Download - ICFST 2007 Presentation[1]
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Comparative Evaluation of Rheological Models used for Evaluating Dark Chocolate
Viscosity
Emmanuel Ohene Afoakwa1,2 *, Alistair Paterson1 & Mark Fowler2
1 Center for Food Quality, University of Strathclyde, Glasgow
2 Nestle Product Technology Centre, York
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INTRODUCTION
Chocolate manufacturing is a complex physical and chemical process.
Requires numerous technological operations and the addition of different ingredients.
To achieve products of suitable physical and chemical attributes and an attractive appearance and taste.
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Chocolate manufacturing process:
Mixing
Refining
Conching
1st Stage: Dry Conching2nd Stage: Pasty Phase
3rd Stage: Liquid Conching
Tempering
MouldingEnrobing Panning (b)
Packaging
Agglomeration of ingredients in thick paste, continuously or with batch mixers
Size reduction of mix via 2, 3 or 5 roll refiner
Final flavour development, final viscosity of the sample with conche
rotations for 4 to 24 hours
Most stable form of cocoa butter crystals – form V via
heating/cooling systems (maintained at 35°C)
SugarCocoa liquorCocoa butter
Skimmed milk powder (SMP)(a)
Addition of the remaining ingredients:
surfactants, cocoa butter
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Chocolate viscosity:
Chocolate behaves as a non-Newtonian liquid exhibiting non-ideal plastic behaviour, with shear-thinning. Parameters of interest are yield value and plastic viscosity.
Influential factors: Particle size distribution (PSD)
Conching Time Temperature
Ingredient variation Fat content Emulsifiers (Lecithin, PGPR)
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Models for evaluating chocolate viscosity:
(τ: shear stress; τ0: yield stress; ηpl: plastic viscosity; τCA: Casson yield value; ηCA:
Casson plastic viscosity; γ: shear rate; η: viscosity of the suspension; n: flow
viscosity index)
Models Equation
Herschel-Bulkley
Casson
Bingham
τ = τ0 + ηpl . (γ)n
√τ = √ τCA + √ηCA . √γ
τ = τ0 + ηpl . γ
Model equations for characterizing chocolate viscosity
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Casson’s model and ICA Recommendations:
Since 1973, the flow behaviour of molten chocolate has been evaluated using Casson’s model by extrapolation techniques.
In 2000, the ICA (IOCCC) recommended use of interpolation data.
The ICA recommendations:
i. the value of the stress at a shear rate of 5 s-1 be used to represent yield stress.
ii. the value of the viscosity at a shear rate of either 30 s-1, 40 s-1, or 50 s-1 could be used depending on product characteristics, but recommended the use of viscosity value at a shear rate of 40 s-1 to represent apparent viscosity due to it relative reproducibility.
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Limitations on Casson’s model:
i. Mathematical models uses only a small set of parameters, and are limited in accuracy as chocolate flow properties do not exactly fit the Casson equation.
ii. At lower shear rates, the rheology data generally do not well fit the Casson equation.
i. Low degree of repeatability in inter-laboratory analysis of chocolate viscosity.
Currently, the National Confectioners Association/Chocolate Manufacturers Association (NCA/CMA) use the Casson’s model whilst the ICA uses the new recommendations for evaluating chocolate viscosity; thus the need for further investigation, for the appropriate harmonization.
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Main research objective:
This work compares the efficiency and relationship between the two models in defining the rheological properties of dark chocolate systems.
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MATERIALS AND METHODS
Main Ingredients:
Cocoa liquor of Central West African Origin
Sucrose (pure cane extra fine granulated sugar)
Pure prime pressed cocoa butter,
Soy lecithin
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EXPERIMENTAL DESIGN:
A 4 x 3 x 2 factorial experimental design was used and the principal factors were:
i. Particle size (D90): 18, 25, 35 and 50 µmii. Fat content: 25, 30 and 35 %iii. Lecithin content: 0.3 and 0.5%
All other variables including refiner temperature and pressure, conching time and temperature, and cocoa butter (5%) were held constant.
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RECIPE FORMULATIONS:
Table 1. Recipes used for the formulation of the dark chocolate
Ingredient 25% Fat (% w/w) 30% Fat (% w/w) 35% Fat (% w/w)
Sucrose (%) 58.8 59.0 49.7 49.9 40.7 40.8
Cocoa liquor (%) 35.9 35.5 45.0 44.6 54.0 53.7
Cocoa butter (%) 5.0 5.0 5.0 5.0 5.0 5.0
Lecithin (%) 0.3 0.5 0.3 0.5 0.3 0.5
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CHOCOLATE MANUFACTURE:
Mixing of ingredients (Crypto Peerless Mixer K175)
Refining (Buhler 3-roll refiner)
Conching (Lipp Conche) at for 4 h at 60°C
Storage at ambient temperature (20-22 °C)
Samples were analysed for fat and moisture content to ensure they within the stipulated level.
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ANALYTICAL METHODS:
Particle size distribution (Malvern MasterSizer® Laser
Diffraction Particle Size Analyzer)
Fat content (ICA, 1990)
Moisture content (ICA, 1988)
Rheological properties Casson models ICA (2000) Recommendations
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RHEOLOGICAL PARAMETERS:
Casson plastic viscosity - Cason model Casson yield value - Cason model Yield stress - ICA Apparent viscosity - ICA Thixotropy - ICA
DATA ANALYSIS:
Statgraphics Plus 4.1 (Graphics Software System, STCC, Inc,
Rockville, USA)
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RESULTS
Particle size distribution of (A) 18 & 25 µm D90
Particle Diameter (µm.)
Vol
ume
(%)
0
10
0
10
20
30
40
50
60
70
80
90
100
0.1 1.0 10.0 100.0 1000.0
Particle Diameter (µm.)
V ol u m e( % )0
10
0
10
20
30
40
50
60
70
80
90
100
0.1 1.0 10.0 100.0 1000.0
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Particle size distribution of (C) 35 & (D) 50 µm D90
Particle Diameter (µm.)
0
10
0
10
20
30
40
50
60
70
80
90
100
0.1 1.0 10.0 100.0 1000.0
Vol
ume
(%)
Particle Diameter (µm.)
0
10
0
10
20
30
40
50
60
70
80
90
100
0.1 1.0 10.0 100.0 1000.0
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CASSON PLASTIC VISCOSITY:
2530
3518
2535
50
0
5
10
15
20
25
Ca
ss
on
pla
sti
c v
isc
os
ity
(P
as
)
Fat (%) Particle size (D90, µm)
18 25 35 500.5% Lec
0.3%Lec
0.3% Lec
0.5%Lec
0.3% Lec
0.5% Lec
0.3% Lec
0.5% Lec
Legend: Particle size (D90, µm) & Lecithin (%)
RESULTS
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CASSON YIELD VALUE:
2530
35 1825
3550
0
50
100
150
200
250
300
350
400
450
Cas
son
Yie
ld V
alu
e (P
a)
Fat (%) Particle size (D90, µm)
18 25 35 500.3% Lec
0.3% Lec
0.3% Lec
0.3% Lec
0.5% Lec
0.5% Lec
0.5% Lec
0.5% Lec
Legend: Particle size (D90, µm) & Lecithin (%)
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APPARENT VISCOSITY:
2530
35 1825
3550
0
10
20
30
40
50
60
70
Ap
par
ent
Vis
cosi
ty (
Pas
)
Fat (%) Particle size (D90, µm)
18 25 35 50
Legend: Particle size (D90, µm) & Lecithin (%)
0.3% Lec
0.3% Lec
0.3% Lec
0.3% Lec
0.5% Lec
0.5% Lec
0.5% Lec
0.5% Lec
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YIELD STRESS:
25 3035 18
2535
50
0
100
200
300
400
500
600
700
800
900
1000
Yie
ld S
tres
s (P
a)
Fat (%) Particle size (D90, µm)
18 25 35 50
Legend: Particle size (D90, µm) & Lecithin (%)
0.3% Lec
0.3% Lec
0.3% Lec
0.3% Lec
0.5% Lec
0.5% Lec
0.5% Lec
0.5% Lec
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THIXOTROPY:
0
100
200
300
400
500
600
700
Th
ixo
tro
py
(Pa)
25 30 35
Fat (%)
18 25 35 50
Legend: Particle size (D90, µm) & Lecithin (%)
0.3% Lec
0.3% Lec
0.3% Lec
0.3% Lec
0.5% Lec
0.5% Lec
0.5% Lec
0.5% Lec
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RELATIONSHIP BETWEEN CASSON MODELS AND ICA RECOMMENDATIONS:Table 6. Regression and correlation analyses
Parameter Analysis Casson plastic
viscosity
Casson yield value
Apparent
viscosity
Yield stress
Thixotropy
Casson plastic viscosity
Regression
- 0.8368* 0.9053* 0.8919*
0.9021*
Correlation
- 0.8903* 0.9467* 0.9349*
0.9447*
Casson yield value
Regression
- - 0.9582* 0.9694*
0.9665*
Correlation
- - 0.9786* 0.9844*
0.9823*
Apparent viscosity
Regression
- - - 0.9898*
0.9955*
Correlation
- - - 0.9941*
0.9977*
Yield stress Regression
- - - - 0.9939*
Correlation
- - - - 0.9957*
Thixotropy Regression
- - - - -
Correlation
- - - - -
* Significant at P< 0.05
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CASSON YIELD VALUE AND CASSON PLASTIC VISCOSITY:
Casson plastic viscosity (Pa.s)
Ca
sson
yie
ld v
alu
e (P
a)
0 4 8 12 16 20 24
0
100
200
300
400
500
R2 = 0.84
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CASSON PLASTIC VISCOSITY AND APPARENT VISCOSITY:
Apparent viscosity (Pa.s)
Ca
sso
n p
last
ic v
isco
sity
(P
a.s
)
0 20 40 60 80
0
4
8
12
16
20
24
R2 = 0.91
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CASSON YIELD VALUE AND YIELD STRESS:
R2 = 0.91
Yield stress (Pa)
Ca
sson
yie
ld v
alu
e (P
a)
0 200 400 600 800 10000
100
200
300
400
500
R2 = 0.97
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YIELD STRESS AND APPARENT VISCOSITY:
Apparent viscosity (Pa.s)
Yie
ld s
tres
s (P
a)
0 20 40 60 800
200
400
600
800
1000
R2 =0.99
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YIELD STRESS AND THIXITROPY:
Thixotropy (Pa)
Yie
ld s
tres
s (P
a)
0 20 40 60 80
0
200
400
600
800
1000
R2 =0.99
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APPARENT VISCOSITY AND THIXITROPY:
Thixotropy (Pa)
Ap
par
ent
visc
osit
y (P
a.s)
0 20 40 60 80
0
20
40
60
80
R2 =0.99
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PRINCIPAL COMPONENT ANALYSIS:
Biplot
Component 1
Co
mp
on
ent
2
Fat contentLecithin content
D(v,90)
Specific surface area
D[3,2]
D[4,3]D(v,50)
Casson plastic viscosity
Casson yield value
Yield stressApparent viscosity
Thixotropy
-9 -6 -3 0 3 6
-2.9
-0.9
1.1
3.1
5.1
PC1, PC2 and PC3 respectively account for 74.2%, 13.7% and 7.3% of the variability. PSD, fat content and lecithin content, together accounting for ~ 95% of the variability in the rheological data.
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CONCLUSIONS:
The Casson reference parameters (Casson yield value and Casson plastic viscosity) and the new recommendations by ICA recommendations (yield stress and apparent viscosity) for evaluating chocolate rheology are very closely related, and could be effectively used independently.
The ICA method is relatively more efficient than the Casson’s model but has some limitations as well.
Both rheological models are dependent on PSD, fat and lecithin, as key factors influencing chocolate rheology under controlled processing conditions.
For purposes of global harmonization, the use of Casson reference parameters could be maintained for routine (industrial) quality control purposes, while the new recommendations by ICA are used for research purposes where wide variations in component viscosity occur.
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ACKNOWLEDGEMENT
Government of Ghana
Nestle PTC (York)
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THANK YOUTHANK YOU