Rheology and sensory texture of biopolymer gels

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1 Current Opinion in Colloid & Interface Science 12 (2007) Rheology and sensory texture of biopolymer gels E. Allen Foegeding Department of Food Science, North Carolina State University, Box 7624, Raleigh, NC , USA Received 11 July 2007; accepted 18 July 2007 Available online 24 July 2007 Abstract Sensory texture perception is based on food structure and the mastication process. Real-time observations of crack growth and rheological measurements have shown different patterns of microstructural fracture. This has allowed for a reductive approach in consolidating a range of gels into characteristic microstructures and fracture patterns that can be linked to sensory texture Elsevier Ltd. All rights reserved. Keywords: Sensory texture; Rheology; Fracture mechanics; Microstructure; Gels; Proteins; Polysaccharides 1. Introduction The quality and desirability of food products depends on their flavor and texture. Food texture has historically been considered those properties that are not covered in the classical definitions for taste and flavor compounds. This includes the mechanical properties evaluated from force deformation relationships, tactile sensations such as adhesion, in addition to visual and auditory stimuli [1]. Textural properties are most accurately measured by sensory analysis techniques that use panelists trained to detect and evaluate specific textural attributes such as hardness and stickiness. Indeed, a case has been made that texture is a sensory property that cannot be simply measured by analytical tests [2]. There are several reasons why one wants to understand the relationships between food structure and sensory texture. We clearly want to have a basic understanding of what makes one texture more desirable than another, as well as the flexibility to use various ingredients to make a similar food structure (texture) from different molecular compositions. Bearing in mind that sensory analysis is the most comprehensive way to evaluate texture, a series of sensory attribute descriptors can be generated that provide a sensory texture fingerprint of a product (Table 1 and Fig. 1). If two food products have the same sensory textural fingerprint, will they have the same texture? If not, what elements are missing from the fingerprint? These Tel.: ; fax: address: allen_foegeding@ncsu.edu. questions can only be answered by understanding the relationships between food structure and textural properties. This is required if one wants to take an a priori approach to changing composition while maintaining a desired texture. Although much progress has been made, there remain many challenges in relating food structure to sensory texture [3,4]. In a discussion session at the Food Summit 2002 meeting, the following three questions were proposed [5]: 1. For which dynamic changes do we have evidences that they are actually perceived during mastication of gels and matter for sensory perception? 2. Which gel properties can we actually predict from basic principles? 3. Is it possible to understand the physics and chemistry of food attribute descriptors cold, sticky, fatty, Vaseline-like, creamy, smooth, chewy, chalky, etc? These questions provide a good framework for the following discussion. Relating textural perception to physical measurements of food structure, such as fundamental mechanical properties, is not a simple task. One problem is the lack of consistent definitions for texture terms [6,7]. Indeed, this presents problems not only within one language, but becomes more complex when comparing across languages. The only reasonable approach is to have clear definitions that describe the process used to evaluate the term and realize that hardness in one manuscript may be firmness in another. Another /$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi: /j.cocis

2 E.A. Foegeding / Current Opinion in Colloid & Interface Science 12 (2007) Table 1 Sensory terms used for descriptive analysis of gels Evaluation phase Term Definition Technique Pre-fracture Smoothness b Degree to which sample was perceived as smooth Move gel in mouth without chewing when evaluated with tongue Small-strain force c Force required to cause 10% deformation Compress to 10% using molars First bite Firmness a,b (Fracture force) c Force required to fracture sample with molars Completely bite through sample using molars Moisture release b Extent of moisture released Same as above Crumbliness/fracturability a,b Degree to which the sample fractures into pieces Same as above Deformability c Degree of deformation prior to fracture Same as above Chew down Degree of breakdown a,b (Particle breakdown) c Amount of breakdown Chew the sample for 3 times, c 5 times, a or 8 to 10 times b Particle size b Size after chews (small to large) Same as above Particle size distribution b Degree of homogeneity in distribution Same as above Particle shape b Degree of irregular particle shape (irregular means distinct edges) Same as above Cohesiveness a,b Degree to which the chewed mass holds together Same as above Adhesiveness a,b Degree to which the chewed mass sticks to mouth surfaces Same as above Smoothness a,b Degree to which the chewed mass surface is smooth Same as above Number of chews b (Chewiness) c Number of chews required for swallowing Complete chewing Residual Smoothness of mouth coating a Smoothness felt after expectoration Evaluate after swallowing A superscript letter indicated that the term was evaluated in the investigations of Brown et al. a [47] Gwartney et al. b [46] or Barrangou et al. c [41] Definition for terms used in Gwartney et al. [46]. challenge is in understanding the breakdown pathway of foods during mastication. This involves the specific pattern of food being transformed into a series of particles [2], the mouth sensing the physical changes [6 ] and ultimately making the decision to swallow. Hutchings and Lillford [8 ] noted the importance of understanding all elements of texture perception. They indicated that a model for the in-mouth perception of texture must account for: 1) behavior of food during eating conditions, 2) describe the breakdown pathway and 3) account for changes in the breakdown resulting from the food, the consumer and the occasion. They proposed a general model for the breakdown path based on: 1) degree of structure, 2) degree of lubrication and 3) time. This model provides a comprehensive framework from which to view texture and various elements of this model will be addressed in subsequent sections. Many empirical methods have been developed to imitate the human analysis of texture and thereby circumvent the need for human subjects. The most widely used imitative method is the Texture Profile Analysis Method (often called the Instron Texture Profile Analysis Method to separate it from sensory texture profile analysis and the General Foods Texturometer), that consists of a two-cycle uniaxial compression [9]. While not questioning the usefulness of this approach, it has limitations; one being the empirical nature of the method. If the goal is to understand the chemical interactions and physical structures responsible for texture, then fundamental mechanical tests are required. The modern approach of viewing foods as soft condensed materials has offered new avenues for probing the complex molecules and structures that provide the appearance, flavor and texture of foods [10,11,12,13 ]. In theory, a complete model of a particular food structure could be developed if one could precisely measure the chemical interactions among molecules and accurately describe the network structure of interacting molecules from molecular to macroscopic length scales. The mechanical properties of such defined foods could be probed at Fig. 1. Sensory scores from evaluation of 38 day old Mozzarella (black) Monterey Jack (grey) cheese. Asterisk ( ) signifies a significantly greater sensory score for that property. Definitions for terms are listed in Table 1 and the data is form Brown et al. [47].

3 244 E.A. Foegeding / Current Opinion in Colloid & Interface Science 12 (2007) different strain levels to provide a complete analysis from nondestructive deformations to fracture. Would this array of mechanical properties be sufficient to describe the overall structure of a food? If so, which mechanical property, or collection of properties, predict specific sensory texture terms or an overall textural sensation? Those questions will be explored in this review. Research on food texture is extensive and therefore some boundary conditions must be placed on this review. First, the main but not exclusive focus will be on protein and polysaccharide gels that are firm enough to be self-supporting and show fracture at large deformations. This restricts the macromolecular concentration to levels that produce gels that show clear fracture stresses and strains and rules out those softer gels that tend to yield or flow. From a sensory evaluation stand point, the texture of firm, self-supporting gels is evaluated by placing a sample between the molars and biting. This is distinguished from softer materials that can be ruptured between the tongue and palate. Self-supporting gels are reasonable models for some specific types of processed meats and cheese, and directly related to texture of cooked egg white and gelatinbased desserts. Those interested in a thought provoking coverage of the texture of fluid foods should consider Malone et al. [14,15]. One associated area that will not be covered is flavor release from gels. This is a very active area of research and deserves to be addressed as an individual topic. The elements of texture discussed will be restricted to those mechanical and tactile sensations perceived during mastication, omitting any visual or auditory stimuli. 2. Food texture evaluation The human evaluation of texture is a coupling of the material properties (rheology, fracture mechanics and adhesion) related to food structure with the oral processing of the food (physiology) to produce a sensory (psychological) evaluation of texture [16 ]. In many ways, this is the combination of three different areas of science and seldom approached in a unified manner. As stated by Lucas et al. [17 ], There is still a considerable gap between the methods that food scientists use to gauge the perception of food texture in the human mouth and those which oral physiologists employ to investigate ingestion, mastication and swallowing. This is basically because food scientists are focused on understanding what consumers want and how to make food products with desirable textures, while physiologists are more interested in the biology of the process. However, as emphasized by Lucas et al. [17 ], there is much to be gained by a blending of the sciences. Material properties that are important to texture include those related to deformation and flow (i.e. rheology) and adhesion. In general, rheological properties are usually considered those that are evaluated prior to fracture and fracture properties are those determined at fracture; however, this is not strictly adhered to in the literature and rheological properties are often used as an all encompassing term. Food structure is a general term that includes how molecules are assembled into structures that collectively are responsible for the physical properties of a food. The fracture pattern of foods is highly dependent on food structure [18]. For gels, this ranges from macroscopic appearance (opaque or clear) to the nanometer scale of interactions between individual molecules and at all length scales in-between. Microscopic techniques are most commonly used to characterize food structure. One of the challenges in understanding gel texture is defining when gels have distinctively different types of structure, as opposed to more of the same structure. As will be seen in the following sections, there has been recent progress on establishing taxonomy of gel structures. Oral processing is a key element of food texture that has often been overlooked in food texture investigations and therefore will be elaborated on in more detail. Finally, sensory analysis of texture involves choosing among different testing approaches and conditions, panelists characteristics (e.g., trained or un-trained) and often complex statistical methods. This review will focus on the interlinking among all the elements of food texture evaluation, rather than an in-depth evaluation of any one component. 3. Oral processing Oral processing of food can be divided into the following three phases [16,19]: 1. Preparation: movement of the food from the front of the mouth in preparation for chewing without teeth occlusion. 2. Chewing: reduction of particle size in preparation for swallowing. 3. Pre-swallowing: movement of the food to the back of the tongue by tongue palate movement with no characteristic jaw movement. The time from intake until swallowing will vary among individuals and food quality [20], therefore one needs some means to normalize the process to allow for comparisons. Scientists interested in oral processing of food tend to focus on stage two (chewing) and evaluate masticatory performance, generally defined as the subject's ability to pulverize food [21 ]. Masticatory performance or chewing efficiency is generally determined by chewing a test food for a fixed number of chews then analyzing the particles (this could be number of particles or particle size distribution) [20,22]. By evaluating masticatory performance, the mouth is reduced to a mechanical testing device that exerts a force per chew and is regulated by a constant feed-back system designed to prevent tooth damage from rapid contact. As discussed in Mioche at al. [23], the rhythmic activity of the jaw-opening and-closing muscles during chewing is adaptively modulated by sensory inputs throughout the entire masticatory sequence in accord to the physical and textural demands of the bolus. Modifications are made in mandibular movements and forces [24], and the duration and number of chewing cycles [25]. This requires information on the position and velocity of the jaw, forces acting on the teeth and jaw, and the activity of muscles involved in chewing [17,20]. Foods can be classified on the basis of particle size after chewing to the point just prior to swallowing. Particle size for 10 natural foods

4 E.A. Foegeding / Current Opinion in Colloid & Interface Science 12 (2007) Fig. 2. Hypothetical stress strain plot for a biopolymer gel. Grey bars indicated boundary regions of linear viscoelasticity and fracture. Inserted cartoons depict a microstructure that is intact (I), showing micro-cracks (II) and fractured (III). Grey-filled elements of the microstructure indicate separation from the network. ranged from 0.82 to 3.04 mm with b2 mm being a general requirement unless the food particles were soft enough to allow for larger size during swallowing [26]. Chewing pattern can alter sensory perception of texture. Brown et al. [27 ] evaluated the chewing pattern of 52 people and was able to cluster them into 5 groups. When presented with the same series of whey protein gels, there were significant differences among the groups in their evaluation of firmness and rubberiness. This emphasized the importance of individual variation in the chewing process when evaluating texture. There are various approaches that can be used to measure the chewing process. Mioche et al. [23] demonstrated that electromyography (EMG) measures are sensitive to differences in the physical properties of natural foods. For example, the summary measures of number of chews, chew duration (time), total muscle work, and mean muscle work per chew are known to increase in value with physical parameters that reflect food firmness or toughness. Foster et al. [28 ] investigated the effect of food texture on muscle activity and jaw movements during mastication. They compared elastic (gelatin-based confectionery product) and plastic (caramel) model foods, each made such that there were four levels of similar sensory hardness. In agreement with the prior discussion, there were direct links between EMG measures and level of hardness. Differences between elastic and plastic textures were not apparent in EMG measures; however, they were clearly distinguished by aspects of jaw movement (vertical amplitude, lateral amplitude and closing velocity) [28,29]. This clearly shows that EMG measurements are sensitive to food hardness and suggests that other sensory properties, such as adhesiveness, may be derived from jaw movement or a combination of jaw movement and EMG parameters. It opens the possibility that a combination of EMG and jaw movement parameters can be used to classify different types of food textures. However, it should be remembered that the use of a physiological measure, i.e., EMG, does not preclude complications due to differences among individuals. Electromyography outputs can be different due to trained versus un-trained subjects [30] and personality type [31]. 4. Sensory analysis and mechanical testing Descriptive sensory analysis attempts to quantify textural properties over the three phases of oral processing listed above, in addition to evaluating residual effects after swallowing. Terms used in evaluating cheese and agar gels can be used for comparison (Table 1). In the preparation phase, surface smoothness or roughness is evaluated (Table 1). During chewing, a variety of parameters relating to the breakdown of particles (size and shape), adherence of particles to the mouth's surfaces and each other, and properties of the chewed mass are evaluated. Finally, properties of the bolus (cohesiveness and smoothness) are evaluated, along with the residue in the mouth after swallowing. This evaluation takes place during the chewing and pre-swallowing phase. As such, the sensory panelist is attempting to make periodic judgments along the course of a dynamic process. In contrast, mechanical tests determine relationships among force, deformation and time up to and at the point of fracture. This is illustrated in Fig. 2 where a stressstrain curve is separated into regions of linear viscoelastic (LVE), non-linear and fracture, based on strain levels. Values that can be obtained from mechanical tests are: viscoelastic properties determined at non-destructive strains (within LVE region), maximum strain for the LVE region, extent of strain

5 246 E.A. Foegeding / Current Opinion in Colloid & Interface Science 12 (2007) hardening or weakening within the non-linear region and stress and strain at fracture. There are a range of tests that can be used to determined fracture properties and the differences among tests have been addressed [32 34]. It is apparent that sensory terms such as hardness (firmness) and deformability are directly measured by the human and machine and are expected to be correlated if the testing conditions (e.g., deformation rate and temperature), sensitivity and resolution of the human and machine are similar. However, sensory properties evaluated during the chew down phase (Table 1) are not directly imitated with fundamental mechanical tests. Since mechanical properties reflect differences in gel structure, it is possible that mechanical properties measured up to and at fracture predict changes occurring during the chewing process. For example, the degree of fragmentation of food into particles is related to ratio of toughness (work needed to generate a unit of surface area during fracture) to the Young's modulus (E, normal stress/normal strain) [17,35 ]. Tactile properties, such as adhesiveness and smoothness, may not be reflected in force-deformation relationships. Understanding the physics and chemistry of tactile properties is important because, as illustrated in Fig. 1, Mozzarella and Monterey Jack cheese are very similar in firmness but different in tactile properties. The testing conditions in a rheometer can be precisely controlled concerning contact surface area, deformation rate (strain rate) and temperature. In contrast, testing conditions will vary among humans. Subjects whose teeth have larger surface areas available for chewing tend to have fewer chew cycles before swallowing [36]. There is also variation in the chewing (deformation) rate. A group of 10 subjects evaluating the hardness of ten different cheeses had biting velocities ranging from 16.6 to 39 mm/s [37]. It is logical to assume that deformation rates during mechanical testing should be matched with deformation rates during mastication. However, when comparing instrumental hardness with sensory hardness of 26 commercial cheeses, the level of deformation is more important than the rate of deformation. A deformation rate between 1 and 10 mm/s, with 10 mm/s being the maximum investigated, maintained optimal correlations (r N0.7) as long as the deformation level was 70% [38]. It should be noted that the significance of deformation rate may be coupled with the viscoelastic properties of the food and the contrast in deformation rates between the mechanical test and chewing. Foods that are relatively elastic should in theory be deformation rate independent. This topic needs further investigation with foods showing a wide range of viscoelastic properties. 5. Relating sensory texture to microstructural, rheological and fracture properties One of the challenges of relating sensory texture to rheological properties is in the choice of a model system. Ideally, the gelation mechanism should be reasonably understood and gels can be made into various shapes and remain homogeneous and isotropic. Protein and polysaccharide gels are logical choices because they can be made such that they fit the fore mentioned requirements and, in some cases, are actual food products (e.g., cooked egg white and gelatin desserts). The key in designing a model system is to be able to set boundary conditions such that a wide range of rheological and fracture properties are produced. Agarose and agar gels are appropriate model gels because they are formed by predominately one interaction (hydrogen bonding) and do not melt in the temperature range encountered during chewing. Gels containing 1 to 2.5% (w/w) agarose have the general trends of fracture stress increasing and fracture strain decreasing with increased agarose concentration [39]. This is a common trend observed in other gels, such a whey protein gels [40]. Addition of 0 to 60% w/w glycerol causes a similar increase in fracture stress but also increases fracture strain. This independent variation of fracture properties produces a series of gels with fracture stress values ranging from 25 kpa to 78 kpa, and fracture strain values ranging from 0.48 to 1.0. None of the rheological properties determined in the LVR (G, G, or G ) are significantly correlated with any of the sensory texture properties determined by compressing gels between the first two fingers and thumb. In contrast, hand small-strain force (10% compression) and fracture force are most significantly predicted by the fracture modulus (fracture stress/fracture strain; respective r-values of 0.98 to 0.99), and less so by fracture stress (respective r-values of 0.76 to 0.82). Hand fracture deformation is significantly correlated with fracture strain (rvalue of 0.98). While agarose gels are less complex than agar gels, the former are not food-grade, so a subsequent investigation used agar gels to evaluated textural properties during mastication. Similar to the agarose gel investigation, a range of agar (1 to 4% w/w) and glycerol (0 or 60% w/w) concentrations were used to produce gels with fracture stress values ranging from 20 to 120 kpa and fracture strain values ranging from 0.64 to 1.22 [41]. Small-strain force and fracture force (Table 1) are similarly correlated with fracture stress (respective r-values of 0.95 and 0.96) and fracture modulus (respective r-values of 0.93 and 0.94). This is different than what was observed for agarose gels, where fracture modulus was more highly correlated than fracture stress [39]. This could be due to the differences in the materials, or, as pointed out by Lucas et al. [17 ], due to differences in the sensing ability of the hand and mouth. The particle breakdown and chewiness (Table 1) are correlated with fracture stress (respective r-values of 0.97 and 0.97) and fracture modulus (respective r-values of 0.86 and 0.95). Gel deformability is more correlated with a constant indicating the degree of strain hardening (r-value of 0.93) than fracture strain (r-value of 0.88). This observation concerning gel deformability presents an interesting contrast between mechanical and sensory measurements of texture that may be related to the physiology of chewing. Panelist could not be trained to be sensitive enough to directly detect differences in strain hardening during first bite (this involved sensing the relative increase in force required as the sample was compressed), yet the mechanical property appears to be part of the detection of deformability. The role of strain hardening in sensory texture requires further investigation. The sensory texture properties of agar gels are therefore predictable based on

6 E.A. Foegeding / Current Opinion in Colloid & Interface Science 12 (2007) fracture stress, fracture modulus and a measure of strain hardening. However, one major difference between agar gels and soft solid foods, such as cheese, is that tactile properties evaluated during chewing, such as adhesiveness, are not present in agar gels at sufficient levels to be important to the characteristic texture [42]. Whey protein gels can be made under varied solution conditions (ph, ionic strength) to produce different gel structures. Fine-stranded gels are clear or translucent and generally hold fluids during deformation, whereas particulate gels are opaque and exude water during deformation. Gelation mechanisms and rheological properties of these gels have been investigated [4,43 45]. There are major differences in the sensory properties of fine-stranded and particulate whey protein isolate gels. Fine-stranded gels are characterized as being springy (recoverable deformation) with a smooth, slippery surface and slowly breakdown into large particles, with minimal release of fluid during mastication [46]. These gels are very similar to agar gels [41]. Particulate gels release large amounts of fluid, rapidly breakdown into small particles during mastication, and adhere to teeth [46]. These properties are more similar to cheese [47]. The role of microstructural variation in particulate gels was investigated by Langton et al. [48]. Microstructural properties (pore size, particle size and amount of threads) were correlated with oral textural properties of gritty, creamy, sticky and falling apart. Good correlations were found between the volume of particles and pores and the textural properties. An increase in particle volume was associated with increased grittiness, and as pore volume increased, the gels tended to fall apart or disintegrate more rapidly during chewing. These investigations clearly suggest the changes in microstructure alter the breakdown pattern such that it alters textural properties. Microscopic evaluation of gel networks during fracture have provided new insight into the initial fracture process. Biopolymer gels that have networks visible using confocal laser scanning microscopy (CLSM) can be deformed by means of a tension/compression stage so that changes in stress, stain and microstructure can be recorded dynamically [49 ]. The fracture pattern of particulate β-lactoglobulin gels changes with protein concentration [50]. Gels containing 6% protein, have lower fracture stress, higher fracture strain, and the pores grow and protein clusters turn during deformation. In comparison, gels containing 12% protein have higher fracture stress, lower fracture strain and the microstructure shows much less changes with deformation. Crack growth shows a smooth progression with the stronger gels and a step-wise growth with the weaker gels. Brink et al. [51 ] showed that crack propagation was different between particulate whey protein isolate gels and whey protein isolate gels filled with gelatin. It was established that fracture starts within the network long before the point of macroscopic fracture, indicating that it would be occurring in the non-linear region of a stress strain plot (Fig. 2). Another interesting observation was that a shift in rheological properties occurred at a critical amount of added gelatin that was not reflected in the microstructure observed at the length scale limits of CLSM. Two types of fracture were observed. A porous fracture was describes as the structure rupturing gradually immediately after applied stress. In contrast, a stretched fracture is where the existing pores and network are stretched but the number of pores do not increase. A combination of the two mechanisms was also observed. It would be interesting to see if the changes in fracture processes observed for protein and mixed protein-polysaccharide gels were associated with differences in sensory texture perception, especially related to properties evaluated during sequential chews. Multicomponent or mixed gels. Gels made with a mixture of proteins and polysaccharides are called multicomponent, mixed or composite gels [49 52,53 ]. These systems are ideal for texture studies because by adjusting the protein and polysaccharide types, concentrations, and processing conditions, a variety of single or two phase gels can be produced. This general approach has been proposed as a means to generate a wide range of textures [13 52,54]. One of the challenges with phase-separated gels is developing a systematic approach to such a varied set of gels. In other words, when are gel structures distinct enough to be considered different types? de Jong and van de Velde [53 ] determined structures and fracture properties of cold-set whey protein isolate gels containing one of nine different polysaccharides and compared properties based on charge density of the polysaccharides. They were able to consolidate the nine polysaccharides into three different groups based on changes in fracture stress and microstructure, with charge density being the major factor determining groupings. The first group is polysaccharides with low charge density. These gels have a sharp boundary between the protein continuous phase and discontinuous polysaccharide phase. An increase in polysaccharide concentration causes an increase in fracture stress to a maximum, then fracture stress decreases and approaches zero as the protein network becomes the discontinuous phase. The second type, observed at intermediate charge density, was a phase-separated structure with less defined phase boundaries, and increasing polysaccharide causes a coarsening of the protein network. This coincides with a decrease in fracture stress. The highest charged polysaccharides produced networks that were homogeneous at the length scale of CLSM (above 0.5 μm). In this case, fracture stress increased with polysaccharide concentration. While the changes in fracture stress with increasing polysaccharide concentration were distinctive for each polysaccharide charge density group, there was overlap in microstructure appearance [53 ]. Thus, the question could be raised, are there common properties associated with specific microstructures that are independent of the specific polysaccharide used to create the microstructure? This was addressed by van den Berg et al. (2007) [55 ], again, using cold-set whey protein isolate gels and a range of polysaccharides. Gels were classified based on microstructure as being homogeneous or one of three phase-separated microstructures (coarse stranded, bicontinuous and protein continuous). With this approach, one polysaccharide could produce different gel types depending on concentration. For example, 0.09% pectin produced a coarse stranded structure while 0.14% pectin produced a protein

7 248 E.A. Foegeding / Current Opinion in Colloid & Interface Science 12 (2007) continuous structure. The fracture path, as observed by confocal microscopy, differed among gel microstructures. In protein continuous gels, micro-cracks developed spontaneously at strains of 0.3, whereas true fracture strains occurred at 1.04 to In contrast, there were no spontaneous micro-cracks in the bicontinuous gel. However, taking into consideration the entire fracture process, the protein continuous and bicontinuous gels had similar fracture mechanisms that were different than the coarse stranded gels. It should be noted that this series of gels were relatively weak, with fracture strains ranged from 1.0 to 1.2 and fracture stresses from 4 to 11 kpa. Therefore, sensory firmness was evaluated by compressing between the tongue and palate rather than between teeth. This should be considered when comparing with textural properties of firmer, selfsupporting gels. Principle components analysis of descriptive sensory analysis showed that two principle components explained 89% of the variation. The first principle component accounted for 75% of the variation and separated gels as being firm and crumbly or spreadable. Phase separate gels (bicontinuous and protein continuous) with a protein area fraction below 50% are firm and crumbly, whereas coarse stranded structures produce spreadable gels. This appears to be associated with the breakdown pattern of the gel network, as previously discussed. Principle component two, accounting for 14% of the variation, distinguished gels based on watery scores. Bicontinuous gels were the most watery, followed by coarse stranded gels, with homogeneous and protein continuous gels not being judged as watery. This property reflects the porosity of the gels. As expected, the sensory term firm (effort to compress the sample between tongue and palate) was correlated with the greatest number of physical properties (Young's modulus, energy for fracture and true fracture stress). Watery was correlated with serum volume released during compression (r = 0.81) and fracture strain (r = 0.87). Spreadability, a property related to gel breakdown after fracture, was not correlated to any physical property. However, spreadability was reflected in the network fracture mechanism observed by microscopy. It is possible that another mechanical test would be more sensitive to spreadability, as yield stress and strain determined by a vane method have been associated with spreadability [56]. One of the common features of phase-separated gels is a low water-holding capacity; when chewed or compressed, fluid flows out of the gel. A detectable release of fluids during mastication therefore becomes a differentiating characteristic of gel texture. Cold-set whey protein isolate gels containing various concentrations of gellan gum, to cause phase separation, were used to investigate the role of serum release in gel texture [57]. Bicontinuous microstructures were observed with interconnected serum-filled pores weaving though the protein gel network. In general, increasing gellan gum concentration caused an increase in phase separation, which increased the protein phase concentration, and thereby increased fracture stress. By correcting for fluid loss, it was shown that homogeneous and phase-separated gels had to achieve a given tensile strain to cause fracture; indicating that the protein network was regulating fracture strain. With a high degree of phase separation and low strain rate, fluid moved out more rapidly such that the critical level of tensile strain was not achieved by the end of compression. Sensory gel firmness was most highly correlated (r = 0.91) with true fracture stress corrected for volume loss during compression, whereas a watery feeling during mastication was correlated (r=0.76) with serum volume released during compression. An interesting observation was that these correlations were observed at strain rates much lower than oral processing conditions. Suggesting at least for this case, that the strain rate did not have to be in the range occurring during mastication. 6. Conclusions Biopolymer gels are excellent model systems for understanding the relationships between food structure and sensory texture. Recent advances in using confocal laser scanning microscopy to characterize microstructure and dynamically observe fracture have shown that different gels can be placed in categories of structures and fracture patterns. What is still needed is a more comprehensive coupling of sensory attributes with microstructural fracture and macroscopic rheological (fracture) properties. Moreover, studies are needed that incorporate all-inclusive measures of food structure, mechanical properties, oral processing and sensory texture attributes. This complete approach is required to understand the complex nature of food texture. Acknowledgements Paper No. FSR of the Journal Series of the Department of Food Science, North Carolina State University, Raleigh, NC Support from the North Carolina Agricultural Research Service, Dairy Management Inc., the Southeast Dairy Foods Research Center and the USDA- National Research Initiative Competitive Grants Program ( ) is gratefully acknowledged. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products named nor criticism of similar ones not mentioned. References and recommended readings [1] Lawless HT, Heymann H. Sensory evaluation of food, principles and practices. New York: Chapman and Hall; [2] Szczesniak AS. Texture is a sensory property. Food Qual Prefer 2002;13: [3] Nishinari K. Rheology, food texture and mastication. J Texture Stud 2004: [4] Renard D, van de Velde F, Visschers RW. The gap between food gel structure, texture and perception. Food Hydrocoll 2006;20: [5] Dejmek P, Visschers RW. Discussion session on food gels. Food Hydrocoll 2006;20: [6] van Vliet T. On the relation between texture perception and fundamental mechanical parameters for liquids and time dependent solids. Food Qual Prefer 2002;13: An excellent overview of the key factors to consider when relating rheological properties with sensory texture. Of special interest. Of outstanding interest.

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