Abstract
Ageing causes retrogradation or recrystallisation of starch, which leads to staling of food products and embrittlement of non-food starch products. Some plasticisers are known to reduce retrogradation, but it is not clear how.
In chapter 1, an overview is given of the present knowledge of starch. In chapter 2, the
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analytical techniques and their applicabilities in starch research are presented.
In chapter 3, retrogradation and sub-Tg physical ageing are described of gelatinised starch, as studied with infrared and NMR relaxation spectroscopy. The influence of processing temperature on initial crystallinity and subsequent recrystallisation (by X-ray diffraction) are described of compression moulded starch, plasticised by water and glycerol.
In chapters 4-6, the interaction between starch and the plasticisers glycerol or ethylene glycol in the absence of water are described. In chapter 4, the interaction is described to cause a strong exothermal DSC transition. With solid state NMR spectroscopy, an immobilisation of the plasticisers and mobilisation of starch were observed. Upon storage at room temperature, the interaction also occurred, but faster for ethylene glycol than for glycerol, and glycerol interacted mainly with amorphous starch. Less plasticiser molecules interacted with more of their hydroxy groups than upon heating.
In chapter 5, the interaction between dry amylopectin and ethylene glycol is described as studied by dielectric relaxation spectroscopy. Ethylene glycol was suggested to form intra-chain H-bonded bridges between the amylopectin chains, increasing chain stiffness and increasing the glass transition. Ethylene glycol was confined to nanometer sized droplets, as the dynamics changed from VFT towards Arrhenius behaviour.
In chapter 6, the interaction was studied by Inverse Recovery Cross Polarisation NMR spectroscopy. At room temperature, the plasticiser mobility decreased and the amylopectin C6 mobility increased. The mobilities of the other amylopectin carbons did not change. The interaction mainly occurs at C6. Upon heating, the interaction develops fast, after which crystal perfection is assumed to take place. Crystal perfection is slower for glycerol than for ethylene glycol.
In chapters 7 and 8, retrogradation is described of fully and partly gelatinised starch with several plasticisers. Due to partial gelatinisation, some granular structure remained, appearing as non-crystalline ghosts. These may act as nuclei for crystallisation. In chapter 7, systems are described with a range of plasticisers, increasing in size and number of hydroxy groups (ethylene glycol, glycerol, threitol, xylitol, glucose and for potato starch also maltose). The larger the number of OH groups, the better the plasticiser reduced the crystallisation inducing effect of ghosts in potato starch. Wheat starch recrystallised to a lesser extent (X-ray crystallinity indices of ~0.4 vs. ~0.5 for potato starch), probably because of the shorter amylopectin chains. Wheat starch did not show clear trends for the influence of plasticiser size and of ghosts.
In chapter 8, retrogradation is described of wheat starch with a range of malto-oligosaccharides (maltose, maltotriose, maltotetraose, maltopentaose and maltohexaose). Malto-oligosaccharides substantially reduced retrogradation (crystallinity indices of ~0.2). No trend was found for the influence of ghosts. The finding that maltose reduced retrogradation substantially better than glucose (chapter 7), is of practical importance for starch based foods. Malto-oligosaccharides consisting of 6 or more glucose residues (6 residues are needed for helix formation) were proposed to increase retrogradation because of co-crystallisation. The smaller malto-oligosaccharides were assumed to reduce retrogradation by intruding between the starch chains.
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