Current Opinion in Colloid & Interface Science 16 (2011) 36–40

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Current Opinion in Colloid & Interface Science

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Rheology of food materials

Peter Fischer ⁎, Erich J. Windhab

Institute of Food, Nutrition and Health, ETH Zurich, 8092 Zurich, Switzerland article info abstract

Article history: Food rheology focuses on the flow properties of individual food components, which might already exhibit a Received 12 July 2010 complex rheological response function, the flow of a composite food matrix, and the influence of processing Accepted 13 July 2010 on the food structure and its properties. For processed food the composition and the addition of ingredients Available online 18 July 2010 to obtain a certain food quality and product performance requires profound rheological understanding of individual ingredients their relation to food processing, and their final perception. Keywords: © 2010 Elsevier Ltd. All rights reserved. Rheology Suspensions Emulsions Foams Microstructure Flow geometries Tribology Squeeze flow Interfacial rheology Soft glasses

1. Introduction already in the early 1950 s. Food processing heavily relies on complex flow processes. Therefore, rheological characterization of the individual Global challenges in food science are the sustainable and safe access ingredients as well as the composed food product found on supermarket to clean water and supply of sufficient energy sources, i.e. food based on shelves is an integral part of food science. Rheological research in food fats, proteins, and carbohydrates for high quality human nutrition. In science is therefore closely linked to the development of food products this context food quality is mostly defined by sensorial characteristics and could address the industrial production of food (stirring, pumping, and consumer-driven preferences selecting the convenience level as dosing, dispersing, spraying), home based cooking as well as consump- well as health supporting properties of the chosen food. Depending on tion of food (oral perception, digestion, well-being). the socio-economical and nutritional background of the consumer, Properties of processed food products are increasingly tailored to individual diets might be different but will be, in particular in the so- meet consumers' requirements and benefits. Tailored product called Western Diet, based on food products that are partially or entirely properties are designed along structure–property and process– processed. The resulting decomposition and subsequent re-composition structure guidelines, considering structure from the molecular to of food materials allows us to design food according to nutritional the macroscopic scales and its consequences on processing and guidelines and to add ingredients for enhancing the nutritional benefits perception. Rheology comes into play in the context of structure as of the final product (e.g. fortification with micronutrients). The newly one of its most prominent dynamic properties. The close link of tailored or designed food might be stabilized by the same mechanism as rheology and structure also introduces the relationship to flow the original food components, but removed or added components will processing, which determines the dynamic conditions under which need additional stabilizing methods. It is not surprising that along with the food material flows. Particularly for food systems rheology plays the emergence of processed foods, food science has devoted significant an important role because (i) flow properties define food structure research to the role of individual ingredients, in particular to stabilizing during manufacturing (factory) or preparation (kitchen) and (ii) agents. Journals focusing on food hydrocolloids, carbohydrate biopoly- physiologically in mouth, stomach, and intestine where food structure mers, or food hydrocarbons and on interactions of ingredients with the is perceived and digested. Rheology impacts directly on perception food matrix were established in the 1980 s, while research on non- and digestion by influencing the flow characteristics during mastica- composed food such as starch-based products nucleated journals tion and digestion but also triggers other quality characteristics such as flavor or nutrient release at specific sites. Food rheology is not an unified discipline, but its practice can be fi ⁎ Corresponding author. subdivided into three categories. A rst category is represented by E-mail address: peter.fi[email protected] (P. Fischer). food product developers mostly based on a food technology

1359-0294/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2010.07.003 P. Fischer, E.J. Windhab / Current Opinion in Colloid & Interface Science 16 (2011) 36–40 37 background and aimed at comparative characterization of food 2.1. Rheology and human perception products and rheology–property relationships. Typical ‘properties’ of interest correlated with rheology are (i) sensory / perception During mastication and swallowing the tongue and mouth senses characteristics (e.g. texture), (ii) stability, (iii) convenience aspects only those aggregates greater than 20 μm [33]. Structured food (e.g. portioning, scoping, dosing, filling) and (iv) nutritive character- products either benefit from this size limit (structure breakdown is istics (e.g. release kinetics, satiety). The second category are associated with textural sensation and is coupled with flavor and represented by food engineers, who try to develop rheology–process nutrient release and therefore clearly links to eating pleasure) or relationships of the food and use rheological data for process or should be avoided (sandy mouth feel in chocolate is associated with a product optimization. Rheological measurements are also used in insufficiently refined product). Since many food systems, e.g. emul- analytical to semi-empirical modelling as well as in numerical flow sions and food suspensions, do have aggregate sizes of several μm, process simulations. Typical flow processes in food processing include food manufacturing operations aim at changing the microstructure on mixing/stirring, dispersing, extrusion, spinning, coating, injection this length-scale. In classical food technology and food engineering it moulding and spraying. The third category is represented by material is therefore of main interest to control the final food structure and its scientists or physicists who focus on rheology–structure relationships perception and texture [37–45]. For food quality control, relating of soft materials. They are mostly interested in model food systems, textural perception to physical measurements of food structure, such rheometric model flows as well as analytical to semi-empirical as rheological properties, requires understanding of the breakdown modelling and simulations. pathway of food during mastication and the correlation of instru- In a recent review we have attempted to give a brief overview of mental readings to decisions taken by humans. In this framework, current food rheology based on structural criteria, including phenom- biopolymer gels are considered as a suitable model systems to link enological, processing-related and molecularly-based approaches [1]. food structure and texture to rheological properties [40] because the Here, this brief overview intends to shed some light on rheological gelation mechanism of biopolymers composed out of proteins, techniques as well as on some recent research trends in food rheology. polysaccharides and combinations thereof is considerably known so that samples meet the requirements for parameter variations in a 2. Dealing with a hierarchical material sensorial test. The limited capacities of any rheological technique (including rheometers, viscometers, texture analyzers, and consist- Food products may be simple liquids or solids but the vast majority ometers) to elucidate texture and perception requires additional of food materials belong to the category of soft condensed matter descriptions of the product. To close this gap, combinations of sensory composed of a range of hierarchical nanostructures and microstruc- evaluation and structure analysis utilizing fluorescence microscopy, tures [2–8]. Accordingly, suspension, emulsion and interfaces [9–14], confocal laser scanning microscopy, NMR, and numerical modeling foams [15–20], biopolymer gels and mixtures [21–27] can be the food are use in order to characterize both microstructure and fracture encountered in rheological investigations. The rheology of such evolution during mastication [46–49]. It is to be expected that in the complex products is governed by the main ingredients and their future combined approaches measuring food structure, rheological interactions on a wide variety of length and time scales. For example, properties, oral processing, and sensory properties will addressed the droplets and particles in a typical food emulsion or food suspension, topic in detail. e.g. in a salad dressing and or in chocolate, are primarily interacting on A phenomena linked to perception and food structure is the recent the non-colloidal level, whereas the proteins, surfactants, cell walls, society-driven trend in food science for the replacement or the lipids, polysaccharides stabilizing the dispersed system interact on the reduction of fat. In case of a prominent candidate of a non-colloidal colloidal length scale. Moreover, in industrial scale food processing food material, chocolate, this would be the creation of low calorie length scales in order of meters are relevant. The corresponding time product with the same perceived properties as the full fat original. The scales may be in the sub-millisecond regime during aggregation of the easiest way to achieve this goal is the reduction of the cocoa and milk ingredients or up to years during the long-term shelf life of canned fat but this would lead to an increase of the chocolate viscosity causing food products [6,12,13,24,28–36]. problems during manufacturing [33]. Rheological characterization of The aim of rheological characterization is to quantify the the flow properties as well as modifications of the ingredients are functional relationships between deformation, stresses, and the required to optimize the chocolate suspension. In the first case, the resulting rheological properties such as viscosity, elasticity, or particle size distribution and the particle shape can be adapted to keep viscoelasticity. A prerequisite for proper rheological data is rheo- the chocolate melt viscosity acceptable for processing [33,50–53] metric flow conditions, i.e. a defined laminar deformation field. while for the later case addition of biopolymer gels or oils has been Consideringstrawberryyoghurtoranyotherheterogeneously proposed [23,54]. In both cases, rheological investigations support the structured material it is clear that for many food products classical optimization the final food product by helping to understand the role rheological devices will fail due to non-homogeneous flow fields. As a of other relevant ingredients, i.e. oil and fat as matrix fluid, cocoa, consequence, partly strange measuring devices for food characteriza- sugar, and milk powder as dispersed materials. In addition, phospho- tion were developed in the past simply because the measurements are lipids such as lecithin used in chocolate have significant impact on the motivated by quick and reliable evaluation during food processing or rheological properties. This indicates that the interaction of the by the fact that, literally spoken, a whole apple does not fit into a dispersed particles can be controlled by the surface coating with self- Couette geometry and even if it would, the resulting flow profile assembled colloidal compounds. would most likely not be rheometrical in the strictest sense. For practical purposes, the latter example can be avoided by using 2.2. Complex flow phenomena in multiscale food systems different mechanical analysis techniques to tackle the hierarchical structure of food systems such as fruits, cheese, dough, meat beside Beside the non-trivial link between instrumental readings provid- others. On the other hand, rheological experiments on individual ed by rheometers and human perception of food, the rheological ingredients in aqueous or lipid-based solvents neglect the complexity response of complex food materials can be challenging by its own. of the real food matrix but provide understanding of the self-assembly One inherent problem of concentrated and structured food materials of food ingredients on the colloidal level. Considering the mentioned is the occurrence of ‘yielding’: an apparent solid-to-liquid flow approaches it becomes clear that food rheology is defined by its transition is observed, depending on the material structure and the application rather than by a straightforward physical classification of applied shear stress. Examples can be found in food products such as materials. ketchup, sauces, mayonnaise, yogurt, margarine and in many other 38 P. Fischer, E.J. Windhab / Current Opinion in Colloid & Interface Science 16 (2011) 36–40 systems [7,55–57]. Yielding phenomena and yield stresses have methods [72–77]. A considerable amount of publications address the received considerable attention over the last decades and we will characterization of the expanding bubble technique (biaxial extension not repeat the discussion in full detail. However, it is safe to say that flow) and its numerical simulation [78–81]. Dough rheology is a good for hierarchical food materials showing a solid-like behavior at rest example of an industrial-based characterization method with strong the internal nano- and microstructure resists the applied stress and standing in food rheology; it cannot be easily replaced, even though it reversibly deforms. Structural breakdown will lead to both a structure might be hampered by non-ideal flow properties. Approaches that different to the original one and to flow. The stress necessary to aim to introduce extensional rheometry [82–86] are extremely useful initiate a flow transition is called yield stress and distinguishes elastic to understand the material but are complicated by the fact that deformation and viscous or viscoelastic flow. It is important to keep in measuring devices are expensive, do not operate in the production mind that the material will retain its chemical composition, but can be time scale, and do not provide instrumental readings easily be present in very distinct structures. For example, a classical semi-solid transferred to the process or to the baker at four o'clock in the yoghurt exhibits elastic response best tested with a vane geometry in morning. its original container, or using small amplitude oscillatory shear Tribology, lubrication, triborheometry, and micro-gap rheometry deformation if adequate sample preparation is possible, whereas has recently received considerable attention to study the effect of yoghurt under shear shows a viscous response and can be tested in shear, squeeze flow and the role of confinement at length scales that more traditional rheometrical geometries. are approaching the length scale of the food microstructure [87–90]. There is always interaction between the applied flow field, the For example, the lubricating properties of human saliva is influenced measuring device, the measuring conditions and, of course, the by health and diet conditions but also depends on the food material it sample material. In classical rheometric geometries (plate/plate, interacts with [91–94]. Saliva lubricates and protects surfaces in the concentric cylinders), proper choice of the surface topography of the mouth, supports transport of food materials and aids in taste shearing surfaces is important [58]. Additionally, to minimize the perception. Using tribological methods to study the situation in the effect of yielding and slip, several specialized rheometrical devices mouth it was found that saliva, depending on the food eaten, exhibits have been proposed in the past. All those geometries aim to reduce a very high elasticity for a generally low viscous fluid. As explanation the surface area where slippage occurs, jeopardize a well-defined flow it is rationalized that high molecular weight glycoprotein form field by doing so. Commercial viscosimeter geometries, helical ribbon aggregated clusters leading to the elastic response [94]. and pin mixers, ball measuring systems as well as vane geometries Another kind of ‘thin film’ rheology is encountered in interfacial [59–62] are widely used. Amongall, the vane is probably the one for rheology where mobile gas–liquid and liquid–liquid interfaces which the most ‘solid’ fluid mechanical analysis is available, i.e. the present in foams, emulsions, and blends (e.g. drug delivery systems, one that is closest to a ‘rheometric’ flow [60,63–65]. It should be kept functionalized food and health related products such as parenteral in mind that all geometries provide not shear rate and shear stress, and enteral feeding) are investigated [95,96]. These interfaces are but torque and rotational speed. To obtain rheological data, a mixer often stabilized by adsorption of surfactants [97,98], proteins and analysis using model fluids showing similar flow properties as the partially hydrophobic biopolymers [99–103] or colloidal particles. The unknown sample (e.g. yielding, power law behavior, thixotropy) must adsorption layer can exhibit viscoelastic, elastic or even rigid solid- be performed prior any analysis [62]. For heterogeneous materials like rheological response function under lateral shear and dilatational such as strawberry yoghurt the mentioned geometries might be the stresses. In case of emulsions droplets, the deformation and breakup only chance to obtain rheological data at all. Faced with ‘no data’ or behavior of protein-covered emulsion drops is influenced by the ‘relative data with errors’; using a mixer geometry means opting for rheological properties of the adsorption layer, which prevents the latter one, ideally keeping in mind that no absolute values can be coalescence and rupture of the droplet of foam bubble [103–106]. obtained. Beside the mentioned flow geometries adapted to rotational Further focus areas in interfacial rheology for food-related systems rheometers, in-line rheometry can be used to obtain process-related are: surface interactions of small molecular weight surfactants with rheological data. In recent years non-invasive inline methods such as proteins or other polyelectrolytes [107–110], ‘fluidization’ of protein resonator-based and ultrasonic-Doppler based devices have been layers by competitive adsorption with surfactants [111,112], and proposed and utilized for the characterization of fat crystal suspen- chemical or enzymatic interfacial cross linking of proteins [113–115]. sions, salad dressings, chocolate, and cheese [66,67]. Assembly of micron-sized colloidal particles at liquid interfaces has Associated with the complex flow field for yielding materials as been extensively studied, especially in relation to the extraordinary well with the perception of food and break down of food structure stability increase for particle-stabilized emulsions (Pickering emul- during mastication, squeeze flow and lubrication flow offer flow fields sions) [116–119]. Other applications of interfacial rheology addressed that have been claimed to be close to the flow in the mouth. The use of the flow of saliva proteins in presence of compounds commonly found squeeze flow rheometry to overcome yielding and slippage as well as in oral health and beverage products [93]. special squeeze flow devices such as inverted filament stretching set- ups to study bi-axial elongational flow have been discussed recently 3. Summary and perspectives [68–71]. It is important to memorize that while squeezing flow between a pair of parallel discs or a plate and a sphere seems to be a Characterizing and, ideally, understanding the rheology of food simple experiment, the choice of the appropriate model, data analysis materials is essential for numerous aspects of food science and and boundary conditions (slip or no slip, rough or smooth surface, ...) technology, such as the standardized characterization of raw is extremely important and the wrong choice can easily ruin entire set materials and innovative products, or for optimized industrial of experiments. processing. Classical rheometrical techniques as well as methods Another application of squeeze flow is seen in dough rheology adapted for the food material and the purpose of the measurement where both tradition and a complex viscoelastic sample complicate have received considerable attention in the past decades providing a the use of standard shear rheological investigations. Dough can be deeper understanding of the raw material, its processing and its seen as a starch particle suspension dispersed in a concentrated underlying task in a complex food matrix. biopolymer solution where interactions on the molecular to the Besides the discussed research activities some trends that have micron length scale determine the overall rheological properties, been nucleated in the last years should be mentioned. Along with the baking process, and final bread quality. 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