Radiation of Biopolymers Jaejoon Han
INTRODUCTION Polymers are macromolecules composed of many small molecules added to each other to form linear, branched, or cross-linked structures. Synthetic origin - Teflon, nylon, polyvinylchloride, polystyrene, polyurethanes, etc. Natural origin (biopolymers) Protein, polysaccharides, polynucleotides, or natural rubber.
Pollution due to plastics is a result of the slow rate of disappearance of the synthetic polymers from the environment. Biopolymers degrade rapidly in the biological medium via the enzymatic route. Biopolymers from renewable sources (proteins, carbohydrates, etc.) have gained considerable research interests. - Used for food coating or stand-alone film wrap. - Retard unwanted mass transfer in food products.
Biopolymeric Protein Films Positive: Excellent O 2 and CO 2 barriers. Negative: (1) Highly hydrophilic and tend to absorb large quantities of water under elevated relative humidity conditions. (2) Mechanical properties are weakened and their water vapor permeabilities are increased.
To Extend Functional and Mechanical Properties of Protein Films (1) Optimization of the interactions between polymers. - protein protein interactions. - charge charge electrostatic complexes between proteins and polysaccharides. (2) Formation of cross-links through physical, chemical, or enzymatic treatments.
Protein Cross-links Confer elastomeric properties to the materials. Improve the mechanical strength and water resistance of materials.
Protein Cross-links Enzymes (e.g. transglutaminase) have been used to cross-link many food protein including caseins. However, the use of enzymes is - High cost. - Application is limited on a large scale. Physical treatment such as γ-irradiation improved the mechanical strength of biopolymeric films at much lower costs.
Radiation of Biopolymers γ-irradiation can also affect proteins by causing conformational changes, formation of protein free radicals, and recombination/polymerization reactions (Urbain,1986). γ-irradiation has been reported to be useful in the cross-linking of proteins, which increase the tensile strength of the product (Ressouany et al., 1998).
Tyrosine-Tyrosine cross-links - Protein cross-linking by γ-irradiation - Improved the mechanical resistance, and water resistance of the biopolymeric films. (Mezgheni et al., 1998: Ressouany et al., 1998) Higher irradiation doses in combination with Ca 2+ could further increase the mechanical strength of the protein films by forming a dense network. (Brault et al. 1997)
Attempts have been made to establish the suitability of irradiation for the development of cross-linked protein solutions for edible or biodegradable packaging applications. Furthermore, γ-irradiation generates sterile biomaterials which could be used in pharmaceutical or biomedical application.
MATERIALS AND METHODS
Biopolymeric Film preparation Different protein sources were used for the film formations. Solutions were irradiated at several total doses from 4 to 128 kgy in a 60 Co underwater calibrator unit (UC-15; 17.33 kgy/h). Film forming solution Cast onto Petri dishes Spread evenly Dry in a climatic chamber Peel dried films Optichromic and Gammachrome dosimeters were used to validate the dose distribution throughout the samples.
Analysis & Measurements Film thickness Mechanical properties of films (Puncture tests) Insoluble matter determination Microstructure observations Fluorescence measurements for bityrosine formation Permeability measurement (Water vapor permeability)
RESULTS AND DISCUSSION
Mechanical properties of films (Puncture tests) Mechanical properties of cross-linked films improved significantly the puncture strength for all types of films. (Lacroix et al., 2002) Fig 1. Puncture strength of unirradiated and irradiated caseinate films.
Insoluble matter determination The proportion of the insoluble fraction increased with the irradiation dose up to 32 kgy. (Vachon et al., 2000) Fig 2. Fraction of insoluble matter as a function of irradiated dose.
Microstructure Observations Observed by transmission electronic microscopy. Microstructures - (a) slicker and porous - (b) Clearly more dense (Vachon et al., 2000) Fig 3. Cross sections of (a) unirradiated or (b) irradiated (32 kgy) calcium caseinate films
Microstructure Observations Cross-link, present in irradiated films, increased the molecular proximity of the protein chains. Increased molecular proximity and additional molecular bonds directly influenced on Mechanical Strength and Water Resistance.
Fluorescence Measurements - bityrosine formation Fig 4. Effect of CaCl 2 and irradiation dose on bityrosine fluorescence.
Fluorescence Measurements Bityrosine increased with irradiation dose, resulting in a higher number of cross-links between tyrosine units. Ca 2+ enhanced the formation of bityrosine. CaCl 2 - Indirect effect by forming salt bridges between adjacent protein molecules. - Reduce the molecular distances, thus make the formation of bityrosine easier. (Ressouany et al., 1998)
Permeability measurement (Water vapor Permeability) WVP = (WX) / AT (P1-P2) W: weight gain (g) T : time (d) X : film thickness (mm) A : exposed area of the film (m 2 ) P2-P1 : water vapor pressure differential across the film.
Permeability measurement (Water Vapor Permeability) γ-irradiation treatment reduced significantly (P<0.05) the WVP of various protein films. (Lacroix et al., 2002 and Ouattara et al., 2002) Resulted from the formation of high molecular weight proteins and extensive cross-linking.
CONCLUSION γ-irradiation is efficient for inducing cross-links in biopolymeric protein films. γ-irradiation can enhance physico-chemical properties of biopolymers.