Computer simulation of sugar crystallization in confectionery products.pdf

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Computer simulation of sugar crystallization in confectionery products

Eyal Ben-Yoseph a,, Richard W. Hartel b,1

a 15 Hurshat Tal Street, Yokneam, Israelb Food Science Department, University of Wisconsin, 1605 Linden Drive, Madison, WI, USA

Received 11 May 2005; accepted 2 December 2005

Abstract

A computer simulation was developed to model drying, moisture sorption and crystallization, during processing and storage of confectionery

products. The three-dimensional model analyzed simultaneously heat and moisture transfer, accounting for shrinkage, with temperature and

moisture-dependent transport properties. Kinetics of crystal growth at different solution conditions were determined experimentally. Predicted

crystal growth rates were evaluated from crystallization kinetics based on supersaturation, temperature, and state of the solution. The influence of

growth on the solution concentration and drying rate were taken into account. The model predicted quality and storage stability based on

correlations with the calculated sugar concentration, crystallinity and phase state. Simulation results were in agreement with theory and

experimental results.

2006 Elsevier Ltd. All rights reserved.

Keywords: Sugar; Crystallization; Confectionery; Drying; Modeling

Industrial relevance: The goal of this work was to develop a model for quality and storage stability determination of confectionary products using crystallization of

sugar shell in hard panning process as example. The model developed can successfully predict the concentration profile and growth of sugar crystals in a thin film

and its industrial relevance is evident by the industrial financial support.

1. Introduction

Controlling crystallization of sugars in confectionery pro-

ducts is critical to development of their proper appearance,

texture and storage stability (Hartel, 2001). Sugar crystals in

the product may be desirable (e.g. sugar shell, fondant, toffee

and fudge), or undesirable (e.g. hard candy, jelly and caramel).

However, our understanding of the parameters that influence

those attributes is severely lacking, since heat transfer, masstransfer, and phase transition phenomena, which determine

crystallinity, are complex and interrelated. A computer

simulation can model these phenomena, and predict crystalli-

zation during the production process and storage of confec-

tionery products. The use of computer simulation can result in

faster and cheaper process development, help optimize existing

processes, and improve our understanding of the product

properties.

The objective of this work was to develop a model that

determines quality and storage stability of confectionery pro-

ducts. We chose to model the simplest confectionery system,

i.e., sucrose solution, which is applied as a thin film on a solid

surface as might be found in a panned confection. The quality

attribute modeled was the appearance of the coating, which wasdirectly related to the surface area coverage of crystallinity of

the coating. Storage stability was predicted based on the water

activity and glass transition temperature. The model in its

current configuration can be used to model crystallization of

sugar shell in Hard Panning process. For more complex

systems, the model needs to be modified. For example, to

model a system of sucrose, corn syrup and water (e.g. Hard

Candy) the governing equations need to be changed; namely for

diffusivity of sucrose and water, glass transition temperature

equation, water activity equation and sucrose crystallization

kinetics in the presence of corn syrup.

Innovative Food Science and Emerging Technologies 7 (2006) 225232

www.elsevier.com/locate/ifset

Corresponding author. Tel.: +972 4 993 7052; fax: +972 4 959 7631; mobile:

+972 5 378 2036.

E-mail addresses: [email protected] (E. Ben-Yoseph),

[email protected] (R.W. Hartel).1 Tel.: +1 608 263 1965; fax: +1 608 2626872.

1466-8564/$ - see front matter 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.ifset.2005.12.003
mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.ifset.2005.12.003http://dx.doi.org/10.1016/j.ifset.2005.12.003http://dx.doi.org/10.1016/j.ifset.2005.12.003mailto:[email protected]:[email protected]


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2. Theory/modeling

The modeling phase consisted of the following eight major

steps:

1. The temperature of the film as function of time was

calculated from the heat transfer equation.2. The sugar concentration as function of time and location was

calculated from the unsteady state mass transfer equation.

3. Crystal growth rate at each spatial point in the film was

predicted based on local supersaturation, film temperature,

and growth kinetics. Rate of crystal growth as function of

supersaturation and temperature was used from previous data

(Howell & Hartel, 2001)

4. Crystal size was calculated based on the crystal growth rate

at each point in space and time.

5. The influence of crystal growth on the film concentration and

temperature was evaluated.

6. Glass transition concentration was calculated from the filmtemperature, and local amorphous areas within the film were

defined.

At this point we were able to define three different regions in

the film: (a) crystalline region its amount, location, and

crystals number and size; (b) solution region amount, loca-

tion, and the concentration profile; and (c) amorphous region

its amount, location, concentration, and the glass transition

temperature. This information was used in the last 2 steps of

model development:

7. Crystal area coverage was predicted from the crystal size and

distribution in the film.8. Storage stability was predicted based on glass transition

temperature and water activity, which were calculated from

the sugar concentration of the solution phase and the amor-

phous phase.

The working steps are illustrated in Fig. 1.

2.1. Heat and mass transfer

The system of nonlinear partial differential equations for heat

transfer in time and for mass transfer in time and three dimen-

sions was solved numerically. The sucrose solution was sub-

divided into many small rectangular elements in order to apply

the finite difference method. A detailed description of the

methodology used to predict the temperature and concentration

in the film are found in Ben-Yoseph, Hartel and Howling

(2000). Many of the physical parameters within the film are

functions of local temperature and/or concentration. These

parameters were defined explicitly and are listed in Table 1.

2.2. Predicting the state of solution, and crystal growth rate,size, shape and distribution

The information about temperature and concentration at any

location in the solution during the drying process located each

point in the solution in the phase diagram; either it was an

undersaturated, metastable, labile or glassy solution. A region

was considered glassy if the temperature at that point was below

the glass transition temperature, Tg. Roos and Karel, 1991 sug-

gested a method to calculate Tg for sucrosewater by applying

the Gordon and Taylor equation:

Tg w1Tg1 kw2Tg2

w1 kw21

where w1 and w2 are weight fractions of component com-

pounds, Tg1 and Tg2 are the absolute glass transition

(8)(7)

(1)

(5)

(4,6)

(1)

(3)

(2)

Correlation of physicalproperties to solution conditions

Crystallization

kinetics

Crystal coverage

C (t,x,y,z)

S(t)Experiments ofcrystal growth(with Howell)

Growth

influence on

solution

Numerical solution

of Heat TransferEquation

Numerical solution

of Mass TransferEquation

T(t)

Processconditions

Crystals size, shape andlocation, glass transition

concentration

aw, Tg

Initialconditions

Fig. 1. Schematic illustration of the working steps, as described in this section. The numbers in parenthesis correspond to the steps described in Section 3.

226 E. Ben-Yoseph, R.W. Hartel / Innovative Food Science and Emerging Technologies 7 (2006) 225 232


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temperatures (K) of the component compounds, and k is

constant. In our case, Tg1=138 K (Tg of water), Tg2=335 K (Tgof pure sucrose), and kfor sucrosewater equal to 4.7 (Roos &

Karel, 1991).

Growth kinetics of sucrose crystals were studied in a

method described by Howell, Ben-Yoseph, Rao and Hartel

(2002), and the results were correlated to the solution

conditions using the following equation (Wey, 1981):

G 2DAB

rsdCCi 2

where is the thickness of the surface sublayer, s is the

density of sucrose crystal, C is the bulk concentration, Ci is

the solute concentration at crystalsolution interface, and

DAB is the mutual diffusion coefficients in watersucrose

system, which is a function of solution temperature and

concentration.

Eq. (2) includes two terms that depend on the solution

concentration. The diffusion coefficient, DAB, decreases withconcentration, while the supersaturation (CCi) increases

with concentration. The growth rate is zero when the

supersaturation equals zero or when the diffusivity equals

zero. Plotting the growth rate as function of concentration

gives a convex curve with a maximum growth rate at a

specific solution concentration.

During growth, when the volume of a seed reached the element

volume, growth continued in the adjacent elements (as long as

there were no crystals in these elements). Growth continued from

the center of the adjacent element (Fig. 2) with growth rateproportional to the surface concentration in this volume element.

One or more seeds can be placed at any point in the matrix.

2.3. Evaluation of the influence of crystal growth on the

temperature and concentration

Sugar crystals growing within the solution exert an influence

on the surrounding environment. The following phenomena

were taken into account in the simulation program:

1) Reduction in concentration: When a crystal grew, sucrose

molecules were removed from the solution and the con-centration around the crystal decreased. The change in

Table 1

Physical properties of water, sucrose, sucrose solution and air, as a function of temperature and/or concentration

Properties (source) Units Ranges and parameters units Correlation

Density of sucrose solution (Honig, 1953) g/mL 333373 K =[1.23517.52* T* 104+(2.74+0.002625* T) *

TS*103+(2.350.00175* T)*TS2* 105]*10660100% TS

Density of pure water (Geankoplis, 1983) g/m3 50100 C w=[1.00451.9582*104* T2.6589*106* T2]*106

Density of pure sucrose (Reiser, Birch,& Mathlouthi, 1995)

g/m3

C s=(1.5887

4.4285*105

* T)*106

Solubility of sucrose in water (Pancoast

& Junk, 1980)

% w/w

sucrose

090 C S=64.397+0.07251* T+

0.0020569* T29.035*106* T3

Vapor pressure of pure water (Perry & Chilton, 1973) mm Hg 323373 K Pw0 = 10 (12.73475940.0089/(230+T))

Relative vapor pressure of water above sucrose

solution (Norrish, 1966)

mm Hg 323373K Ps,w=[0.6710.3355*log10(TS)]*Pw0

0100% TS

Heat capacity of sucrose (Honig, 1953) cal/g/C 0100 C cs=0.2619+1.2858*103* T

Heat capacity of water (Geankoplis, 1983) cal/g/C 50100 C cw=1.00171.5754*104* T+2.1607*106* T2

Heat of fusion for sucrose (Pancoast & Junk, 1980) cal/g 65.693.3 C Hf=0.0551*TS+0.1988*T4.0902

5080% TS

Heat of vaporization of water (Felder & Rousseau, 1986) cal/g 10100 C rw=598.340.58456* T

Air density (Geankoplis, 1983) g/mL 0100 C g=1.2910*1034.4026*106* T+

9.4193*109* T2

Air viscosity (Geankoplis, 1983) g/cm/s 40120 C g=1.7355*104+4.4309*107* T

Air thermal conductivity (Geankoplis, 1983) cal/s/cm/K 0100 C =5.7889*105

+1.7840*107

* TDiffusivity of water vapor in air (McCabe, Smith,

& Harriot, 1985)

ft2/h p =1 atm Dv=3.22*105* T1.81

T=[K]

a) b) c)

Fig. 2. Illustration of the way growth was modeled in a discrete matrix of volume element (in the xzplane). (a) Seed is placed in a center of volume element. (b) When

the volume of the seed reaches the element volume, growth continued in the adjacent element. (c) Growth rates are function of the conditions in each volume element,therefore some faces grow faster than others. In this case the face in the top of crystal grew faster, therefore growth continued in adjacent elements around this face.

227E. Ben-Yoseph, R.W. Hartel / Innovative Food Science and Emerging Technologies 7 (2006) 225232


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concentration was calculated from a mass balance on sucrose

and water.

2) Generation of fusion heat: Crystal growth generated heat that

raised the candy temperature. A term was added to the heat

transfer equation, relative to the mass that was built on the

crystal.

3) Decrease in diffusivity: The volume of a crystal acted as a

barrier for diffusion of sucrose and water. To model this

influence, a mass transfer of zero was used between two

points when a crystal was present between them.

4) Decrease in drying rate: Crystals that grew on the surface

decreased the surface area for water migration from the

solution to the air. In order to take this into account, the

drying surface area was multiplied by one minus the ratio of

crystal cross sectional area to solution cross sectional area in

the direction of water flux.

2.4. Predicting the appearance

The overall abundance of sugar crystal coating was deter-

mined in a visual descriptive sensory analysis by 33 panelists.

The panelists were asked to indicate their assessment of the

appearance, where 1 = sugar crystal coating not apparent (absent,

coating looks clear as the solution is glassy) and 7=extensivesugar crystal coating (extremely abundant, coating appears

Fig. 3. Predicted and experimental growth rates (m/min) of sucrose crystals during drying of thin sucrose films on microscope slides in different drying temperatures

as function of air relative humidity (left chart) and initial film concentration (right chart). The lines represent the predicted results, and the marks with the error bars

the experimental results (data from Howell and Hartel, 2001).

8.3

41.7

75.

0

108.3

141.

7

175.0

8.2

57.6

107.0

156.4

88.088.589.089.590.090.591.091.592.092.593.0

93.594.094.595.095.596.096.597.097.598.098.599.099.5

100.0

X direction (m)Distance from

bottom (m)

%TS

Fig. 4. Concentrationfields (in % totalsolids) around growing crystals after 115 s

of drying of thin sugar film. Seed crystals were initially placed in 3 locations in

film. Sucrose solution conditions: initial concentration 83% TS, temperature

71.0 C. Film thickness 103.8

m. Drying air conditions: temperature 80 C,velocity 1.0 m/s, relative humidity 10%.

0 10 2

0 30 4

0 50 6

0 70 8

0 90

100

110

120

413

3048

6582

99

80

82

84

86

88

90

92

94

96

98

100

82-84 84-86 86-88 88-90

90-92 92-94 94-96 96-98

%T

Sofsucrose

Drying time (sec)

Distance in film

from bottom (m)

Fig. 5. Sucrose solution (TS) concentration vs. time and location during drying

of thin sugar film with initial dense layer of seed crystals 48 m from bottom.

Sucrose solution conditions: initial concentration 83% TS, temperature 71.0 C.

Film thickness 103.8

m. Drying air conditions: temperature 80C, velocity 1.0m/s, relative humidity 10%.



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white, as most sugar is crystallized). The crystal area coverage,

A, was correlated to the film crystallinity, x, to get Eq. (3):

A 3:8912x2 9:0155x 1 R2 0:9285 3

This correlation was combined into the model. Since the

model predicted the crystallinity of the coating, using Eq. (3) it

can determine the appearance of the coating.

3. Results

3.1. Simulating the effect of process conditions on crystal-

lization rate

The program was used to determine growth rate of sucrose in

thin films under various process conditions. Results were com-

pared to experimental work conducted by Howell and Hartel

(2001). In general, good agreement between predicted and

experimental results was achieved. Both experimental and sim-

ulated results showed the significant impact of temperature and

relative humidity on growth rate and the minor impact of air

velocity and initial film concentration (Fig. 3).

The growth rate increased substantially with air temperature.

Above 40 C volume diffusion governs sucrose crystal growth

rates and as the temperature increased, the diffusivity of sucrose

molecules was higher and the growth rate increased. On the

other hand, increasing the temperature decreased the supersat-

uration due to lower solubility, which would tend to inhibit

growth. However, in this case, temperature elevation also

increased the drying rate and the concentration in the film, so a

decrease in supersaturation was not obtained. Air velocity had

almost no effect on crystal growth because it had no effect on

drying rate (under the simulated conditions internal water

diffusion determined the drying rate) and had no effect on the

internal conditions in the film. Relative humidity of the airdirectly affected the film concentration. When the air was moist

(4050% RH) the film concentration was low (80%),

resulting in lower supersaturation and slower growth. When

the air was very dry, the film concentration became so high that

the decrease in mobility inhibited crystal growth. At the optimal

35% RH the film concentration was approximately 90%, which

gave maximum growth rate at 80 C. Initial concentration in the

range of 75% to 85% had only a minor effect on crystal growth.

Observing the concentration profiles in the film in the

simulation output gave an explanation: drying occurred so

Fig. 6. Simulation of temperature during drying of thin sucrose film of 104 m thickness with dense crystal seeding. Air temperature was 80 C, velocity of 1 m/s, and

relative humidity of 25%. Initial film concentration was 83% TS.

Table 2

Process conditions for intense drying of sugar-coated substrate (process A), and the simulation results

Stage Description Process conditions Results (end of stage)

Time

(min)

Air temperature

(C)

Air velocity

(m/s)

Air humidity

(g w/g da)

Sugar coating

phase state

Crystal area

coverage

Tg (C) aw

1 Transport to dryer 0.2 26 0.11 0.009 (40% RH) 99% solution (81% TS) 1.06 41.6 67%

1% crystalline

No glassy material

2 Drying 4.0 80 1.0 0.003 (1.5% RH) 44% solution (99% TS) 4.81 44.8 0.3%

56% crystalline

No glassy material

3 Cooling 2.0 21 1.0 0.004 (30% RH) 0% solution 4.97 29.7 5%

59% crystalline

41% glass at 96.4% TS



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rapidly that the solution approached high supersaturation veryquickly regardless of initial concentration.

3.2. Growth velocity of the crystals faces and concentration

fields

A simulation of growth of three 10 m seed crystals, that

were virtually placed in a sugar film, was performed in order to

follow the change of crystal size and shape and the

concentration fields. The crystal faces grew at rates proportional

to their respective surface concentration. At initial stages of

drying, when the film concentration was at its lowest, the crystal

faces that were closer to the air interface grew faster since the

concentration at the surface was higher. Later, when the filmconcentration was high and molecular mobility restricted

growth, the face closer to the bottom of the film grew faster

since the concentration at that surface was now lower. The

concentration below the crystals was low compared with other

regions in the same horizontal layers due to a combination of

growth desaturation and the crystals acting as a barrier for

moisture migration upward (Fig. 4).

Fig. 5 describes the concentration in a sucrose solution when

a dense layer of seeds was placed in a horizontal layer in the

center of the film. Simulation showed that the crystals grew in

all directions, forming a crystalline layer between two liquid

layers. Initially, the concentration at the bottom of the filmincreased because of the drying; but after 40 s, once a closed

solid layer of crystals had formed, the concentration near the

bottom of the film decreased because the solid layer blocked the

water movement upward and sugar was removed from solution

during growth.

3.3. Appearance of coating

For a given seed density and location, the drying conditions

had the same effect on the crystal area coverage as on growth

rate (Section 4.1). An initial high density of seeds in the film

led to more crystal growth and resulted in more crystalline

material. Therefore, dense seeding is an advantage in order to

get maximum crystalline appearance. The simulation resultsshowed that the best location for seeds, in order to maximize

crystal growth, was at the bottom of the film. During drying of

thin films, the film concentration closer to the surface

increased above the optimal concentration for growth and

inhibited crystallization. At the bottom of the film, the

concentration was the lowest and was closer to the optimal

concentration for growth. Therefore, the growth rate was faster

at the bottom. Also, when seeds are located at the bottom of the

film, a larger layer of crystalline material that inhibits further

drying of the film was not formed.

3.4. Temperature of the sugar film

Four factors affect the temperature of the sugar film during

drying: the amount of heat absorbed from the air, the amount of

latent heat released by water evaporation, the amount of heat

that is generated by the growing crystals, and the thermal

properties of the sugar solution. Fig. 6 shows the temperature

change during simulation of drying and dense film seeding. An

initial decrease of 4 C occurred because of evaporative cooling.

After 20 s, the film temperature increased mostly by the heat

absorbed from the drying air and partly by the heat generated

during growth. After 70 s of drying, the temperature of the film

increased above the drying temperature (due to the release of

latent heat) and reached

81 C.

Table 3

Process conditions for optimized drying of sugar-coated substrate (process B) and the simulation results

Stage Description Process conditions Results (end of stage)

Time

(min)

Air temperature

(C)

Air velocity

(m/s)

Air humidity

(g w/g da)

Sugar coating phase

state (end of stage)

Crystal area

coverage

Tg(C)

aw

1 Transport to dryer 0.2 26 0.11 0.009 (40% RH) 99% solution (81% TS) 1.06 41.6 66%

1% crystallineNo glassy material

2 Intense short drying 1.0 80 1.0 0.003 (1.5% RH) 91% solution (95% TS) 1.78 24.9 9.2%

9% crystalline

No glassy material

Mild long drying 3.0 80 1.0 0.046 (15% RH) 3% solution (94% TS) 6.09 17.7 14%

97% crystalline

No glassy material

3 Cooling 2.0 26 1.0 0.08 (30% RH) 1% solution (95% TS) 6.12 23.6 10%

99% crystalline

No glassy material

Table 4

Conditions for simulating storage of Cotton candy

Conditions Exposure to medium

air humidity

Exposure to high

temperature

Initial candy temperature 21 C 21 C

Initial candy floss thickness 1 mm 1 mm

Initial sugar concentration 99.9% TS 99.9% TS

Storage air temperature 21 C 82 C

Storage air humidity 45% RH 0% RH

Storage air velocity 0.1 m/s 0.1 m/s

In both cases we assumed that the air is moving past the candy at a speed of0.1 m/s.



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3.5. Prediction of crystal area coverage and storage stability of

sugar-coated substrate

Two drying processes of sugar-coated substrate at 80 C

were simulated. Both processes started with applying warm

aqueous sugar solution over substrate in a thin film and seeding.The main difference between the processes was that process (A)

included intense drying, while process (B) included drying in 2

steps, optimized to achieve highest crystal growth rate.

Drying conditions for process B were chosen in the following

way: Based on Eq. (2), maximum growth rate at 80 C is

achieved at 94%. Process (B) included step of fast drying to

reach this concentration, and a longer step of mild drying to

maintain this concentration, while the crystals grew. The 94%

concentration was controlled by the humidity of the drying air.

The Norrish equation (Norrish, 1966) gives the equilibrium film

concentration for a given relative humidity of the surrounding

air. To achieve a concentration of 94%, the relative humidity ofthe drying air was adjusted to 15.4%.

The process conditions and simulation results are described

in Tables 2 and 3. While conveying the product at room

temperature (step 1), the films were cooled and the seed crystals

started to grow. The drying duration was 4 min in both

processes (step 2), but process B gave higher crystallinity (97%

compared to 56%), and higher crystal coverage area because of

its optimal conditions for growth. In addition the film in process

A reached higher concentration (98.8% TS), which correspond

to high Tg (44.8 C). When drying process ended, film A cooled

below this temperature, it became glassy and no more crystal

growth occurred. Film B had lower concentration, lower Tg, anddid not become glassy. At the end of the process (step 3), film B

was in a more stable condition for storage: 99% was

crystallized, compared to film A where only 59% was

crystallized, and the remaining solution had aw of 5% and Tgof 29.7 C. With this low water activity, film A would probably

pick up moisture from the air and become sticky. Even if it was

wrapped with a moisture barrier, at temperatures above 29.7 C

its glassy structure would collapse and it would start to

crystallize.

3.6. Simulation of cotton candy storage

Cotton candy is an amorphous sugar that is very prone tomoisture sorption and high storage temperatures. To make cotton

candy, sugar is heated to a point above the melting temperature

(190 C) in a spinning head. The melted sugar is forced through

tiny holes into a bowl that catches the candy floss. The fast

cooling of the floss ensures that the sugar is amorphous.

0

20

40

60

80

100

120

2163

146 229313

396479

82

84

86

88

90

92

94

96

98

100

Duration

(min.)

Distance in thread

from center (m)

Center

Surface

%T

Sofsucrose

Fig. 7. Predicted sugar concentration profile in a Cotton candy exposed to air

with 45% relative humidity. Initial sugar concentration was 99.9%.

0

10

20

30

40

50

60

70

80

60 65 70 75 80 85 90 95 100

Sucrose Concentration (% Total Solids)

Temperature(C)

Glass

Supersaturated

Solution

Unsaturated

Solution

Glass

Transition

Curve

Solubility

Curve

1

2

3

5

10

20

60

120240

Fig. 8. Phase diagram for watersucrose system. The dotted line on the chart represents the predicted average sugar concentration of a Cotton candy exposed to

medium air humidity. The dashed line represents the predicted temperature change of a Cotton candy exposed to high temperature. The numbers on the line representthe time duration in minutes to get to that point.



8/8

Simulations of cotton candy storage were done under 2 en-

vironmental conditions: exposure of candy to medium air hu-

midity (45%) at room temperature, and exposure of candy to

high temperature (82 C) without moisture transfer. The simu-

lation conditions are detailed in Table 4.

3.6.1. Exposure to medium air humidityFig. 7 shows the reduction in sugar concentration when cotton

candy was exposed to air at 45% RH. The surface of the amor-

phous sugar absorbed water rapidly, until reaching the equi-

librium concentration of 88% TS. The sugar and water mobility

increased, and water penetrated toward the center of the floss.

After 4 h, the entire floss was at concentration below 90% TS.

Fig. 8 shows the sucrosewater phase diagram, and the dashed

line represents the change in the concentration of the candy.

When the concentration of the candy decreased to below 94.6%

(Cg at 21 C) the sugar started to crystallize. This happened after

approximately 70 min. Simulation showed that after 2 h the

majority of the floss had crystallized and the amorphous struc-ture collapsed.

3.6.2. Exposure to high air temperature

After exposure of the floss to high temperature, it took

approximately 20 min for the candy to reach 82 C (Fig. 8, dotted

line). However, the candy reached the glass transition point of

62 C quickly, to become a sugar melt. As the mobility of the

sugar molecules increased with viscosity increase, the sugar

started to crystallize. The crystal growth rate at 82 C is predicted

by Eq. (2) approximately 90 m/min.

4. Summary

A model to predict the concentration profile and growth of

sugar crystals in a thin film has been developed and utilized to

study various conditions of sucrose crystal growth. The model

solves the unsteady state mass transfer equation coupled with

an appropriate growth kinetic model to predict crystal size

during drying of a thin film of sucrose with seed crystals

imbedded in the film. By coupling this model with sensory

measurements of film appearance based on area of crystal

coverage, the crystal area coverage, as related to sensory

appearance, can be modeled for different operating and storage

conditions.

Acknowledgments

Financial support for this study was provided by Kellogg's

Company. The authors would like to express their gratitudeto Mr. David Howlling and Mr. Terry Howell for their

assistance.

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