No abstract

19Bacterial cellulose (BC) has several current and potential future uses in the food 20 industry because of its ability to form hydrogels with distinctive properties. The texture of 21 BC hydrogels is determined by both the cellulose fibre network and the internal dispersed 22 water. In this study, mechanical properties of hydrated BC synthesised by six different strains 23 of Komagataeibacter genus were investigated with regards to their extensibility, compressive 24 strength, relaxation ability, viscoelasticity and poroelasticity. The stress/strain at failure and 25 Young's modulus were assessed by uniaxial tensile testing. The compressive strength, 26 relaxation ability and viscoelasticity were measured via a series of compression and small 27 amplitude oscillatory shear steps. A poroelastic constitutive modelling simulation was used to 28 investigate the mechanical effects of water movement. The morphology of the BC fibril 29 network under compression was observed via scanning electron microscopy. Results showed 30 that the mechanics of BC were highly dependent on the cellulose concentration, as well as the 31 morphology of the fibril network. BC synthesised by ATCC 53524 was the most 32 concentrated (0.71 wt%), and exhibited high tensile properties, stiffness and storage moduli; 33 whereas the comparatively low mechanical properties were noted for BC produced by ATCC 34 700178 and ATCC 10245, which contained the lowest cellulose concentration (0.18 wt%).35 Small deformation responses (normal stress, G') scaled with cellulose concentration for all 36 samples, whereas larger deformation responses (Young's modulus, poroelasticity) depended 37 on both cellulose concentration and additional factors, presumably related to network 38 morphology. Increasing concentration and compressive coalescence of fibres in the integrated 39 BC network reduced both the relaxation of the normal stress and the movement of water. This 40 research aids the selection of bacterial strains to modulate the texture and mechanical 41 properties of hydrated BC-based food systems. 42 43 44 45 Keywords bacterial cellulose hydrogel; tensile test; compression test; small amplitude 46 oscillatory rheology; poroelasticity 47 Food gels based on cellulose, like Nata-de-coco, are valued for their juicy mouthfeel and 51 chewable texture (Zhang et al., 2017). The Nata-de-coco is derived from fermentation of 52 certain bacterial strains which produces ultra-fine fibres of bacterial cellulose (BC) in the 53 form of a hydrogel. BC hydrogels are characterised by a randomly oriented three-dimensional 54 swollen fibril network (typically above 99 wt% water). The chemical structure of BC is 55 identical to plant cellulose, i.e. β-1-4-linked glucan chains. These chains are arranged into 56 relatively crystalline BC fibres (also called ribbons), containing a large amount of hydroxyl 57 groups on their surfaces. Recent X-ray and neutron scattering analyses suggest that BC fibres 58 have a core-shell structure built up from microfibril units of ca. 3.4 nm di...

The aim of this study was to identify the contributions made by size fractions of four milled rice (i.e., waxy, white, black and brown rice) to structural and in vitro starch digestion properties after cooking. Rice grains were hammer-milled in a controlled manner, and the coarse (300-450 μm), medium (150-300 μm) and fine size (<150 μm) fractions were segregated through vertical sieving. All samples displayed monophasic digestograms, and starch digestion rate and extent for size fractionated rice flours were predicted through the Logarithm of Slope model. It was found that digestion rate and extent were markedly reduced with increasing particle size within each rice variety. Of the four rice varieties, non-waxy rice flour fractions showed lower digestion rate and extent compared to the waxy counterpart, possibly due to the formation starch-lipid complexes as judged by XRD with ca. 4%-8% V-type crystalline structure remained after cooking. We suggested that the less rigid morphological structure and almost amorphous conformation lead to the high digestion rate and extent during simulated intestinal starch digestion. These findings will help develop functional rice ingredients with desirable digestion behaviour and attenuated postprandial glycemic responses.