The impact of food structure on taste and digestibility.

The modern food chain depends on complex interactions between businesses from farming to retail. Until recently their success depended upon providing consumers with safe, convenient food which was pleasant to eat, at a reasonable value for money. This has required detailed research into how food structures deliver recognisable and preferred types of foods, from hard solids to thick liquids. Fortunately the consumer is able to detect and report sensations of texture and flavour which can be related to the composition, structure and breakdown of food in the mouth. Chemists, physicists and engineers can attempt to build mechanistic models of how structures relate to perception. The state of the art in our understanding and design capabilities are reviewed. In the developed world, the success is self evident as food prices (as a proportion of income) have decreased and there is a surfeit of choice on the supermarket shelf. More recently, the requirement to add a balanced healthy diet to the simple pleasure of eating has become the new target. This is a different type of challenge. The effects of diet on health are long term, and not easily reported by the consumer. Whilst we know something of how the digestive tract works in breaking down foods, we know little of how food structure impacts upon this process, and even less of how the neural and metabolic feedback systems operate to relate food structures to satiety and satiation. Therefore, in the absence of causal models relating structures to eating habits, structures designed to achieve both immediate pleasure and long term healthy eating are much more speculative. What we think we know, and what we need to know are reviewed. There is no doubt that other skills, in nutrition, physiology, neuroscience, and molecular biology etc. will need to be added to the classical approaches of food materials science and engineering if these challenges are to be met.

The physico-chemical characterization of a boiling stable antifreeze protein from a perennial grass (Lolium perenne).

We have characterized a cold-induced, boiling stable antifreeze protein. This highly active ice recrystallization inhibition protein shows a much lower thermal hysteresis effect and displays binding behavior that is uncharacteristic of any AFP from fish or insects. Ice-binding studies show it binds to the (1 0 1 0) plane of ice and FTIR studies reveal that it has an unusual type of highly beta-sheeted secondary structure. Ice-binding studies of both glycosylated and nonglycosylated expressed forms indicate that it adsorbs to ice through the protein backbone. These results are discussed in light of the currently proposed mechanisms of AFP action.

Investigation of bacterial spore structure by high resolution solid-state nuclear magnetic resonance spectroscopy and transmission electron microscopy.

High resolution solid-state nuclear magnetic resonance spectroscopy (NMR) in combination with transmission electron microscopy (TEM) of spores of Bacillus cereus, an outer coatless mutant B. subtilis 322, an inner coatless mutant B. subtilis 325 and of germinated spores of B. subtilis CMCC 604 were carried out. Structural differences in the coats, mainly protein of spores were reflected by NMR spectra which indicated also differences in molecular mobility of carbohydrates which was partially attributed to the cortex. Dipicolinic acid (DPA) of spores of B. cereus displayed a high degree of solid state order and may be crystalline. Heat activation was studied on spores of B. subtilis 357 lux + and revealed a structural change when analysed by TEM but this was not associated with increases in molecular mobility since no effects were measured by NMR.

Understanding the mechanism of ice binding by type III antifreeze proteins.

Type III antifreeze proteins (AFPs) are present in the body fluids of some polar fishes where they inhibit ice growth at subzero temperatures. Previous studies of the structure of type III AFP by NMR and X-ray identified a remarkably flat surface on the protein containing amino acids that were demonstrated to be important for interaction with ice by mutational studies. It was proposed that this protein surface binds onto the (1 0 [\bar 1] 0) plane of ice with the key amino acids interacting directly with the water molecules in the ice crystal. Here, we show that the mechanism of type III AFP interaction with ice crystals is more complex than that proposed previously. We report a high-resolution X-ray structure of type III AFP refined at 1.15 A resolution with individual anisotropic temperature factors. We report the results of ice-etching experiments that show a broad surface coverage, suggesting that type III AFP binds to a set of planes that are parallel with or inclined at a small angle to the crystallographic c-axis of the ice crystal. Our modelling studies, performed with the refined structure, confirm that type III AFP can make energetically favourable interactions with several ice surfaces.

Structure and solution properties of tamarind-seed polysaccharide.

The major polysaccharide in tamarind seed is a galactoxyloglucan for which the ratios galactose:xylose:glucose are 1:2:25:2.8. A minor polysaccharide (2-3%) contains branched (1----5)-alpha-L-arabinofuranan and unbranched (1----4)-beta-D-galactopyranan features. Small-angle X-ray scattering experiments gave values for the cross-sectional radius of the polymer in aqueous solution that were typical of single-stranded molecules. Marked stiffness of the chain (C infinity 110) was deduced from static light-scattering studies and is ascribed partially to the restriction of the motion of the (1----4)-beta-D-glucan backbone by its extensive (approximately 80%) glycosylation. The rigidity of the polymer caused significant draining effects which heavily influenced the hydrodynamic behaviour. The dependence of "zero-shear" viscosity on concentration was used to characterise "dilute" and "semi-dilute" concentration regimes. The marked dependence on concentration in the "semi-dilute" region was similar to that for other stiff neutral polysaccharide systems, ascribed to "hyper-entanglements", and it is suggested that these may have arisen through a tenuous alignment of stiffened chains.

The effect of mechanical deformation on the movement of water in foods.

This study has shown the importance of relating NMR spectroscopic information on the water in model food structures to the mechanical properties of those structures. Analysis of the NMR relaxation data can be used to examine the distribution of water domain sizes, and this has been related to the mechanical properties of the samples. A novel NMR probe-head has been designed, which allows both the NMR and the mechanical data to be simultaneously measured during compression of the sample. This probe-head allows compressive stress/strain data to be obtained directly from the NMR sample, allowing changes in the distribution of the water to be directly correlated to changes in mechanical properties.

Salt-induced swelling of meat: The effect of storage time, pH, ion-type and concentration.

The effect of hypertonic salt solutions on meat fibres has been studied as a function of post-mortem storage. Rabbit longissimus dorsi fibres (after about 20 h post-mortem storage at 4°C), were found to swell in hypertonic salt solutions, such as 0·25m KI or 0·6m KCl, to two to three times their original diameter. This swelling occurred in the fibre transverse plane only. X-ray diffraction shows swelling occurs by a combination of an increase in the myofilament lattice spacing and a loss of myofilament order. Fibre swelling is highly co-operative. At pHs below the isoelectric point of the myofibrillar proteins, hypertonic salt solutions induce fibre shrinkage. The post-mortem time course of response shows a peak at 18-35 h bounded by periods of minimal or zero response. We propose that the collagenous endomysial sheath acts as a restraint to myofibril swelling and that the characteristics of post-mortem swelling are a balance of the myofibrillar propensity to swell and the constraint of the endomysium.