present, the ten-

dency for crystallites to act as platelets aligned in the

plane of the ﬁlm. When performed correctly, orientation is

an effective means of improving barrier properties and

reducing material requirements.

POLYMER COMPOSITION

Although a case has been made for a situational deﬁnition

for barrier polymers, common practice still focuses on a

rather small set of polymers. Figure 2 contains schematic

chemical structures of polymers with low permeabilities to

permanent gases, especially oxygen.

Finding common traits among this diverse group is

difﬁcult. However, each has some polarity which leads to

chain-to-chain interactions that give good packing in the

amorphous phase. Also, frequently, sufﬁcient symmetry

exists to allow crystallinity to develop. Again, this can lead

to good packing in the amorphous phase.

AVAILABILITY

Barrier polymers are available as resins for extrusion,

resins for dissolution and coating, and latices for coating.

Each form has its own advantages and disadvantages.

Resins for extrusion can be made into monolayers or multi-

layers with thicknesses to give adequate barrier for de-

manding applications. However, extrusion can give a severe

thermal stress to the polymer which could lead to degrada-

tion. For semicrystalline polymers the extrusion tempera-

ture must exceed the melting temperature. When more

modest total barrier is needed, a coating may be adequate.

Typically, coatings more than 3 mm (0.1 mil) are difﬁcult to

achieve. However, semicrystalline polymers can be used in

solvents well below the melting temperature. Hence, poly-

mers with marginal thermal stability may be used. Solvent

recovery and management must be considered. For latices,

the particles are too small to allow crystallinity to develop;

hence, semicrystalline polymers remain amorphous until

after coating and drying. Latices cannot be stored indeﬁ-

nitely and must be used before coagulation occurs.

CONCLUSION

The choice of barrier polymer(s) for any given application

is determined by economics, performance requirements,

and the package format. A wide variety of barrier

Table 7. Effect of Orientation on Oxygen Permeability

for Certain Polymers

Polymer

Degree of

Orientation, %

Oxygen permeability,

nmol/m s GPa)

Polypropylene 0 300

300 160

Polystyrene 0 840

300 600

Polyester 0 20

500 10

Copolymer of 70%

acrylonitrile and

30% styrene

0 2.0

300 1.8

Reference (2).

At 231C.

See Table 1 for unit conversions.

108 BARRIER POLYMERS

polymers are available for use, and the number of material

options will continue to grow. In extreme cases, barrier

requirements can be met without the use of foil by

combining the correct barrier polymers, with technologies

such as gas ﬂushing and active packaging scavengers.

BIBLIOGRAPHY

1. P. T. DeLassus, ‘‘Barrier Polymers’’ in J. I. Kroschwitz,

ed., Kirk–Othmer Encyclopedia of Chemical Technology,

4th edition Vol. 3, John Wiley & Sons, New York, 1992,

pp. 931–962.

2. M. Salame and S. Steingiser, Polymer-Plastics Technol. Eng.

8(2), 155–175 (1977).

3. J. Lange and Y.Wyser, Packag. Technol. Sci. 16, 149–158

(2003).

4. L. Massey, Permeability Properties of Plastics and Elastomers,

2nd edition, Plastics Design Library/William Andrew Publish-

ing, 2003.

5. M. Alexandre and P. Dubois, Mater. Sci. and Eng. 28, 1–63

(2000).

Figure 2. Chemical structures of barrier polymers. (a)

Vinylidene chloride copolymers; (b) hydrolyzed ethylene–

vinyl acetate (EVOH); (c) acrylonitrile barrier polymers;

(d) nylon-6; ( e) nylon-6,6; (f) amorphous nylon (Selar PA

3426), y = x + z;(g) nylon-MXD6; (h) poly(ethylene ter-

ephthalate); and (i) poly(vinyl chloride).

BARRIER POLYMERS 109

BIOBASED MATERIALS

LINSHU LIU

Eastern Regional Research

Center, Agricultural Research

Service, U.S. Department of

Agriculture, Wyndmoor,

Pennsylvania

JOSEPH KOST

Department of Chemical

Engineering, Ben-Gurion

University, Beer-Sheva, Israel

The preﬁx ‘‘bio-’’ means life in Greek. Biobased materials

indicate the substances obtained from living or dead

animals or plants. Biobased materials are a large group

of loosely related processing (or engineering) materials,

which are mainly derived from substances originally

existing in nature, such as in living tissues or organisms,

but may also be obtained by synthetic methods. Accord-

ingly, biobased materials include common commodities

such as leather and wood, as well as those that have

undergone more extensive processing such as pectin, oleic

acid and carboxymethyl cellulose. In literature, the term

‘‘biobased material’’ is often used synonymously with the

words biomass or biomaterial. The three words differ from

each other slightly in deﬁnition and mostly in habitual

uses. Biomass refers to animal or plant matter grown for

uses for production of chemicals and ﬁbers, but does not

include their products after processing; and more com-

monly, biomass is frequently used in literature of energy

production, emphasizing its status in the carbon cycle as a

renewable fuel. The term ‘‘biomaterial’’ has an additional

meaning. As a result of its long use for biomedical applica-

tions, it refers to materials that can perform biological

functions and are biocompatible when in contact with

living tissues. The study of biobased materials is now a

large sector in material science and agrobusiness, as are

(a) the study of biomaterials in biomedical and pharma-

ceutical sciences and (b) the study of biomass in renewable

energy.

Based on their sources and production, biobased mate-

rials can be divided into three major categories (1, 2):

. Category 1 includes those removed (extraction, exud-

ing, isolation after milling, etc.) from biomass. Ex-

amples are (a) polysaccharides such as starch and

cellulose and (b) proteins such as collagen and casein.

. Category 2 includes polymers produced by a chemical

method using renewable biobased monomers. Exam-

ples are some polyesters such as poly(lactic acid),

which is produced from the polymerization of lactic

acid, a fermentation product of carbohydrate feed-

stock. Category 2 biobased materials also include

monomers or low-molecular-weight chemicals (so-

called ‘‘building blocks’’) obtained from biobased feed-

stocks by chemical or biochemical methods, such as

rosin, castor oil, and terpene.

. Category 3 includes materials produced by microor-

ganisms or genetically modiﬁed bacteria. Examples

are bacterial cellulose and polyhydroxyalkonoates

such as polyhydroxybutyrate.

Because of their overwhelming presence in the world and

the versatility of their chemistry and architecture, bio-

based materials are the sources of many industrial pro-

ducts, such as medicinal, chemicals, ﬁbers, and paint, as

well as plastics, and so on. Although most of them can be

used for packaging purpose, the consumption of biobased

materials in the packaging industry is only about 1%; a

large portion of these products are currently produced

from petroleum-derived materials. The limitation of pet-

roleum resources and the awareness of environmental

protection have raised a new prospect that biobased

materials may be once again the major contributor to

the industries. Biobased materials are less dense than

metal, and some petroleum-derived thermal plastics, are

ideal components for many structural materials. Most

biobased polymers perform in a fashion similar to that of

conventional polymers. Unlike petroleum-derived materi-

als, most biobased materials are biodegradable. This

property enables the end-use products of biobased materi-

als to be disposed of upon completion of their useful life

without causing any environmental concerns. This is

attractive in the production and applications of packaging

materials and has become their main focus. The use of

biobased materials also addresses other economic issues:

the use of surplus stocks and the production of higher

value-added material from agricultural products and by-

products. Therefore, the use of biobased materials pro-

motes agrobusiness development.

Scientists and engineers are developing new technolo-

gies that will provide competitive cost for products from

biobased materials, meet the standards of various applica-

tions, and optimize their performance (3–9). Presently,

important research areas include: (I) Reproducibility

and quality of biobased materials. These not only depend

on the methods and processing conditions of separation,

puriﬁcation and fabrication, but also dramatically rely on

the sources of raw materials, such as where grown, when

harvested, and how and how long they are stored. All

these variations make product quality control more com-

plicated and difﬁcult. (II) In composites, water absorption

can be considered a disadvantage. Migration of water

through the polymer can lead to disturbance of the ﬁller/

matrix interface, reducing the overall strength of the

composites. Most biopolymers are hydrophilic. It is a

challenge to improve the water resistance of biobased

materials in order to retain good mechanical properties

when the composites are exposed to highly humid condi-

tions. (III) The durability of biobased materials is related

to their biodegradability. The degradation of biobased

products should be controllable and their properties

should be constant during the time of their useful life.

(IV) Gas barrier properties have speciﬁc signiﬁcance in

packaging materials. In particular, food packaging re-

quires speciﬁc atmospheric conditions to sustain food

freshness and overall quality during storage. Biobased

materials mimic quite well the oxygen permeability of a

wide range of the conventional petroleum-derived thermal

plastics. As many of the biobased materials are

110 BIOBASED MATERIALS

hydrophilic, their gas barrier properties are very depen-

dent on environmental humidity. (V) Thermal stability.

Most category 1 biobased materials are not stable at

higher temperature, limiting applications and choices of

processing methods. (VI) Safety. Biobased materials, par-

ticularly those come from biological processes, may sup-

port microorganism growth. Concerns have risen over the

spread of novel trails in existing populations and the

introduction of modiﬁed species.

CATEGORY 1 BIOBASED MATERIALS

There are a large amount of polysaccharides and proteins

that are directly isolated from agricultural or marine

plants and animals, and they can be used in packaging

applications. Examples of category 1 biobased materials

include starch, cellulose, pectin, alginate, collagen, soy-

bean ﬂour protein, and zein. Most category 1 biobased

materials exhibit useful gas barrier properties; but they

are hydrophilic and unstable at higher temperature,

causing problems in processing.

Starch

Starch is a widely available raw material suitable for a

variety of applications, including packaging (10–12).

Starch is a storage polysaccharide found in the cytoplas-

mic granules of plant cells. Starch is a composite consist-

ing mainly of amylose and amylopectin, and it is primarily

derived from corn, wheat, potatoes, and rice. Due to its

huge availability and low cost in processing, starch is

economically competitive with petroleum-derived materi-

als and is therefore a promising candidate for preparing

compostable plastics. Starch is nontoxic, biologically ab-

sorbable, resistant to passage of oxygen, and semiperme-

able to carbon dioxide. On the other hand, starch granules

are rigid, and starch blends are brittle. Plasticizers,

such as glycerol, low-molecular-weight polyhydroxy com-

pounds, polyethers, or ureas, can be included in starch

blends to reduce the intermolecular hydrogen bonding and

thus increase the ﬂexibility. Because of the hydrophilic

nature of starch, the performance of starch blends

changes during and after processing as a result of water

content changes with the changes in humidity. Side-chain

modiﬁcation to obtain more hydrophobic starch deriva-

tives is a strategy to overcome this challenge.

Cellulose

Cellulose is the most abundant naturally occurring poly-

mer. Cellulose is a high-molecular-weight 1-4-beta-

linked polymer of

D-glucopyranose, which displays a

diverse range of conformations and crystalline packing

arrangements, as well as ﬁber structure. Because of its

regular linear structure and array of hydroxyl groups,

cellulose-based ﬁlms tend to be tough and ﬂexible and

resistant to fat and oil. Cellulose is hydrophilic; it swells,

but does not dissolve in water. To produce cellulose ﬁlms,

an aggressive process is required that involves several

strong basic, strong acid and toxic solvents. Although, the

resultant ﬁlms possess good mechanical properties, they

are hydrophilic and moisture-sensitive (1, 13).

Cellulose derivatives are used for ﬁlm forming, coating,

and encapsulating applications. Cellulose derivatives,

such as ethyl cellulose, methyl cellulose, carboxy methyl

cellulose, hydroxyethyl or hydroxypropyl cellulose, and

cellulose acetate, are commercially available. Cellulose

acetate possesses relatively low gas and moisture barrier

properties (1, 10, 14–16).

Pectin

Pectin is a cell wall polysaccharide. The majority of

the pectin structure consists of homopolymeric partially

methylated poly a-(1-4)-

D-galacturonic acid residues

(‘‘smooth’’ regions), but there are substantial ‘‘hairy’’

regions of alternating a-(1-2)-

L-rhamnosyl-a-(1-4)-D-

galacturonosyl sections containing branch-points with

mostly neutral side chains (1–20 residues) of mainly

L-

arabinose and

D-galactose (rhamnogalacturonan I). Pec-

tins may also contain rhamnogalacturonan II with

side chains containing other residues such as

D-xylose, L-

fucose,

D-glucuronic acid, D-apiose, 3-deoxy- D-manno -2-

octulosonic acid, and 3-deoxy-

D-lyxo-2-heptulosonic acid

attached to poly a-(1-4)-

D-galacturonic acid regions. The

types and amounts of substructural entities in pectin

preparations depend on their source and extraction meth-

odology. Commercial pectin is mainly derived from citrus

peels and apple pomace. It can also come from sugar beet

pulp and sunﬂower heads. Commercial extraction causes

extensive degradation of the neutral sugar-containing

side chains. The pectin molecule does not adopt a straight

conformation in solution, but is extended and curved

with a large amount of ﬂexibility. The carboxylate groups

tend to expand the structure of pectin. Methylation of

these carboxylic acid groups forms their methyl esters,

which are much more hydrophobic and have a different

effect on the structure of surrounding water. Thus, the

properties of pectin depend on the degree of esteriﬁcation

(D.E.). High D.E. pectin (W40% esteriﬁed) tends to gel

through the formation of hydrogen-bonding and hydro-

phobic interactions at low solution pH (pH B3.0) to reduce

electrostatic repulsions, or in the presence of sugars

(W70% esteriﬁed). Low D.E. pectin (o40% esteriﬁed)

gels by calcium divalent cations that bridge adjacent

twofold helical chains to form the so-called ‘‘egg-box’’

junction zone structures so long as a minimum of 14–20

residues can cooperate (17).

In addition to its gelling properties, pectin is a well-

established ﬁlm-forming material. In isolated form, pectin

readily reassociates or aggregates to form networks, and it

interacts with proteins, other polysaccharides, and syn-

thetic hydrocolloids via hydrogen bonding, ionic, or hydro-

phobic interactions. This character has led to applications

of pectin in encapsulation, coating, packaging, and wrap-

ping for food and pharmaceutical products (18–20).

Alginate

Alginates are mainly derived from seaweed. The alginic acid

family of linear 1-4-linked glycuronans are copolymers

composed of beta-

D-mannopyranuronic acid (M) residues

BIOBASED MATERIALS 111

and alpha-L-gulupyranuronic acid residues (G) that are

arranged in homopolymeric blocks (GG and MM) and

heteropolymeric (GM) sequences in varying proportions

and distribution patterns. Alginates possess good ﬁlm-form-

ing properties that make them particularly useful in food

packaging applications . Divalent cations, such as calcium,

magnesium, manganese, and aluminum, are used as gelling

reagents in alginate ﬁlm formation. Calcium ion appears

to be more effective in gelling alginate than other divalent

ions; calcium propionate provides acceptable ﬂavor. Desir-

able properties of alginate ﬁlms include the improved

product texture, juiciness, color, odor, and appearance, along

with moisture retention and shrinkage reduction. Sodium

alginate is water soluble. Sodium alginate coatings are used

to extend the shelf life of foods and fruits (21, 22).

Chitin and Chitosan

In its structure, chitin is a cellulose analogue and is

comprised of 1-4-beta-linked

N-acetyl-D-glucosamine

units. Chitin is the second most abundant glycan after

cellulose, being present in the exoskelton of invertebrates.

Chitin occurs in several crystalline polymorphic forms, of

which the alpha-chitin is the most common. Like cellulose,

chitin chains adopt a 2

screw axis. All of the hydroxyl

groups are hydrogen bonded, and the bonding between

sheets accounts for the fact that chitin does not swell in

water. The fully or partially N-deacetylated derivative of

chitin, chitosan, has received considerable attention. Some

desirable properties of chitosan include its ﬁlm forming

properties, its antimicrobial activity, and the ability to

absorb heavy metal ions. Chitosan ﬁlms exhibit good

oxygen and carbon dioxide permeability, as well as good

mechanical properties. Chitosan ﬁlms and coatings show

activity against bacterial yeasts and molds, and they inhibit

the growth of a number of microorganisms. The cationic

nature of chitosan allows for electronic interactions with

anionic compounds during processing and can lead to the

incorporation of speciﬁc properties into products (23–25).

Collagen and Gelatin

Collagen is a major structural protein in vertebrates. Most

of the ectodermal and mesodermal tissues are composed of

collagen. In this sense, collagen is similar to cellulose and

pectin, which serve a somewhat analogues role in plants.

Collagen has triple helix architecture, thus collagen is

insoluble and difﬁcult to process. Commercially available

collagens are extracted from animal skin, bone, and

tendons. By taking advantages of biotechnology and ge-

netic engineering, collagen can be obtained not only from

animal tissues but also from bacterial cultures and even

from genetically modiﬁed plants. Collagen ﬁlms have

strong mechanical properties and have been proposed for

use in food packaging. Edible collagen ﬁlms can become an

integral part of meat products, and thus they function to

reduce shrink loss and to increase juiciness and smoke

permeability to the meat products (26–28).

Gelatin is a substantially pure product obtained

by either partial acid or alkaline hydrolysis of collagen.

The denaturation treatments disrupt the tight, helical

structure of collagen and release water-soluble fragments

that can form stiff gels and ﬁlms. Gelatin is a highly

processable material and is moisture-sensitive. Gelatin

gels or ﬁlms show thermally reversible behavior and melt

below body temperature. Gelatin ﬁlms and coatings have

been used to carry ﬂavors or antioxidants in food packa-

ging applications (29, 30).

Soybean Flour Protein

Soy proteins are commercially available as soy ﬂour (50%

protein), soy concentrate (70% protein), and soy isolate

(90% protein). Flours are made by grinding and sieving

ﬂakes. Concentrates are prepared by extracting and re-

moving the soluble sugars from defatted ﬂakes, by leach-

ing with diluted acid at pH 4.5, or by leaching with

aqueous ethanol. Isolate soy proteins are obtained by

extracting the soluble proteins with water at pH 8–9,

precipitating at pH 4.5, followed by centrifugation and

drying. Soy proteins consist of two major protein fractions,

7S (conglycinin, 35%) and 11S (glycinin, 52%). Both frac-

tions are considered as storage proteins and contain

cysteine residues leading to disulﬁde bridge formation.

Soy proteins are adhesive- and moisture-sensitive. Soy

protein coatings can reduce moisture loss and control lipid

oxidation in coated samples. Soy proteins are also used as

binders for aqueous inks and as pigmented coatings on

paperboard. Furthermore, soy ﬂour proteins are proposed

for the use in food coating, encapsulation, and active

packaging (18, 19, 31, 32).

Zein

Zein is a class of alcohol-soluble prolamine proteins, ob-

tained from corn gluten meal. Pure zein forms a hard,

edible, clear, odorless , tasteless , and water-insoluble mate-

rial, making it invaluable in processed foods and pharma-

ceuticals. Zein is now used as a coating for candy, nuts, fruit,

pills, and other encapsulated foods and drugs. Zein can be

further processed into resins and other bioplastic polymers

by extrusion or rolling into a variety of plastic products .

Zein-based ﬁlms can function as water barriers and, thus,

have potential use for packaging materials (33, 34).

Casein and Whey

Casein and whey are milk-derived proteins. Both have

high nutritional values, excellent mechanical, and emul-

sion and barrier properties, and they are available in large

volume worldwide. They have been used in the manufac-

ture of edible ﬁlms. Due to its random coil structure,

casein is easily processable. By controlling the types of

plasticizers, casein ﬁlms can be made with very different

mechanical properties varying from stiff and brittle to

ﬂexible and tough materials. Casein and whey ﬁlms can

reduce moisture loss, delay lipid oxidation, and reduce

peroxide value of packaged food products (35–37).

CATEGORY 2 BIOBASED MATERIALS

The use of classical chemical methods to produce poly-

mers, monomers, and other chemical ‘‘building block’’

112 BIOBASED MATERIALS

materials from biobased feed stocks have generated a wide

spectrum of category 2 products. Theoretically, all the

conventional packaging materials currently derived from

petroleum can be produced from monomers obtained from

biobased materials; however, due to their high cost, the

production of those monomers is not economically feasible.

Thus, these costs have been an obstacle on the road to

broadening the application of biobased monomers. An

exception may be poly(lactic acid) (PLA) production. As a

result of the increase in both the efﬁciency of lactic acid

fermentation and the market price of raw oil, the cost of

making poly(lactic acid) from biobased materials is gra-

dually approaching the price of PLA obtained from fossil

fuel.

Poly(Lactic acid)

Poly(lactic acid) or polylactide (PLA) is a thermoplastic,

aliphatic polyester derived from fermentation of agricul-

tural products and byproducts such as corn starch and

other starch-rich substances like maize, sugar, or wheat.

Bacterial fermentation is used to produce lactic acid,

which is oligomerized and then catalytically dimerized

for ring-opening polymerization. It can be easily produced

in a high-molecular-weight form, most commonly using a

stannous octoate catalyst. The properties of PLA strongly

depend on the ratio of the two mesoforms (

L and D) of the

lactic acid monomer. Poly

L-lactide (PLLA) is the product

resulting from polymerization of

L-lactide. PLLA has

crystallinity around 37%, a glass transition temperature

) of 50–801 C, and a melting temperature (T

) of 173–

1781 C. If a mixture of

D- and L-PLA is used, a polymer

with adjustable T

and T

, PDLLA, can be obtained. The

physical blends of PDLA and PLLA are useful for produ-

cing loose-ﬁll packaging, compost bags, microwavable

trays, food packaging and disposable tableware. PLA can

also be plasticized by blending it with its monomer or

oligomer. The resultant blends possess lower T

and T

PLA resembles conventional petrochemical-based plastics

in its characteristics and has good water vapor barrier

properties and relatively low gas transmittance. PLA can

be processed into ﬁbers, blown ﬁlms, injected molded

objects, and coatings on standard equipment that already

exist for the production of conventional thermoplastics. To

date, PLA has shown the highest potential for a commer-

cial major-scale production of biobased packaging materi-

als (38–41).

Monomers and Chemical ‘‘Building Blocks’’

A number of monomers and chemical ‘‘building blocks,’’

low-molecular-weight organic compounds or chemical pre-

cursors for preparing polymeric materials, can be obtained

from biobased materials (1, 2, 42, 43). Examples include

castor oil, oleic acid, molasses, furfural, multifunctional

alcohols, multifunctional acids, and terpenes.

Castor oil is used for the preparation of polyurethane.

The resultant polyurethane is water-resistant and widely

used in the electronics industry. Unsaturated fatty acids

and oils, such as oleic acid, linoleic acid, and ricinoleic

acid, can be recovered from seed crops, castor beans,

coconut, ﬂax, and other agricultural origins and have

found applications in water-proof coatings and multi-

layered packaging materials. Furthermore, oleic acid can

be chemically transformed to multi-functional alcohols

and acids, amines and esters. The resultant azelaic acid

and azelaic diacid are used in polyamide synthesis.

Furfural can be produced from woody biomass and

molasses. Furfural can be transformed to furfuryl alcohol

and to furan resin and a wide range of furan chemicals.

Levulinic acid also can be produced from woody materials.

Levulinic acid is a precursor for the synthesis of various

lactones, furans, and other functional building blocks,

which are used in the production of packaging materials.

Succinic acid and 1,3-propanediol are two examples of

chemicals, which are prepared by fermentation of carbo-

hydrate-rich materials using selected microorganisms.

Succinic acid and 1,3-propanediol can be used to make

polyesters, which, in turn, are used for preparation of

packaging materials. Terpene chemicals are isolated from

pine trees and have resulted in a number of terpene-based

products, which are used to prepare resin materials, or as

fragrances incorporated into resin materials for active

packaging (1, 42).

At present, biobased monomers and ‘‘building blocks’’

may not be commercially attractive. However, with the

progress in biotechnology and genetic engineering, these

represent promising alternatives to petroleum-derived

materials.

CATEGORY 3 BIOBASED MATERIALS

This group includes polymers produced directly from

biomass by natural or modiﬁed organisms.

Bacterial Cellulose

Cellulose is an important starting material in many

industries. Plants are the main source of cellulose. A

harsh chemical treatment is required to isolate plant

cellulose from lignin, hemicellulose, and pectins. The

treatment severely impairs the chemical and physical

characteristics of plain cellulose, such as molecular depo-

lymerization and changes in crystal structure. Bacterial

strains of Acetobacter xylinum and A. pasteurianus are

able to produce pure cellulose that originally formed in

plants (homo-beta-1,4-glucan) under ambient conditions.

Bacterial cellulose is highly crystalline (70% in cellulose I

form) and has a molecular weight 15 times higher than

that isolated from wood pulp. Although the bacterial

cellulose shows outstanding mechanical properties, the

production cost of bacterial cellulose is high and hampers

its application in current packaging industries (44, 45).

Poly(hydroxyalkanoates)

Poly(hydroxyalkanoates) (PHAs) are a family of polyesters

that are produced by a large number of bacteria, in the

form of intracellular particles, functioning as energy and

carbon reserve. The properties of PHAs are dependent on

their monomer composition. The speciﬁc monomer compo-

sition of PHAs depends on the nature of carbon sources

used and the bacterial strains selected, providing a tool to

BIOBASED MATERIALS 113

control the properties of the resultant ﬁnal products.

PHAs of medium-chain length are elastomers with low

melting point and low crystallinity. Poly(hydroxybutyrate)

(PHB) is an important member of PHAs. PHB is a highly

crystalline thermoplastic. PHB mimics the mechanical

behavior of poly(isopropylene). The incorporation of hy-

droxyvalerate can remarkably change the mechanical

properties of PHB, such as a decrease in stiffness and

tensile strength and an increase in toughness. PHAs

possess a low water vapor permeability, which is close to

LDPE. The functional groups in the side chains of PHAs

make it possible to chemically modify the polymers. PHA

also can be produced from genetically modiﬁed crops, such

as switchgrass. All these provide an enormous potential of

the polymer for packaging applications (46, 47). A com-

mercially available PHA product, Mirel

, can be ob-

tained from Metabolix (Cambridge, MA, USA).

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BIOFILM DEVELOPMENT ON PACKAGING

MATERIALS

PAUL TAKHISTOV

Department of Food Science,

Rutgers University, New

Brunswick, New Jersey

INTRODUCTION

The term ‘‘microbial bioﬁlm’’ or ‘‘bioﬁlm’’ refers to the

complex aggregation of microorganisms growing on a solid

substrate such as a packaging material, not a ﬁlm that is

made of biopolymer. This article provides an introductory

understanding of bioﬁlm and its formation. Since a major

function of packaging is to protect the product from

microbial contamination and its associated heath risks,

an understanding of bioﬁlm is important, particularly for

products which require aseptic packaging and retortable

packaging.

Bacteria on the surface develop a bioﬁlm-associated

community with higher resistance to toxic compounds

(1, 2) than their planktonic counterparts in the bulk. In

general, bioﬁlms result from physicochemical conditions

and interactions in the bacteria/environment complex

(3, 4). A bioﬁlm consists of a living microbial biomass

surrounded by an exopolysaccharide envelope (EPS), pro-

teins, and nucleic acids, which the bioﬁlm microorganisms

produce. These components help bacteria to attach to

surfaces, stabilize local environment, and spatially orga-

nize communities that need to collaborate to use the

substrate effectively (5). The process of the microorgan-

ism’s attachment to a surface is very complex, and the

nature of both the microbial cell surface and the support-

ing surface (substratum) is critical for successful attach-

ment (6). Surface adherence is an important survival

mechanism for microorganisms. Moreover, the adhesion

kinetics is the unique characteristic of a speciﬁc micro-

organism, differing even among phenotypes and strains

(7). Several major factors affect attachment and conse-

quently bioﬁlm formation: the nature of the cell surface,

the chemistry and texture of the attachment surface, the

nature of the surrounding medium, and the temporal and

spatial distribution of available nutrients (8–10).

A bioﬁlm can be deﬁned as a layer of microorgan-

isms immobilized at a substratum held together in a

multi-nature matrix polymer matrix (11). This matrix

consists mainly of water (97%), microbial cells (2–5%),

polysaccharides (neutral and polyanionic) (1–2%), proteins

[including enzymes (1–2%)], and DNA and RNA from lysed

cells (1–2%) (12). Usually, a mix consortium of microbes

makes up this ecosystem, and they ‘‘team up’’ in order to

protect themselves from stress and maximize nutrient

uptake. One of the main components of a bioﬁlm is the

exopolysaccharide (EPS), which often consists of one or

more family of different polysaccharides produced by at

least some of the bioﬁlm microorganisms. These compo-

nents aid the attachment of cells to surfaces, stabilizing the

local environment and spatial organization of the microbial

communities, which may need to cooperate with each other

to effectively use the available substrate (5).

BIOFILM ARCHITECTURE

A bioﬁlm is a multiphase system. It consists of the bioﬁlm

itself, the overlying gas and/or liquid layer, and the

substratum on which it (the bioﬁlm) is immobilized. This

system can be classiﬁed in terms of phases and compart-

ments. The phases consist of the solid, liquid, and gas

components, whereas the compartments consist of the

substratum, the base ﬁlm, the surface ﬁlm, the bulk liquid

and the gas. Bioﬁlms are heterogeneous by nature, how-

ever, having stacks of cells scattered in a glycocalyx net-

work with ﬂuid-ﬁlled channels (13). Structurally, a bioﬁlm

is approximately two-dimensional, with its thickness ran-

ging from a few micrometers to millimeters (5). This

structure allows for the diffusion of nutrients and meta-

bolic substances within the matrix.

Bioﬁlm Organization. The microbes in a bioﬁlm are

typically organized into microcolonies embedded in the

EPS polymer matrix (5). These microcolonies attain dis-

tinct 2-D or 3-D structural patterns. Initially, microcolo-

nies are separated by void spaces, but ultimately they

merge into unique structures forming a mature bioﬁlm.

This spatial organization is very important to the biologi-

cal activity of the bioﬁlm.

Extrapolysaccharide (EPS). EPS may vary in chemical

and physical properties, but it is primarily composed of

polysaccharides (11). Some of these polysaccharides can be

neutral or polyanionic. Generally, this polymer can accom-

modate considerable amount of water into its structure by

hydrogen bonding. Overall, the EPS has an important role

of holding the bioﬁlm together (14). As the EPS layer

thickens, the bioﬁlm microenvironment changes due to

the activities of the bacteria. Therefore, a mature bioﬁlm

is a heterogeneous matrix. This heterogeneity concept is

descriptive for both mixed and pure culture bioﬁlms com-

mon on abiotic surfaces, including medical devices (15).

Quorum Sensing. Quorum-sensing gene expression has

been proposed as an essential component of bioﬁlm phy-

siology, since bioﬁlm typically contains high concentration

of cells (16). Generally, the irreversible attachment of

bacteria to a substratum triggers alteration to an array

BIOFILM DEVELOPMENT ON PACKAGING MATERIALS 115

of gene expression and phenotypes of these cells (17). In

the quorum-sensing process, cell–cell communication is

accomplished through the exchange of extracellular sig-

naling molecules (16). For most gram-negative bacteria,

the quorum-sensing regulation involves a freely diffusible

auto-inducer, acylhomoserine lactone (AHL) signaling

molecule. For instance, the quorum-sensing ability in P.

aeruginosa is dependent upon two distinct but interre-

lated systems, las and rhl (18), which directs formation of

the AHL. In gram-positive bacteria, structurally diverse

peptides act as quorum sensing regulators (19). The QS

system is not necessarily involved in the initial attach-

ment and growth stages of bioﬁlm formation but is very

important in the overall bioﬁlm differentiation process

(20). During bioﬁlm formation, QS signaling molecule

mutants may develop thicker, more acid-resistant (21) or

‘‘abnormal’’ bioﬁlms (22) than do the wild-type strains.

BIOFILM LIFE CYCLE

Once immobilized on a contact surface, microorganisms

have the potential to form a bioﬁlm. Attachment to the

surface is beneﬁcial to the microbe for a number of

reasons. First, the surface represents important microbial

habitats because in the microenvironment of a surface,

nutrient levels may be much higher than they are in the

bulk solution (23). Second, it increases the microbes’

resistance to mechanical and chemical stresses. Overall,

bioﬁlm formation is a dynamic process, comprised of four

main stages (24): migration of cells to the substratum,

adsorption of the cells to the substratum, growth and

metabolic processes within the bioﬁlm, and detachment

of portions of the bioﬁlm (see Figure 1). These steps can be

divided into three phases: initial events, exponential

accumulation and steady state.

Surface Conditioning Film Formation. The conditioning

ﬁlm is created when organic materials (polysaccharides

and proteins) settle on the surface (11). It can be derived

from the microbes in the vessel or from the bulk ﬂuid.

Adsorption of a conditioning ﬁlm is relatively quick com-

pared to the other steps. This ﬁlm has the potential to

alter the physicochemical properties of the substratum

and thus greatly impacts bacterial attachment.

Cell Migration to the Surface. Migration of microorgan-

ism to the substratum is considered the second step in

bioﬁlm formation. This process can be mediated by differ-

ent mechanisms depending on the system under consid-

eration. Thus, transport can be (a) active, facilitated by

ﬂagella (25), or (b) passive, facilitated by Brownian diffu-

sion, convection, or sedimentation. In quiescent systems

(batch culture), sedimentation rates for bacteria are gen-

erally low due to their size and speciﬁc gravity (11), and

microbes with a diameter of 1–4 mm

have small Brownian

diffusivity. Therefore, motility may be the limiting factor

of transport in such systems. In a laminar ﬂow system,

although motility affects transport, diffusion remains the

controlling factor. In a turbulent ﬂow system, Brownian

diffusion has minute contributions to transport, but forces

such as frictional drag force, eddy diffusion, lift force, and

turbulent bursts are signiﬁcant.

Factors that Impact Interactions of Bacteria with a Substrate

Microbial adhesion is mediated by speciﬁc interactions

between cell surface structures and speciﬁc molecular

groups on the substratum. Moreover, the adhesion process

is determined by physicochemical and molecular interac-

tions. It is believed that primary adhesion between bac-

teria and abiotic surfaces is generally determined by

nonspeciﬁc (e.g., hydrophobic) interactions, whereas ad-

hesion to living or devitalized tissue is accomplished

through speciﬁc molecular (lectin, ligand, or a adhesion)

mechanisms (26).

Physicochemical Interactions. Generally, two types of

physicochemical interactions are used to describe the

adhesion of a microorganism to a planar surface. The

DLVO approach relates to the interaction energies (attrac-

tion and repulsion)—primarily to electrostatic and van der

Waals forces—but chemical forces can operate (27). Typi-

cally, attraction between microbe and surface occurs

either at a long range (5–8 nm), a secondary minimum,

or at a shorter range, the primary minimum. Thus,

adhesion can be reversible (at the secondary minimum)

20μm

EPS

Cells

II.

III.

IV.

Figure 1. Listeria bioﬁlm on the packa-

ging surface and schematic representation

of bioﬁlm formation. Step I: Conditioning

ﬁlm formation. Step II: Bacteria migration

to the conditioned surface. Step III: The

cells start to produce extracellular polysac-

charides (EPS), which cause an irreversible

attachment. Step IV: Gradually, the bioﬁlm

increases through growth of the irreversibly

attached cells and new ones from the solu-

tion. Step V: Cells near the outer surface

can dislodge from the bioﬁlm and escape to

colonize new microenvironments.

116 BIOFILM DEVELOPMENT ON PACKAGING MATERIALS

or irreversible (toward the primary minimum) (11). In the

DLVO approach, the ionic charge of the medium, the

physicochemistry of both the bacteria and substratum

surface, and the physicochemistry of biosurfactant deter-

mine the extent of adhesion.

Alternatively, in the thermodynamic approach, adhe-

sion is described as the formation of a new interface

between the substratum surface and adhering bacteria

at the expense of (a) the interfaces between bacteria and

the suspending liquid and (b) the substratum–liquid inter-

face (5). Each interface contains a speciﬁc amount of

interfacial energy (or surface tension). The extent of

adhesion is determined by the surface properties of all

three phases, the surface tension of adhering particles, of

the substratum and of the medium (28). The more hydro-

philic a substrate, the higher is its surface tension.

Molecular Interactions. Bacterial adherence is also

mediated by molecular mechanisms. Bacteria are able to

adhere to animal cells (29), such as muscle meats through

protein–protein interactions on the surface. These pro-

teins sometimes function as ligands to receptors when the

bacteria invade target cells and/or have speciﬁc afﬁnity for

host components (30). The colony-opacity-associated (Opa)

outer-membrane proteins or ligands (often called an a

adhesion) confer intimate bacterial association with mam-

malian cells. Two classes of cellular receptors for Opa

protein receptors have been identiﬁed: a adhesio-sulfate

proteoglycan (HSPG) receptors and members of the carci-

noembryonic antigen (CEA) or CD66 family (31). Listeria

monocytogenes surface proteins Internalin A (InlA) and B

(InlB) are involved in the attachment of this bacterium to

host cells (32).

The magnitude of the cell substratum interaction

forces, the chemical heterogeneity and the roughness of

the substratum surface greatly affect the extent of micro-

bial adhesion. As food traverses from the farm to the table,

it comes in contact with fabricating equipments, utensils,

gaskets, conveyor belts, packaging materials, storage con-

tainers, and chopping boards. These surfaces are usually

metallic, plastic, rubber, or wood. Food processing equip-

ments are often made of stainless steel, transport crates of

high-density polyethylene (HPDE), conveyor belts of rub-

ber, chopping boards of wood, and packaging materials of

aluminum. Other storage and packaging-type materials

include polypropylene, PVC, and Teﬂon. Sometimes the

contact time between foods and surface may be 24–48

hours depending on the processing conditions including

design of equipment cleaning and sanitation techniques.

Substratum Surface Hydrophobicity. The hydrophobicity

of the substratum has substantial effect on bacterial

adhesion. Typically, hydrophilic surfaces such as stainless

steel and glass have a high free surface energy and thus

allow greater bacteria attachment than do hydrophobic

surfaces such as Teﬂon (25). For instance, a general trend

of decreasing colonization density was observed for Sta-

phylococcus epidermis and Pseudomonas aeruginosa with

an increase in substratum hydrophobicity (33). In the

above-mentioned study, the packaging materials used

were stainless steel, poly(vinyl chloride), polystyrene,

and glass. Likewise, bioﬁlm formation by L. monocyto-

genes LO28 was faster on hydrophilic (stainless steel)

than on hydrophobic polytetraﬂuoroethylene (PTFE) (34).

Substratum-Surface Roughness/Topography. Many re-

ports have indicated that metal surfaces with a high

degree of roughness serve as a better substrate for bacter-

ial attachment than do smooth ones, since the surface

area of the former is greater (33, 35). Arnold and others

discovered that resistance to bacteria attachment de-

creased in the following order: Electropolished > Sanded >

Blasted > Untreated Stainless Steel. Bower and Daeschel

(1) illustrated that surface topography is extremely im-

portant in bioﬁlm formation and resistance. Still studies

such as that of Barnes et al. (36) claimed that the

difference in bacteria attachment due to difference in

surface topography is minimal.

Substratum Coverage with Organic Material. The layer of

organic substances present on the surface can be favorable

or unfavorable to bacteria adhesion. Barnes et al. (36)

discovered that proteins that adsorbed to a stainless steel

surface inhibited bacterial attachment. The dominating

mechanism was suspected to be competitive inhibition,

since the proteins were able to interact with the hydro-

philic surface. The adhesion process begins, provided that

the conditioning ﬁlm is favorable to bacterial attachment.

Bulk Nutrient Composition. Generally, bulk ﬂuid condi-

tions inﬂuence surface hydrophobicity, adhesion expres-

sions, and other factors that affect adhesiveness (37).

Microbes are usually exposed to a range of nutrients

concentrations from as low as 1 mg/L to 500 g/L, and this

range has an effect on bioﬁlm growth (5). At the highest

nutrient

concentrations, bioﬁlms

can appear to be uniform

with few or no pores. This is common of bioﬁlms associated

with animal and food surfaces. Various reports suggest

that the lower the concentration of nutrients, the greater

the rate of attachment and bioﬁlm development (38–40).

Escherichia coli O157:H7 bioﬁlms developed in minimal

salts medium (MSM) developed faster and had thicker

extracellular matrix, and cells detached much slower

compared to those grown in trypticase soy broth (TSB)

(38).

Temperature. Temperature effect is particularly impor-

tant in the food industry, since food will experience

differentials in temperature (temperature abuse) from

farm to fork. This abuse is a consequence of changes in

cell wall and attachment factors (41). Stopforth et al. (42)

showed that a greater number of Listeria monocytogenes

cells adhered to stainless steel templates at 59 1F and

77 1F compared to 41 1F and 95 1F. Moreover, Stepanovic

et al. (43) suggested that a microaerophilic environment

supports bioﬁlm formation. Presently, there are few con-

clusive reports that support this claim.

Ionic Strength. The atomic ions present in the medium

can indirectly affect attachment of the bacteria to other

substratum. These ions may act as chelator, forming

bridges between protein molecules on the bacteria surface

BIOFILM DEVELOPMENT ON PACKAGING MATERIALS 117