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Home , Food rheology

ANNUAL TRANSACTIONS OF THE NORDIC SOCIETY, VOL. 16, 2008

Food rheology: A personal view of the past and future

Richard Ipsen1

1 University of Copenhagen/ Faculty of Life Sciences, Copenhagen, Denmark

ABSTRACT well as in industry. Hence I felt the need to In the last 20 years or so, rheology has look back become an increasingly important tool for science as well as the and is now used for a wide range of purposes, ranging from routine analysis in industry to more complex investigations of e.g. macromolecular interactions. The following is a subjective summary and status of the development within the various areas of food rheology. The focus is on the rheology of dairy products, and most of the examples illustrating the development are from dairy science and industry. Figure 1. Number of papers containing Also included is a view into the possible the term ‘food rheology’ (bars) as well as future of food rheology, i.e. a commented ‘food’ and ‘rheology’ (black circles) found description on some of the probable by searching Google™ Scholar, May 2008. directions and applications and evaluate the contribution rheology has INTRODUCTION made to , and also hazard a A rough estimate of the interest in food guess as to what future directions food rheology can be seen in Fig. 1, which shows rheology can be imagined to take. results from searching Google™ Scholar for Some of the areas that have received papers with the term ‘food rheology’ as well major attention are: as papers with ‘food’ and ‘rheology’. There is little doubt that the amount of published Rheology as a probe for macromolecular material dealing with the area has been interactions in steadily increasing in the last 20 years or so Relation between measured instrumental – and no decline in the interest seems (rheological) parameters and sensory evident. perception This estimate coincides well with my Using rheology to predict final food own experience: That food rheology has product quality. become a well established part of the Rheology as an integrated part of food toolbox for food scientists in academia as ingredient development and verification

Interfacial rheology for and There are numerous examples of studies foams dealing with the relation between measured Food process rheology instrumental (rheological and fracture) Relations between rheology and food parameters and sensory perception of food. microstructure Within our group we have paid much Rheology and oral processing attention to establishing such relations for a variety of dairy foods. An example dealing In the following I’ll provide a few with stirred yoghurt4 will suffice to examples from the above areas to illustrate illustrate the complexity involved. We found the applicability of rheological methods in that the sensory perception of creaminess in food science. low- yoghurt depended mainly on oral and smoothness. Creaminess was RHEOLOGY AS A PROBE FOR not only a result of textural properties and MACROMOLECULAR INTERACTIONS hence was difficult to predict solely from Rheology is a bulk method, usually, and rheological properties. does not provide direct evidence of In the same study much improved macromolecular interaction. It is, however, a predictions of oral viscosity measured using good investigative tool, illustrating the Posthumus funnel were found by phenomena that can subsequently be probed recording the mass of yoghurt exiting the by other methods. funnel, illustrating that simple empirical When rheology was applied initially to measurements can be useful when dealing follow the acidification of milk, one of the with more straight forward textural questions was where the somewhat puzzling parameters than creaminess. ‘bump’ in the phase angel (δ) came from1. Certainly more dedicated data treatment Immediately after the gel point, δ decreases can be used to extract information from to below 20o and then gradually increases to empirical tests. The sensory crunchy- or a maximum when the pH is approximately 5 crispiness of food can be related to the and then steadily declines to a final value of jaggedness of the force-displacement curve o 13-15 . This was later resolved to be a obtained in compression or cutting5. consequence of network formation A recent review6 dealing with facilitated by heat treated whey prior biopolymer gels as model systems for to total depletion of calcium from the casein understanding the relationships between micelles. In fact, it appears2 that if network food structure and sensory texture notes the (i.e. gel) formation, irrespective of how it is need for more comprehensive coupling of induced, occurs at pH values above ~ 5, sensory attributes with microstructural where almost all calcium will be on ionic fracture and macroscopic rheological form, the ‘bump’ in δ is evident, illustrating properties. Thus, in order to probe and continuing depletion of calcium from the understand the complex nature of food casein micelles. texture, studies are needed that incorporate Another case, where rheology gave the analysis of food structure, mechanical initial evidence for a surprising properties and oral processing with sensory phenomenon, is our own discovery of texture attributes6. nanotubular structures induced by limited proteolysis3. USING RHEOLOGY TO PREDICT FINAL FOOD PRODUCT QUALITY RHEOLOGY AND SENSORY It is, of course, of major interest to food PERCEPTION manufacturers to be able to predict final from tests on raw materials or partially processed products. Consider, for the quality of many food products. example, frozen dough, where a recent Numerous studies have been conducted on study7 has been able to correlate dough simplified model systems in order to textural behaviour in compression with the understand the behaviour of various surface quality of final bread samples. This gives active components at interfaces. producers the ability to intervene in the It is often not possible to directly relate bread making process and make necessary the interfacial rheological properties to adjustments. functionality such as foam or Our own recent research shows that stability15. However, even in highly rheological measurements (oscillatory complex systems, interfacial rheology can shear) performed on cheese powder be used to probe the mechanisms underlying suspensions can be used to predict the phenomena observed in bulk. An example of functionality of the cheese powders in this is the effect of adding phospholipase to cheese cracker manufacture8. cheese milk, where the resultant improved emulsification of fat into the cheese matrix RHEOLOGY IN FOOD INGREDIENT and the observed higher (~ 3 %) yield can DEVELOPMENT AND VERIFICATION be related to interactions between protein As many ingredients are added to food and lyso-phospholipid and the increased products in order to control textural ability of lyso-phospholipid to decrease properties and ensure long shelf-life, it is surface tension12. not surprising that much research and development effort has been applied to FOOD PROCES RHEOLOGY using rheology for understanding and Food process rheology is often confined verifying the efficacy of various food to the behaviour of liquid foodstuffs, though ingredients. the tendency is increasingly to consider the Just considering our group, we have used response of both solid and liquid rheology to probe food ingredients such as materials18. (e.g. transglutaminase, proteases Monitoring viscosity online or in-line including rennet, during poses a number of phospholipases)9,10,11,12, milk protein challenges due to the complex rheological preparations (e.g. whey protein concentrates nature of typical fluid foods. Foods can for and isolates, purified whey , casein, example be multiphase, elastic, shear caseinates)4,13,14,15 and polysaccharides thinning, fibrous, particulate and highly (exopolysaccharides and viscous. In addition, high temperature and carrageenans)16,17. pressure demands specific design of Rheology has, in combination with other equipment and sanitary requirements must methods, most certainly contributed in a also be met19. A number of approaches major way to our understanding of the have, however, proven successful for mechanism behind how ingredients act in process viscometry in the food industry, the very complex systems that foods are. either in pilot or full industrial scale. Methods used include tube, vibrational, INTERFACIAL RHEOLOGY FOR rotational, hot wire, mixer viscometry, EMULSIONS AND FOAMS Ultrasonic Doppler Velocimetry, ultrasonic Not only has bulk rheology become an reflectance and diffusing wave integrated part of the toolbox of food spectroscopy19. scientists and developers. Interfacial It would appear that ultrasonics-based phenomena are increasingly being in-line rheometry could be developed into a recognized as being of major importance to valid method of process monitoring as it can be used for studying in-line the rheological it interacts with the protein matrix in properties of e.g. starch suspensions and yoghurt and hence also the resulting gels20. A non-invasive Doppler ultrasound- microstructure. This is again reflected in the based technique has for example been used rheological and sensory properties, e.g. in for investigation of non-stationary flow in the characteristic ropiness normally chocolate pre-crystallization21 and the associated with EPS producing LAB. High influence of seeded crystals, temperature, ropiness is associated with direct interaction and flow velocity on the rheology of between EPS and protein, evident in the chocolate suspension was monitored on- microstructure and reflected in the line. rheology25.

RELATIONS BETWEEN RHEOLOGY RHEOLOGY AND ORAL PROCESSING AND FOOD MICROSTRUCTURE The complex flow, deformation and Much effort has gone into establishing fracture taking place orally during relations between rheology and food mastication and mixing with saliva is a microstructure. While the search for generic major challenge to food rheologists. Food is relations between microstructural elements subjected to a range of mechanical and in foods and the resultant textural properties chemical processes in the mouth. It is has perhaps not been very successful, the fractured, diluted and broken down by combination of the two approaches had led saliva, heated or cooled, formed into a bolus to major insights into the factors governing and swallowed26. In addition people vary structure formation in a wide array of substantially in eating behaviour, and diverse food products. This is even more certainly oral physiology plays a role in apparent when the two methodologies are food texture perception26. combined and used to probe dynamic A useful simplification is to consider changes in food structure. that oral processing of most semisolid and The fracture dynamics of biopolymer solid foods can be summarized as two gels, for example, can be characterized by opposing mechanical influences: forces that using a tensile stage coupled to a confocal fracture food particles versus those that laser scanning microscope and image make them adhere to each other27. During analysis22,23. ingestion or the early stages of mastication, In general, application of more advanced the aim of processing solid food particles is image analysis, including multivariate usually to fracture them. Later on adhesion methods applied to whole images, is is desirable to form a bolus that can clear the necessary in order to unravel to complex mouth of isolated food fragments27. These rheological behaviour of even simple food processes are not function of any particular systems. Just visually differentiating force (or stress) nor displacement (or strain) between micrographs can be almost on food particles, but is instead controlled impossible and we have used multivariate by energy. methods to aid in this task17,24. It may, in fact, be incorrect to define e.g. An example of rheology and hardness in terms of a given force. When microstructure providing separate but food is assessed in the mouth, hardness supplementary information on dairy systems appears to be associated with the conditions is the case of exopolysaccharide (EPS) present just as a food particle starts to producing strains of lactic acid bacteria in fracture27. Sensory hardness and production of yoghurt. The specific crunchiness of fruit and vegetables are thus chemistry (i.e. monosaccharide closely related to the stress required to composition) of produced EPS affects how initiate a crack running through the sample cases not yet elucidated. The ability to and to the resistance a material has to crack control assembly of food macromolecules propagation. The implication is that in hard and minor components into a resultant food or crunchy foods the main fracture events matrix is, in fact, rapidly becoming an must take place in the relatively small zone integral part of food product design. Much at the tip of a crack-starting notch28. Such a less is known, however, about how food highly localized event could be identified microstructure influences the break-down of very effectively by muscle spindles not food, e.g. during mastication or in the GI possessing the ability to monitor he force tract, or about the influence of the itself27. microstructure on the delivery and Thus, in order to make additional bioavailability of nutrients. In investigation headway in the area of oral processing and of these issues rheology and understanding how it affects sensory texture perception, a of fracture properties of food plays a key multidisciplinary approach involving food role. physics (especially rheology), thermal Food microstructure affects physics, physiology, and psychology is bioavailability of nutrients, which are 29 usually located in natural cellular called for . compartments or within assemblies produced during processing. They thus need THE FUTURE DIRECTIONS OF FOOD to be released during digestion in order to be RHEOLOGY absorbed in the gut. Bioactive ingredients as Health issues have become an important well have to be protected in order to achieve point of concern in our society, as a large site-specific delivery and increase increase of -related diseases has bioavailability. Carriers can be used to evolved. This will naturally also influence protect against degradation and to control the future directions and developments in the rate of release. The properties of carrier food rheology. and encapsulation systems in relation to Future foods should not only be a good their performance in the GI tract are not as source of nutrients and have high sensory yet well described, and rheology can aid in appeal, but also contribute to the general the elucidation of this. well-being and health of individuals, However, looking in isolation at specific indicating a strong need to control bio food microstructures is not enough. The accessibility and target release of food influence of industrial processing on the components during gastrointestinal transit. interaction between generated food A major challenge for food rheology in the microstructures and nutrient bioavailability future is thus moving rheology into the has not been very well researched. There is human body, i.e. the gastrointestinal (GI) a high degree of linkage between tract in order to understand the interplay 30 between food structure, oral processing and bioavailability and processing , but limited nutrient bioavailability and other insight into how food microstructures can be phenomena related to human health such as designed and processed to ensure maximum survival of probiotic bacteria and delivery of bioavailability. bioactive ingredients. The focus areas for food rheology in the A lot of effort has already been invested past will of course move with us into the in understanding the microstructure of future and bring novel insights as well as model food systems. It is now known how to serve as a standard method for analysis in create a variety of well defined the industry. There are also several areas not microstructures even though the precise discussed here that deserves attention, e.g. mechanism of their formation is in many the rheology of individual food structures (nano-rheology as e.g. made possible by stirred yoghurt", Journal of Texture Studies, atomic force microscopy). 37, 276–299.

CONCLUSION – A CHALLENGE 5. Ipsen, R., Andersen, R.V., Jespersen, L., The challenge for future food rheology Lao, M.L., and Lohman, V. (1999) is to aid in elucidating how human health "Evaluation of crispness in food systems: and wellbeing is influenced by food An example using meringue made with egg structure and the break down of structure or milk protein", Annual Transactions of the during oral processing and passage through Nordic Rheology Society, 7, 39-42. the GI tract. This entails further developing our 6. Foegeding, A.E. (2007) "Rheology and understanding of how food structure is sensory texture of biopolymer gels", generated on the molecular and Current Opinion in Colloid & Interface supramolecular level as well as how it is Science, 12, 242–250. influenced and controlled by processing. It also necessitates knowledge of how such 7. Giannou, V. and Tzia, C. (2007) "Frozen generated structures behave during storage, dough bread: Quality and textural behaviour during oral processing and in the GI tract. during prolonged storage – Prediction of Rheology, in my opinion, can play a role final product characteristics", Journal of in all of these areas but can not stand alone. , 79, 929–934. As always a multidisciplinary approach is called for, and I trust future collaborations 8. Torres, I. C. (2008) "Rheology and will include – but not be restricted to - food Functionality of Cheese Powders", M.Sc. physics and chemistry, pharmacology, Thesis, Department of Food Science, medicine, physiology, microbiology, Faculty of Life Sciences, Copenhagen anthropology and psychology. University.

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