LEARNING OUTCOMES

1
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
2
Levels of organisation sugars macromolecules
assembles growth rings granules grains/
tuber
3
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
• During this
• redistribution of water in the granule
• stresses in the granule
• Swelling and melting are a cooperative process

This is non reversible
4
Temperature of the cooperative disassociation of
the crystallites

• depends on the botanical source
• depends on the water (platisicer) present

i.e. the water content matters
Starch
Gel range
C
Corn
regular
62-80
waxy
63-74
Potato
56-69
Rice
61-80
Sago
60--74
Cassava
52-64
Wheat
53-72
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Polarised Light potato starch
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Measuring loss of order

• Polarized light microscopy
• X-ray order
• FTIR (Fourier Transform infra-red)
• DSC (Differential scanning calorimtery)
• NMR (Nuclear magnetic resonance)

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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...
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The RVA
9
Native starches, pasted in excess water
10000
120
9000
100
8000
Potato
7000
80
6000
Wheat
5000
60
Viscosity (cP)
Temperature (C)
Maize
4000
WMS
40
3000
Rice
Maize
2000
20
Cassava
1000
0
0
0
5
10
15
20
25
30
35
Time (min)
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Starch conversion
Crystalline granules Non crystalline
granule Some polysaccharide lost from
granule Decreasing molecular weight
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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
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Methods to monitor starch conversion
Starch conversion - a continuum of changes from
the native granule to dextrin!
Starch conversion
13
Stages of granular dispersion of starch in baked
goods
Swollen Gelatinized Disrupted Dispersed Enz
yme degraded
Scottish short bread
biscuits
cakes
bread
wafers
From Greenwood, 1976
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Heating starches in limitedwater
Donovan, 1979
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Starch conversion
Crystalline granules Non crystalline
granule Some polysaccharide lost from
granule Decreasing molecular weight
HEAT MECHANICAL ENERGY ENZYMIC CHEMICAL
ENVIRONMENT
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Processed samples
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Shearing the starch

• Shearing swollen granules
• Milling of wheat
• Damaged starch
• Thermomechanical extrusion
• Milling
• Rework
• Sample preparation

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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. disassociati
on of the crystallites and then break down the
macromolecules
19
SME(specific mechanical energy) (torquescrew
speed no of screws) / throughput
1Whr/kg3.6kJ/kg
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SME and Starch parameters
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• Raw maize grits Wheat ball pellets 3D
  pellets

22
Comparison of mills

• Impeller mill
• Tecator 1093 Cyclotec mill

• Disc mill
• Tecator 1990 Cemotec mill
• Perten FN mill 3303

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RVA profiles of samples milled with two
different mills

Raw maize grits
14000
120
12000
100
10000
80
8000
Viscosity (cP)
60
6000
Temperatute (C)
Wheat ball pellets
40
4000
20
3000
120
2000
0
0
2500
100
0
5
10
15
20
25
30
35
2000
80
Time (min)
Viscosity (cP)
1500
60
Temperature (C)
3D pellets
1000
40
500
20
1400
120
1200
0
0
100
0
5
10
15
20
25
30
35
1000
80
Time (min)
800
60
Viscosity (cP)
Temperature (C)
600
Yellow disc mill Pink impeller mill
40
400
20
200
0
0
0
5
10
15
20
25
30
35
Time (min)
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Heating without shear

• Limited water

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Quiescent conversion of maize grits Particle
structure of maize grits before and after heating
(ESEM)
10mm

• SEM 2000x, non-heated maize endosperm particle
• - starch granules (10?m) covered with protein
  matrix

SEM 2000x of maize endosperm particle heated 20
min at 140C - starch granules in place - no loss
of their granular integrity
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Quiescent conversion of maize grits Loss of
molecular order and hot water absorption and
solubility indices (HWAI/HWSI)

• - not detectable ¹Values are mean of duplicates
  ²Values are mean of four repetitions

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Quiescent conversion of maize grits X-ray
diffraction pattern
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Chemical dimension

• Redox agents affects starch depolymerisation

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Starch and Sulfite
40
Low levels of sulfite radicals Excess oxygen Low
depolymerisation
Excess sulfite Oxygen scavenged Low
depolymerisation
35
30
25
Swelling volume (ml/100ml)
20
15
10
5
0
0.00
0.01
0.1
Sulfite concentration () in log scale
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Starch solubility when heated in excess water at
95C for 60 minutes
Starch heated in excess water at 95C for 60
minutes then total material solubilised in
hydroxide.
120
250
100
80
200
Solubility ()
60
150
Intrinsic viscosity (ml/g)
40
100
20
0
50
rice
sago
potato
wheat
maize
cassava
0
wheat
potato
cassava
Paterson et al, Food Hydro. 8, 259-263
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Measurement of starch pastes
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
No additives
Viscosity (Pa.s)
0.01
1
0.1
0
2
4
6
8
10
12
Time (h)
Taken from Valles-Pamies et al, 1997
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(No Transcript)
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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