Thermodynamic Properties of Water in Amorphous Carbohydrates: Physical and Chemical Considerations Johanna Claude Job Ubbink Lausanne - Switzerland
Outline Amorphous carbohydrates in foods : phase behavior and physical properties Polymer physics approach Water : a key parameter Probing the molecular level Thermodynamic nanostructure Chemical stability Heating in sealed cell maltodextrin Color change Concluding remarks
Phase Behavior and Physical Properties of Amorphous Carbohydrates Determine Food Quality Amorphous state Aqueous solution of carbohydrates concentration dissolution Rubbery state Glass-rubber transition Glassy state
Polymer Physics of Amorphous Carbohydrates Model food polymer: maltodextrin Hydrolysis product of starch Mainly α-(1 4) bonded glucose residues Some branching because of α-(1 6) linkages Amorphous Glucose Hydrolysis determines Molecular weight (DE1 = starch; DE100 = glucose) Polydispersity (often Mw/Mn > 5) http://www.poco.phy.cam.ac.uk/research/starch/0
Hydrogen Bonding in Amorphous Carbohydrates and the plasticization by Water Material Mw Tg (dry) [K] Glucose 180 304 MD DE35 1600 353 MD DE21 7000 388 MD DE6 86000 425 Starch 520 In comparison: Material Mw Tg [K] Polystyrene 4000 313 Polystyrene 300,000 372 Polyethylene 140 Poly(vinyl)chloride 358 High dry Tg of biopolymers because of extensive hydrogen bonding in dry state But consequently water is a highly efficient plasticizer! Tg Rubbery state Glassy state d(tg)/d(a w ) Material d(tg)/d(a w ) [K] MD DE35-182 MD DE21-176 MD DE6-176 a w
Study Combines Thermodynamics With nanostructural Investigations Thermodynamics Sorption behavior PVT properties Nanostructure Defects in molecular packing Hydrogen bonding Chemical stability Kinetics of caramelization in glassy and rubbery matrices
Annealing of Polymer Matrices Method 1 Annealing Method 2 DRY POLYMER Potential problems: above Tg under vacuum Annealing of polymers equilibrated + WATER POLYMER ANNEALED Water is key element in carbohydrates + Chemical degradation POLYMER + WATER initial water activity Hermetically sealed sample container POLYMER + WATER ANNEALED Potential problem: Chemical degradation final water activity =
Determination of Carbohydrate Degradation How to follow degradation? Detection of degradation products Changes in molecular weight profile Color changes
Caramelization Reactions Generally Studied in Dilute Solutions Caramelization comprises a whole range of (complex) chemical reactions: Initial step: Loss of water; formation of the anhydro form of the sugar Second step Further loss of water; formation of water-soluble caramelan (color); some formation of flavor compounds Subsequent steps Formation of very dark, insoluble pigments; formation of flavor compounds arising from sugar fragmentation and dehydration
Hermetically-sealed Stainless Steel Cell Maltodextrin IT-12, a w 0.15, Tg = 88 C O ring 1 cm Temperatures tested in an oven: 58 C 88 C 118 C 188 C Thermo couple
Color Measurements Macbeth Color-Eye XTH Spectrophotometer CIE standard daylight illumination source D 65 Standard observer 10 Maltodextrin IT-12 188 C Light source Observer object 30 min 60 min 120 min
Color Measurements L*a*b* Hunter-Scotfield equation E * = * 2 ( a ) + ( b ) + ( L * 2 * ) 2 black L* white green 0 100 - a* + red blue - 0 b* + yellow 0
Color Evolution at Four Temperatures Browner DE* 25 20 15 10 5 0 58 C 88 C 118 C 188 C TEMPERATURE other color indices * * 2 * 2 * 2 E = ( a ) + ( b ) + ( L ) Time [h] 0 0.5 1 2 4 8 16 72
Arrhenius Plot Indicates Absence of Tg Effects Color evolution at 118 C Arrhenius plot D E* 10 9 8 7 6 5 4 3 2 1 0 R2 = 0.99 slope 0 5 10 15 20 Heating time [h] ln(k) 1/T [1/K] Tg 0.0020 0.0025 0.0030-4 -6 188 C -8 118 C -10 88 C -12 slope 58 C -14-16 ln( k) = ln( k o ) E R a 1 T E a = 80 KJ/mol
Annealing leads to water release and change of physical state of matrix Norm Absorbances 0.010 0.005 0.000-0.005-0.010-0.015-0.020-0.025 NIR spectra Water 58 C 88 C 1850 1900 1950 2000 2050 2100 2150 2200 2250 118 C 188 C Carbohydrate Water release Wavelength [nm] Water activity 0.25 0.2 0.15 0.1 Water activity Rotronic sensor Maltodextrin IT 12 at 188 C + initial 30 min 60 min 120 min Heating time [min] Normalized Differences absorbance with initial maltodextrin IT-12 Diff Norm data from the InfraAlyzer 500 QS
Concluding remarks Annealing behavior has to be clear before thermodynamic and nanostructural studies Annealing via heat induces degradation / caramelization in carbohydrates Color and water measurements as proof of thermal degradation Color formation already happens below glass transition and is Tg independent. Increase of a w by water release
Acknowledgements P.Joshi V. Meunier M.-I. Alonso & J.Strassburg Food Materials group