Extensional Rheology of Bread Dough

We investigated the extensional properties of wheat flour dough on the Filament Stretching Rheometer (FISER), in which the sample approaches uniform uni-axial extension flow at a constant strain rate over a large portion of the experiment, thereby allowing us to directly probe the transient material function characterizing the behavior of dough in extension. The large dynamic range of this Rheometer permitted us to investigate the visco-elastic nature of the dough well into the baking/proofing range. Special experimental protocols and modifications to the rheometer fixtures were designed and built to overcome problems in sample preparation. Parameters such as water content, base flour type and mixing conditions were varied to determine their respective effects on the extensional properties. Ultimately we would like to develop a constitutive equation describing the evolution of stress during extension and arrive at a model for the stability against rupture in these doughs. This will form the basis for developing protocols to map results from the true uni-axial extension experiments onto empirical measurements obtained from existing and widely utilized industrial standard testing devices. As an example in this first stage, we focused on the Mixograph, which is a widely accepted method of testing dough in the food industry, and considered how its output can be related to the true material functions generated in filament stretching rheometry. Thesis Supervisor: Gareth H. McKinley Title: Professor of Mechanical Engineering Acknowledgements I will like to thank my thesis advisor Professor Gareth H. McKinley for his support and guidance, especially in taking a chance with a student from course 16 with relatively little experience in rheology. Also special thanks must go to my family, especially my parents for instilling a love of science and engineering, and also for providing such perfect role models. Finally, I will like to thank Kraft foods and Mahesh Padmanabhan for the financial support and my colleagues at the Non-Newtonian Fluids lab for many interesting conversations and brilliant ideas A BSTRA C T .......................................................................................................................................................... 2 A C K N O W LED G EM EN TS .............................................................................................................................. 4 IN TR O D U C TIO N .............................................................................................................................................. 7 INGREDIENTS ..................................................................................................................................................... 7 W heat Flour .................................................................................................................................................. 7 Com position .................................................................................................................................................. 8 S ta rch ............................................................................................................................................................ 9 P ro tein ......................................................................................................................................................... 1 1 W a ter ........................................................................................................................................................... 1 2 O th ers ......................................................................................................................................................... 1 3 THE STRUCTURE OF D OUGH .......................................................................................................................... 14 M OLECULAR STRUCTURE OF G LUTENIN N ETW ORK ................................................................................... 15 Elastic netw ork ........................................................................................................................................... 16 A lignm ent ................................................................................................................................................... 17 Entanglem ent ............................................................................................................................................. 18 LITERATURE REVIEW MECHANICAL TESTING AND FUNCTIONALITY OF DOUGH.21 SHEAR RHEOLOGY .......................................................................................................................................... 22 Steady shear experim ent ............................................................................................................................ 22 Start-up of steady shear ............................................................................................................................. 23 Sm all A m plitude O scillatory Shear test ................................................................................................... 23 Capillary Rheom etry .................................................................................................................................. 27 Com parison betw een different m ethods of rheom etry .............................................................................. 28 EXTENSIONAL RHEOLOGY ............................................................................................................................. 30 U ni-axial Com pression .............................................................................................................................. 30 Extensigraph ............................................................................................................................................... 32 Bubble inflation technique ......................................................................................................................... 35 CONCLUSION .................................................................................................................................................. 40 C O N STITU TIV E EQ U A TIO N S ................................................................................................................... 42 RHEOLOGICAL INVARIANCE ......................................................................................................................... 43 D EFORM ATION TENSORS ............................................................................................................................... 44 U PPER C ONVECTED M AXW ELL AND O LDROYD-B M ODEL ........................................................................ 44 PHA N-THEIN M ODEL ..................................................................................................................................... 46 POM -POM M ODEL ........................................................................................................................................... 49 POW ER LAW M ODEL ...................................................................................................................................... 50 C ONCLUSION .................................................................................................................................................. 51 FILA M EN T STR ETC H IN G R H EO M ETR Y .............................................................................................. 52 K INEM ATICS .................................................................................................................................................... 53 D YNA M ICS ....................................................................................................................................................... 55 FIXTURES AND PROTOCOLS ........................................................................................................................... 58 FILAM ENT STRETCHING EXPERIM ENT .......................................................................................................... 61 RESULTS ........................................................................................................................................................... 61 STRAIN RATE V ISCO-ELASTICITY OF DOUGH ............................................................................................. 63 CONSTITUTIVE EQUATION AND CHARACTERISTIC TimE-SCALE ............................................................... 67 C ONSIDtRE C RITERION .................................................................................................................................. 70 N ON-DIM ENSIONAL REPRESENTATION OF RESULTS .................................................................................... 76 D iff erent Flour Types ................................................................................................................................ 80 M IXIN G .............................................................................................................................................................. 85 INTRODUCTION ............................................................................................................................................... 85 D EFORM ATION OF DOUGH IN THE M IXOGRAPH ......................................................................................... 85 SEPARATING THE TW O COM PONENTS .......................................................................................................... 91 CONCLUSION .................................................................................................................................................. 95 C O N C LU SIO N ................................................................................................................................................. 96 R EFER EN C ES .................................................................................................................................................... 99 Introduction Wheat flour dough has the unique ability to form cohesive elastic doughs when mixed with water under the certain conditions. Because of this characteristic, such doughs has the ability to retain leavening gas during the bread-making process to yield a light, fine cell structure and a soft yet resilient response to chewing. Doughs formed from other flours are significantly less elastic and extensible, and yield products which are coarser and denser. We begin by first discussing the various components which go into forming the dough.