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)
10m
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 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