522 Novel food packaging techniques 24.2.4 Native starch Starch is nature's primary means of storing energy and is found in granule form in seeds, roots and tubers as well as in stems, leaves and fruits of plants. Starch is totally biodegradable in a wide variety of environments and allows the levelopment of totally degradable products for specific market needs. The two main components of starch are polymers of glucose: amylose(MW 10-10),an essentially linear molecule and amy lopectin(MW 10'-10%), a highly branched molecule. Amylopectin is the major component of starch and may be considered as one of the largest naturally occurring macromolecules. Starch granules are semi-crystalline, with crystallinity varying from 15 to 45% depending on the ource.The term ' native starch' is mostly used for industrially extracted starch It is an inexpensive(<0.5 Euro/kg)and abundant product, available from potato, maize, wheat and tapIoca 9 24. 2.5 Thermoplastic starch Thermoplastic starch(TPS) or destructurised starch(DS) is a homogeneous hermoplastic substance made from native starch by swelling in a solvent (plasticiser)and a consecutive ' extrusion'treatment consisting of a combined kneading and heating process. Due to the destructurisation treatment, the starch undergoes a thermo-mechanical transformation from the semi-crystalline starch granules into a homogeneous amorphous polymeric material. Water and glycerol re mainly used as plasticisers, with glycerol having a less plasticising effect in TPS compared to water, which plays a dominant role with respect to the properties of thermoplastic starch 24.2.6 Water resistance of starch-based products Thermoplastic starch behaves as a common thermoplastic polymer and can be processed as a traditional plastic. TPS shows a very low permeability for oxygen (43cm /m2/min/bar compared to 1880cm/m2/min/bar of LDPE)which makes this material very suitable for many packaging applications. In contrast, the permeability of TPs for water vapour is very high(4708cm/m- compared to 0.7cm/m" of LDPE). This sensitivity to humidity(highly hydrophilic)and the quick ageing due to water evaporation from the matrix makes thermoplastic starch as such unsuitable for most applications. Due to this drawback there are no products available at the moment made from pure thermoplastic starch which are form-stable (or even hydrophobic)in a wet atmosphere and mechanically stable over a sufficiently long period of time Producers of starch-based products overcome this problem by blending the thermoplastic starch with hydrophobic synthetic polymers(biodegradable polyesters)or by the production of more hydrophobic TPs derivatives(starch ester). Unfortunately, all theses production processes make the starch-based products rather expensive in comparison to the common plastic alternatives24.2.4 Native starch Starch is nature’s primary means of storing energy and is found in granule form in seeds, roots and tubers as well as in stems, leaves and fruits of plants. Starch is totally biodegradable in a wide variety of environments and allows the development of totally degradable products for specific market needs. The two main components of starch are polymers of glucose: amylose (MW 105 –106 ), an essentially linear molecule and amylopectin (MW 107 –109 ), a highly branched molecule. Amylopectin is the major component of starch and may be considered as one of the largest naturally occurring macromolecules. Starch granules are semi-crystalline, with crystallinity varying from 15 to 45% depending on the source. The term ‘native starch’ is mostly used for industrially extracted starch. It is an inexpensive (< 0.5 Euro/kg) and abundant product, available from potato, maize, wheat and tapioca.9 24.2.5 Thermoplastic starch Thermoplastic starch (TPS) or destructurised starch (DS) is a homogeneous thermoplastic substance made from native starch by swelling in a solvent (plasticiser) and a consecutive ‘extrusion’ treatment consisting of a combined kneading and heating process. Due to the destructurisation treatment, the starch undergoes a thermo-mechanical transformation from the semi-crystalline starch granules into a homogeneous amorphous polymeric material. Water and glycerol are mainly used as plasticisers, with glycerol having a less plasticising effect in TPS compared to water, which plays a dominant role with respect to the properties of thermoplastic starch. 24.2.6 Water resistance of starch-based products Thermoplastic starch behaves as a common thermoplastic polymer and can be processed as a traditional plastic. TPS shows a very low permeability for oxygen (43cm3 /m2 /min/bar compared to 1880cm3 /m2 /min/bar of LDPE) which makes this material very suitable for many packaging applications. In contrast, the permeability of TPS for water vapour is very high (4708cm3 /m2 compared to 0.7cm3 /m2 of LDPE). This sensitivity to humidity (highly hydrophilic) and the quick ageing due to water evaporation from the matrix makes thermoplastic starch as such unsuitable for most applications. Due to this drawback there are no products available at the moment made from pure thermoplastic starch, which are form-stable (or even hydrophobic) in a wet atmosphere and mechanically stable over a sufficiently long period of time. Producers of starch-based products overcome this problem by blending the thermoplastic starch with hydrophobic synthetic polymers (biodegradable polyesters) or by the production of more hydrophobic TPS derivatives (starch ester). Unfortunately, all theses production processes make the starch-based products rather expensive in comparison to the common plastic alternatives. 522 Novel food packaging techniques