Food Structure and Health
Carbohydrate Biopolymer Structure
Primary Objectives
- To identify and explain novel starch granule structures that can be used to moderate the digestion of starch. This will allow the identification of new isolated starches or novel vegetables and cereals which can be used to generate starch-based foods that reduce blood glucose levels and enhance the resistant starch content of foods.
- To test molecular hypotheses for the bioactivity of natural plant cell wall polysaccharides. This knowledge will establish health claims for the consumption of fruits and vegetables.
- To understand the rational self-assembly of polysaccharide-protein multilayers and to develop novel food structures with controlled barrier and release properties following consumption and digestion.
Polysaccharides are a major component of the human diet: they include the macronutrient starch, non-starch cell wall polysaccharides (dietary fibre) and food additives. Polysaccharides contribute to the structure of foods and this determines the mode and site of breakdown of food structure on digestion, and the nature and form of absorption of the released fragments. Understanding this fragmentation and absorption provides a basis for identifying mechanisms for the bioactivity of carbohydrates including roles as prebiotics. Natural carbohydrates can also be used for the fabrication of novel coatings, barriers and release systems for site-specific delivery of components within the GI tract. These projects will identify the structural characteristics of self-assembled natural plant carbohydrates which enhance their bioactivity and nutrient value and the structural basis for self-assembly and functionality of fabricated food structures. Modern experimental in vitro techniques, physical modelling and simulations, together with in vivo human studies will be used to study these systems.
One example of how new insights can be gained which contribute to developing new and healthier foods is the study of starch structure. Over the last 25 years we have produced a systematic molecular description of the processing of starch-based foods as a basis for interpreting starch digestion and colonic fermentation. Currently the digestibility of starch in starch-based foods is determined by the retrogradation of starch following gelatinisation and storage: the crystalline structure of the granule is destroyed and new crystalline structures are created in the food. In the last 5 years newly developed methodology for AFM imaging of starch granule structure and the understanding of contrast in AFM images has allowed study of the effect of biosynthetic mutations on starch structure and functionality. These have identified how mutations in biosynthesis alter the crystalline structure within granules generating 'intrinsic resistant starch': the granular structure survives processing and influences digestion. For isogenic pea starch mutants it has been shown that high-amylose mutants (r loci) contain new crystalline structures that raise the resistant starch (RS) content of foods. High-resolution AFM of starch granules in seeds (fig.2.1a-b), plus use of a wide range of microscopic methods has been used to define the crystalline structures within individual starch granules. These methods have identified commercial pea varieties containing such mutated starches. Combined microscopic studies reveal that the seeds contain a blend of 'immature' granules that gelatinise normally and novel 'mature' granules that convey intrinsic resistance to digestion, even after cooking or processing. Preliminary human studies show reductions in blood glucose levels on consumption of foods based on these pea varieties.
Fig 2.1. High-resolution AFM images of crystallinity in (a) WT pea & (b) r mutant. Heterogeneity of granule structure in ae maize (c) iodine staining & (d) birefringence.
The role of dietary carbohydrates in cancer progression and metastasis is a growing field of clinical importance. There is strong emerging evidence that extracts of β-glucans and pectin may act as anti-cancer agents; they may be effective for all types of cancer and affect various stages of tumour development. There are testable hypotheses for the molecular basis of this action which, in both cases, involve interactions of carbohydrate fragments with mammalian lectins. If dietary carbohydrates can be used as anti-cancer agents, then there is potential for exploring the possibility that oral consumption of foods containing these molecular species can provide protection against the progression of a range of cancers. If suggested molecular mechanisms are correct, then it is necessary that oral consumption of foods allow appropriate fragments to be released into the lymph systems to enable attachment to mammalian lectins expressed on cell surfaces. In the case of pectin bioactive fragments are suggested to bind to and inhibit the role of galectin 3 (Gal3), a galactose-binding protein expressed on tumour cell surfaces: Gal3 is implicated in various stages of tumour development including angiogenesis, emboli formation, cell adhesion and apoptosis. New AFM methodology has been developed to probe carbohydrate-lectin interactions at the molecular level (Fig 2.2a). Feasibility studies suggest binding of pectin-derived galactans or arabinogalactans to Gal3 (Fig 2.2b) are responsible for the bioactivity of pectin.
Figure 2.2 Pectin-Gal3 binding. (a) Schematic of the method used to probe pectin-derived fragments to Gal3. (b) Force-distance curve indicating specific binding.
There is also a need to further investigate the breakdown of plant cell walls on digestion, not only to understand and manipulate the release of bioactive carbohydrates, but also to understand the role of cell wall breakdown on the release and bioavailability of nutrients and other bioactive components such as phytochemicals.
A third aspect of carbohydrate polymers research relates to controlled site-specific release of active food components in the mouth and GI tract. This requires materials which respond to the local environment (pH, ionic strength, osmotic pressure or enzymatic effects). Novel coatings and barriers can be produced by generating mixed multilayers of natural polyelectrolyte biopolymers (pectin, alginate, chitosan, gelatin and other food proteins). Through control of the charge density and distribution it is possible to rationally assemble structures that respond to the differing environments of the GI tract: they can be fabricated to function as barrier/resistant layers which can respond and release active components under defined conditions. Layer-by-layer (lbl) electrostatic assembly is a versatile and robust means of fabricating nanoscale surface and interfacial multilayers comprising polyelectrolytes, polyampholytes and charged colloidal particles. A mechanistic description for predicting the growth pattern a particular polyanion/polycation pair will adopt remains unknown. We are studying how the characteristic growth behaviour and functional properties of biopolyelectrolyte multilayers depend on the irregular charge distribution and chain structure of the constituent biopolymers. We are investigating the detailed steady state and dynamical mechanisms, operating at a molecular level, by which reversibly adsorbed biopolyelectrolyte multilayers assemble and subsequently respond to changes in environmental conditions. Such an understanding will facilitate more effective knowledge-based means of designing and fabricating such mutilayer structures for delivery in the GI tract environment.
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