The functionality of bovine milk during acid-induced gelation is governed by the interplay between protein genetic variants and the overall protein composition. This study evaluates the influence of bovine milk κ-casein (κ-CN; AA, AB) and β-casein (β-CN; A1A1, A1A2, A2A2) genetic variants on the microstructure and physicochemical properties of set and stirred yoghurts prepared from 5% (w/w) rehydrated skim milk.The κ-CN(AB) milk contained smaller casein micelles compared to κ-CN(AA), attributed to the larger surface area and higher proportion of κ-CN. Smaller micelles allowed more interaction points for whey protein complexation, leading to the formation of larger protein aggregates and a firmer gel network. This was associated with improved gel strength and lower syneresis after storage in set yoghurts, and increased resistance to shear in stirred yoghurts.While the β-CN genetic variants influenced initial acidification rates and kinetics, results suggest its influence was likely dependent on the β-lactoglobulin (β-Lg) to κ-CN ratio and it is hypothesised that the β-(A1) allele may facilitate more efficient early-stage acidification when β-Lg levels are limited. However, these differences in behaviour observed between milks of varying β-CN genetic variants were not sustained after 24 h of refrigerated storage, as the protein network continued to rearrange towards equilibrium.These results suggest the κ-CN genotype primarily governs yoghurt viscoelastic properties, while β-CN genotype may influence early-stage gelation kinetics. Results also indicate that yoghurt quality is best measured under refrigerated conditions and at least 24 h post-production as measurements taken during fermentation may not reflect the final consumer experience.

The crystallisation behaviour of milk fat plays an important role in the functionality and sensory properties of fat rich dairy products. In this study, we investigated the impact of tempering to 25 degrees C on the viscoelastic properties, particle size and thermal behaviour of 20% w/w unprocessed and homogenised creams prepared from bovine milk. The crystallisation properties were examined by synchrotron X-ray diffraction (XRD) at small (SAXS) and wide angle (WAXS) and differential scanning calorimetry (DSC). Oscillation rheology was performed to characterise the cream's viscoelastic properties. Homogenisation (35 MPa) reduced the average droplet size from 4.4 to 1.3 mu m. After 24 h storage at 4 degrees C, milk fat structures showed triacylglycerol (TAG) 2L and 3L((001, 002, 003, 005)) lamellar stacking orders associated predominantly with the alpha and beta' polymorphic forms. Tempering to 25 degrees C induced the complete melting of the 3L crystals and led to an irreversible loss in the elastic modulus (G') and a reduction in the viscous modulus (G'') once returned to refrigerated conditions, due to changes in the particle-particle interactions and structure of the reformed milk fat crystals. The results demonstrate that crystallisation behaviour of milk fat is influenced by droplet size and the rearrangement of triacylglycerol (TAG) upon tempering, and lead to changes in the viscoelastic behaviour of dairy products containing a high level of milk fat.

Cheddar cheese was prepared using milk separated into large (d(4,3) = 5.07 +/- 0.06 gm) and small (d(4,3) = 2.76 +/- 0.07 mu m) milk fat globule (MFG) size distributions using microfiltration. MFG size did not influence the moisture-adjusted yield or the amount of protein and lactose. However differences in the texture, microstructure, lipolysis and other compositional factors were found between cheeses. Notably, the small MFG milk (SMM) cheese retained more salt and contained more moisture and polar lipids, observed through a novel confocal staining regime, compared with the large MFG milk (LMM) cheese. Fewer intact MFG and more pockets of coalesced milk fat were observed in the LMM cheese. However both SMM and LMM cheese were softer, and less resistant to melting than the Control. These findings show that MFG separation can influence cheese mechanical behaviour and allow for the development of differentiated cheese products. (C) 2016 Elsevier Ltd. All rights reserved.

Protein folding at emulsion interfaces has long been a topic of discussion within the field of emulsion science, the recent development of Synchrotron Radiation Circular Dichroism (SRCD) brings new opportunities to examine this question. Changes in the secondary and tertiary structures of three globular proteins, namely lysozyme, bovine serum albumin (BSA) and myoglobin, upon their adsorption to oil-in-water emulsion interfaces were characterised by far-UV and near-UV SRCD. All three proteins exhibited systematic reductions in alpha-helical structure (by 4-7% for lysozyme, 12-21% for BSA and 19-28% for myoglobin) and increases in beta-sheet contents upon their adsorption to oil/water interfaces. Even though BSA has a high number of disulphide linkages, it underwent greater conformational rearrangement at oil/water interfaces compared to lysozyme which has fewer disulphide linkages. Similar to our previous studies, all three proteins underwent larger conformational (secondary and tertiary) changes upon adsorption to the non-polar hexadecane/water interface than the tricaprin/water interface. The conformation of the proteins (lysozyme and BSA) adsorbed at oil/water interfaces was also more stable to heat induced unfolding compared to their native structure in solution. In spite of reductions in alpha-helical structure in the adsorbed conformation of the proteins compared to their solution structure, the secondary structure of these proteins at oil/water interfaces is still largely dominated by helical structure motifs. This study shows that there appear to be several consistent trends in protein folding at emulsion interfaces which may provide the basis to a fundamental understanding of protein folding in this environment and insight in some causes of (in)stability of protein based emulsions. Crown Copyright (c) 2012 Published by Elsevier Ltd. All rights reserved.

The deliberate design of food structures that impact on lipid digestion has received increasing attention because of the need for solutions to combat nutrition related concerns such as obesity and metabolic syndrome. In this study we examined how the hierarchical structure of foods can impact lipid digestion by incorporating gastric structuring emulsions in different biopolymer networks, namely i) a thermally reversible gelatine network, ii) a colloidal casein network, and iii) a concentrated starch particulate dispersion. The digestive breakdown of these emulsion filled biopolymer gels was followed by fat digestion kinetics in vitro and human clinical study (in vivo), rheological measurements and confocal laser scanning microscopy. The parent caseinate/monoglyceride (CasMag) stabilised emulsion underwent extensive partial coalescence upon exposure to gastric juice and as a result had very slow lipolysis (in vitro and in vivo). When the emulsion was incorporated within the biopolymer networks the rates of lipolysis were strongly correlated with the extent of partial coalescence of the CasMag emulsion, which was directly influenced by the structure and breakdown properties of each different biopolymer network. The way that biopolymer networks alter the digestion of the parent CasMag emulsion is likely affected by; i) how well the digestive juices mixed with the network/emulsion and, ii) the frequency and speed of droplet encounters, both of which have a direct impact on the ability of emulsions to undergo flocculation and (partial) coalescence. This knowledge may have important implications for the design and testing of real foods to understand and control the digestive behaviour of food nutrients. Crown Copyright (c) 2013 Published by Elsevier Ltd. All rights reserved.

The destabilisation of emulsions within the stomach alters their droplet size and surface area, which in turn influences the rate and extent of fat digestion. In this study, we sought to gain further understanding of the mechanisms of the colloidal destabilisation of emulsions during digestion by examining how the composition of the interface impacts on these destabilisation processes. Understanding of emulsion destabilisation within the stomach was then linked to the extent of fat digestion through in vitro lipolysis measurements and in vivo triglyceride absorption studies. Two factors were examined; 1) co-variance of protein and monoglyceride composition at the droplet surface and 2) fat phase composition. Of the two emulsifiers present, caseinate provided the colloidal stability to the emulsion via a combination of electrostatic and steric repulsion. The acidic pH of gastric fluid resulted in a loss of electrostatic charge and a collapse of the casein steric layer, ultimately causing the emulsion to flocculate. The presence of monoglyceride influenced the emulsions susceptibility to flocculation in gastric juice and the resistance of the interface to film rupture which impacted the degree of droplet coalescence. It appeared that there was an optimum ratio between monoglyceride and protein at the interface for emulsion destabilisation. An excessive decrease in protein at the interface as monoglyceride concentration increased limited initial droplet flocculation, because there were fewer junction points for protein bridging between droplets. These changes to emulsion droplet structure had an impact on the in vitro rate and the extent of lipolysis. However triglyceride absorption in vivo was only significantly impacted when the coalesced droplet structure (e.g. emulsion containing solid fat) was maintained until the intestine. The principle cause of the altered lipolysis profile was the destabilisation of the emulsion within the stomach. These results highlight that the complexity of real food systems (i.e. multiple/mixed ingredients) can have an important impact on the digestion of emulsions, and have implications for the creation of functional foods aimed at obesity and/or diabetes. Crown Copyright (c) 2013 Published by Elsevier Ltd. All rights reserved.

The use of ultrasound pre-processing treatment, compared to blanching, to enhance mechanical properties of non-starchy cell wall materials was investigated using carrot as an example. The mechanical properties of carrot tissues were measured by compression and tensile testing after the pre-processing treatment prior to and after retorting. Carrot samples ultrasound treated for 10 min at 60 °C provided a higher mechanical strength (P<0.05) to the cell wall structure than blanching for the same time period. With the addition of 0.5% CaCl(2) in the pre-treatment solution, both blanching and ultrasound treatment showed synergistic effect on enhancing the mechanical properties of retorted carrot pieces. At a relatively short treatment time (10 min at 60 °C) with the use of 0.5% CaCl(2), ultrasound treatment achieved similar enhancement to the mechanical strength of retorted carrots to blanching for a much longer time period (i.e. 40 min). The mechanism involved appears to be related to the stress responses present in all living plant matter. However, there is a need to clarify the relative importance of the potential stress mechanisms in order to get a better understanding of the processing conditions likely to be most effective. The amount of ultrasound treatment required is likely to involve low treatment intensities and there are indications from the structural characterisation and mechanical property analyses that the plant cell wall tissues were more elastic than that accomplished using low temperature long time blanching.

The rheological behaviour and microstructural properties of rehydrated cell wall particle (CWP) dispersions and CWP–xanthan mixtures were investigated using small-deformation rheology and confocal laser scanning microscopy. Dispersions with two different CWP particle sizes were used. CWP dispersions were found to be elastic with a weak-gel type behaviour. The elastic modulus was a function of the CWP concentration cp and depended on the particle size of the CWP. The addition of xanthan to the CWP dispersions was found to affect the rheological behaviour of the CWP–xanthan mixtures at low CWP concentration (cp≤1wt%), due to the increase in the viscoelastic properties of the continuous phase. At high CWP concentrations (cp≥3%), the effect of xanthan on the rheological behaviour of the CWP–xanthan mixtures was marginal, as the viscoelastic behaviour of the mixtures was dominated by the CWP particle network, with xanthan molecules entrapped in the interstitial voids. However, at intermediate CWP volume fractions (e.g. at a CWP concentration cp=2%) both xanthan and CWP phases contributed to the viscoelastic behaviour of the CWP–xanthan mixtures.

Understanding and manipulating how emulsion structure impacts on fat digestion is an important step towards understanding the role of fat in our diet. This article reports on the nature of emulsion structuring within the digestive tract and how it affects the dynamics of fat digestion. Emulsions were designed a priori to have specific structuring behaviours (stable, coalesced, partially coalesced and fully broken) under gastrointestinal conditions, through careful emulsifier selection and control of solid fat composition. The impact these structures had on lipolysis was then assessed in vitro using a digestion model and in vivo by measuring the postprandial change in blood triglyceride concentration as a marker of fat absorption. The major factor controlling the rate of fat digestion in vitro was the droplet surface area available for lipase adsorption, which was governed by emulsion instability. The rate of fat absorption in vivo was only affected by large changes in the droplet surface area, and only if these changes remained until the droplets reached the small intestine. This was most evident in emulsions that had undergone extensive partial coalescence under gastric conditions. Partial coalescence resulted in a dramatic reduction in triglyceride absorption, in part because the network of fat crystals provided the agglomerates with an internal scaffold to resist re-dispersion as they passed through the pylorus. The differences in fat absorption profile achieved by controlling emulsion structural stability during digestion provide a basis for examining the physiological effects of food structure on lipid metabolism, which will be the subject of a follow-up clinical paper.

A range of thermal and mechanical processes were used to create dispersions with different particle morphologies, i.e., systems that contain primarily plant cell wall clusters with an average particle size (d(0.5)) of ∼200 µm, single cells ((d(0.5) = ∼70 µm) or cell fragments (d(0.5) = ∼40 µm). The small and large deformation rheology (viscoelastic properties and flow properties) of these dispersions, with a range of total solid contents covering textures varying from a fluid to a paste, were determined. The particle dispersions showed weak gel-like behaviour. Their elastic modulus (G′) as a function of total solid content exhibited three regions of different rheological behaviours. The particles formed particulate colloidal networks at relatively low solid content. The interaction of particles contributed to the sharp increase in the elastic modulus of the dispersion in the concentrated region. Further packing of particles beyond the critical packing volume fraction, caused the G′ for the ‘cluster-cell’ and the ‘single-cell’ dispersions to reach a plateau value. This is due to particle deformation resulting in changes in their microstructure and their ability to pack closely. Plant particle dispersions displayed abrupt yielding at the critical stress with a 3–4 order decrease in viscosity and high yield stress at low solids due to the formation of particulate colloidal network.

The rheological behaviour of plant cell-wall particle dispersions was investigated using dynamic oscillatory measurements. Two starting plant materials, broccoli stem and carrot were used and two types of particles were obtained by mechanically shearing blanched (80°C, 10 min) or cooked (100°C, 15 min) plant tissues. Blanching resulted in cell-wall particles made up of a collection of clusters of cells with an average particles size of ∼200 μm, while cooking generated nearly all single-cell particles with an average particle size of ∼80 μm. The rheological measurements showed that in the range of weight concentrations considered (∼0.5% to ∼8%) the dispersions behaved as elastic materials with the elastic modulus G' higher than G″ within the frequency range (0.01-10 Hz). This study shows that the behaviour of the complex modulus G* as a function of the effective volume fraction ϕ can be modelled using different theoretical equations. To do so, it is assumed that below a critical volume fraction ϕc a network of plant cell-wall particles was formed and G* as a function of ϕ obeys a power-law relationship. However above ϕc, where the particles were highly packed, G* could be modelled using theoretical equations developed for concentrated emulsions and elastic particle dispersions.

Isolated wheat protein (IWP) is an acidic deamidated wheat protein. The deamidation process enhances the protein solubility at pHs greater than 6, and therefore its potential ability to act as a food emulsifier. The interfacial properties and the mechanism by which this protein stabilises oil-in-water emulsions were investigated by measuring the protein's absorbed layer thickness on latex particles, its interfacial rheology, and the colloidal and thermal stability of IWP stabilised emulsions. IWP forms a relatively thick interfacial layer of 18 nm upon adsorption onto latex beads, suggesting that the protein adsorbed with the long axis perpendicular to the surface, i.e. end-on, at a full protein coverage. The interfacial rheology measurement showed that IWP formed a relatively weak fluid-like interface. Similar to other protein emulsifiers, the colloidal stability of IWP emulsions is provided largely through electrostatic repulsion. Although IWP emulsions were sensitive to salt induced flocculation, the presence of excess protein in the aqueous phase (e.g. 4 wt%) was able to reduce the effect of salt screening (50 mM CaCl2) on a 25 wt% oil-in-water emulsion completely. The emulsions underwent minimal coalescence when droplets were in close contact, e.g. flocculated, because the interfacial layer of IWP provides a barrier to droplet coalescence, even in high salt environments. IWP emulsions were resistant to thermal treatment with no changes in particle size observed when the emulsions were heated (up to 90 degrees C for 20 min) in the absence or the presence of 150 mM NaCl. The heat stability of IWP emulsions is thought to arise from the structure of IWP at the interface. A lack of free cysteines combined with few hydrophobic regions meant that there were minimal interactions between protein molecules adsorbed onto the same droplet or on neighbouring droplets. The unique interfacial properties of IWP, e.g. its physical layer thickness and the structure provide enhanced stability for emulsions against coalescence and heating. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.