biodegradablepolymers by adelaide17madette


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


      polymers for food                                                    (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-
    packaging: a review                                                    cause their good mechanical performance such as tensile
                                                                           and tear strength, good barrier to oxygen, carbon dioxide, an-
                                                                           hydride and aroma compound, heat sealability, and so on. But
      Valentina Siracusaa,*, Pietro                                        nowadays their use has to be restricted because they are not
    Rocculib, Santina Romanib and                                          non-totally recyclable and/or biodegradable so they pose se-
                                                                           rious ecological problems (;
                Marco Dalla Rosab                                          Sorrentino, Gorrasi, & Vittoria, 2007). Plastic packaging ma-
                                                                           terials are also often contaminated by foodstuff and biologi-
 Department of Physical and Chemical Methodology                           cal substance, so recycling these material is impracticable
  for Engineering, Engineering Faculty, University of                      and most of the times economically not convenient. As a con-
     Catania, Viale A. Doria 6, 95125 Catania, Italy                       sequence several thousands of tons of goods, made on plastic
   (Tel.: D39 095 7382755; fax: D39 095 333231;                            materials, are landfilled, increasing every year the problem of
                      e-mail:                     municipal waste disposal (Kirwan & Strawbridge, 2003).
b                                                                          The growing environmental awareness imposes to packaging
 Department of Food Science, Alma Mater Studiorum,
University of Bologna, Cesena (FC), Piazza Goidanich                       films and process both user-friendly and eco-friendly attri-
                               60, c.a.p. 47023, Italy                     butes. As a consequence biodegradability is not only a func-
                                                                           tional requirement but also an important environmental
For a long time polymers have supplied most of common pack-                    The compostability attribute is very important for bio-
aging materials because they present several desired features              polymer materials because while recycling is energy expen-
like softness, lightness and transparency. However, increased              sive, composting allows disposal of the packages in the soil.
use of synthetic packaging films has led to a serious ecological            By biological degradation it produced only water, carbon
problems due to their total non-biodegradability. Although their           dioxide and inorganic compounds without toxic residues.
complete replacement with eco-friendly packaging films is just                  According to the European Bioplastics, biopolymers
impossible to achieve, at least for specific applications like food         made with manufactures renewable resources have to be
packaging the use of bioplastics should be the future. The aim of          biodegradable and especially compostable, so they can
this review was to offer a complete view of the state of the art on        act as fertilizers and soil conditioners. Whereas plastics
biodegradable polymer packages for food application.                       based on renewable resources do not necessary have to be
                                                                           biodegradable or compostable, the second ones, the bio-
                                                                           plastic materials, do not necessary have to be based on re-
                                                                           newable materials because the biodegradability is directly
    The current global consumption of plastics is more than
                                                                           correlated to the chemical structure of the materials rather
200 million tonnes, with an annual grow of approximately
                                                                           than the origin. In particular, the type of chemical bond de-
5%, which represents the largest field of application for crude
                                                                           fines whether and in which time the microbes can biode-
oil. It emphasises how dependent the plastic industry is on oil
                                                                           grade the material. Several synthetic polymers are
and consequently how the increasing of crude oil and natural
                                                                           biodegradable and compostable such as starch, cellulose,
gas price can have an economical influence on the plastic
                                                                           lignin, which are naturally carbon-based polymers. Vice
market ( It is becoming in-
                                                                           versa, same bioplastics based on natural monomer, can
creasingly important to utilize alternative raw materials. Un-
                                                                           loose the biodegradability property through chemical mod-
til now petrochemical-based plastics such as polyethylene
                                                                           ification like polymerization, such as for example Nylon 9
terephthalate (PET), polyvinylchloride (PVC), polyethylene
                                                                           types polymers obtained from polymerization of oleic acid
                                                                           monomer or Polyamid 11 obtained from the polymerization
* Corresponding author.                                                    of castor oil monomer.
0924-2244/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
                              V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643                          635

    Plastics are compounds based on polymers and several                  The speed of biodegradation depends on temperature
other chemicals like additives, stabilizers, colourants, pro-         (50e70  C), humidity, number and type of microbes. The
cessing aids, etc., which quantity and type change from               degradation is fast only if all three requirements are present.
a polymer to another, because each final products have to              Generally at home or in a supermarket biodegradation occurs
be optimized with regard its processing and future applica-           very low in comparison to composting. In industrial com-
tion (Guilbert, Cuq, & Gontard, 1997; Petersen et al.,                posting bioplastics are converted into biomass, water and
1999). For these reasons, manufacture a product using                 CO2 in about 6e12 weeks (
a 100% renewable resources is neither impossible in the                   Polymer-based products are required to biodegrade on
early future and the tendency is to utilize the highest pro-          a controlled way: natural polymer (like rubber, lignin, hu-
portion of renewable resources possible. Until now bioplas-           mus) and synthetic polymer like polyolefins biodegrade fol-
tics contain more than 50% weight of renewable resources              lowing an oxo-biodegradation mechanism (Arvanitoyannis,
( Many bioplastics are                  1999) and consequently cannot satisfy the rapid mineraliza-
mixes or blends containing synthetic components, such as              tion criteria requested for standard biodegradation. Also, at
polymers and additives, to improve the functional proper-             ambient temperature, oxo-biodegradation is a slower pro-
ties of the finished product and to expand the range of ap-            cess than hydro-biodegradation as well described by Scott
plication. If also additives and pigments can be based on             and Wiles (2001). These authors explained that during the
renewable resources, we can obtain a polymer with approx-             oxo-degradation of carboxylic acid, molecules of alcohols,
imately 100% weight of biodegradation compounds.                      aldehydes and ketones biodegradable with low molar mass
    Bioplastics, like plastics, present a large spectrum of ap-       are produced by peroxidation, initiated by heat or light,
plication such as collection bags for compost, agricultural           which are the primary cause of the loss of mechanical prop-
foils, horticultures, nursery products, toys, fibres, textiles,        erties of hydrocarbon polymers. Than bacteria, fungi, en-
etc. Other fields such as packaging and technical application          zymes start the bioassimilation giving rise to biomass and
are gaining importance (                CO2 that finally form the humus. Generally synthetic poly-
   The performance expected from bioplastic materials                 mers contain antioxidants and stabilizers added to protect
used in food packaging application is containing the food             the polymer against mechano-oxidation during the process-
and protecting it from the environment and maintaining                ing operation and to provide the required shelf-life. So,
food quality (Arvanitoyannis, 1999). It is obvious that to            from one hand antioxidant is necessary to improve the per-
perform these functions is important to control and modify            formance of these materials but, on the other hand, for the
their mechanical and barrier properties, that consequently            biodegradation process it is better to not add these mole-
depend on the structure of the polymeric packaging mate-              cules during polymer processing.
rial. In addition, it is important to study the change that               Hydro-biodegradation is the well-known process that
can occur on the characteristics of the bioplastics during            gives bioassimilable products from cellulose, starch, poly-
the time of interaction with the food (Scott, 2000). As de-           esters, etc. Aliphatic polyester is hydrolyzed and bioassimi-
scribed in Biodegradable polymers applications in food                lated rapidly in an aqueous environment in much the same
packaging field section, the study of the literature shows             way as starch and cellulose (Scott & Wiles, 2001).
up that only a limited amount of biopolymers are used                     Polyolefin were selected as a basis for the study of biode-
for food packaging application. Unlike the usual wrap,                gradable polymer because they had already achieved a central
films, labels and laminates came from fossil fuel resources,           position for packaging application, thanks to their combina-
the use of biodegradable polymers represents a real step in           tion of flexibility, toughness, excellent barrier properties, all
the right direction to preserve us from environmental pollu-          at low cost because coming from low value oil fraction.
tion. Several University Research Center in the world (Italy,             Synthetic and natural polymers stand at the opposite
Ireland, France, Greece, Brazil, USA and so on) and several           ends of a spectrum of properties: polyolefins are hydrocar-
Industry like NatureWorks LLC, are focalizing their atten-            bon hydrophobic polymers, resistant to peroxidation, bio-
tion to the study of these bio-based materials.                       degradation, highly resistant to hydrolysis, which is their
    From our point of view, it is important to understand not         main attribute in packaging, and not biodegradable. To
only the physical and mechanical properties of such mate-             make it biodegradable it is necessary to introduce pro-
rials for the task but also the compatibility with the food,          oxidant additives which promote the oxo-biodegradation
which has been recognized as a potential source of loss               by producing low molar mass oxidation compounds bioas-
in food quality properties (Halek, 1988).                             similate from the microorganisms. Natural compounds, like
                                                                      cellulose, starch and so on, are hydrophilic polymers, water
                                                                      wettable or swellable and consequently biodegradable.
Chemistry of degradation                                              They are not technologically useful for food packaging
   The bioplastic aim is to imitate the life cycle of biomass,        where water resistant is required. Between these two ex-
which includes conservation of fossil resources, water and            tremes are the hydro-biodegradable aliphatic polyesters
CO2 production, as described in Scheme 1 (www.                        such as polylactic acid (PLA) and the poly(hydroxyacid)                                             (PHA) (Scott & Wiles, 2001).
636                           V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643

                                          Scheme 1. Life cycle

   Although hydrocarbons polymers make a positive contri-             products. Water vapour and oxygen are two of the main
bution to environment because they can be mechanically re-            permeants studied in packaging applications, because they
cycled if clean, incinerated with energy recovery, with               may transfer from the internal or external environment
a calorific value almost identical to the oil from which they          through the polymer package wall, resulting in a continuous
coming on, they are not compostable. According to the Euro-           change in product quality and shelf-life (Germain, 1997).
pean standard norm UNI EN 13432 (2002), a product to be               Carbon dioxide is now important for the packaging in mod-
defined compostable must be biodegradable and disintegr-               ified atmosphere (MAP technology) because it can poten-
able in brief time, or rather it must be turned from the micro-       tially reduce the problems associated with processed fresh
organisms into water, carbonic and fertile anhydride                  product, leading a significantly longer shelf-life. For exam-
compost. Finally, to be defined compostable, the manufac-              ple, for fresh product respiration rate is of a great impor-
tured article must result compatible with a process of com-           tance in MAP design so identify the best packaging is
posting, that means it must not release dangerous                     a crucial factor. The most important barrier properties of
substances and must not alter the quality of the produced             polymer films used in packaging application are described.
compost. The last Financial Law out as objective the dismiss-
ing of the mono-use pouches not biodegradable, for food staff         Oxygen transmission rate (OTR)
transportation, within the 2010 (Scott & Wiles, 2001).                    The oxygen barrier property of a food packaging container
   The use of long-lasting polymers as packaging materials            for fresh product (e.g. fruits, salad, ready-to-eat meals) plays
for short application is not justified, also because physical          an important role on its preservation. The oxygen barrier is
recycling of these materials is often impractical because             quantified by the oxygen permeability coefficients (OPC) which
food contamination. So there is an increasing demand on               indicate the amount of oxygen that permeates per unit of area
the use of biodegradable polymer which could be easily re-            and time in a packaging materials [kg m mÀ2 sÀ1 PaÀ1]. So,
newable (Kale, Auras, & Singh, 2006). While most of the               when a polymer film packaging has a low oxygen permeability
commercialized biopolymer materials are biodegradable,                coefficients, the oxygen pressure inside the container drops to
these are not fully compostable in real composting condi-             the point where the oxidation is retarded, extending the shelf-
tions, which vary with temperature and relative humidity.             life of the product. Generally the biodegradable polymers pres-
                                                                      ent a value one or more order of magnitude below the synthetic
Barrier properties                                                    polymer used in the same field like PET and OPS. Several
    The determination of the barrier properties of a polymer          authors reported in literature the oxygen permeability coeffi-
is crucial to estimate and predict the product-package shelf-         cients of one of the most commercialized biodegradable poly-
life. The specific barrier requirement of the package system           mer like the PLA (Auras, Harte, & Selke, 2004; Auras, Singh,
is related to the product characteristics and the intended            & Singh, 2006; Auras, Singh, & Singh, 2005; Lehermeier,
end-use application. Generally plastics are relatively per-           Dorgan, & Way, 2001; Oliveira et al., 2004).
meable to small molecules such as gases, water vapour, or-                Together with the permeability coefficient the oxygen
ganic vapours and liquids and they provide a broad range of           transmission rate (OTR), expressed in cc mÀ2 sÀ1 is given.
mass transfer characteristics, ranging from excellent to low          The OPC is correlated to the OTR by the following
barrier value, which is important in the case of food                 equation:
                             V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643                         637

OPC ¼ OTR Â l=DP                                           ð1Þ       nonbiodegradable ones (ASTM D882-02, Standard Test
                                                                     Method for Tensile Properties of Thin Plastic Sheeting).
where l is the thickness of the film (m), DP is the difference        Impact properties test is a method utilized to determine
between oxygen partial pressure across the film [Pa].                 the energy that causes the plastic to fail under specific im-
DP ¼ p1 À p2, where p1 is the oxygen partial pressure at             pact conditions, conducted following the ASTM D1709-03,
the temperature test on the test side and p2 is equal to             Standard Test Methods for Impact Resistance of Plastic
zero on the detector side.                                           Film by the Free-Falling Dart Method.
                                                                        The compression test is normally conducted on thermo-
Water vapour transmission rate (WVTR)                                formed sample, according to the ASTM D642, Standard
    The water vapour barrier properties for the packaged             Test Method for Determining Compressive Resistance of
product whose physical or chemical deterioration is related          Shipping Containers, Components, and Unit Loads. Natu-
to its equilibrium moisture content, are of great importance         rally the compression strength is function of the material
for maintaining or extending its shelf-life. The water va-           and of design (shape and size).
pour barrier is quantified by the water vapour permeability
coefficients (WVPC) which indicate the amount of water                Chemical resistance properties
vapour that permeates per unit of area and time in a packag-            Products that could be packaged in this kind of con-
ing materials [kg m mÀ2 sÀ1 PaÀ1]. For fresh food products           tainers may have weak or strong acid characteristics; so it
it is important to avoid dehydration while for bakery or del-        is necessary to assess the performance and the suitability
icatessen is important to avoid water permeation. The                of biopolymers stored with common food packaging solu-
WVPC of the PLA biodegradable polymer is reported in                 tion as a function of time. The interaction and absorption
the literature (Auras, Harte, Selke, & Hernandez, 2003;              between chemical compounds and polymer may affect the
Auras et al., 2005, 2006).                                           final mechanical properties of a polymer (Auras et al.,
    Together with the permeability coefficient is given the           2005). Normally the chemical resistance is tested measur-
water vapour transmission rate (WVTR), expressed in                  ing the tensile stress, elongation at break and modulus of
cc mÀ2 sÀ1 (or g mÀ2 dayÀ1). The WVPC is correlated to               elasticity of sample submerged in weak and strong acid so-
the WVTR as described up in Eq. (1) for the oxygen                   lutions as a function of time, simulating real conditions, at
parameter.                                                           ambient temperature (23  C) and at À18  C, À23  C and
                                                                     À29  C. The weak acid solution is prepared with acetic
Carbon dioxide transmission rate (CO2TR)                             acid while the strong acid solution is prepared with hydro-
   Like the oxygen and water vapour barrier properties,              chloric acid (Auras et al., 2005).
also the carbon dioxide barrier property is of particular im-
portance on food packaging application. The carbon diox-             Some important production consideration
ide barrier is quantified by the carbon dioxide                           Currently, there are several types of bio-based polymers
permeability coefficients (CO2PC) which indicates the                 on the market: same coming from petrochemical monomer,
amount of carbon dioxide that permeates per unit of area             like certain types of polyester, polyester amides and polyvi-
and time in a packaging materials [kg m mÀ2 sÀ1 PaÀ1]. To-           nyl alcohol, produced by different manufacturer, used prin-
gether with the permeability coefficient is given the carbon          cipally as films or moulding. Four other bio-based polymers
dioxide transmission rate (CO2TR), expressed in cc mÀ2 s             are starch materials, cellulose materials, polylactic acid
    (or g mÀ2 dayÀ1). The CO2PC is correlated to the                 (Polyester, PLA), polyhydroxy acid (polyester, PHA). Until
CO2TR as described up in Eq. (1).                                    now, the PHA polymer is a very expensive polymer because
                                                                     it is commercially available in very limited quantities. PLA
Mechanical properties                                                is becoming a growing alternative as a green food packag-
   It is well-known that the polymer architecture plays an           ing materials because it was found that in many situations it
important role on the mechanical properties, and conse-              performs better than synthetic ones, like oriented polysty-
quently on the process utilized to modelling the final prod-          rene (OPS) and PET materials (Auras et al., 2005).
uct (injection moulding, sheet extrusion, blow moulding,                 Different types of materials can be combined to form
thermoforming, film forming). In addition, many packaging             blend or compounds or semifinished products such as films.
containers are commercially used below room temperature,                 The degradation of the materials is normally studied under
so it is important to assess the mechanical performance un-          real compost conditions and under ambient exposure by dif-
der these conditions (Auras et al., 2005).                           ferent techniques. The polymer degradation rate is deter-
   Tensile test analyses are made to determine the tensile           mined by the nature of the functional groups and the
strength (MPa), the percent elongation at yield (%), the per-        polymer reactivity with water and catalysts. Any factor which
cent elongation at break (%) and the elastic modulus (GPa)           affects the reactivity such as particle size and shape, temper-
of the food polymer packaging material. These values are             ature, moisture, crystallinity, isomer percentage, residual
important to get mechanical information of the biopolymer            monomer concentration, molecular weight, molecular weight
materials to be compared with the commercial                         distribution, 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.,
                                                                       À23  C and À29  C, since fresh produce packaging and
2006). In general high temperature and humidity will degrade
                                                                       deli containers are generally used commercially at this
more rapidly the polymer. By visual inspection the packages
are 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-            Biodegradable polymers applications in food
ness gauge according to the ASTM norm D4166-                           packaging field
99(2004)e1, Standard Test Method for Measurement of                       The field of application of biodegradable polymer in
Thickness of Nonmagnetic Materials by Means of a Digital               food-contact articles includes disposable cutlery, drinking
Magnetic Intensity Instrument, or by micrometer according              cups, salad cups, plates, overwrap and lamination film,
to ASTM D 374-99, Standard Test Methods for Thickness                  straws, stirrers, lids and cups, plates and containers for
of Solid Electrical Insulation.                                        food dispensed at delicatessen and fast-food establish-
    By gel permeation chromatography it is possible to de-             ments. These articles will be in contact with aqueous,
termine the molecular weight of samples dissolved in the               acidic and fatty foods that are dispensed or maintained at
appropriate solvent. Molecular weight variations are an                or below room temperature, or dispensed at temperatures
indication of the degradation rate of the polymers and                 as high as 60  C and then allowed to cool to room temper-
give information about when the main fragmentation oc-                 ature or below (Conn et al., 1995).
curs in a polymer.                                                        In the last few years, polymers that can be obtained from
    By differential scanning calorimetry (DSC) it is possible          renewable resources and that can be recycled and com-
to determine the glass transition temperature (Tg), melting            posted, have garnered increasing attention. Also their opti-
temperature (Tm) and crystallinity of the polymer sample               cal, physical and mechanical properties can be tailored
(ASTM D3418, Standard Test Method for Transition Tem-                  through polymer architecture so as a consequence, biode-
peratures and Enthalpies of Fusion and Crystallization of              gradable polymers can be compared to the other synthetic
Polymers by Differential Scanning Calorimetry). The crys-              polymers used in fresh food packaging field, like the
tallinity is determined according to ASTM D3417-97 and                 most common oriented polystyrene (OPS) and polyethylene
using the following equation, well-known in the literature,            terephthalate (PET).
                                                                          Depending on the production process and on the source,
xc ð%Þ ¼ 100 Â ðDHc þ DHm Þ=DHm                              ð2Þ       biopolymers can have properties similar to traditional ones.
                                                                       They are generally divided into three groups: polyesters;
where DHc is the exothermic enthalpy of cold crystalliza-              starch-based polymer; and others.
tion, DHm is the endothermic enthalpy of fusion, DHc is m
the endothermic heat of melting of purely crystalline poly-            Polyesters
mer under study (for example: for PLA is 135 J gÀ1, Kale                 These materials can be:
et al., 2006; for PET is 125.6 J gÀ1, Auras et al., 2003).
   By thermo-gravimetric analysis (TGA) it is possible to                 i. Polymers directly extracted from biomass like pro-
obtain the decomposition temperature, according to the                       teins, lipids, polysaccharides, etc.
ASTM E1131-03, Standard Test Method for Compositional                    ii. Polymeric materials synthesized by a classical poly-
Analysis by Thermogravimetry.                                                merization procedure such as aliphaticearomatic
   The determination of the pH of the sample surrounding                     copolymers, aliphatic polyesters, polylactide aliphatic
is one of the most important factors of hydrolytic polymer                   copolymer (CPLA), using renewable bio-based mono-
degradation since pH variations can change hydrolysis                        mers such as poly(lactic acid) and oil-based mono-
rates by few order of magnitude. The chemical resistance                     mers like polycaprolactones.
is normally determined exposing the materials to weak                   iii. Polymeric materials produced by microorganisms and
acid (pH ¼ 6, acetic acid solution) and strong acid                          bacteria like polyhydroxyalkanoates.
(pH ¼ 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-              Aliphaticearomatic copolymers
aging application is the determination of the oxygen, car-                These materials are a combination of polyetilene tere-
bon dioxide and water vapour transmission rate (OTR,                   phthalate (PET), resistant to microbial attack, with three or
CO2TR and WVTR, respectively). These tests are per-                    more biodegradable aliphatic polyesters. It is soft, pliable
formed according to the ASTM norm described before.                    with a good touch but with a melting point of around 200  C,
   Concerning the mechanical properties, the samples                   too high for a degradable material. The aliphatic monomer cre-
could be analysed by Impact tests, Tensile properties and              ates a weak spots in the aromatic polymeric chain which makes
Compression Test of Thermoformed Containers. Generally                 them susceptible to degradation through hydrolysis.
these parameters are studies at ambient temperature (22  C)              Also if it is totally biodegradable, coming from fossil
and at frozen food storage temperatures of À18  C,                    fuels such as oil, coal and natural gas, PET production
                              V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643                              639

causes a consume of non-renewable and finite resources                 Poly(lactic acid) (PLA)
and with an heavy impact on global waste disposal. If prop-               One of the most promising biopolymer is the poly(lactic
erly disposed it degrades in 8 weeks but if it is trash without       acid) (PLA) obtained from the controlled depolymerization
any control in the environment, the degradation process can           of the lactic acid monomer obtained from the fermentation
take 50 years to breakdown the structure. The long polymer            of sugar feedstock, corn, etc., which are renewable resources
molecules are cleaved by moisture into smaller ones, which            readily biodegradable (Cabedo, Feijoo, Villanueva, Lagaron,  ´
are naturally consumed by microbes and converted to car-              & Gime  ´ nez, 2006). It is a versatile polymer, recyclable and
bon dioxide and water. This material is commonly used                 compostable, with high transparency, high molecular weight,
for eating utensils (like fork, knife, spoon, dishes and so           good processability and water solubility resistance. In gen-
on) and bottles but it costs twice as much than commercial            eral commercial PLA is a copolymer between poly(L-lactic
ones (Salt, 2002). DuPont (Tennessee) produce the PET/hy-             acid) and poly(D-lactic acid). Depending on the L-lactide/D-
dro-biodegradable polyester under the trade name of Bio-              lactide enantiomers ratio, the PLA properties can vary con-
max, exported in all over the world.                                  siderably from semicrystalline to amorphous ones. Re-
                                                                      searches carried out to improve the performance quality of
                                                                      this material are made on PLA with D-lactide content less
Aliphatic polyesters                                                  than 6%, which is the semicrystalline polymer. However
   These materials have properties similar to PE and PP               the amorphous one, containing 12% of D-lactide enantiomer,
polymers, they are biodegradable but with lack in thermal             is easy to process by thermoforming, which is the actual tech-
and mechanical properties. These materials come from                  nology in the food packaging sector, and it shows properties
polycondensation reaction of glycol and aliphatic dicarbox-           like polystyrene. This material is commercialized by differ-
ylic acid, both obtained from renewable resources. They are           ent companies with different commercial names, like for ex-
odourless and can be used for beverage bottles and they               ample the NatureworksÔ PLA produced by NatureworksÔ
biodegrade in soil and in water giving carbon dioxide and             LLC (Blair, NB). Currently it is used in food packaging appli-
water, in a period of 2 months (e.g. for a 0.04 mm thick              cation only for short shelf-life products.
film) (                                               In Table 1 physical characteristics of PLA obtained from
   A commercially available aliphatic copolyester is pro-             Auras et al. (2006) are reported.
duced by Procter and Gamble Co. (P&G, Cincinnati, OH)                     Kale et al. (2006) studied the compostability of three
with the trade name of Nodax and it can degrade in aerobic            commercially available biodegradable packages made of
and anaerobic environmental conditions. The other one                 PLA, in particular water bottles, trays and deli containers,
is the Eastar bIo, produced from the Eastam Chemical                  to composting and to ambient exposure. They investigate
Company (Hartlepool, UK).                                             the properties breakdown of these packages exposed to
                                                                      compost conditions by several experimental procedure in-
Polylactide aliphatic copolymer (CPLA)                                volving gel permeation chromatography (GPC), differential
   This material is a mixture between renewable resources             scanning calorimetry (DSC), thermal gravimetric analysis
as lactide and aliphatic polyesters like dicarboxylic acid or         (TGA) and visual inspection. The compost pile was pre-
glycol, with hard (like PS) and soft flexible (like PP) prop-          pared with cow manure, wood shaving and waste feed, at
erties, depending on the amount of aliphatic polyester pres-
ent in the mixture. It is easy to process and thermally stable
up to 200  C. The heating value and the quantity of carbon            Table 1. Physical experimental data for PLA (Auras et al., 2006)
dioxide generated during combustion are about the half of              Experimental data                            PLA
that generated from commercial polymer like PE and PP,                       
                                                                       Tg ( C)                                      62.1 Æ 0.7
and incineration does not produce toxic substances. In nat-            Tm ( C)                                     150.2 Æ 0.5
ural environment it starts to degrade in 5e6 months, with              DHc (J gÀ1)
                                                                           m                                        93
a complete decomposition after 12 months. If composted                 Percent crystallinity (xc)                   29.0 Æ 0.5
                                                                       Oxygen transmission rate (OTR)               56.33 Æ 0.12
with food garbage, it begins to decompose after 2 weeks.               (cc mÀ2 day)a
                                                                       Oxygen permeability                          4.33E-18 Æ 1.00E-19
                                                                       rate (OPC) (kg m mÀ2 sÀ1 PaÀ1)b
Polycaprolactone (PCL)                                                 Water vapour transmission                    15.30 Æ 0.04
    It is a fully biodegradable polymer coming from the po-            rate (WVTR) (g mÀ2 dayÀ1)a
lymerization of not renewable raw material, like crude oil.            Water vapour permeability                    1.34E-14 Æ 3.61E-17
It is a thermoplastic polymer with good chemical resistance            rate (WVPC) (kg m mÀ2 sÀ1 PaÀ1)c
to water, oil, solvent and chlorine, with a melting point of               Thickness of 20.0 Æ 0.2.
58e60  C, low viscosity, easy to process and with a very                  OPC ¼ OTR Â l/DP, with l is the thickness in m and DP is the
short degradation time. It is not used for food application            difference in oxygen partial pressure across the film.
                                                                           WVPC ¼ WVTR Â l/DP, with l is the thickness in m and DP is
but if mixed with starch it is possible to obtain a good bio-          the difference in water vapour partial pressure across the film.
degradable material at a low price, used for trash bags.
640                           V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643

a temperature above 60  C. After 3 weeks the pile was put                           randomly, longer PLA chain is more susceptible to cleavage
on asphalt pad. The initial pile temperature, relative humid-                        than the shorter ones. The fragmentation process, which pro-
ity and pH was 65 Æ 5  C, 63 Æ 5% and 8.5 Æ 0.5, respec-                            duces decomposition of the macromolecules into shorter olig-
tively. The packages were subjected to composting for 1,                             omer chains and monomers, took place giving an initial rise of
2, 4, 6, 9, 15 and 30 days and packages exposed to ambient                           the polydispersity index (PDI). Afterwards, polymer frag-
conditions were also studied. This is because it is well-                            mentation took place with a decrease in PDI until 1.00 value,
known that PLA absorb water, resulting in the hydrolysis                             when only oligomers of the PLA chains are present.
and cleavage of the ester linkages, autocatalyzed by the car-                           After the first 4 days where an increase of Tg was ob-
boxylic acid end groups (Scheme 2).                                                  served correlated to a short increments of Mw, the value
    They found that the degradation rate changes with the ini-                       showed a reduction which was obviously associated to
tial crystallinity and L-lactide content of the packages, with                       the molecular weight reduction, starting from about 62  C
lower degradation for the polymer with the highest content                           to 30  C. The Tg reduction of the PLA packages exposed
of L-lactide (96 versus 94%) due to the higher crystallinity,                        to compost conditions followed a linear trend, with a reduc-
which makes more difficult the degradation of the whole                               tion in  C for day which depend on the shape of the con-
structure. During the hydrolysis process the sample decreased                        tainers (Kale et al., 2006). The Tm variation as a function
in size (reduction of the thickness) and became tough (in-                           of time did not follow a linear relationship, with a slight in-
crease in fragility) with a trend that depend to the shape.                          crease of Tm for the samples submitted to compost condi-
The dimension of the containers before and after composting                          tions at the beginning of the composting process.
was calculated by measuring the variation on width, length,                             The decomposition temperature TD, determined by
height and thickness. During the first period of the ambient ex-                      thermo-gravimetric analysis (TGA), associated to depoly-
posure (within the first 15 days) the molecular weight of the                         merization (rapid reduction of polymer mass with a slow
sample Mw showed a small increase probably due to UV or                              reduction in molecular weight) and random degradation
gamma radiation which produces chain cleavage and subse-                             (slow loss of polymer mass with an exponential decrease
quent recombination, which can result in crosslinking and                            in molecular weight) was determined for PLA samples ex-
hence an increase in the Mw. The increase in molecular weight                        posed to ambient and composting conditions. No variation
produced an increase in the glass transition temperature Tg                          of TD as a function of time was observed for the sample ex-
and leading to slower degradation since glassy polymers de-                          posed to ambient condition while for the other ones a reduc-
grade more slowly than rubbery ones. The same trend was ob-                          tion of TD was observed, with a linear variation.
served for the sample exposed to the compost pile; the Mw                               PLA packages will compost in municipal or industrial
increase could be attributed to crosslinking or recombination                        facilities but the PLA degradation is driven by hydrolysis
reactions. Subsequently, during exposure, the degradation                            which needs higher temperatures to take place, so a com-
produced a molecular weight decrease that followed a first or-                        pletely compost will be difficult. Further studies will be
der kinetic associated to a first order hydrolysis process af-                        necessary to find method and techniques that can assess
fected by the initial crystallinity, thickness and shape of the                      the degradability of the biodegradable food packaging un-
sample (Tsuji & Ikada, 1998). Since the hydrolysis occurs                            der real composting conditions.

                                        CH3           O         CH3              O

                                              O                         O
                                   HO                       O                        O poly
                                          O       CH3               O       CH3


                                                                                     CH3                 O       CH3         O

                                                                                               O                       O
                                                                            HO                               O                   O poly
                                                                                           O       CH3           OH    OH CH

                             CH3          O       CH3                                              CH3

                                    O                       O                                                O poly
                        HO                    O                 H            +             HO
                               O        CH3             O                                            O

                                              Scheme 2. PLA hydrolysis and molecular cleavage.
                               V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643                            641

   As reported from Kale et al. (2006), Pometto et al. in a pre-       higher than those for PET and OPS. The modulus of elasticity
vious research studied the banana field exposition of PLA               showed a similar trend with the best value for the OPLA poly-
film in Costa Rica. They found that this film lost its mechan-           mer, while the elongation at break was similar at room tem-
ical properties faster than during exposure in simulated con-          perature for the three polymers but was much higher for PET
ditions in the laboratory, with a degradation enhanced by an           at value below the room temperature. The compression test of
increase in temperature and relative humidity. This data not           thermoformed containers had point out that OPLA and OPS
concern complete packages degradation.                                 have similar compression strength while the PET containers
   Concerning the PLA toxicology for human safety, Conn                showed the best value but in this case it was not possible to
et al. (1995) studied the migration of small molecules com-            give an conclusive information about the overall perfor-
ing from the hydrolytic degradation phenomena of PLA                   mance because the containers had different shapes.
polymer films of food-contact articles, which are lactic                    The results for chemical resistance tests showed that expo-
acid (a safe food substance), the monomer lactide and the              sure to acid and vegetable oil resulted in a minimal strength
linear dimer of lactic acid which is lactoyllactic acid. In            degradation PLA (and also for the other two polymer PET
any case dimers and oligomers hydrolyse in aqueous sys-                and OPS). PLA studied in these conditions has showed that
tem to lactic acid, which is a common food ingredient                  when it is submerged to weak acid solution there is an in-
that has been shown to be safe in food at levels far in excess         crease of tensile strength, it becomes more ductile and there
of any small amount that might result from the intended                is a reduction in the modulus of elasticity as a function of
uses of PLA. They studied the PLA components migration                 time. For sample submerged in strong acid solution there
by extraction tests in which samples of the polymer were               was an increase of tensile stress, no variation in the elonga-
exposed to food-simulating solvents under conditions that              tion at break, it becomes more brittle with an increase in
reproduce the most severe temperature/time conditions to               the modulus of elasticity which is an indication of the brittle-
which food would be exposed while in contact with PLA.                 ness of the sample as a function of time.
The examined contact was with aqueous, acidic and fatty                    The same mechanical properties were measured when
foods. It was found that in any case migrants from PLA                 PLA sample containers were exposed to vegetable oil and
other that lactic acid (dimers, trimers, etc.) represented             it was found that there was a decrease of the tensile stress,
very small and safe amounts. Migrating quantities of these             a reduction of the elongation at break and an increase of the
species hydrolyse to lactic acid in the aqueous and acidic             modulus of elasticity.
media commonly found in food and in the stomach. Lactide                   Based on the experimental researches made until now it
has demonstrated low intrinsic toxicity in testing while the           has been found that PLA is safe and generally recognized
lactoyllactic acid is normally present in commercially                 as safe for its use in food-contact articles. It has the advantage
available lactic acid that is an evidence of its safety.               of easily tailoring their physical properties by changing the
   Concerning the optical, physical and mechanical perfor-             chemical composition (amount of L- and D-isomer) and the
mance of the oriented PLA polymer (OPLA) in food appli-                processing conditions. PLA packages perform, as well as
cation, Auras, Singh & Singh (2006) made a comparison                  other containers made on synthetic polymer like PET, PS,
with two of the commonly used materials used for fresh                 etc., at room and low temperature, suggesting that PLA
food packaging application, which are polyethylene tere-               would also be suitable for the same food application. How-
phthalate (PET) and oriented polystyrene (OPS). The phys-              ever, same properties such as flexural properties, gas perme-
ical experimental data obtained on PLA samples are                     ability, impact strength, processability, etc., are often not
reported in Table 1.                                                   good enough for this application. This material shows good
   Concerning the physical data, OPLA presents the lowest              barrier to aroma but the most important limitation on the
Tg and Tm data respect the PET and OPS polymers, while                 use of PLA for food application packaging is the medium
the crystallinity is very similar to that of PET (OPS is atactic       barrier to gases and vapours and the brittleness properties.
and does not crystallize so it is highly transparent). About the       A possible strategy to decrease the brittleness is to make
oxygen transmission rate it was found that OPLA is a good              a blend between PLA and others polymer. Cabedo et al.
film for food like tomato and other breathable products where           (2006) studied the blend of PLA with polycaprolactone
the oxygen and carbon dioxide barrier requirements are spe-            (PCL), which is also a biodegradable semicrystalline poly-
cifically matched to the respiration rate of the fresh produce.         mer obtained from the polymerization of 3-caprolactone. It
In order to maintain the freshness property and shelf-life of          showed low tensile strength, high elongation at break, and
fruit and vegetable, it is necessary to control their storage          processing temperature similar to the PLA and it can be uti-
conditions, like humidity and quantity of gases (oxygen, car-          lized like plasticizer to increase the gas permeability of the
bon dioxide and ethylene). Usually the specific gas require-            PLA as a consequence of the poor gas barrier properties of
ments are achieved by controlling the type of films used as             PCL. In this research to the PLA/PCL blend they added
packaging materials for different atmospheric conditions.              also kaolinite nanocomposites by melt mixing using a con-
As far as mechanical properties are concerned, it was found            ventional polymer extrusion process. By SEM analysis
that at room temperature the three polymers showed similar             they found that this blend is immiscible across the composi-
tensile strength while at temperature besides that, were               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-                        Yu, Chua, Huang, Lo, and Chen (1998) used different types
mechanical analysis, DMA) and a slight increase in its ther-            of food wastes as carbon source to produce several PHA poly-
mal stability (by Thermo-gravimetric analysis, TGA) with an             mers with different physical and mechanical properties, like
increment of this effect with the PCL amount increment. The             flexibility, tensile strength, melting viscosity. The use of
gas barrier properties showed a significant decrease propor-             food waste is a good way to reduce the cost of bioplastics pro-
tional to the amount of PCL added to the blend, which was               duction, but until now it is only an experimental procedure
compensated in the sample containing kaolinite which shows              without any possibility to have a commercial application.
an increase in the gas barrier properties. Anyway, these
changes were clearly discernible but small. The effect of
                                                                        Starch-based polymer
the nanocomposites is currently under study but it is clear
                                                                           Commercial polymer coming from the synthesis of oil-
that these compounds could be a valid route to decrease the
                                                                        based monomer can be mixed with different percentage
inherent rigidity of some biopolymers and to enhance their
                                                                        (10, 50 and 90%) of starch used as additive. Depending
                                                                        on starch percentage and other materials like additives (col-
    Further studies on PLA products must be performed to de-
                                                                        ouring additives, flame retardant additives) the properties of
termine the range of compatibility of this polymer and to de-
                                                                        these materials can be varied a lot, becoming stable to un-
termine the performance in real shelf-life studies. A study of
                                                                        stable for example in hot/cold water.
the Life-Cycle Assessment (LCA) for the PLA polymer was
                                                                           Starch, consumed by microbial action, accelerates the
just made by Bohlmann (2004), who made a comparison
                                                                        disintegration or fragmentation of polymer chain by pro-
with the most used polypropylene (PP) in food packaging ap-
                                                                        ducing pores in the materials which weaken them. This pro-
plication. He found that PLA is more energy efficient than PP
                                                                        cess is quite slow, it can be accelerated only if the starch
polymer because PLA consumes no feedstock energy. How-
                                                                        added to the mixture exceed 60%. Depending on the type
ever, when it is taken in consideration the uncertainty of the
                                                                        of the thermoplastic starch materials, they can degrade in
estimation, the difference between the two polymers be-
                                                                        5 days in aqueous aerobic environment, in 45 days in con-
comes marginalized. He found also that the PLA and PP
                                                                        trolled compost and in water.
greenhouse gas emission are equivalent.
                                                                           In 1993 LDPE-starch blend were commercialized under
    Fang et al. (2005) studied the possibility to make a multi-
                                                                        the trade name EcostarÒ. Other commercial trade names
layer film with PLA and a natural material like modified starch
                                                                        are BioplastÒ (from Biotec GmbH) and NOVONÒ (from
to have equal or better performance characteristics to those of
                                                                        NOVON International) (
existing product not biodegradable like polyethylene/polyvi-
                                                                           Starch can be transformed also into a foamed material us-
nylidenchloride/polystyrene (PE/PVDC/PS) multilayer films.
                                                                        ing water steam, replacing the polystyrene foam as packaging
Starch is a totally biodegradable polymer coming from agri-
                                                                        material. It can be pressed into trays or disposable dishes and
culture; it is abundant, renewable, safe and economics but as
                                                                        dissolves in water leaving a non-toxic solution, consumed by
a component of biodegradable laminate film, it shows no plas-
                                                                        microbic environment in about 10 days giving only water and
tic behaviour, no adequate mechanical properties and it ther-
                                                                        carbon dioxide as by-products. The commercial trade names
mally degrades at around 260  C. When it is extruded in
                                                                        are BiopurÒ (from Biotec GmbH), Eco-FoamÒ (from Na-
combination with plasticizers it becomes thermoplastic,
                                                                        tional Starch & Chemical) and EnvirofillÔ (from Norel).
mouldable and an amorphous material with an excellent oxy-
gen barrier characteristic, but it is extremely sensitivity to the
environmental humidity giving rapid biodegradation.                     Others biomaterials not used in food application
                                                                           Another natural plastic material, the casein formalde-
Polyhydroxyalkanoates (PHA)                                             hyde, can be generated from a natural protein obtained
    These polymers are produced in nature by bacterial fer-             from milk, horn, soy bean, wheat, etc. It looks like cellu-
mentation of sugar and lipids. They can be thermoplastic or             loid, ivory or artificial horn and it is insoluble in water, in-
elastomeric materials, with a melting point between 40 C               flammable and odourless. This material is used to make
and 180  C, depending on the monomer used in the synthesis.            buttons, pins, cigarette-cases, umbrella handles and so on
These polymers, alone or in combination with synthetic plas-            but not in food application.
tic or starch give excellent packaging films (Tharanathan,                  The cellulose acetate (CA) is an amorphous tough ther-
2003). The most common type is the polyhydroxybutyrate                  moplastic obtained by introducing the acetyl radical of ace-
(PHB), coming from the polymerization of 3-hydroxybuty-                 tic acid into cellulose (cotton or wood). To decrease its
rate monomer, with properties similar to PP but more stiffer            inflammability it is used with additives, with self-extin-
and brittle. The copolymer polyhydroxybutyrate-valerate                 guishing properties. Cellulose acetate is an insulator mate-
(PHBV), used as packaging material, is less stiff and tougher.          rial with a little tendency to electrostatic chargin, brittle
The price is very high but it degrades in 5e6 weeks in a micro-         under freezing point. Horn is an organic thermoplastic ma-
biology active environments, giving carbon dioxide and wa-              terial containing 80% of keratin; it can be pressed in vari-
ter in aerobic condition. In anaerobic environment the                  ous objects and laminas, like buttons, combs, pens, etc.
degradation is faster, with production of methane.                      (
                                  V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643                                         643

Conclusions                                                               Auras, R., Harte, B., Selke, S., & Hernandez, R. (2003). Mechanical,
   Bioplastics development is just beginning; until now it                    physical and barrier properties of poly(lactide) films. Journal of
                                                                              Plastic Film and Sheeting, 19(2), 123e135.
cover approximately 5e10% of the current plastic market,                  Auras, R., Singh, S. P., & Singh, J. (2006). Performance evaluation of
about 50,000 t in Europe. The European countries with the                     PLA against existing PET and PS containers. Journal of Testing and
highest utilization of bioplastics are France, Germany, En-                   Evaluation, 34(6).
gland, Netherland and Italy but other countries like Bel-                 Auras, R., Singh, S. P., & Singh, J. J. (2005). Evaluation of oriented poly(-
gium, Austria, Spain and Switzerland are going to utilize                     lactide) polymers vs. existing PETand oriented PS for fresh food service
                                                                              containers. Packaging Technology and Science, 18, 207e216.
it in individual applications. The principal field regards                 Bohlmann, G. M. (2004). Biodegradable packaging life-cycle assess-
the use of films packaging for food products, loose film                        ment. Environmental Progress, 23(4), 342e346.
used for transport packaging, service packaging like carry                                                                        ´
                                                                          Cabedo, L., Feijoo, J. L., Villanueva, M. P., Lagaron, J. M., &
bags, cups, plates and cutlery, biowaste bags, in agri- and                        ´
                                                                              Gimenez, E. (2006). Optimization of biodegradable nanocompo-
horticultural fields like bags and compostable articles.                       sites based application on a PLA/PCL blends for food packaging
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the benefit of economic scale. The increased utilization of                    LaDu Jr., B. N., et al. (1995). Safety assessment of polylactide (PLA)
biomass as energy source and raw materials is necessary in                    for use as a food-contact polymer. Food and Chemical Toxicology,
the long term due to the fact that the crude oil and natural                  33(4), 273e283.
gas resources are limited, but it is to be keep in mind that these        Fang, J. M., Fowler, P. A., Escrig, C., Gonzalez, R., Costa, J. A., &
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materials have to be found place in a very strong international               films derived from naturally occurring carbohydrate polymers.
market of synthetic ones, with an annual plastics consump-                    Carbohydrate Polymers, 60, 39e42.
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                                                                          Germain, Y. (1997). Conception de films polymer a perm[abilite           ´
a 5% average growth per annum. However, plastics and bio-                            ˆ ´                                             ´e
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                                                                          Guilbert, S., Cuq, B., & Gontard, N. (1997). Recent innovation in
vidual profile, so they present an enormous diversity which                    edible and/or biodegradable packaging materials. Food Additives
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show some limitations in terms of performance like thermal                    packaging materials. Fluid Phase Equilibria, 222/223, 317e324.
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