Ian T. Norton

Oral behaviour of food hydrocolloids and emulsions. Part 1. Lubrication and deposition considerations

Biopolymers (polysaccharides and proteins) and their mixtures are used in the Food Industry to impart stability, texture and appearance to fabricated foods. This paper reviews the physical measures conducted to elucidate textural properties such as creaminess, smoothness, sliminess and thickness of food products and discusses oral processing mechanisms in relation to the behaviour of hydrocolloids and emulsions in the oral cavity during eating. In particular, this article covers the use of Tribology and Evanescent Wave Spectroscopy techniques that enable the study of the lubrication and deposition behaviour of food components. Comparison of the physical measurements with sensory properties indicate that thin film rheology and surface deposition phenomena make an important contribution to sensory properties such as fattiness, smoothness and astringency.

Microstructure design in mixed biopolymer composites

Abstract This paper is an overview of recent work on some particular aspects of the behaviour of biopolymer solution and gel mixtures, the focus being on aspects that are of particular relevance to the materials found in food systems. As such, the following areas are considered. (1) The phase behaviour of such mixtures in terms of the microstructures formed, the ideas of phase volume, phase continuity, surface tension and the use of the Flory–Huggins theory to model the behaviour. (2) The kinetics of phase separation in terms of the evidence for spinodal decomposition, ripening processes and the interplay between the kinetics of phase separation, gelation and molecular ordering. (3) The effects of shear on the composite microstructure in terms of particle size and shape, and how shear can be used to cause phase inversion of the system. (4) The material properties of the composites, including the fracture behaviour, the effect of particle size, and the role of the interface between the phases. It is clear that over the past few years a considerable literature has been built up on mixtures of biopolymers. Despite this growing understanding it remains a fact that we are not capable, as yet, of designing the microstructure of mixed biopolymer composites, although it is clear that there is a considerable technological advantage in being able to do so.

A molecular model for the formation and properties of fluid gels

The effects of biopolymer gelation in a shear field are discussed. Gel particles are produced if the gelation mechanism involves an aggregation step. Particular attention is paid to the molecular events of ordering and aggregation upon cooling, investigating the differences in such processes as a result of shearing during gelation. A model is proposed which follows the conception of the particles, their growth, physical properties and stability.

Influence of thermal history on the structural and mechanical properties of agarose gels.

Using a multitechnique approach, two temperature domains have been identified in agarose gelation. Below 35 degrees C, fast gelation results in strong, homogeneous and weakly turbid networks. The correlation length, evaluated from the wavelength dependence of the turbidity, is close to values of pore size reported in the literature. Above 35 degrees C, gelation is much slower and is associated with the formation of large-scale heterogeneities that can be monitored by a marked change in the wavelength dependence of turbidity and visualised by transmission electron microscopy. Curing agarose gels at temperatures above 35 degrees C, and then cooling them to 20 degrees C, produces much weaker gels than those formed directly at 20 degrees C. Dramatic reductions in the elastic modulus and failure strain and stress are found in this case as a result of demixing during cure. An interpretation, based on the kinetic competition between osmotic forces (in favor of phase separation) and elastic forces (that prevent it) is proposed.

Shear-induced anisotropic microstructure in phase-separated biopolymer mixtures

Abstract This work is concerned with the formation, under flow, of regular, anisotropic particle shapes in phase separated liquid–liquid biopolymer mixtures (water-in-water emulsions) and the fixing of such structures through gelation of one or both phases. Hypothesising that, for the purpose of flow processing, such water-in-water emulsions can be treated in the same manner as conventional emulsions, a shear flow process was chosen. Microstructures produced by this route show a wide spectrum of shape, from spherical particles, developed at the lower end of the shear stress range, to long extended particles, as well as irregularly shaped particles, formed at higher shear stresses. These different microstructures developed are discussed in the context of viscosity and shear rate data recorded during the structuring process. For one of the mixtures, the viscosity ratio and interfacial tension between the two phases were quantified. It was found that the observed particle deformations in this mixture coincided with predictions from droplet deformation theories.

Phase equilibria and gelation in gelatin/maltodextrin systems — Part II: polymer incompatibility in solution

Abstract The effect of thermodynamic incompatibility in mixed solutions of gelatin and Paselli maltodextrins SA-6 and SA-2 has been studied at a temperature (45°C) where the individual polymers are stable as disordered coils. Concentrated mixtures of SA-6 and gelatin showed classic phase separation into two co-existing liquid layers, with compositions lying along a well-defined binodal. On decreasing SA-6 concentration below the binodal, however, a substantial proportion (up to 60%) of the maltodextrin was precipitated, with normal single-phase solutions occurring only at much lower concentrations of both polymers. SA-2 showed a more extreme version of the same behaviour, with precipitation of up to 100% of the maltodextrin and no evidence of co-existing liquid phases at any accessible concentrations. In both cases, the amount of maltodextrin precipitated was proportional to the square of its initial concentration and to the first power of gelatin concentration, indicating that gelatin drives self-association and aggregation of maltodextrin when both polymers are present in a single liquid phase. 1 H NMR showed the precipitated maltodextrin to be higher in molecular weight and in degree of branching than the material remaining in solution, and particlesize analysis indicated that the volume of the individual maltodextrin particles increased linearly with the total mass precipitated.

Phase equilibria and gelation in gelatin/maltodextrin systems — Part IV: composition-dependence of mixed-gel moduli

Abstract The storage moduli ( G ′) of phase-separated co-gels formed by quench-cooling mixed solutions of gelatin and potato maltodextrin (Paselli SA-2 and SA-6) have been related quantitatively to the experimentally-determined concentration-dependence of G ′ for the constituent polymers. Distribution of water between the phases was examined explicitly by using polymer blending laws to derive calculated moduli for gelatin-continuous and maltodextrin-continuous networks over the entire range of solvent partition. Allowance was made for the direct contribution of polymer chains, and for density differences between the phases, in calculating relative phase-volumes. The effect of gel formation prior to phase-separation was calculated using classical theory for network de-swelling. Good agreement with observed moduli for more than 30 gelatin/maltodextrin combinations was achieved using a single adjustable parameter, p (the ratio of solvent to polymer in the gelatin phase divided by the same ratio for the maltodextrin phase), with an optimum value of p ≈ 1·8 for both SA-6 and SA-2.

Understanding food structuring and breakdown: engineering approaches to obesity

Many of the processing operations of the food industryhave been undertaken for millennia and many of the prob-lems are equally old. For example, Garnsey points out thatin classical Rome, most of the population ate cheap meatfrom roadside stalls or taverns, rather than cooking itthemselves (1).Many food products are structurally complex. Thisstructure, and its breakdown in the mouth, determines thetaste, texture and eating pleasure of each product. Theirmanufacture is also complex. For example, bread involvesthe creation of microstructure, coupled with heat andmass transfer, and the flow and deformation of highly non-Newtonian materials of which our engineering understand-ing is limited (2). Food products need to be metastable todeliver taste and flavour on consumption. For example, theconfectionary fats in chocolate are highly polyphasic, withsix polymorphs melting within 20 ° C of one another, andthe form that the consumer enjoys is not the thermodynam-ically stable one (3).To make these products efficiently, a combined under-standing of chemistry and material science is needed,together with knowledge of how the processing the mate-rial receives affects its structure, chemistry and attractive-ness. This type of understanding can be used to developstructured foods that are inherently more healthy.It is our hypothesis that one way forward will be for thefood manufacturer to encourage a change in lifestyle andto develop the next generation of foods, designed to beconvenient, cheap, nutritionally balanced and enjoyable toeat. These foods will be structured in such a way as tocontrol the rate of release of macronutrients and slow therate of the stomach emptying, thus limiting the amount offood that people consume. We have recently discussed theways in which food science and engineering might be usedto develop healthier foods (4). Here, we suggest howenhanced understanding of food structuring and break-

Phase equilibria and gelation in gelatin/maltodextrin systems — Part III: phase separation in mixed gels

Abstract Co-gels of potato maltodextrin Paselli SA-6 with gelatin were prepared by rapid quenching of mixed solutions from 90°C. At fixed setting temperature and fixed concentration of gelatin, the time required to form a self-supporting network showed an initial steady decrease with increasing concentration of SA-6 (as expected from polymer exclusion), but then increased dramatically before again decreasing. The interpretation of this behaviour as phase inversion from a gelatin-continuous network with SA-6 inclusions to a (more slowly-forming) SA-6 network with gelatin inclusions was confirmed by differential scanning calorimetry (showing both components melting separately, with no evidence of specific interaction), mechanical spectroscopy (showing that the mixed gel network was destroyed completely by melting of the gelatin component at low concentrations of SA-6, but only weakened at SA-6 concentrations above the inversion point) and by light microscopy (showing the expected changes in distribution of the two polymers). In similar studies using the faster-gelling potato maltodextrin Paselli SA-2, microscopy and gel-melting profiles again showed phase-inversion from a gelatin-continuous network at low concentrations of SA-2 to a maltodextrin-continuous network at higher concentrations. Inversion, however, occurred at a lower concentration of maltodextrin than in the gelatin/SA-6 systems, and the accompanying change in gelation rate was confined to a sharp decrease in the dependence of gel-time on SA-2 concentration.

Gelation of casein-whey mixtures: effects of heating whey proteins alone or in the presence of casein micelles

The aim of the present work was to investigate the role of whey protein denaturation on the acid induced gelation of casein. This was studied by determining the effect of whey protein denaturation both in the presence and absence of casein micelles. The study showed that milk gelation kinetics and gel properties are greatly influenced by the heat treatment sequence. When the whey proteins are denatured separately and subsequently added to casein micelles, acid-induced gelation occurs more rapidly and leads to gels with a more particulated microstructure than gels made from co-heated systems. The gels resulting from heat-treatment of a mixture of pre-denatured whey protein with casein micelles are heterogeneous in nature due to particulates formed from casein micelles which are complexed with denatured whey proteins and also from separate whey protein aggregates. Whey proteins thus offer an opportunity not only to control casein gelation but also to control the level of syneresis, which can occur.