Starch and its role in texture Sandra Hill • LEARNING OUTCOMES – Appreciation of what happens when starch is heated • In high water environments • Low water environments – Understand the concept of starch conversion – Recognise the techniques used to measure starch conversion and appreciate what is being assessed Nov 2005 1 Levels of organisation sugars macromolecules assembles growth rings granules grains/ tuber 2 Starch, Water and Heat • water associated with the crystallites • water in the amorphous phase • melting of the crystallites • hydration and swelling of the molten crystallite region and the amorphous region This is non reversible During this: • redistribution of water in the granule • stresses in the granule Swelling and melting are a cooperative process 3 Temperature of the cooperative disassociation of the crystallites •depends on the botanical source •depends on the water (platisicer) present Starch i.e. the water content matters Gel range C Corn regular waxy Potato Rice Sago Cassava Wheat 62-80 63-74 56-69 61-80 60--74 52-64 53-72 4 Polarised Light: potato starch 58.3 66.5 59.9 68.2 62.3 70.0 64.0 71.8 5 Measuring loss of order • • • • • Polarized light microscopy X-ray order FTIR (Fourier Transform infra-red) DSC (Differential scanning calorimtery) NMR (Nuclear magnetic resonance) 6 Starch gelatinisation and Starch pasting Native starch granules in excess water ( water 4* the amount of starch) temperatures above the cooperative disassociation of the crystallites Gelatinisation Water into the granule - granule swells -loss of some polymer PASTING If subjected to shear... 7 The RVA 8 Native starches, pasted in excess water 10000 Peak 8000 Potato 7000 Viscosity (cP) Set Back Final viscosity 100 80 Cold water 6000 Wheat 5000 Maize 60 4000 WMS 3000 40 Rice Maize 2000 Temperature (C) 9000 120 20 Cassava 1000 0 0 0 5 10 15 20 Time (min) 25 30 35 9 Starch conversion Crystalline granules Non crystalline granule Some polysaccharide lost from granule Decreasing molecular weight 10 Temperature and changes, starch in excess water 50 60 70 80 90 100 0C Disappearance of Birefringence DSC endotherm Disappearance of X-ray Swelling power Solubilisation Morphological changes Rheological behaviour 11 Starch conversion - a continuum of changes from the native granule to dextrin! Methods to monitor starch conversion Maltese crosses DSC endotherms X-ray order water absorbance solubility enzyme digestibility alkaline viscosity RVA final viscosity ? Starch conversion 12 Stages of granular dispersion of starch in baked goods Swollen Scottish short bread biscuits Gelatinized cakes Disrupted bread Dispersed wafers Enzyme degraded From Greenwood, 1976 13 Heating starches in limited water Donovan, 1979 14 Starch conversion Crystalline granules Non crystalline granule Some polysaccharide lost from granule Decreasing molecular weight HEAT MECHANICAL ENERGY ENZYMIC CHEMICAL ENVIRONMENT 15 Processed samples 10000 120 9000 Temperature 100 7000 80 6000 GM 5000 60 RM 4000 40 3000 EM 2000 Tem perature (C) Viscosity (cP) 8000 20 1000 TCM 0 0 0 5 10 15 20 25 30 35 Tim e (m in) RM GM EM TCM Raw maize grits Gelatinised maize Extruded maize Thermally converted maize 16 Shearing the starch • Shearing swollen granules • Milling of wheat – Damaged starch • Thermomechanical extrusion • Milling – Rework – Sample preparation 17 What happends if low water contents and the sample is sheared? i.e. extrusion (thermalmechanical cooking) low water high mechanical energies Would seem that the mechanical input can be even more important in breaking starch than thermal energy. disassociation of the crystallites and then break down the macromolecules Starch Water Barrel Screw Feeding Heaters Kneading Die Cooking 18 2500 120 2000 100 228kJ/kg 80 1500 276kJ/kg 60 361kJ/kg 1000 40 433kJ/kg 494kJ/kg 500 20 0 Tem perature (C) Viscosity (cP) SME(specific mechanical energy) = (torque*screw speed* no of screws) / throughput 0 0 5 10 15 20 Tim e (m in) 25 30 35 1Whr/kg=3.6kJ/kg 19 SME and Starch parameters SME [kJ/kg] Crystal line pattern V GE [J/g] 228 MC [%, dwb] 30.60 - Alk. visc. [mPas] 1.47 276 25.55 V - 1.45 361 20.20 V/E - 1.40 433 16.89 E - 1.31 494 13.58 E - 1.23 2500 Viscosity (cP) 2000 1500 Cold water viscosity 1000 500 0 150 Final viscosity 250 350 SME (kJ/kg) 450 550 20 Raw maize grits Wheat ball pellets 3D pellets 21 Comparison of mills Disc mill: Tecator 1990 Cemotec mill Perten FN mill 3303 Impeller mill: Tecator 1093 Cyclotec mill 22 Viscosity (cP) RVA profiles of samples milled with two different mills 120 12000 100 10000 80 8000 60 6000 40 4000 20 3000 120 2000 2500 100 0 2000 80 1500 60 1000 40 500 20 10 15 20 25 Time (min) Yellow disc mill Pink impeller mill 30 35 15 20 25 30 35 1400 120 1200 100 1000 80 800 60 600 40 400 Temperature (C) 5 10 3D pellets 0 0 5 Time (min) Viscosity (cP) 0 0 0 Temperature (C) Viscosity (cP) Wheat ball pellets 14000 Temperatute (C) Raw maize grits 20 200 0 0 0 5 10 15 20 25 30 35 Time (min) 23 Heating without shear • Limited water 24 Quiescent conversion of maize grits Particle structure of maize grits before and after heating (ESEM) 10m SEM 2000x, non-heated maize endosperm particle - starch granules (10m) covered with protein matrix SEM 2000x of maize endosperm particle heated 20 min at 140°C - starch granules in place 25 - no loss of their granular integrity Quiescent conversion of maize grits Loss of molecular order and hot water absorption and solubility indices (HWAI/HWSI) Heating time Birefringence Relative Gelatinisation (min) () crystallinity (%)¹ enthalpy (J/g) ¹ HWAI² HWSI² 0 ++ 23.66 13.30 1197 7.38 5 ++ 19.77 13.78 1113 7.19 10 + 12.58 4.32 907 8.17 15 - - - 649 12.70 20 - - - 708 10.34 30 - - - 545 20.67 - - - 962 22.74 amorphous - not detectable ¹Values are mean of duplicates ²Values are mean of four repetitions 26 Quiescent conversion of maize grits X-ray diffraction pattern 0 min 5 min Intensity 10 min 15 min 20 min 30 min amorphous 0 10 20 2 Theta Scale 30 40 27 Chemical dimension • Redox agents affects starch depolymerisation 28 Starch and Sulfite Swelling volume (ml/100ml) 40 35 30 25 Low levels of sulfite radicals Excess oxygen Low depolymerisation Excess sulfite Oxygen scavenged Low depolymerisation High numbers of sulfite radicals Many superoxide radicals Some oxygen High depolymerisation 20 15 10 5 0 0.00 0.01 0.1 Sulfite concentration (%) in log scale 29 Intrinsic viscosity (ml/g) Starch solubility when heated in Starch heated in excess water at excess water at 95C for 60 minutes 95C for 60 minutes then total material solubilised in hydroxide. Solubility (%) 120 100 80 60 40 20 0 250 200 150 100 50 0 no additive 0.01% sulfite Paterson et al, Food Hydro. 8, 259-263 30 Measurement of starch pastes 0.10 Starches (5%) gelatinised at 90C for 30 min in phosphate buffer. Sheared using Silverson at max speed for 2 min. Samples mixed with additives and held at 60C. Cooled to 25C before measurement at 27s-1 on Bohlin CS10. pH differences less than 0.3 units sulfite Viscosity (Pa.s) 0.08 0.06 No additives 0.04 0.01% 1% 0.02 0.1% 0.00 0 2 4 6 8 10 12 Time (h) Taken from: Valles-Pamies et al, 1997 31 without ascorbic with ascorbic 0.01% 32 Starch changes in processing • Starch gelatinisation – disruption of the structure of the starch granules or its components as a result of heating with or without shear in an excess water environment. • Starch melting – the loss of ordered regions in a granule as a result of heating and/or shear at low water contents • Starch conversion- disruption of the structure of the starch granule and/or its components as a consequence of a process involving heat and/or mechanical energy. It can occur at high or low water contents 33