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Hard solid foods encompass a large variety of dry products as well as products with high water content. Most of these foods have a cellular structure, which is generally characterized by connected fairly rigid cell walls, enclosing a fluid material that may be liquid-like (fruit and vegetables) or a gas (mainly manufactured cellular foods). Typical for many hard solid products is their brittle fracture behavior, mostly accompanied by acoustic emission. The latter characteristic is essential for their crispy or crunchy character. Other main texture attributes are hardness and brittleness and for fruits and vegetables juiciness. The latter requires that the liquid content of the cells is released during mastication. Aspects of fracture behavior of cellular food products, oral processing of these products, and the interplay between product characteristics and perception of some main texture attributes (hardness and crispness) will be discussed. PRACTICAL APPLICATIONS It has become more and more clear that texture perception by consumers of hard solid (brittle) foods is based on the interplay between product structure, fracture behavior, oral processing and final grading by the brain. Better understanding of the relations between these aspects is essential for the production of healthy, tasteful food that is liked by consumers. This article focus on the interplay between physical properties of hard solid foods and oral processing in relation to the perception of selected texture attributes (hardness and crispness)

Step-wise and oscillatory gravimetric sorption experiments were used to study the equilibrium and dynamic water sorption behavior of bread crust. Water uptake kinetics is strongly related to crispness retention of composite products consisting of a dry crispy part and a more humid and soft part. We show that oscillatory sorption experiments of bread crusts could be very well described by a Fickian diffusion model. Many essential features, such as the shape of the oscillatory sorption curves, and the dependency of water sorption rates on time, time-interval between successive step-wise changes in relative humidity, and particle size are understood now to a great detail.

The effect of the ratio between the modulus of the oil droplets and that of the gel matrix (varied by changing gelling agent concentration and oil droplet size) on the large deformation properties of gelatine, ¿-carrageenan and whey protein isolate (WPI) gels was studied at different compression speeds. The effect of gelling agent concentration and oil droplet size on strain-dependency of modulus and viscoelastic properties was also studied. An increase in the concentration of gelling agent resulted in denser gels with more bonds between structural elements. This induced an increase of both Young's modulus and fracture stress for all gels. With increasing gelling agent concentration, polymer gels (gelatine and ¿-carrageenan) became less strain-hardening, and the particle gels (WPI) even became strain-softening. The effect of a decrease in the oil droplet size on the Young's modulus was generally according to the Van der Poel theory, unless when the oil droplets were aggregated. Moreover, a decrease in oil droplet size induced a decrease of the fracture strain in gels with non-aggregated bound droplets. The extent of these changes was shown to depend on the gelling agent concentration. The effect of a decrease of the oil droplet size on other fracture parameters and in other gel systems was minor. With decreasing oil droplet size gelatine gels with unbound droplets and WPI gels became more viscous and less elastic. Keywords: Carrageenan gels; Gelatine gels; WPI gels; Emulsions

The large deformation properties of gelatine, κ-carrageenan and whey protein isolate (WPI) gels filled with bound and unbound oil droplets were studied as a function of compression speed. The rheological properties of the gel matrices controlled the compression speed-dependency of the gels containing oil droplets. Polymer gels (gelatine and κ-carrageenan gels) showed a predominantly elastic behaviour. Their Young's modulus was not affected by the compression speed. The increase of fracture stress and strain observed with increasing compression speed was related to friction between the structural elements of the gels and, for gelatine, to the unzipping of physical bonds. Particle gels (WPI gels) showed a more viscoelastic behavior. Their Young's modulus and fracture stress increased with compression speed. This was attributed to the viscous flow of the matrix and friction phenomena between structural elements of the gel. The effect of an increase in the oil volume fraction ( φ ) on the Young's modulus was for all gels according to the Van der Poel theory. In addition, oil droplets embedded in the gel matrix acted as stress concentration nuclei and increased friction. The relative impact of these two effects was related to the viscoelastic properties of the gels and to droplet–matrix interaction. For polymer gels and gels with bound droplets, stress concentration phenomena played a relatively larger role. For particle gels and gels with unbound droplets, friction phenomena were relatively more important, increasing the viscoelastic character of the gels. As a result, an increase in φ resulted in a decrease of both fracture stress and fracture strain for polymer gels and in an increase of the fracture stress and a decrease of fracture strain for particle gels.

Very little is known on the rate dependency of the fracture behavior of crispy products as a function of water activity (Aw). Therefore, the effect of deformation speed on instrumental and sensory crispness was studied as a function of Aw. Deformation speed clearly affects the transition Aw range from crispy to non-crispy. At low deformation speed, the critical Aw was 0.40 ± 0.02 and at high deformation speed between 0.5 and 0.6. The transition was completed at an Aw = 0.56 ± 0.10 and 0.74 ± 0.00 for the low and high speed, respectively. The transition of sensory crispness at regular biting speed started at Aw = 0.48 and was completed at Aw = 0.75. The deformation speed effect is assumed to be caused by the viscoelastic nature of the material in the transition region causing it to behave locally relatively more viscous at low deformation speed and relatively more elastic at high deformation speed. This behavior depends on the plasticizer content, thus, Aw. For the product used in this study, the instrumental deformation speed that best correlate to sensory data is 10 to 40 mm/s. Sensory tests show that crispy attributes are perceived more intense when biting at higher speed

Water sorption and dynamical properties of bread crust were studied using gravimetric sorption experiments. Water uptake and loss were measured while relative humidity (RH) was step-wise in- or decreased. Experimental results were compared with Fickian diffusion models and empirical models like the exponential and power-law model. From comparison of experimental sorption curves and the power-law model for short times it followed for all bread crust that the diffusional coefficient n is close to one. It turned out that this is not due to so-called case II diffusion and water transport that is limited by relaxation of the solid material but due to the fact that RH did not instantaneously but gradually increased to the set value. Sorption curves of isotherm experiments could be best described by the Fickian diffusion model for low RH and by the exponential model for large RH. Transport rates depend on moisture content and show a maximum around RH = 0.7, corresponding to a water mass fraction ω1 = 0.12. Diffusion rates could be well described by free volume theory up to the maximum, but this theory could not explain the strong decrease at higher ω1. Indications for a local glass-rubber transition at room temperature were found near a water mass fraction ω1 ≈ 0.09. This corresponds very well to the start of the crisp–non-crisp transition as measured by a sensory panel, but not to the glass-rubber transition at ω1 ≈ 0.12 as measured by other techniques like Differential Scanning Calorimetry. So it seems that more than one glass-rubber like transitions may be important to describe the properties of heterogeneous cellular food systems.

Based on earlier reported surface rheological behaviour two factors appeared to be important for the functional behaviour of mixed protein/polysaccharide adsorbed layers at air/water interfaces: (1) protein/polysaccharide mixing ratio and (2) formation history of the layers. In this study complexes of β-lactoglobulin (positively charged at pH 4.5) and low methoxyl pectin (negatively charged) were formed at two mixing ratios, resulting in negatively charged and nearly neutral complexes. Neutron reflection showed that adsorption of negative complexes leads to more diffuse layers at the air/water interface than adsorption of neutral complexes. Besides (simultaneous) adsorption of protein/polysaccharide complexes, a mixed layer can also be formed by adsorption of (protein/)polysaccharide (complexes) to a pre-formed protein layer (sequential adsorption). Despite similar bulk concentrations, adsorbed layer density profiles of simultaneously and sequentially formed layers were persistently different, as illustrated by neutron reflection analysis. Time resolved fluorescence anisotropy showed that the mobility of protein molecules at an air/water interface is hampered by the presence of pectin. This hampered mobility of protein through a complex layer could account for differences observed in density profiles of simultaneously and sequentially formed layers. These insights substantiated the previously proposed organisations of the different adsorbed layers based on surface rheological data. © 2007 Elsevier Inc. All rights reserved. Chemicals / CAS: beta lactoglobulin, 9045-23-2; Lactoglobulins; Membranes, Artificial; pectin, 9000-69-5; Pectins; Water, 7732-18-5

Establishing relationships between physical and sensorial properties of semi-solid foods is essential to develop tailored products. Whey protein/polysaccharide mixed gels were used to model both natural and fabricated semi-solid foods. The presence of various polysaccharides modulated the microstructure and large deformation properties of the mixed gels. The gels exhibited a large spectrum of sensorial properties as evaluated by panellists in a quantitative descriptive analysis. Mouthfeel attributes that discriminated best between the gels were wateriness, crumbliness, and spreadability. Wateriness strongly correlated with the amount of exuded phase (serum) measured during uniaxial compression. Serum release may have a positive effect on, for instance, the juiciness of a product. Large deformation measurements showed that highly crumbly gels fracture readily via a free-running crack. Low serum release is a requirement for that. Low crumbly gels fracture slowly, often releasing a high amount of serum. Spreadability related to the occurrence of multiple microcracks during deformation as observed by confocal laser scanning microscopy, which resulted in a large number of pieces after oral processing.

The recoverable energy (RE), defined as the ratio of the work exerted on a test specimen during compression and recovered upon subsequent decompression, has been shown to correlate to sensory profiling of protein-based food products. Understanding the mechanism determining the time-dependency of RE is primordial. This work aims to identify the protein-specific impact on the recoverable energy by stress dissipation via relaxation of (micro)structural rearrangements within protein gels. To this end, caseinate and gelatin gels are studied for their response to time-dependent mechanical deformation as they are known to develop structurally distinct network morphologies. This work shows that in gelatin gels no significant stress relaxation occurs on the seconds timescale, and consequently no time-dependency of the amount of energy stored in this material is observed. In caseinate gels, however, the energy dissipation via relaxation processes does contribute significantly to the time-dependency of reversible stored energy in the network. This can explain the obtained RE as a function of applied deformation at slow deformation rates. At faster deformation, an additional contribution to the dissipated energy is apparent, that increases with the deformation rate, which might point to the role of energy dissipation related to friction of the serum entrapped by the protein-network. This work shows that engineering strategies focused on controlling viscous flow in protein gels could be more effective to dictate the ability to elastically store energy in protein gels than routes that direct protein-specific aggregation and/or network-assembly.

This article describes sound properties that are special for crispy foods and relates them to general knowledge on the acoustic emission and mechanical properties of fracturing materials. From properties like the occurrence of sound, the sound energy, the duration of single sound waves, the time interval between the sound pulses and from the force drops involved, it was possible to calculate the morphological constraints for dry, crispy cellular foods. We present data for the minimum and maximum sizes of the pores and of the sizes of the solid material element surrounding them. The loss of crispness in foods caused by aging under deteriorating circumstances was found not to be caused by changes in the type of elements that fracture.

Compression and wire-cutting experiments on dispersions of fully hydrogenated palm oil in sunflower oil with varying fraction solid fat were carried out to establish which parameters are important for the large deformation behavior of fat crystal networks. Compression experiments showed that the apparent Young's modulus, yield stress, yield deformation and Bingham extensional viscosity scaled with the volume fraction solid fat according a power law. For the Young's modulus and yield stress, the exponent was about 3.8¿3.9. The scaling exponent is close to the one relating the storage modulus and the fraction solid fat at deformations within the linear region. The specific fracture energy obtained from wire cutting also increased with volume fraction solid fat. That of a 10% solid fat dispersion was about 4 J/m2. Combination of the energy input from compression experiments and the specific fracture energies from wire-cutting experiments yielded defect lengths of the order of 20 ¿m. This length scale corresponds to the size of a fat crystal aggregate at the gel point.

We studied the whipping of artificial creams composed of a blend of sunflower oil and hydrogenated palm fat stabilized by protein or a mixture or protein and low molecular weight (lmw) surfactant. It was found that an increased whipping speed, decreased protein concentration, and the addition of lmw surfactant leads to shorter whipping times. Further, shorter whipping times were observed for WPI-stabilized cream compared to cream stabilized by sodium caseinate. In all cases, the decrease in whipping time was due to a decrease in the length of the second stage of whipping, the stage characterized by the adhesion of fat droplets to the air bubble surface. The decrease in whipping time could be accounted for by considering the influence of the experimental variables on the fraction of bubble surface area at which fat droplet spreading is possible. The same changes in parameters that promote droplet spreading at the air/water interface cause a decrease in the whipping time of our model creams. Correlating the whipping time of cream with the spreading behavior of fat droplets at the air/water interface represents a new insight into the mechanisms involved in the whipping of cream.

A previously proposed explanation for the change in gluten properties on addition of pentosans to doughs was based on data for only one wheat cultivar. Using three wheat cultivars, Scipion, Soissons and Amazon, differing in technological quality from weak to strong we have obtained results that support the previous explanation. In addition to standard techniques for characterizing gluten and glutenin macropolymer (GMP) yield, composition and properties, a new technique (particle size analysis) was applied that provides further detail on GMP particle size distribution. For each of the three wheat cultivars the effect of WUS and xylanase on gluten and GMP yield, composition and properties followed the trend previously observed. However, WUS and xylanase affected gluten yield and properties more strongly for Scipion and Soissons than for Amazon. Amazon flour contains more protein and less pentosans. The analysis of GMP particles demonstrates that the volume surface average particle diameter D3,2 of GMP particles from Amazon wheat is larger than those from Scipion and Soissons. Amazon has the ability to form larger and stickier particles. These factors may explain why the effects of pentosans and xylanase on gluten yield and properties are smaller for this wheat. © 2004 Elsevier Ltd. All rights reserved.

Literature on the crispy/crunchy behavior of cellular solid foods with a crust is discussed. The emphasis is on products with a dry crispy or crunchy crust as bread and various snacks and especially on mesoscopic and macroscopic aspects. Successively, the sensory sensations involved, the mechanical and fracture behavior of crispy/crunchy products, morphological aspects, and the relation between crispy and crunchy behavior and mobility of the macro-molecules and plasticizer (primarily water) involved, are discussed. Finally, some ideas for an integrated approach of crispy/crunchy behavior of cellular solid foods with a dry crust will be discussed.

Previously we reported that the SDS insoluble gel-layer: the Glutenin Macro Polymer (GMP) can be considered as a gel consisting of protein particles. These glutenin particles have a size of about 10-1-102 μm and consist of HMW-GS and LMW-GS only. In GMP isolates from flour, the particles are spherical. In isolates from dough, glutenin particles have lost this shape. This seems relevant, since mixing disrupts the particles and the mixing energy required for dough development correlated with the glutenin particle size in flour. The question studied in this paper is how changes at a glutenin particle level affected the subsequent process of gluten network formation during dough rest and if this could be used to explain resulting dough rheological properties. To this end, we studied how various mixing regimes affected the dough properties during and after resting (elasticity). We cannot fully explain the differences in the final dough properties observed using parameters such as the quantity of GMP in flour, the quantity of re-assembled GMP in dough and the size of re-assembled glutenin particles. However, other parameters were found to be important: (1) the Huggins constant K′ reflecting the tendency of glutenin particles to interact at level II of the Hyperaggregation model; (2) the composition of glutenin particles affecting the potential to form smaller or larger particles and (3) for over-mixed dough, covalent re-polymerisation at the so-called level I of hyperaggregation. Using these parameters we can better explain dough viscoelasticity after resting. © 2004 Elsevier Ltd. All rights reserved.

Dispersions of hydrogenated palm oil (HP) in sunflower oil were crystallized in a scraped-surface heat exchanger (SSHE), where local strain rates are high. The HP crystallized completely or partially under shear, possibly followed by crystallization at rest. The Young's moduli scaled with the fraction solid fat to powers 2.5 and 1.5 for dispersions partially and completely crystallized under shear, respectively. Dispersions completely crystallized under shear leave the SSHE as a dispersion of compact aggregates in which the crystals are strongly sintered. The aggregates attract each other by relatively weak Van der Waals forces. Possibly, recrystallization causes a slow increase in consistency over about 16 days as a result of sintering between the aggregates. Dispersions partly crystallized under shear and partly at rest obtained their final consistency within 1 day. This is explained by ongoing crystallization at rest, which causes rapid sintering between aggregates and thereby formation of strong bonds. Any additional recrystallization then did not result in an increased consistency. The final consistency of the dispersions was comparable for both crystallization conditions.

Brownian Dynamics (BD) simulations have been performed to study structure and rheology of particle gels under large shear deformation. The model incorporates soft spherical particles, and reversible flexible bond formation. Two different methods of shear deformation are discussed, namely affine and non-affine deformation, the second being novel in simulation studies of gels. Also two dynamic descriptions of the model are presented, with and without inertia effects. Non-affine deformation resulted in a slower increase of the stress at small deformation than affine deformation. At large deformation both models gave similar stress responses, although the inertia model resulted in lower stresses. The particle gels, regardless of the model used, were observed to fracture into lumps that compactified due to local reorganization. A reversible yielding transition, as observed in polymer gels, was not found. Fractal properties of the gels were irreversibly lost at large deformation

The influence of crystalline fat on the amount and rate of oil spreading out of emulsion droplets onto either a clean or a protein-covered air/water interface was measured for β-lactoglobulin stabilized emulsions prepared with either anhydrous milk fat or a blend of hydrogenated palm fat and sunflower oil. At a clean interface, liquid oil present in the emulsion droplets was observed to completely spread out of the droplets unimpeded by the presence of a fat crystal network. Further, the presence of a fat crystal network in the emulsion droplets had no effect on the rate of oil spreading out of the droplets. At a protein-covered interface, the spreading behavior of emulsion droplets containing crystalline fat was evaluated in terms of the value of the surface pressure ( Π AW ) at the point of spreading; Π AW at spreading was unaffected by the presence of crystalline fat. We conclude it is unlikely that the role of crystalline fat in stabilizing aerated emulsions such as whipped cream is to reduce oil spreading at the air/water interface. However, the temperature of the system did have an effect: spontaneous spreading of emulsion droplets at clean air/water interfaces occurred for systems measured at 5 °C, but not for those measured at 22 or 37 °C. Thus, temperature may play a more important role in the whipping process than commonly thought: the entering and spreading of emulsion droplets was favored at lower temperatures because the surface pressure exerted by protein adsorbed at the air/water interface was reduced. This effect may facilitate the whipping process.

Rheological and fracture properties of optimally mixed flour doughs from three wheat cultivars which perform differently in cereal products were studied in uniaxial and biaxial extension. Doughs were also tested in small angle sinusoidal oscillation. In accordance with previously published results the linear region was found to be very small. The rheological properties at small deformations hardly depended on the cultivar. A higher water content of the dough resulted in a lower value for the storage modulus and a slightly higher value for tan ?. For both uniaxial and biaxial extension a more than proportional increase in stress was found with increasing strain, a phenomenon called strain hardening. In uniaxial extension (i) stresses at a certain strain were higher and (ii) the stress was less dependent on the strain rate than in biaxial extension. This indicates that in elongational flow orientational effects are of large importance for the mechanical properties of flour dough. This conclusion is consistent with published data on birefringence of stretched gluten. Fracture stress and strain increased with increasing deformation rate. The observed time-dependency of fracture properties can best be explained by inefficient transport of energy to the crack tip. Presumably, this is caused by energy dissipation due to inhomogeneous deformation because of friction between structural elements, e.g. between dispersed particles and the network. Differences in the rheological properties at large deformations between the cultivars were observed with respect to (i) stress, (ii) strain hardening, (iii) strain rate dependency of the stress, (iv) fracture properties and (v) the stress difference between uniaxial and biaxial extension. keyword(s) Dough rheology, Strain hardening, Uniaxial extension, Biaxial extension, Fracture properties

Recent studies on gel formation and rheology of milk gels are reviewed. A distinction is made between gels formed by aggregated casein, gels of ‘pure’ whey proteins and gels in which both casein and whey proteins contribute to their properties. For casein–whey protein mixtures, it has been shown that both the fraction whey protein aggregated with the casein particles and whey protein aggregates dispersed in the continuous phase have a large impact on the structural and mechanical properties of heated and acidified milk gels. The availability of site-directed mutants of β-lactoglobulin for aggregation and denaturation studies is an important development in the field of whey protein gelation. Besides cold gelation by whey proteins has received much attention. Major recent advances : During recent years, the dynamic character of casein gels has been much better appreciated. For gels of heated milk, the important role of the whey protein that stays dispersed in the continuous phase after heating has been shown for acid-induced gel formation. Some advances has been made in understanding the molecular details of the protein–protein interactions.

This paper describes an experimental comparison of microstructure, rheology, and demixing of bridging- and depletion-flocculated oil-in-water emulsions. Confocal scanning laser microscopy imaging showed that bridging-flocculated emulsions were heterogeneous over larger length scales than depletion-flocculated emulsions. As a consequence, G' as determined from diffusing wave spectroscopy (DWS) corresponded well with G' as measured macroscopically for the depletion-flocculated emulsions, but this correspondence was not found for the bridging-flocculated emulsions. The heterogeneity of bridging-flocculated emulsions was confirmed by DWS-echo measurements, indicating that their structure breaks up into large fragments upon oscillatory shear deformation larger than 1%. Depletion- and bridging-flocculated emulsions showed a different scaling of the storage modulus with the volume fraction of oil and a difference in percolation threshold volume fraction. These differences will be discussed on the basis of the two types of droplet-droplet interactions studied. Gravity-induced demixing occurred in both emulsions, but the demixing processes differed. After preparation of bridging-flocculated emulsions, serum immediately starts to separate, whereas depletion-flocculated systems at polysaccharide concentrations in the overlap regime usually showed a delay time before demixing. The delay time was found to scale with the network permeability, B; the viscosity, eta, of the aqueous phase; and the density difference between oil and water, Deltarho, as tdelay approximately B(-1)etaDeltarho(-1). The results are in line with the mechanism proposed by Starrs et al. (J. Phys.: Condens. Matter 2002, 14, 2485-2505), where erosion of the droplet network leads to widening of the channels within the droplet networks, facilitating drainage of liquid.

Recent studies on gelation and interfacial properties of vegetable protiens are reviewed. Attention is focused on legume proteins, mainly soy proteins, and on wheat proteins. The rheological properteis of vegetable protein gels as a function of heating time or temperature is discussed as well as the interfacial gelation upon adsorption of soy and wheat proteins at the air/water interface. It is shown that modification of proteins improves functionality and application. ©2002 Elsevier Science Ltd. All rights reserved. Chemicals/CAS: glycinin, 9007-93-6; soybean protein, 9010-10-0; water, 7732-18-5

The mechanical properties of food play an important role during manufacturing, storage, handling, and last but not least, during consumption. For an adequate understanding of the mechanical properties of liquid, liquid-like, soft solid, and solid foods, a basic understanding of relevant aspects of rheology and fracture mechanics is essential. Focusing on basic principles, Rheology and Fracture Mechanics of Foods examines how rheological and fracture behavior in food relates to product structure. Divided into three parts, the book reviews basic concepts and emphasizes aspects relevant for studying food products, raw materials, and intermediate products. It then introduces measuring methods and the equipment used for studying mechanical properties of food products, highlighting tests that deliver reproducible and interpretable data. The final part investigates the relation between rheological and fracture behavior of matter and physical structure at the relevant molecular, mesoscopic, and macroscopic length scales. In addition to the basics of rheology and fracture mechanics, the book explores the relationship between measured mechanical properties and the structure of the different types of food and how they may determine texture perception

Most recent experimental and theoretical work on large deformation and fracture behaviour of gels has focused on fracture stress and strain. Other parameters characterizing fracture behaviour, such as fracture energy and inherent defect length, were much less studied. Progress has been made in the development of theories relating fracture stress and strain to gel matrix structure.

Recently, considerable attention has been given to the understanding of texture attributes that cannot directly be related to physical properties of food, such as creamy, crumbly and watery. The perception of these attributes is strongly related to the way the food is processed during food intake, mastication, swallowing of it and during the cleaning of the mouth after swallowing. Moreover, their perception is modulated by the interaction with other basic attributes, such as taste and aroma attributes (e.g. sourness and vanilla). To be able to link the composition and structure of food products to more complicated texture attributes, their initial physical/colloid chemical properties and the oral processing of these products must be well understood. Understanding of the processes in the mouth at colloidal length scales turned out to be essential to grasp the interplay between perception, oral physiology and food properties. In view of the huge differences in physical chemical properties between food products, it is practical to make a distinction between solid, semi-solid, and liquid food products. The latter ones are often liquid dispersions of emulsion droplets or particles in general. For liquid food products for instance flow behaviour and colloidal stability of dispersed particles play a main role in determining their textural properties. For most solid products stiffness and fracture behaviour in relation to water content are essential while for semi-solids a much larger range of mechanical properties will play a role. Examples of colloidal aspects of texture perception will be discussed for these three categories of products based on selected sensory attributes and/or relevant colloidal processes. For solid products some main factors determining crispness will be discussed. For crispiness of dry cellular solid products these are water content and the architecture of the product at mesoscopic length scales (20-1000 μm). In addition the distribution of water at mesoscopic length scales was found to be important. For semi-solid foods, sensory characteristics as spreadability, watery and crumbliness are primarily determined by food properties at mesoscopic length scales. Crumbliness is directly related to the formation of free running cracks that occur during eating of the product. Exudation of the continuous liquid phase of gels during compression gives rise to watery/juicy sensory attributes. For liquid food products, colloidal interactions of emulsion droplets, particles, proteins, and polysaccharides with saliva and oral surfaces were found to affect texture characteristics as creaminess, fattiness, roughness and astringency. © 2009 Elsevier B.V. All rights reserved.

A detailed study was performed of the fracture behavior of toasted rusk rolls, a cellular solid food, at different water activities and morphologies. We find that the energies of the emitted sound pulses follow Gutenberg-Richter power laws with characteristic exponents $b\ensuremath{\sim}1.5$. The scaling exponents varied only within a range of 0.2 when the method of fracture, humidity, or morphology was changed. However, differences in $b$ were observed, indicating nonuniversal behavior, that seems to be related to morphology and water activity. Also, power law scaling behavior was observed for the waiting time distributions with an exponent $a\ensuremath{\sim}1.9$.

The formation and properties of a crust during and after deep frying are difficult to study. Batter pickup (the amount of batter adhering to a product) and core properties affect crust formation and properties of the crust in such way that it is difficult to compare batters of different viscosity or cores with different properties. Moreover, it is often difficult and laborious to separate the crust/batter from the core. Another problem is the poor reproducibility of many fried products. A deep-fried model (DFM) was designed, making it possible to study crust formation and crust properties without the difficulties stated above. Two different batter types and three cores have been used to test the system. Crusts obtained from the DFM were evaluated on several physiochemical properties and compared with crusts found around commercial deep-fried products. Results show that crusts obtained with the DFM system are comparable to crusts of commercial products. The good reproducibility of the DFM crusts resulted in low variance in analytical results compared with commercial crusts. This high reproducibility, the versatility of the system, and the ease with which the system can be used offer clear benefits for many potential applications.

The effect of preheating milk (10 min 80 [degree sign]C) at pH values from 6.20 to 6.90 on formation of acid skim milk gels was studied by dynamic oscillation measurements. Up to pH 6.65 a higher pH of heating (pHheating) resulted in a higher G'. Since below pH 4.9 the development of G'(pH)/G'(pH=4.9) and tan [delta] (pH) was similar we assume that the rearrangement processes happening below pH 4.9 were similar for all pHheating values studied. Based on data for tan [delta], G'(pH) and the pH of gel formation, it was hypothesized that the differences in moduli with pHheating stem from differences in the aggregation and rearrangement processes occurring before and just after gelation (pH>4.9). These differences are partly due to greater involvement of S-S interactions at higher pHheating. Furthermore, the soluble whey protein aggregates formed during preheating are in part not incorporated in the network, the extent depending on pH.

The modulus, fracture stress and fracture strain of acid-induced skim milk gels prepared from preheated skim milk that was aged for 1200 min depended strongly on pH heating . The role of gel structure regarding the effect of pH heating on gel modulus and fracture stress and strain is discussed. Milk preheated at pH 6.20 gave finer stranded gels likely containing more curved strands with probably less S–S interactions than gels from pH heating 6.90 milk. The lower stiffness of gels made from pH heating 6.20 milk could be explained by a combined effect of differences in strand curvature, gel coarseness and S–S interactions, and the lower fracture stress by a combined effect of gel coarseness and S–S interactions whereby the first effect is opposite to and overruled by that of the S–S interactions. The lower modulus and fracture stress of gels from unheated milk could be explained by a combination of strand curvature and less S–S interactions, and by less S–S interactions alone, respectively.

Various popular food products consist of a crispy crust and a non-crispy moist inner part. Well-known examples are many bread types and deep-fried battered snacks like fried fish. For the first type the crust has originally the same chemical composition as the rest of the food product, while for the...

AFM measurements in the force volume mode were performed over the total penetration depth for different positions on casein aggregates adsorbed to a graphite surface in a liquid cell. The stiffness of the force curves was correlated to indentation depths, layer depth and lateral position within the aggregates with the aim of arriving at a credible explanation for the shapes of the force curves. The commonly used Hertz-based models did not fit the experimental data. The ratio between the height and diameter of the adsorbed casein aggregates was found to be linear, suggesting surface energy dominated liquid droplet behaviour. To investigate the possibility, numerical simulations were performed using the Surface Evolver, an interactive finite element program for the study of surfaces shaped by surface tension and other energies. Simulated force curves were in good agreement with experimental findings, both with respect to slope as a function of indentation as well as describing the variation with indentation position on the aggregate due to interfacial and geometric effects. By comparing the simulated force curves to the measurement data it was found that there would have been an interfacial energy equivalent to 10 mJ/m2.

A large problem for fried snack products with a crispy crust and a soft moist interior is that the crust often loses its crispy character fast, in the order of 3–20 min. As known during cooling of these snacks adherent oil will be sucked into the crust. The presence of oil in a cellular solid crispy material was found to affect the sound emitted on fracture dramatically, while it did not affect mechanical characteristics at least for shorter ageing times. Both the number of sound events and the acoustic energy released during fracture decreased. This could be explained by reflection of emitted sound at the oil–air interface. Both the smaller number of acoustic events and the lower loudness will cause a decrease in crispy perception of the crust and in that way oil uptake during cooling of fried snacks can explain their fast decrease in crispness after frying.

Crispy foods like biscuits and breads crusts are cellular solids with relatively large pores of a wide size distribution. Important for the crispy character of such products are both the fracture behaviour and the acoustic emission (Luyten et al. [1]). With aging under deteriorating conditions, these products become less crispy. This is often ascribed to an increase in water activity of the crusts resulting in a change in the mobility of the different molecules, and thus a change in the solid material properties. However, it is difficult to relate directly the change in molecular properties to changes in crispy behaviour. One of the reasons for this is the product morphology, dry crispy foods are irregular built cellular solids. An example is given in Fig. 1.

Stable and homogeneous emulsion-filled gels were prepared by cold gelation of whey protein isolate (WPI) emulsions. A suspension of heat-denatured WPI (soluble WPI aggregates) was mixed with a 40% (w/w) oil-in-water emulsion to obtain gels with varying concentrations of WPI aggregates and oil. For emulsions stabilized with native WPI, creaming was observed upon mixing of the emulsion with a suspension of WPI aggregates, likely as a result of depletion flocculation induced by the differences in size between the droplets and aggregates. For emulsions stabilized with soluble WPI aggregates, the obtained filled suspension was stable against creaming, and homogeneous emulsion-filled gels with varying protein and oil concentrations were obtained. Large deformation properties of the emulsion-filled cold-set WPI gels were determined by uniaxial compression. With increasing oil concentration, the fracture stress increases slightly, whereas the fracture strain decreases slightly. Small deformation properties were determined by oscillatory rheology. The storage modulus after 16 h of acidification was taken as a measure of the gel stiffness. Experimental results were in good agreement with predictions according to van der Poel's theory for the effect of oil concentration on the stiffness of filled gels. Especially, the influence of the modulus of the matrix on the effect of the oil droplets was in good agreement with van der Poel's theory.

In order to understand foaming behaviour of mixed protein/anionic polysaccharide solutions, we investigated the effect of β-lactoglobulin/pectin interaction in the bulk on β-lactoglobulin adsorption to the air-water interface. Adsorption kinetics were evaluated by following surface pressure development in time of several pure protein solutions and of mixed protein/polysaccharide solutions using an automated drop tensiometer (ADT). It was found that complexation of proteins with polysaccharides can slow down the kinetics of surface pressure development by at least a factor 100, and greatly diminish foam formation. In contrast, a five times acceleration in the increase of surface pressure was observed in other cases. We propose a mechanism for protein adsorption from mixed protein/polysaccharide solutions. Effects of ionic strength, pH and mixing ratio on this mechanism were studied for mixtures of β-lactoglobulin and low methoxyl pectin, whereas other proteins and anionic polysaccharides were used to explore the role of protein and polysaccharide charge density and distribution. Whereas the possibilities to change system parameters like ionic strength or pH are limited in food related systems, selecting a suitable combination of protein and polysaccharide offers a broad opportunity to control protein adsorption kinetics and with that foam formation. © 2005 Elsevier Ltd. All rights reserved.

The relation between mechanical film properties of various adsorbed protein layers at the air/water interface and intrinsic stability of the corresponding proteins is discussed. Mechanical film properties were determined by surface deformation in shear and dilation. In shear, fracture stress, sigma(f), and fracture strain, gamma(f), were determined, as well as the relaxation behavior after macroscopic fracture. The dilatational measurements were performed in a Langmuir trough equipped with an infra-red reflection absorption spectroscopy (IRRAS) accessory. During compression and relaxation of the surface, the surface pressure, Pi, and adsorbed amount, Gamma (determined from the IRRAS spectra), were determined simultaneously. In addition, IRRAS spectra revealed information on conformational changes in terms of secondary structure. Possible correlations between macroscopic film properties and intrinsic stability of the proteins were determined and discussed in terms of molecular dimensions of single proteins and interfacial protein films. Molecular properties involved the area per protein molecule at Pi approximately 0 mN/m (A(0)), A(0)/M (M = molecular weight) and the maximum slope of the Pi-Gamma curves (dPi/dGamma). The differences observed in mechanical properties and relaxation behavior indicate that the behavior of a protein film subjected to large deformation may vary widely from predominantly viscous (yielding) to more elastic (fracture). This transition is also observed in gradual changes in A(0)/M. It appeared that in general protein layers with high A(0)/M have a high gamma(f) and behave more fluidlike, whereas solidlike behavior is characterized by low A(0)/M and low gamma(f). Additionally, proteins with a low A(0)/M value have a low adaptability in changing their conformation upon adsorption at the air/water interface. Both results support the conclusion that the hardness (internal cohesion) of protein molecules determines predominantly the mechanical behavior of adsorbed protein layers.

Food Colloids: Interactions, Microstructure and Processing describes the principles and practice underlying the formulation of food emulsions, dispersions, gels, and foams. Emphasis is on understanding how the functional properties of biopolymers and surfactants determine the texture and shelf-life of multiphase food materials. This book provides essential new findings by experts in the field on specific topics including: the interfacial rheological properties of proteins; the use of microscopy and image analysis to probe structure and phase transitions; the control of colloidal stability during thermal and mechanical processing; the interactions of proteins with polysaccharides and emulsifiers; the incorporation of neutraceuticals into food colloids; and the consumer perception of taste and texture. Food Colloids: Interactions, Microstructure and Processing provides a link between current research on the fundamental physical chemistry of colloidal systems and the requirements of the food technologist to use modern colloid science in new product formulation. It is suitable for postgraduates and researchers, both in industry and academia.