Microstructures, rheological and mechanical properties of gelatin–starch blends

Zhang, N 2016, Microstructures, rheological and mechanical properties of gelatin–starch blends, Doctor of Philosophy (PhD), Science, RMIT University.


Document type: Thesis
Collection: Theses

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Title Microstructures, rheological and mechanical properties of gelatin–starch blends
Author(s) Zhang, N
Year 2016
Abstract Gelatin exhibits good film-forming and gas barrier properties, and it has been used widely in the food and pharmaceutical industries. However, the shortcomings of gelatin films, such as being an animal-derived ingredient, lower softening temperature and the instability of moisture content in gelatin, have led to attempts to use replacement substances. Starch is a common food ingredient and it has a facile film-forming behavior. Both gelatin and starch have separately been widely used to develop edible films. Therefore, development of starch–gelatinbased blends and overcoming the shortcomings of gelatin-only products has both scientific and commercial importance. Blends of gelatin with up to 50 % hydroxypropylated high amylose content (80 %) corn starch was developed forcapsule materials. Poly(ethylene glycol) (PEG) was used as both a plasticizer and a compatibilizer in the blends. To prepare hard capsules for pharmaceutical applications using the well-established method of dipping stainless steel mold pins into solution, solutions with higher solids concentrations (up to 30 %·w/w) were developed. The solutions, films and capsules of different gelatin–starch blends were characterized by viscosity, transparency, tensile testing, water contact angle and scanning electron microscopy (SEM). The linear microstructure of the high amylose starch, and the flexible and more hydrophilic hydroxylpropylene groups grafted onto the starch improved the compatibility between the gelatin and starch. SEM revealed a continuous phase of gelatin on the surface of films from all blends. The water contact angle of pure gelatin and the different blends were similar, indicating a continuous phase of gelatin. By optimizing temperature and incubation time to control viscosity, capsules of various blends were successfully developed. PEG increased the transparency and toughness of the various blends. The complex issue of compatibility between starch and gelatin was investigated based on their interface and phase composition using synchrotron Fourier transform infrared (FTIR) micro-spectroscopy. A high amylose (80 %) corn starch grafted with flexible and hydrophilic hydroxpropyl groups and plasticized by PEG was used throughoutthis work. The FTIR beam focused on a 5 μm × 5 μm detection region and the micro-spectroscopy was scanned across the gelatin–starch interface. It was found that there was about a 20 μm thick layer where gelatin and starch were in co-existence, indicating that gelatin and starch are compatible to a certain degree in theseblends. The ratio of the areas of the saccharide C–O bands (1180–953 cm−1) and the amide I and II bands (1750–1483 cm−1) was used to monitor the relative distributions of the two components of the blends. FTIR 2 and 3-dimensional maps indicated that gelatin constituted the continuous phase to 80 % of starch content. The PEG was homogeneously distributed in both gelatin and starch phases, and it blurred the interface between gelatin and starch in the chemical maps, indicating that PEG acted as a plasticizer andas a compatibilizer for the gelatin–starch blends. Morphologies and phase compositions of different starch–gelatin blends were investigated by various microscopies: optical, SEM and synchrotron FTIR micro-spectroscopy. SEM revealed that the surface became smoother after adding PEG. Optical microscopy (OM) observation revealed that compatibility between gelatin and starch was improved by adding PEG. An FTIR beam focused on a 5 x 5 μm detection area by the micro-spectrometerwas used to map chemical composition. The ratio of areas of the saccharide bands (1180–953 cm-1) and the amide I and II bands (1750–1483 cm-1) was used to monitor the relative distributions of the two components in the blends. All of the FTIR spectra showed contributions from both starch and gelatin absorptions, therefore indicating that complete phase separationintopure starch and gelatin domains did not occur. The PEG improved the compatibility of the gelatin–starch blends. Because of the need to have a rapid test to identify gelatin and hydroxypropyl starch (HPS) in theblendedfilms, a simple technique ofidentifying the HPS in the blend under an optical microscope (OM)throughvisualizingHPS with iodine was established. This method offered a directand definitiveway to study the approxximate phase distribution of starch in the blends. By adopting this observationmethod and combining with SEM, FTIR and extensograph, the phase transition, miscibility and mechanical properties of theblends were studied systematically, and arelationship between phase transition, miscibility and film properties was eatablished. Research usingOM showed that phase inversionoccured when HPS ratio was80 % and interphase mixing was observed, which proved that theseblends showed compatibility to some extent. FTIR and SEM further proved the compatibility of theseblends. Contact angle showed a sharp change at an HPS ratio of 80 % and modulus showed aninflection at this blending ratio, which were due to the phase inversion. The influence of plasticizers on the multilevel structure, mechanical properties and transparency was studied. Plasticizermechanismsactingon the mechanical properties were established. All the plasticizers increased the crystallinity, among which, glycerol had the most profound effect, followed by PEG, then propylene glycol (PG). The influence of plasticizers on the structure of suspended microcells depended on the type of plasticizer. PEG decreased the compactness of the self-similar structure, PG increased the compactness of the self-similar structure of all blendsexcept for pure gelatin. Glycerol plasticized blendsdid not possess self-similar structure, but showeda lamellar structure with15.7 nm spacing. The order of the extent of the influence of plasticizers on decreasing Tgwas PEG>glycerol>PG, indicating that PEG showed the greatestefficiency in increasing mobility withinthe amorphous region, followed by glycerol, then PG. PEG improved the mechanical properties of the blends to the greatest extent, followed by glycerol, then propylene glycol. The order of extent of the influence of plasticizers on increasing the transparency was glycerol>PEG>PG.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Science
Subjects Physical Chemistry of Materials
Food Chemistry and Molecular Gastronomy (excl. Wine)
Rheology
Organic Green Chemistry
Keyword(s) Gelatin
Starch
Edible films
Polymer
Capsule
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