Practical Food Rheology

An Interpretive Approach

Ian T. Norton, Fotios Spyropoulos, Philip Cox, Ian T. Norton, Fotios Spyropoulos, Philip Cox

• Publisher: Wiley-Blackwell
• Year: 2010
• ISBN:
  9781444391053
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Food Science
• Index: Technology & Engineering

Rheology is fundamentally important in food manufacturing in two major senses. Understanding the way in which a substance moves and behaves is essential in order to be able to transport and mix it during processing. Secondly, the rheology of a product dictates much of the consumer experience, e.g. in relation to texture and mouthfeel.

This book doesn't overwhelm the reader with complex mathematical equations but takes a simple and practically-focused approach, interpreting the implications of rheological data for use in different food systems. Through this approach industry-based food developers / rheologists, students, and academics are given clear, concise interpretation of rheological data which directly relates to actual perceived functionality in the food.The functionality may relate to texture, structure and mouthfeel, and may result as a function of temperature, pH, flocculation, concentration effects, and mixing.

The interpretative view is based on the principle that the food rheologist will produce a graph, for example of viscosity or gelation profiling, and then have to extract a practical meaning from it.For example, if viscosity falls with time as a function of pH, this knowledge can be used to tell the customer that the viscosity can be followed with just a pH meter and a stopwatch. Rheological measurements have shown that once the pH has dropped 1 unit after 10 minutes, the viscosity has been halved.This is the type of practical and valuable information for customers of the industrial food rheologist which the book will enable readers to access.

Key features:

• A uniquely practical approach to the often difficult science of food rheology
• Includes chapters introducing the basics of food rheology before moving on to how data can be usefully and easily interpreted by the food scientist
• Can be used as a teaching aid on academic or industry-based courses

1 Introduction – Why the Interpretive Approach?

Niall W. G. Young

The interpretive approach is a new way of looking at rheology, and it stems from the benefits to be gained from the multidisciplinary approach (Wassell and Young, 2007). Sole focus on single aspects – viscosity, shear rate and shear stress – can blink the view of the data and cause either misconstrued interpretations or incomplete analysis of the system. The interpretive approach, whilst placing demands on the rheologist, however allows for a fuller interpretation and analysis of the system. The rheologists must maintain their scientific foundation, but also possess the skills of the journalist and studio presenter. In short, today's rheologists are tasked with being required to deliver their message to the professional and layman in clear, concise terms. The interpretive approach is the key to value creation for the industrial rheologist's customers, or the academic's funding applications, and as such equips the rheologist with ‘alchemic’ powers to turn ‘worthless’ rheological graphs into ‘gold’. This sentiment is echoed in the analogy that, for most food ingredient and food producing companies, rheology is not a key business, but it is the key to the business. Successful implementation of the interpretive approach into the role of the industrial rheologist (Young, 2007) ultimately pays dividends for the company. But what is rheology?

1.1 RHEOLOGY – WHAT IS IN IT FOR ME?

Classically, the term ‘rheology’ dates back to the late 1920s and is credited to Bingham, who expanded on the ancient Greek philosophers’ musings of

or everything flows. The definition of rheology therefore came to be the study of the deformation and flow of matter. Barnes et al. (1989) claim, quite rightly, that rheology is a difficult subject, and one can lose one's audience quickly. Focusing on the key words of the definition in isolation – deformation, flow and matter – suggests that rheology is, by nature, multidisciplinary. Hence, it is rooted in mathematics, physics, mechanics, but it is also rooted in life, and therefore everybody has fundamental experience with the basic concepts of rheology. Here, one can think of the daily non-food application of shampoo to one's hand before rubbing it into the scalp. Subconsciously, one measures the viscosity as the shampoo moves between the fingers and, quite wrongly, associates this viscosity with quality – the thicker the shampoo, the better it cleans one's hair. Hence, most shampoos on the market are viscous in nature. It is this ‘human rheometer’ experience, which fills us with perceptions as to what the world around us should feel like, that drives the rigorous rheological control of materials. This perception is so strong that if the rheological control fails we enter into the abnormal, and the world about us feels wrong. Demands are therefore placed on the food industry to control the flow and viscoelastic properties of all our foods as much for practical reasons as for upholding consumer perceptions.

Achieving this rheological control over the food materials requires a dialogue between the rheologist and the food developer, and for them to agree on the conditions of the measurements to be made. Therefore, this prompts a list of questions that need answers before the measurements can be started:

i. Why should the measurement be performed, and to what aim? Here, the purpose and need of the measurement are established, and begin to shape the manner in which the results can be used. Decision as to small or large deformation is taken, as is holding temperature constant or varied, or stress or shear rate, or frequency of oscillation. Does the measurement require a viscosity curve or a viscoelastic profile? Essentially, the experimental set-up is founded in this stage, and competent understanding here leads to a greater degree of first-time success with the measurement.

ii. Are there any special criteria that should be considered? One has probed the manner in which the material is to be tested. But now consideration of the material itself is required, i.e. does the material undergo a state change as a function of temperature? This could have implications as to the choice of measuring geometry. Other examples could include the effect of pH over time; does the material have particles (large or small) present; how fast should temperature or shear rate be ramped to reflect reality or account for temperature or shear gradients? It is worth noting that we may not be interested in obtaining measurements in the steady state, because this may not describe the ‘reality’ we wish to explain, and this can raise questions as to the validity of results. Thus, through additional fine-tuning to the measurement profile, positive influence is exerted on the results. This can be thought of as an extra lens available to the magnifying glass analogy to sharpen the image.

iii. How will the results be used? Here, one considers the final audience who will receive the results, should they be journal article readers, conference delegates/fellow rheologists, colleagues, customers or the layman. Whichever group, the intended audience alters the manner in which the data are presented, to what level it is discussed and which form the interpretive style will take. This requires the rheologist to possess sound and robust communication skills to be adept at presenting material in the different styles needed.

Assimilating all the information from the questions asked earlier, the rheologist should now be able predict what the outcome of the results will look like. This can be sketched out as a guide, and if the measured results match the predicted results, then the theory and set-up of the rheometer were likely correct. From these basic tenets, the rheologists are now ready to begin measuring their sample, and subsequently explore their data to explain their findings using the interpretive method. In order to demonstrate this approach, a worked case study is presented in the following section.

1.1.1 Case study

Wine gums and the art of making them are governed by the setting temperature of the system. High-ester pectin can be used to make excellent wine gums, but the setting temperature is high, typically around 70–80°C. This in itself presents possible application limits on the use of wine gum syrup, i.e. it makes filling it into chocolate cups unrealistic. How can this problem be solved, quantified and controlled?

High-ester pectin sets in the presence of a co-solute (sugar) as a function of temperature or pH (acidity). Cold setting of pectin is possible and has been documented, but has focused on low-ester pectin (Gilsenan et al., 2000; Lootens et al., 2003), typically not used in wine gums. Controlling the acidification of the wine gum mass by means of glucono-δ-lactone (GDL) allows gelation of high-ester pectin to occur over ambient temperatures (Madsen and Thulin, 2002; Young, 2007).

Discussion now, as regards the rheology, centred on how to measure this gelation profile. Small deformation oscillation is performed with constant small strain; within the linear viscoelastic region, temperature is kept constant – room temperature (23°C) – and the frequency of oscillation is kept constant, 0.5 Hz.

Predictions were made of the type of curve to be seen. Focusing on the phase angle curve against time, we expect to see a high phase angle at time zero, forming an initial plateau – parallel to the x-axis – before dropping off as time progresses, and finally showing signs of levelling off, again parallel to the x-axis. G′ will start below G′ with the gap between them being constant. G′ and G″ then converge and cross, and finally deviate from each other until there is again a constant gap. The real results are given in Fig. 1.1.

Fig. 1.1 The gelation profile of high-ester pectin with 2% GDL at 23°C. Open and closed symbols indicate replicate experiments. For a colour version of this figure, please see the colour plate section.

Immediately apparent is that the predicted results conform to the actual results obtained, suggesting good agreement between theory – measurement – and material. Interpretation of the graph allows characterisation of the gelling process: from time 0 to 1800s little or no structure formation is occurring. The 1800s mark the onset of gelation, which proceeds until 5500s, where the G′ and G″ profiles cross, the phase angle value is 45°, and this is termed the gel point. After 5500s, the gel is building in structure and the graph does not extend far enough to show the gelled plateau.

In terms of the material and what this means for the customer, one can reinterpret the graph to say that between 0 and 1800s the wine gum syrup can here be pumped, moved and deposited into the chocolate cups or moulds. Between 1800s and 5500s, it is recommended that the material is not disturbed, so that the gel is allowed to form. Beyond 5500s, the system has set, and a fruit flake, chocolate bead or other material can be deposited on the surface where it will remain without sinking into the wine gum.

Questions may now arise from the customer about the time allowed for filling and depositing, namely 1800s. Could this be altered to optimise the process? The gelation process is controlled by the speed at which the pH is reduced, and therefore governed by the concentration of GDL. Hence, by varying the GDL concentration, the initial 1800s plateau can be shortened or extended. This can be monitored simply by following the pH decrease over time immediately after the addition of GDL to the system, simultaneously with the rheology measurements. Fig. 1.2 presents the pH reduction curves over time for four GDL concentrations. The beauty and simplicity of this pH reduction measurement is that, after coupling to and aligning with the rheology measurements, it allows the gelation process to be accurately followed using only a pH meter and a stopwatch – equipment every confectionery manufacturer will have access to. Thus, the need for expensive rheological equipment investments is not required.

Fig. 1.2 pH curves versus time for varying GDL dosages at 23°C indicating the onset of gelation and gel point taken from rheology measurements. For a colour version of this figure, please see the colour plate section.

This example demonstrates two important points, which highlight the elegance and simplicity of the interpretive approach, and how this benefits food rheology:

i. Simple measurements can lead to extensive process characterisation and material understanding.

ii. Rheological measurements can be monitored by simpler techniques, allowing the non-specialist to manage the process.

Step I is the initial value creation step where earnings are generated. Here, the process can be described and the response of the material under investigation during the process is explained. Such knowledge brings understanding and therefore creates flexibility, which in turn generates earnings. Step II demonstrates that simpler methods can be used to follow rheological processes – given the right conditions – which allows for ease of operation and does not require extensive investments. This is particularly beneficial in industry. Therefore, industry benefits from the interpretive approach through increased process understanding, leading to flexibility and boosted earnings. Academia, in turn, benefits from the interpretive approach through demonstrating their ability to deliver the knowledge and the understanding to the industry, either directly or through scientific and popular publications. Indeed, a successful interpretive approach is a key to successful knowledge transfer schemes of the university.

This book, with chapters on diverse topics including ultrasound-based rheology, hydrocolloids, dairy systems, emulsions and the link between rheology control and health, aims to provide the readers with the tools and the evidence to take on the interpretive role in their own work and to see the difference.

REFERENCES

Barnes, H.A., Hutton, J.F. and Walters, K. (1989) An Introduction to Rheology. Amsterdam, the Netherlands: Elsevier.

Gilsenan, P.M., Richardson, R.K. and Morris, E.R. (2000) Thermally reversible acid-induced gelation of low-methoxy pectin. Carbohydrate Polymers 41, 339–349.

Lootens, D., Capel, F., Durand, D., Nicolai, T., Boulenguer, P. and Langendorff, V. (2003) Influence of pH, Ca concentration, temperature and amidation on the gelation of low methoxyl pectin. Food Hydrocolloids 17, 237–244.

Madsen, O.T. and Thulin, R. (2002) Cold setting of HE pectin. Patent, PCT 0242 US 60/428747.

Wassell, P. and Young, N.W.G. (2007) Food applications of trans fatty acid substitutes. International Journal of Food Science and Technology 42, 503–517.

Young, N.W.G. (2007) Industrial rheology applied: the role of the rheologist. Food Science and Technology 21, 21–23.

2 Viscosity and Oscillatory Rheology

Taghi Miri

2.1 INTRODUCTION

Th...

Table of contents

• Cover
• Half Title Page
• Title Page
• Copyright
• Preface
• Contributors
• 1: Introduction – Why the Interpretive Approach?
• 2: Viscosity and Oscillatory Rheology
• 3: Doppler Ultrasound-Based Rheology
• 4: Hydrocolloid Gums – Their Role and Interactions in Foods
• 5: Xanthan Gum – Functionality and Application
• 6: Alginates in Foods
• 7: Dairy Systems
• 8: Relationship between Food Rheology and Perception
• 9: Protein-Stabilised Emulsions and Rheological Aspects of Structure and Mouthfeel
• 10: Rheological Control and Understanding Necessary to Formulate Healthy Everyday Foods
• Color Plate
• Index
• Food Science and Technology