Radiation modification of the functional properties of the edible films prepared using starch and starch lipid system. K.Cieśla, A Nowicki, M. Buczkowski Institute of Nuclear Chemistry and Technology, Dorodna 16 str. 03-195 Warszawa, Poland, kciesla@orange.ichtj.waw.pl Edible and biodegradable films and coatings based on proteins, polysaccharides and lipids serves as a barrier for water and gas transfer in food system. Using of such packaging enable to prolong shelf-life of food and to fulfil demand of environmental protection at the relatively low cost. Gamma irradiation was already found to be an effective method for improvement of barrier and mechanical properties of the material based on food proteins. The present studies constitute the continuation of the previously carried work concerning milk proteins films: K. Cieśla, S. Salmieri, M. Lacroix: Modification of the milk proteins films properties by gamma irradiation and starch polysaccharides addition, J. Sci. Food Agric. 86 908 914 (2006). K. Cieśla, S. Salmieri, M. Lacroix, Gamma irradiation influence on the structure and properties of caseinate-whey protein based films. Part I. Radiation effect on the structure of proteins gels and films, J. Agric.Food Chem 54 6374 6384 (2006) K. Cieśla, S. Salmieri, M. Lacroix, Gamma irradiation influence on the structure and properties of calcium caseinate-whey protein isolate based films. Part II: influence of polysaccharide addition and radiation treatment on structure and functional properties of the films. J. Agric.Food Chem, 54 8899 8908 (2006). 1
Starch is an abundant and cheap biopolymer. It is known to have good film forming ability and forms films with good mechanical resistance. Disadvantage of starch is high hydrophilicity. Therefore, chemical modification of starch is recommended in purpose to improve the films properties. In particular, oxidised and partially degraded starch is applied for the films preparation. As irradiation induces starch degradation and oxidation, it seemed to be useful for improvement of the films properties. In purpose to reduce the starch hydrophilicity, the addition of hydrophobic compounds (for example lipids) might be introduced to the films composition. The purposes of the present studies: The studies concerned the effect of irradiation of starch (60Co gamma rays, 5-30 kgy dose) on the properties of the films, prepared using: - Potato starch (B type, no lipids included) - Potato starch and admixed lipids / surfactants - Wheat starch (A-type, containing naturally occurring lipids) The films were prepared with addition of glycerol as a plasticizer (0, 20 or 30% in terms of starch mass). The influence of the prolonged storage and conditioning in atmosphere characterized by the various humidity was determined on the properties of the films with modified compositions. EXPERIMENTAL Irradiations with Co-60 gamma radiation were carried out for solid native starchin the gamma cell Issledovatiel in the Institute of Nuclear Chemistry and Technology, Poland. Films were prepared on the way of casting from the gelatinised starch or starch lipid solutions. Starch gelatinisation was performed on the way of heating of 2% starch solutions at 100 o C during 2 h. Potato starch lipid (surfactant) films were obtained on the basis of starch lipid connections (complexes), obtained on the way of thorough mixing of lipid solution with pregelatinised starch solution and further heating at temperature of 90 o C for several hours. 2
The following lipids were tested: sodium laurate, sodium palmitate, cetyl-trimethyl-ammonium bromide (CTAB). Some preliminary studies with lauricacid and palmiticacid were performed. The films contained 0,016 0,075 g of lipid per 1 g of starch (0,082 0,339 mmol per 1 g of starch). Glycerol was introduced to the film forming solution: - - after gelatinisation and complexation prior to the films casting (procedure I) - before of the starch gelatinisation (procedure II) The films were conditioned before testing in the controlled atmosphere of the relative humidity of 56, 44 and 33 %. Mechanical tests and the wetting angle measurements were performed in purpose to characterize the films. RESULTS In the first step, the influence of storage conditions on the properties of the potato starch and potato starch - sodium laurate films, non-irradiated and irradiated using a 30 kgy dose, was checked. It was therefore decided to carry out the further measurements for the films stored at relative humidity of 56 %. The values of Young modulus of potato starch films were in the range predicted for this type of material. Potato starch films Preliminary results obtained for the films, prepared using the commercial potato starch show on decrease in hydrophilicity of the films after irradiation accompanied by improvement of the mechanical properties. The following values of wetting angle was obtained at RH = 88%. 0 kgy: 40.2 ± 5.5 o 30 kgy: 66,7 ± 5.3 o These results were confirmed in the case of the films, obtained in the systematic experiments using the starch product of Sigma. The similar values of wetting were obtained. 3
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Potato starch sodium laurate (0,049 g/g, 0,222 mmol/g of starch) Potato starch CTAB (0,075 g/g, 0,206 mmol/g of starch) 5
Potato starch sodium palmitate (0,038g/g, 0,136 mmol/g of starch) The lower values of wetting angle than expected might result due to the porosity of the films, therefore addition of glycerol result in increase of the hydrophobic properties Wheat starch films The irradiated films are characterised by the higher strength accompanied by the lower elasticity and lower hydrophilicity: Dose, kgy: 0 30 Wetting angle: 57.2 o 83.9 o CONCLUDING REMARKS: The results show radiation induced improvement of hydrophobic properties of the films prepared using potato and wheat starch, and the selected potato starch lipid composition. The improvement of strength and elasticity was obtained in the case of potato starch films after irradiation, while in the case of wheat starch films, the increase of strength was accompanied by decrease in elasticity. Introduction of sodium laurate to the potato starch films composition results in decreased hydrophilicity and application of the irradiated starch lead to obtaining of the hydrophobic films. 6
The increase in the plasticizer content results in decrease in hydrophilic properties of starch lipid films, due to the increase in their compactness (decrease in porosity). Analysis of the obtained results shows the way of modification of the films parameters by modification of their composition and modification of the irradiation condition. The sensitivity to storage condition of mechanical properties of the hydrophobic films based on the irradiated potato starch and sodium laurate suggests, that these films can be useful as packaging materials for the products stored at high moisture level and/or during relatively short time, thus the films appear appropriate for food systems. The presentation is based on the following reports: 1. K. Cieśla et al.: Wpływ promieniowania gamma na właściwości skrobi: oddziaływania z wodą i z lipidami, folie skrobiowo-lipidowe, Project 2 P06T 026 27 Scientific Report 2007. 2. K.Cieśla, A Nowicki, M. Buczkowski, Radiation modification of the functional properties of the edible films prepared using starch and starch lipid system. Annual Report of the Institute of Nuclear Chemistry and Technology 2007, 42-45. The work was sponsored in the frame of Ministry of Scientific Research and Information Technology research grant 2P06T 026 27. 7