Trends in Food Science & Technology 19 (2008) 634e643

Review

* Corresponding author.

0924-2244/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.tifs.2008.07.003

Biodegradable

polymers for food

packaging: a review

Valentina Siracusaa,*, Pietro

Rocculib, Santina Romanib and

Marco Dalla Rosab

aDepartment of Physical and Chemical Methodology

for Engineering, Engineering Faculty, University of

Catania, Viale A. Doria 6, 95125 Catania, Italy

(Tel.: D39 095 7382755; fax: D39 095 333231;e-mail: vsiracus@dmfci.unict.it)

bDepartment of Food Science, Alma Mater Studiorum,

University of Bologna, Cesena (FC), Piazza Goidanich

60, c.a.p. 47023, Italy

For a long time polymers have supplied most of common pack-

aging materials because they present several desired features

like softness, lightness and transparency. However, increased

use of synthetic packaging films has led to a serious ecological

problems due to their total non-biodegradability. Although their

complete replacement with eco-friendly packaging films is just

impossible to achieve, at least for specific applications like food

packaging the use of bioplastics should be the future. The aim of

this review was to offer a complete view of the state of the art on

biodegradable polymer packages for food application.

IntroductionThe current global consumption of plastics is more than

200 million tonnes, with an annual grow of approximately5%, which represents the largest field of application for crudeoil. It emphasises how dependent the plastic industry is on oiland consequently how the increasing of crude oil and naturalgas price can have an economical influence on the plasticmarket (www.european-bioplastics.org). It is becoming in-creasingly important to utilize alternative raw materials. Un-til now petrochemical-based plastics such as polyethyleneterephthalate (PET), polyvinylchloride (PVC), polyethylene

(PE), polypropylene (PP), polystyrene (PS) and polyamide(PA) have been increasingly used as packaging materials be-cause their large availability at relatively low cost and be-cause their good mechanical performance such as tensileand tear strength, good barrier to oxygen, carbon dioxide, an-hydride and aroma compound, heat sealability, and so on. Butnowadays their use has to be restricted because they are notnon-totally recyclable and/or biodegradable so they pose se-rious ecological problems (www.european-bioplastics.org;Sorrentino, Gorrasi, & Vittoria, 2007). Plastic packaging ma-terials are also often contaminated by foodstuff and biologi-cal substance, so recycling these material is impracticableand most of the times economically not convenient. As a con-sequence several thousands of tons of goods, made on plasticmaterials, are landfilled, increasing every year the problem ofmunicipal waste disposal (Kirwan & Strawbridge, 2003).The growing environmental awareness imposes to packagingfilms and process both user-friendly and eco-friendly attri-butes. As a consequence biodegradability is not only a func-tional requirement but also an important environmentalattribute.

The compostability attribute is very important for bio-polymer materials because while recycling is energy expen-sive, composting allows disposal of the packages in the soil.By biological degradation it produced only water, carbondioxide and inorganic compounds without toxic residues.

According to the European Bioplastics, biopolymersmade with manufactures renewable resources have to bebiodegradable and especially compostable, so they canact as fertilizers and soil conditioners. Whereas plasticsbased on renewable resources do not necessary have to bebiodegradable or compostable, the second ones, the bio-plastic materials, do not necessary have to be based on re-newable materials because the biodegradability is directlycorrelated to the chemical structure of the materials ratherthan the origin. In particular, the type of chemical bond de-fines whether and in which time the microbes can biode-grade the material. Several synthetic polymers arebiodegradable and compostable such as starch, cellulose,lignin, which are naturally carbon-based polymers. Viceversa, same bioplastics based on natural monomer, canloose the biodegradability property through chemical mod-ification like polymerization, such as for example Nylon 9types polymers obtained from polymerization of oleic acidmonomer or Polyamid 11 obtained from the polymerizationof castor oil monomer.

635V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643

Plastics are compounds based on polymers and severalother chemicals like additives, stabilizers, colourants, pro-cessing aids, etc., which quantity and type change froma polymer to another, because each final products have tobe optimized with regard its processing and future applica-tion (Guilbert, Cuq, & Gontard, 1997; Petersen et al.,1999). For these reasons, manufacture a product usinga 100% renewable resources is neither impossible in theearly future and the tendency is to utilize the highest pro-portion of renewable resources possible. Until now bioplas-tics contain more than 50% weight of renewable resources(www.european-bioplastics.org). Many bioplastics aremixes or blends containing synthetic components, such aspolymers and additives, to improve the functional proper-ties of the finished product and to expand the range of ap-plication. If also additives and pigments can be based onrenewable resources, we can obtain a polymer with approx-imately 100% weight of biodegradation compounds.

Bioplastics, like plastics, present a large spectrum of ap-plication such as collection bags for compost, agriculturalfoils, horticultures, nursery products, toys, fibres, textiles,etc. Other fields such as packaging and technical applicationare gaining importance (www.european-bioplastics.org).

The performance expected from bioplastic materialsused in food packaging application is containing the foodand protecting it from the environment and maintainingfood quality (Arvanitoyannis, 1999). It is obvious that toperform these functions is important to control and modifytheir mechanical and barrier properties, that consequentlydepend on the structure of the polymeric packaging mate-rial. In addition, it is important to study the change thatcan occur on the characteristics of the bioplastics duringthe time of interaction with the food (Scott, 2000). As de-scribed in Biodegradable polymers applications in foodpackaging field section, the study of the literature showsup that only a limited amount of biopolymers are usedfor food packaging application. Unlike the usual wrap,films, labels and laminates came from fossil fuel resources,the use of biodegradable polymers represents a real step inthe right direction to preserve us from environmental pollu-tion. Several University Research Center in the world (Italy,Ireland, France, Greece, Brazil, USA and so on) and severalIndustry like NatureWorks LLC, are focalizing their atten-tion to the study of these bio-based materials.

From our point of view, it is important to understand notonly the physical and mechanical properties of such mate-rials for the task but also the compatibility with the food,which has been recognized as a potential source of lossin food quality properties (Halek, 1988).

Chemistry of degradationThe bioplastic aim is to imitate the life cycle of biomass,

which includes conservation of fossil resources, water andCO2 production, as described in Scheme 1 (www.european-bioplastic.org).

The speed of biodegradation depends on temperature(50e70 �C), humidity, number and type of microbes. Thedegradation is fast only if all three requirements are present.Generally at home or in a supermarket biodegradation occursvery low in comparison to composting. In industrial com-posting bioplastics are converted into biomass, water andCO2 in about 6e12 weeks (www.european-bioplastic.org).

Polymer-based products are required to biodegrade ona controlled way: natural polymer (like rubber, lignin, hu-mus) and synthetic polymer like polyolefins biodegrade fol-lowing an oxo-biodegradation mechanism (Arvanitoyannis,1999) and consequently cannot satisfy the rapid mineraliza-tion criteria requested for standard biodegradation. Also, atambient temperature, oxo-biodegradation is a slower pro-cess than hydro-biodegradation as well described by Scottand Wiles (2001). These authors explained that during theoxo-degradation of carboxylic acid, molecules of alcohols,aldehydes and ketones biodegradable with low molar massare produced by peroxidation, initiated by heat or light,which are the primary cause of the loss of mechanical prop-erties of hydrocarbon polymers. Than bacteria, fungi, en-zymes start the bioassimilation giving rise to biomass andCO2 that finally form the humus. Generally synthetic poly-mers contain antioxidants and stabilizers added to protectthe polymer against mechano-oxidation during the process-ing operation and to provide the required shelf-life. So,from one hand antioxidant is necessary to improve the per-formance of these materials but, on the other hand, for thebiodegradation process it is better to not add these mole-cules during polymer processing.

Hydro-biodegradation is the well-known process thatgives bioassimilable products from cellulose, starch, poly-esters, etc. Aliphatic polyester is hydrolyzed and bioassimi-lated rapidly in an aqueous environment in much the sameway as starch and cellulose (Scott & Wiles, 2001).

Polyolefin were selected as a basis for the study of biode-gradable polymer because they had already achieved a centralposition for packaging application, thanks to their combina-tion of flexibility, toughness, excellent barrier properties, allat low cost because coming from low value oil fraction.

Synthetic and natural polymers stand at the oppositeends of a spectrum of properties: polyolefins are hydrocar-bon hydrophobic polymers, resistant to peroxidation, bio-degradation, highly resistant to hydrolysis, which is theirmain attribute in packaging, and not biodegradable. Tomake it biodegradable it is necessary to introduce pro-oxidant additives which promote the oxo-biodegradationby producing low molar mass oxidation compounds bioas-similate from the microorganisms. Natural compounds, likecellulose, starch and so on, are hydrophilic polymers, waterwettable or swellable and consequently biodegradable.They are not technologically useful for food packagingwhere water resistant is required. Between these two ex-tremes are the hydro-biodegradable aliphatic polyesterssuch as polylactic acid (PLA) and the poly(hydroxyacid)(PHA) (Scott & Wiles, 2001).

Scheme 1. Life cycle www.european-bioplastics.org.

636 V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643

Although hydrocarbons polymers make a positive contri-bution to environment because they can be mechanically re-cycled if clean, incinerated with energy recovery, witha calorific value almost identical to the oil from which theycoming on, they are not compostable. According to the Euro-pean standard norm UNI EN 13432 (2002), a product to bedefined compostable must be biodegradable and disintegr-able in brief time, or rather it must be turned from the micro-organisms into water, carbonic and fertile anhydridecompost. Finally, to be defined compostable, the manufac-tured article must result compatible with a process of com-posting, that means it must not release dangeroussubstances and must not alter the quality of the producedcompost. The last Financial Law out as objective the dismiss-ing of the mono-use pouches not biodegradable, for food stafftransportation, within the 2010 (Scott & Wiles, 2001).

The use of long-lasting polymers as packaging materialsfor short application is not justified, also because physicalrecycling of these materials is often impractical becausefood contamination. So there is an increasing demand onthe use of biodegradable polymer which could be easily re-newable (Kale, Auras, & Singh, 2006). While most of thecommercialized biopolymer materials are biodegradable,these are not fully compostable in real composting condi-tions, which vary with temperature and relative humidity.

Barrier propertiesThe determination of the barrier properties of a polymer

is crucial to estimate and predict the product-package shelf-life. The specific barrier requirement of the package systemis related to the product characteristics and the intendedend-use application. Generally plastics are relatively per-meable to small molecules such as gases, water vapour, or-ganic vapours and liquids and they provide a broad range ofmass transfer characteristics, ranging from excellent to lowbarrier value, which is important in the case of food

products. Water vapour and oxygen are two of the mainpermeants studied in packaging applications, because theymay transfer from the internal or external environmentthrough the polymer package wall, resulting in a continuouschange in product quality and shelf-life (Germain, 1997).Carbon dioxide is now important for the packaging in mod-ified atmosphere (MAP technology) because it can poten-tially reduce the problems associated with processed freshproduct, leading a significantly longer shelf-life. For exam-ple, for fresh product respiration rate is of a great impor-tance in MAP design so identify the best packaging isa crucial factor. The most important barrier properties ofpolymer films used in packaging application are described.

Oxygen transmission rate (OTR)The oxygen barrier property of a food packaging container

for fresh product (e.g. fruits, salad, ready-to-eat meals) playsan important role on its preservation. The oxygen barrier isquantified by the oxygen permeability coefficients (OPC) whichindicate the amount of oxygen that permeates per unit of areaand time in a packaging materials [kg m m�2 s�1 Pa�1]. So,when a polymer film packaging has a low oxygen permeabilitycoefficients, the oxygen pressure inside the container drops tothe point where the oxidation is retarded, extending the shelf-life of the product. Generally the biodegradable polymers pres-ent a value one or more order of magnitude below the syntheticpolymer used in the same field like PET and OPS. Severalauthors reported in literature the oxygen permeability coeffi-cients of one of the most commercialized biodegradable poly-mer like the PLA (Auras, Harte, & Selke, 2004; Auras, Singh,& Singh, 2006; Auras, Singh, & Singh, 2005; Lehermeier,Dorgan, & Way, 2001; Oliveira et al., 2004).

Together with the permeability coefficient the oxygentransmission rate (OTR), expressed in cc m�2 s�1 is given.The OPC is correlated to the OTR by the followingequation:

637V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643

OPC¼ OTR� l=DP ð1Þ

where l is the thickness of the film (m), DP is the differencebetween oxygen partial pressure across the film [Pa].DP¼ p1� p2, where p1 is the oxygen partial pressure atthe temperature test on the test side and p2 is equal tozero on the detector side.

Water vapour transmission rate (WVTR)The water vapour barrier properties for the packaged

product whose physical or chemical deterioration is relatedto its equilibrium moisture content, are of great importancefor maintaining or extending its shelf-life. The water va-pour barrier is quantified by the water vapour permeabilitycoefficients (WVPC) which indicate the amount of watervapour that permeates per unit of area and time in a packag-ing materials [kg m m�2 s�1 Pa�1]. For fresh food productsit is important to avoid dehydration while for bakery or del-icatessen is important to avoid water permeation. TheWVPC of the PLA biodegradable polymer is reported inthe literature (Auras, Harte, Selke, & Hernandez, 2003;Auras et al., 2005, 2006).

Together with the permeability coefficient is given thewater vapour transmission rate (WVTR), expressed incc m�2 s�1 (or g m�2 day�1). The WVPC is correlated tothe WVTR as described up in Eq. (1) for the oxygenparameter.

Carbon dioxide transmission rate (CO2TR)Like the oxygen and water vapour barrier properties,

also the carbon dioxide barrier property is of particular im-portance on food packaging application. The carbon diox-ide barrier is quantified by the carbon dioxidepermeability coefficients (CO2PC) which indicates theamount of carbon dioxide that permeates per unit of areaand time in a packaging materials [kg m m�2 s�1 Pa�1]. To-gether with the permeability coefficient is given the carbondioxide transmission rate (CO2TR), expressed in cc m�2 s�1 (or g m�2 day�1). The CO2PC is correlated to theCO2TR as described up in Eq. (1).

Mechanical propertiesIt is well-known that the polymer architecture plays an

important role on the mechanical properties, and conse-quently on the process utilized to modelling the final prod-uct (injection moulding, sheet extrusion, blow moulding,thermoforming, film forming). In addition, many packagingcontainers are commercially used below room temperature,so it is important to assess the mechanical performance un-der these conditions (Auras et al., 2005).

Tensile test analyses are made to determine the tensilestrength (MPa), the percent elongation at yield (%), the per-cent elongation at break (%) and the elastic modulus (GPa)of the food polymer packaging material. These values areimportant to get mechanical information of the biopolymermaterials to be compared with the commercial

nonbiodegradable ones (ASTM D882-02, Standard TestMethod for Tensile Properties of Thin Plastic Sheeting).Impact properties test is a method utilized to determinethe energy that causes the plastic to fail under specific im-pact conditions, conducted following the ASTM D1709-03,Standard Test Methods for Impact Resistance of PlasticFilm by the Free-Falling Dart Method.

The compression test is normally conducted on thermo-formed sample, according to the ASTM D642, StandardTest Method for Determining Compressive Resistance ofShipping Containers, Components, and Unit Loads. Natu-rally the compression strength is function of the materialand of design (shape and size).

Chemical resistance propertiesProducts that could be packaged in this kind of con-

tainers may have weak or strong acid characteristics; so itis necessary to assess the performance and the suitabilityof biopolymers stored with common food packaging solu-tion as a function of time. The interaction and absorptionbetween chemical compounds and polymer may affect thefinal mechanical properties of a polymer (Auras et al.,2005). Normally the chemical resistance is tested measur-ing the tensile stress, elongation at break and modulus ofelasticity of sample submerged in weak and strong acid so-lutions as a function of time, simulating real conditions, atambient temperature (23 �C) and at �18 �C, �23 �C and�29 �C. The weak acid solution is prepared with aceticacid while the strong acid solution is prepared with hydro-chloric acid (Auras et al., 2005).

Some important production considerationCurrently, there are several types of bio-based polymers

on the market: same coming from petrochemical monomer,like certain types of polyester, polyester amides and polyvi-nyl alcohol, produced by different manufacturer, used prin-cipally as films or moulding. Four other bio-based polymersare starch materials, cellulose materials, polylactic acid(Polyester, PLA), polyhydroxy acid (polyester, PHA). Untilnow, the PHA polymer is a very expensive polymer becauseit is commercially available in very limited quantities. PLAis becoming a growing alternative as a green food packag-ing materials because it was found that in many situations itperforms better than synthetic ones, like oriented polysty-rene (OPS) and PET materials (Auras et al., 2005).

Different types of materials can be combined to formblend or compounds or semifinished products such as films.

The degradation of the materials is normally studied underreal compost conditions and under ambient exposure by dif-ferent techniques. The polymer degradation rate is deter-mined by the nature of the functional groups and thepolymer reactivity with water and catalysts. Any factor whichaffects the reactivity such as particle size and shape, temper-ature, moisture, crystallinity, isomer percentage, residualmonomer concentration, molecular weight, molecular weightdistribution, water diffusion, metal impurities from the

638 V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643

catalyst, will affect the polymer degradation rate (Kale et al.,2006). In general high temperature and humidity will degrademore rapidly the polymer. By visual inspection the packagesare observed for colour, texture, shape and change in dimen-sion. Generally a digital camera is used to take the pictures.

The thickness of the packages is determined by a thick-ness gauge according to the ASTM norm D4166-99(2004)e1, Standard Test Method for Measurement ofThickness of Nonmagnetic Materials by Means of a DigitalMagnetic Intensity Instrument, or by micrometer accordingto ASTM D 374-99, Standard Test Methods for Thicknessof Solid Electrical Insulation.

By gel permeation chromatography it is possible to de-termine the molecular weight of samples dissolved in theappropriate solvent. Molecular weight variations are anindication of the degradation rate of the polymers andgive information about when the main fragmentation oc-curs in a polymer.

By differential scanning calorimetry (DSC) it is possibleto determine the glass transition temperature (Tg), meltingtemperature (Tm) and crystallinity of the polymer sample(ASTM D3418, Standard Test Method for Transition Tem-peratures and Enthalpies of Fusion and Crystallization ofPolymers by Differential Scanning Calorimetry). The crys-tallinity is determined according to ASTM D3417-97 andusing the following equation, well-known in the literature,

xcð%Þ ¼ 100� ðDHcþDHmÞ=DHcm ð2Þ

where DHc is the exothermic enthalpy of cold crystalliza-tion, DHm is the endothermic enthalpy of fusion, DHc

m isthe endothermic heat of melting of purely crystalline poly-mer under study (for example: for PLA is 135 J g�1, Kaleet al., 2006; for PET is 125.6 J g�1, Auras et al., 2003).

By thermo-gravimetric analysis (TGA) it is possible toobtain the decomposition temperature, according to theASTM E1131-03, Standard Test Method for CompositionalAnalysis by Thermogravimetry.

The determination of the pH of the sample surroundingis one of the most important factors of hydrolytic polymerdegradation since pH variations can change hydrolysisrates by few order of magnitude. The chemical resistanceis normally determined exposing the materials to weakacid (pH¼ 6, acetic acid solution) and strong acid(pH¼ 2, hydrochloric acid solution) for a period of 0, 1,3, 5 and 7 days.

The most important analysis for film used in food pack-aging application is the determination of the oxygen, car-bon dioxide and water vapour transmission rate (OTR,CO2TR and WVTR, respectively). These tests are per-formed according to the ASTM norm described before.

Concerning the mechanical properties, the samplescould be analysed by Impact tests, Tensile properties andCompression Test of Thermoformed Containers. Generallythese parameters are studies at ambient temperature (22 �C)and at frozen food storage temperatures of �18 �C,

�23 �C and �29 �C, since fresh produce packaging anddeli containers are generally used commercially at thisconditions.

Biodegradable polymers applications in foodpackaging field

The field of application of biodegradable polymer infood-contact articles includes disposable cutlery, drinkingcups, salad cups, plates, overwrap and lamination film,straws, stirrers, lids and cups, plates and containers forfood dispensed at delicatessen and fast-food establish-ments. These articles will be in contact with aqueous,acidic and fatty foods that are dispensed or maintained ator below room temperature, or dispensed at temperaturesas high as 60 �C and then allowed to cool to room temper-ature or below (Conn et al., 1995).

In the last few years, polymers that can be obtained fromrenewable resources and that can be recycled and com-posted, have garnered increasing attention. Also their opti-cal, physical and mechanical properties can be tailoredthrough polymer architecture so as a consequence, biode-gradable polymers can be compared to the other syntheticpolymers used in fresh food packaging field, like themost common oriented polystyrene (OPS) and polyethyleneterephthalate (PET).

Depending on the production process and on the source,biopolymers can have properties similar to traditional ones.They are generally divided into three groups: polyesters;starch-based polymer; and others.

PolyestersThese materials can be:

i. Polymers directly extracted from biomass like pro-teins, lipids, polysaccharides, etc.

ii. Polymeric materials synthesized by a classical poly-merization procedure such as aliphaticearomaticcopolymers, aliphatic polyesters, polylactide aliphaticcopolymer (CPLA), using renewable bio-based mono-mers such as poly(lactic acid) and oil-based mono-mers like polycaprolactones.

iii. Polymeric materials produced by microorganisms andbacteria like polyhydroxyalkanoates.

Aliphaticearomatic copolymersThese materials are a combination of polyetilene tere-

phthalate (PET), resistant to microbial attack, with three ormore biodegradable aliphatic polyesters. It is soft, pliablewith a good touch but with a melting point of around 200 �C,too high for a degradable material. The aliphatic monomer cre-ates a weak spots in the aromatic polymeric chain which makesthem susceptible to degradation through hydrolysis.

Also if it is totally biodegradable, coming from fossilfuels such as oil, coal and natural gas, PET production

639V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643

causes a consume of non-renewable and finite resourcesand with an heavy impact on global waste disposal. If prop-erly disposed it degrades in 8 weeks but if it is trash withoutany control in the environment, the degradation process cantake 50 years to breakdown the structure. The long polymermolecules are cleaved by moisture into smaller ones, whichare naturally consumed by microbes and converted to car-bon dioxide and water. This material is commonly usedfor eating utensils (like fork, knife, spoon, dishes and soon) and bottles but it costs twice as much than commercialones (Salt, 2002). DuPont (Tennessee) produce the PET/hy-dro-biodegradable polyester under the trade name of Bio-max, exported in all over the world.

Aliphatic polyestersThese materials have properties similar to PE and PP

polymers, they are biodegradable but with lack in thermaland mechanical properties. These materials come frompolycondensation reaction of glycol and aliphatic dicarbox-ylic acid, both obtained from renewable resources. They areodourless and can be used for beverage bottles and theybiodegrade in soil and in water giving carbon dioxide andwater, in a period of 2 months (e.g. for a 0.04 mm thickfilm) (www.designinsite.dk).

A commercially available aliphatic copolyester is pro-duced by Procter and Gamble Co. (P&G, Cincinnati, OH)with the trade name of Nodax and it can degrade in aerobicand anaerobic environmental conditions. The other oneis the Eastar bIo, produced from the Eastam ChemicalCompany (Hartlepool, UK).

Table 1. Physical experimental data for PLA (Auras et al., 2006)

Experimental data PLA

Tg (�C) 62.1� 0.7Tm (�C) 150.2� 0.5DHc

m (J g�1) 93Percent crystallinity (xc) 29.0� 0.5Oxygen transmission rate (OTR)(cc m�2 day)a

56.33� 0.12

Polylactide aliphatic copolymer (CPLA)This material is a mixture between renewable resources

as lactide and aliphatic polyesters like dicarboxylic acid orglycol, with hard (like PS) and soft flexible (like PP) prop-erties, depending on the amount of aliphatic polyester pres-ent in the mixture. It is easy to process and thermally stableup to 200 �C. The heating value and the quantity of carbondioxide generated during combustion are about the half ofthat generated from commercial polymer like PE and PP,and incineration does not produce toxic substances. In nat-ural environment it starts to degrade in 5e6 months, witha complete decomposition after 12 months. If compostedwith food garbage, it begins to decompose after 2 weeks.

Oxygen permeabilityrate (OPC) (kg m m�2 s�1 Pa�1)b

4.33E-18� 1.00E-19

Water vapour transmissionrate (WVTR) (g m�2 day�1)a

15.30� 0.04

Water vapour permeabilityrate (WVPC) (kg m m�2 s�1 Pa�1)c

1.34E-14� 3.61E-17

a Thickness of 20.0� 0.2.b OPC¼OTR� l/DP, with l is the thickness in m and DP is the

difference in oxygen partial pressure across the film.c WVPC¼WVTR� l/DP, with l is the thickness in m and DP is

the difference in water vapour partial pressure across the film.

Polycaprolactone (PCL)It is a fully biodegradable polymer coming from the po-

lymerization of not renewable raw material, like crude oil.It is a thermoplastic polymer with good chemical resistanceto water, oil, solvent and chlorine, with a melting point of58e60 �C, low viscosity, easy to process and with a veryshort degradation time. It is not used for food applicationbut if mixed with starch it is possible to obtain a good bio-degradable material at a low price, used for trash bags.

Poly(lactic acid) (PLA)One of the most promising biopolymer is the poly(lactic

acid) (PLA) obtained from the controlled depolymerizationof the lactic acid monomer obtained from the fermentationof sugar feedstock, corn, etc., which are renewable resourcesreadily biodegradable (Cabedo, Feijoo, Villanueva, Lagaron,& Gimenez, 2006). It is a versatile polymer, recyclable andcompostable, with high transparency, high molecular weight,good processability and water solubility resistance. In gen-eral commercial PLA is a copolymer between poly(L-lacticacid) and poly(D-lactic acid). Depending on the L-lactide/D-lactide enantiomers ratio, the PLA properties can vary con-siderably from semicrystalline to amorphous ones. Re-searches carried out to improve the performance quality ofthis material are made on PLA with D-lactide content lessthan 6%, which is the semicrystalline polymer. Howeverthe amorphous one, containing 12% of D-lactide enantiomer,is easy to process by thermoforming, which is the actual tech-nology in the food packaging sector, and it shows propertieslike polystyrene. This material is commercialized by differ-ent companies with different commercial names, like for ex-ample the Natureworks� PLA produced by Natureworks�LLC (Blair, NB). Currently it is used in food packaging appli-cation only for short shelf-life products.

In Table 1 physical characteristics of PLA obtained fromAuras et al. (2006) are reported.

Kale et al. (2006) studied the compostability of threecommercially available biodegradable packages made ofPLA, in particular water bottles, trays and deli containers,to composting and to ambient exposure. They investigatethe properties breakdown of these packages exposed tocompost conditions by several experimental procedure in-volving gel permeation chromatography (GPC), differentialscanning calorimetry (DSC), thermal gravimetric analysis(TGA) and visual inspection. The compost pile was pre-pared with cow manure, wood shaving and waste feed, at

640 V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643

a temperature above 60 �C. After 3 weeks the pile was puton asphalt pad. The initial pile temperature, relative humid-ity and pH was 65� 5 �C, 63� 5% and 8.5� 0.5, respec-tively. The packages were subjected to composting for 1,2, 4, 6, 9, 15 and 30 days and packages exposed to ambientconditions were also studied. This is because it is well-known that PLA absorb water, resulting in the hydrolysisand cleavage of the ester linkages, autocatalyzed by the car-boxylic acid end groups (Scheme 2).

They found that the degradation rate changes with the ini-tial crystallinity and L-lactide content of the packages, withlower degradation for the polymer with the highest contentof L-lactide (96 versus 94%) due to the higher crystallinity,which makes more difficult the degradation of the wholestructure. During the hydrolysis process the sample decreasedin size (reduction of the thickness) and became tough (in-crease in fragility) with a trend that depend to the shape.The dimension of the containers before and after compostingwas calculated by measuring the variation on width, length,height and thickness. During the first period of the ambient ex-posure (within the first 15 days) the molecular weight of thesample Mw showed a small increase probably due to UV orgamma radiation which produces chain cleavage and subse-quent recombination, which can result in crosslinking andhence an increase in the Mw. The increase in molecular weightproduced an increase in the glass transition temperature Tg

and leading to slower degradation since glassy polymers de-grade more slowly than rubbery ones. The same trend was ob-served for the sample exposed to the compost pile; the Mw

increase could be attributed to crosslinking or recombinationreactions. Subsequently, during exposure, the degradationproduced a molecular weight decrease that followed a first or-der kinetic associated to a first order hydrolysis process af-fected by the initial crystallinity, thickness and shape of thesample (Tsuji & Ikada, 1998). Since the hydrolysis occurs

nHO

O

O

O

O

OO

OCH3

CH3

CH3

CH3

H2O

HO

HO

O

O

O

H

OO

OCH3

CH3

CH3

n

+

Scheme 2. PLA hydrolysis an

randomly, longer PLA chain is more susceptible to cleavagethan the shorter ones. The fragmentation process, which pro-duces decomposition of the macromolecules into shorter olig-omer chains and monomers, took place giving an initial rise ofthe polydispersity index (PDI). Afterwards, polymer frag-mentation took place with a decrease in PDI until 1.00 value,when only oligomers of the PLA chains are present.

After the first 4 days where an increase of Tg was ob-served correlated to a short increments of Mw, the valueshowed a reduction which was obviously associated tothe molecular weight reduction, starting from about 62 �Cto 30 �C. The Tg reduction of the PLA packages exposedto compost conditions followed a linear trend, with a reduc-tion in �C for day which depend on the shape of the con-tainers (Kale et al., 2006). The Tm variation as a functionof time did not follow a linear relationship, with a slight in-crease of Tm for the samples submitted to compost condi-tions at the beginning of the composting process.

The decomposition temperature TD, determined bythermo-gravimetric analysis (TGA), associated to depoly-merization (rapid reduction of polymer mass with a slowreduction in molecular weight) and random degradation(slow loss of polymer mass with an exponential decreasein molecular weight) was determined for PLA samples ex-posed to ambient and composting conditions. No variationof TD as a function of time was observed for the sample ex-posed to ambient condition while for the other ones a reduc-tion of TD was observed, with a linear variation.

PLA packages will compost in municipal or industrialfacilities but the PLA degradation is driven by hydrolysiswhich needs higher temperatures to take place, so a com-pletely compost will be difficult. Further studies will benecessary to find method and techniques that can assessthe degradability of the biodegradable food packaging un-der real composting conditions.

O poly

O

O

O

O poly

poly

O

O

OCH3

CH3

CH3

CH3OH OHn

O

O

CH3

HO

d molecular cleavage.

641V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643

As reported from Kale et al. (2006), Pometto et al. in a pre-vious research studied the banana field exposition of PLAfilm in Costa Rica. They found that this film lost its mechan-ical properties faster than during exposure in simulated con-ditions in the laboratory, with a degradation enhanced by anincrease in temperature and relative humidity. This data notconcern complete packages degradation.

Concerning the PLA toxicology for human safety, Connet al. (1995) studied the migration of small molecules com-ing from the hydrolytic degradation phenomena of PLApolymer films of food-contact articles, which are lacticacid (a safe food substance), the monomer lactide and thelinear dimer of lactic acid which is lactoyllactic acid. Inany case dimers and oligomers hydrolyse in aqueous sys-tem to lactic acid, which is a common food ingredientthat has been shown to be safe in food at levels far in excessof any small amount that might result from the intendeduses of PLA. They studied the PLA components migrationby extraction tests in which samples of the polymer wereexposed to food-simulating solvents under conditions thatreproduce the most severe temperature/time conditions towhich food would be exposed while in contact with PLA.The examined contact was with aqueous, acidic and fattyfoods. It was found that in any case migrants from PLAother that lactic acid (dimers, trimers, etc.) representedvery small and safe amounts. Migrating quantities of thesespecies hydrolyse to lactic acid in the aqueous and acidicmedia commonly found in food and in the stomach. Lactidehas demonstrated low intrinsic toxicity in testing while thelactoyllactic acid is normally present in commerciallyavailable lactic acid that is an evidence of its safety.

Concerning the optical, physical and mechanical perfor-mance of the oriented PLA polymer (OPLA) in food appli-cation, Auras, Singh & Singh (2006) made a comparisonwith two of the commonly used materials used for freshfood packaging application, which are polyethylene tere-phthalate (PET) and oriented polystyrene (OPS). The phys-ical experimental data obtained on PLA samples arereported in Table 1.

Concerning the physical data, OPLA presents the lowestTg and Tm data respect the PET and OPS polymers, whilethe crystallinity is very similar to that of PET (OPS is atacticand does not crystallize so it is highly transparent). About theoxygen transmission rate it was found that OPLA is a goodfilm for food like tomato and other breathable products wherethe oxygen and carbon dioxide barrier requirements are spe-cifically matched to the respiration rate of the fresh produce.In order to maintain the freshness property and shelf-life offruit and vegetable, it is necessary to control their storageconditions, like humidity and quantity of gases (oxygen, car-bon dioxide and ethylene). Usually the specific gas require-ments are achieved by controlling the type of films used aspackaging materials for different atmospheric conditions.As far as mechanical properties are concerned, it was foundthat at room temperature the three polymers showed similartensile strength while at temperature besides that, were

higher than those for PETand OPS. The modulus of elasticityshowed a similar trend with the best value for the OPLA poly-mer, while the elongation at break was similar at room tem-perature for the three polymers but was much higher for PETat value below the room temperature. The compression test ofthermoformed containers had point out that OPLA and OPShave similar compression strength while the PET containersshowed the best value but in this case it was not possible togive an conclusive information about the overall perfor-mance because the containers had different shapes.

The results for chemical resistance tests showed that expo-sure to acid and vegetable oil resulted in a minimal strengthdegradation PLA (and also for the other two polymer PETand OPS). PLA studied in these conditions has showed thatwhen it is submerged to weak acid solution there is an in-crease of tensile strength, it becomes more ductile and thereis a reduction in the modulus of elasticity as a function oftime. For sample submerged in strong acid solution therewas an increase of tensile stress, no variation in the elonga-tion at break, it becomes more brittle with an increase inthe modulus of elasticity which is an indication of the brittle-ness of the sample as a function of time.

The same mechanical properties were measured whenPLA sample containers were exposed to vegetable oil andit was found that there was a decrease of the tensile stress,a reduction of the elongation at break and an increase of themodulus of elasticity.

Based on the experimental researches made until now ithas been found that PLA is safe and generally recognizedas safe for its use in food-contact articles. It has the advantageof easily tailoring their physical properties by changing thechemical composition (amount of L- and D-isomer) and theprocessing conditions. PLA packages perform, as well asother containers made on synthetic polymer like PET, PS,etc., at room and low temperature, suggesting that PLAwould also be suitable for the same food application. How-ever, same properties such as flexural properties, gas perme-ability, impact strength, processability, etc., are often notgood enough for this application. This material shows goodbarrier to aroma but the most important limitation on theuse of PLA for food application packaging is the mediumbarrier to gases and vapours and the brittleness properties.A possible strategy to decrease the brittleness is to makea blend between PLA and others polymer. Cabedo et al.(2006) studied the blend of PLA with polycaprolactone(PCL), which is also a biodegradable semicrystalline poly-mer obtained from the polymerization of 3-caprolactone. Itshowed low tensile strength, high elongation at break, andprocessing temperature similar to the PLA and it can be uti-lized like plasticizer to increase the gas permeability of thePLA as a consequence of the poor gas barrier properties ofPCL. In this research to the PLA/PCL blend they addedalso kaolinite nanocomposites by melt mixing using a con-ventional polymer extrusion process. By SEM analysisthey found that this blend is immiscible across the composi-tion range studied, but is was observed a plasticization effect

642 V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643

of the blend compared to the PLA matrix (by Dynamic-mechanical analysis, DMA) and a slight increase in its ther-mal stability (by Thermo-gravimetric analysis, TGA) with anincrement of this effect with the PCL amount increment. Thegas barrier properties showed a significant decrease propor-tional to the amount of PCL added to the blend, which wascompensated in the sample containing kaolinite which showsan increase in the gas barrier properties. Anyway, thesechanges were clearly discernible but small. The effect ofthe nanocomposites is currently under study but it is clearthat these compounds could be a valid route to decrease theinherent rigidity of some biopolymers and to enhance theirapplications.

Further studies on PLA products must be performed to de-termine the range of compatibility of this polymer and to de-termine the performance in real shelf-life studies. A study ofthe Life-Cycle Assessment (LCA) for the PLA polymer wasjust made by Bohlmann (2004), who made a comparisonwith the most used polypropylene (PP) in food packaging ap-plication. He found that PLA is more energy efficient than PPpolymer because PLA consumes no feedstock energy. How-ever, when it is taken in consideration the uncertainty of theestimation, the difference between the two polymers be-comes marginalized. He found also that the PLA and PPgreenhouse gas emission are equivalent.

Fang et al. (2005) studied the possibility to make a multi-layer film with PLA and a natural material like modified starchto have equal or better performance characteristics to those ofexisting product not biodegradable like polyethylene/polyvi-nylidenchloride/polystyrene (PE/PVDC/PS) multilayer films.Starch is a totally biodegradable polymer coming from agri-culture; it is abundant, renewable, safe and economics but asa component of biodegradable laminate film, it shows no plas-tic behaviour, no adequate mechanical properties and it ther-mally degrades at around 260 �C. When it is extruded incombination with plasticizers it becomes thermoplastic,mouldable and an amorphous material with an excellent oxy-gen barrier characteristic, but it is extremely sensitivity to theenvironmental humidity giving rapid biodegradation.

Polyhydroxyalkanoates (PHA)These polymers are produced in nature by bacterial fer-

mentation of sugar and lipids. They can be thermoplastic orelastomeric materials, with a melting point between 40�Cand 180 �C, depending on the monomer used in the synthesis.These polymers, alone or in combination with synthetic plas-tic or starch give excellent packaging films (Tharanathan,2003). The most common type is the polyhydroxybutyrate(PHB), coming from the polymerization of 3-hydroxybuty-rate monomer, with properties similar to PP but more stifferand brittle. The copolymer polyhydroxybutyrate-valerate(PHBV), used as packaging material, is less stiff and tougher.The price is very high but it degrades in 5e6 weeks in a micro-biology active environments, giving carbon dioxide and wa-ter in aerobic condition. In anaerobic environment thedegradation is faster, with production of methane.

Yu, Chua, Huang, Lo, and Chen (1998) used different typesof food wastes as carbon source to produce several PHA poly-mers with different physical and mechanical properties, likeflexibility, tensile strength, melting viscosity. The use offood waste is a good way to reduce the cost of bioplastics pro-duction, but until now it is only an experimental procedurewithout any possibility to have a commercial application.

Starch-based polymerCommercial polymer coming from the synthesis of oil-

based monomer can be mixed with different percentage(10, 50 and 90%) of starch used as additive. Dependingon starch percentage and other materials like additives (col-ouring additives, flame retardant additives) the properties ofthese materials can be varied a lot, becoming stable to un-stable for example in hot/cold water.

Starch, consumed by microbial action, accelerates thedisintegration or fragmentation of polymer chain by pro-ducing pores in the materials which weaken them. This pro-cess is quite slow, it can be accelerated only if the starchadded to the mixture exceed 60%. Depending on the typeof the thermoplastic starch materials, they can degrade in5 days in aqueous aerobic environment, in 45 days in con-trolled compost and in water.

In 1993 LDPE-starch blend were commercialized underthe trade name Ecostar�. Other commercial trade namesare Bioplast� (from Biotec GmbH) and NOVON� (fromNOVON International) (www.designinsite.dk).

Starch can be transformed also into a foamed material us-ing water steam, replacing the polystyrene foam as packagingmaterial. It can be pressed into trays or disposable dishes anddissolves in water leaving a non-toxic solution, consumed bymicrobic environment in about 10 days giving only water andcarbon dioxide as by-products. The commercial trade namesare Biopur� (from Biotec GmbH), Eco-Foam� (from Na-tional Starch & Chemical) and Envirofill� (from Norel).

Others biomaterials not used in food applicationAnother natural plastic material, the casein formalde-

hyde, can be generated from a natural protein obtainedfrom milk, horn, soy bean, wheat, etc. It looks like cellu-loid, ivory or artificial horn and it is insoluble in water, in-flammable and odourless. This material is used to makebuttons, pins, cigarette-cases, umbrella handles and so onbut not in food application.

The cellulose acetate (CA) is an amorphous tough ther-moplastic obtained by introducing the acetyl radical of ace-tic acid into cellulose (cotton or wood). To decrease itsinflammability it is used with additives, with self-extin-guishing properties. Cellulose acetate is an insulator mate-rial with a little tendency to electrostatic chargin, brittleunder freezing point. Horn is an organic thermoplastic ma-terial containing 80% of keratin; it can be pressed in vari-ous objects and laminas, like buttons, combs, pens, etc.(www.designinsite.dk).

643V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643

ConclusionsBioplastics development is just beginning; until now it

cover approximately 5e10% of the current plastic market,about 50,000 t in Europe. The European countries with thehighest utilization of bioplastics are France, Germany, En-gland, Netherland and Italy but other countries like Bel-gium, Austria, Spain and Switzerland are going to utilizeit in individual applications. The principal field regardsthe use of films packaging for food products, loose filmused for transport packaging, service packaging like carrybags, cups, plates and cutlery, biowaste bags, in agri- andhorticultural fields like bags and compostable articles.

Their development costs are high and yet they do not havethe benefit of economic scale. The increased utilization ofbiomass as energy source and raw materials is necessary inthe long term due to the fact that the crude oil and naturalgas resources are limited, but it is to be keep in mind that thesematerials have to be found place in a very strong internationalmarket of synthetic ones, with an annual plastics consump-tion of approximately 200 million tons, with approximatelya 5% average growth per annum. However, plastics and bio-plastics cover an abundance of types, each with its own indi-vidual profile, so they present an enormous diversity whichmakes them so successful in numerous applications.

It was shown that polyolefins present the same oxo-bio-degradability of biopolymers, but they are more economicaland effecting during use, so certain they will remain thematerials of choice for packaging application.

Bio-based polymers have already found important appli-cations in medicine field, where cost is much less importantthan function. It seems very unlikely that biodegradable oil-based polymers will be displaced from their current role inpackaging application, where cost is more important for theconsumer market than environmental acceptability.

Biopolymers fulfill the environmental concerns but theyshow some limitations in terms of performance like thermalresistance, barrier and mechanical properties, associatedwith the costs. Then, this kind of packaging materials needsmore research, more added value like the introduction ofsmart and intelligent molecules (which is the nanotechnologyfield) able to give information about the properties of the foodinside the package (quality, shelf-life, microbiological safety)and nutritional values. It is necessary to make researches onthis kind of material to enhance barrier properties, to ensurefood properties integrity, to incorporate intelligent labelling,to give to the consumer the possibility to have more detailedproduct information than the current system.

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