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

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.

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

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)

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

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.

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.

Main research objective:

This work compares the efficiency and relationship between the two models in defining the rheological properties of dark chocolate systems.

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

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.

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

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.

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

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)

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

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

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

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 (%)

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

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

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

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

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

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

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

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

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

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

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.

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.

ACKNOWLEDGEMENT

Government of Ghana

Nestle PTC (York)

THANK YOUTHANK YOU