Indian Journal of Chemical Technology Vol. 10, July 2003, pp. 386-390

Articles

Kinetic study of crystal colouration during crystallization of sucrose

Kaman Singh* & Sudhanshu Mohan

Department of Physical Chemistry, National Sugar Institute, Mini stry of Food & Consumer Affairs, Kanpur 208 017, India

Received 27 August 2001; revised received 21 April2003; accepted 20 May 2003

Although, existence of the coloured syrup layer surrounding the crystal surface is fail accompli, there is no definite explanation for the colour inside the crystal. In the present investigation, therefore, whether the colour is adsorbed or occluded or both, three types of artificial colouring matters; c:aramel, iron-phenol complex and reducing sugar-amino acid browning reaction products are used to examine the effect of each on crystal colouration. By static and dynamic analysis, it is concluded that different colourants have different affinities for the crystals and cause different colouration under the same condition; viz. caramel is the slowest, RS-AA browning product is intermediate and the iron-phenol complex is the fastest. At a slow rate of the crystallization (k=0.86-6.05xl05 mg/m2/h), colouration is caused mainly by adsorption. However, when the rate increases, this may also be accompanied by another mechanism such as occlusion, although adsorption is still in effect. The quantity of colouration decreases with rise in crystallization temperature, the extent of which decreases with increase in super-saturation. By reducing the amount of these colourants of higher affinity which are present in the ·mother liquor and under varying crystallization conditions, different effective measures may be adopted to overcome the colouration. Thermodynamic data of the crystal colouration are also evaluated and data indicate a loss i111 the entropy change in the process.

The phenomenon of crystal colouration is commonly referred to in sugar literature as caramelization which affects quality of commercial sugar and is the main cause of its deterioration and therefore, this problem has attracted the attention of all the concerned. Investigation on sugar colour has revealed that the sugar products and process materials contain many natural colourants as well as colourants formed in the manufacturing process. The former include delphinidine, chlorophyll, xanthophyll, carotene, tannin and so on, while the latter include caramel, formation of iron-phenol complex, reducing sugar­amino acid browning reaction products, slow alkaline caramelization, formation of melanodines, and excessive moisture adsorption by the crystal, etc. These substances contain chromophore groups and are also rich in auxochromes so, when present in greater quantities, the easier it is for colour to develop and hence deterioration of sugar crystal.

Although, the existence of coloured layer surrounding the crystal surface is a fait accompli, there is no definite explanation for the existence of

*For correspondence (Department of Chemistry, Government Postgraduate Leading College, Tikamgarh 472 001 , India E-mail : drkamansingh @yahoo.com)

colour · inside the sugar crystal. However, three theories have been proposed; adsorption, occlusion and co-crystallization. It has been reported 1-

2 that major concentration of colourants is to be contained on the b-axis of sugar crystal (Fig. 1) where approxi ­mately 25% of the colourants are adsorbed on the surface of the sugar crystal. They are also found in the region of crystal defects. The sucrose crystallizat ion process takes place by layer upon-layer deposition at

- b

+c

00)

101 /

//a loo /

-c

/ /

HO

/

/ /

, /

/

+ b

Fig. l-5ucrose crystal (VanHook, Chapmann & Hall London. 1961 ).

Singh & Mohan: Kinetic study of crystal colouration during crystallization of sucrose Articles

the crystal surface and as such the crystal molecules move from the bulk of the massecuite for deposition at different sugar crystal faces . Impurities affect such movements and hence the crystal growth and quality. Freundlich3 suggested that the influence of the impurities may be ascribed to the formation of an adsorbed layer which impeded the otherwise normal crystallization. At a given temperature the rate of crystal growth may be expressed4 as,

dw/dt=As/ K,

where w=Weight of the solid deposited, t=time, s=saturation coefficients and A=crystal surface area. K=overall mass transfer coefficients and as

!IK=liKct +liKr

where K1=Surface resistance and Kct=diffusional resistance.

In a diffusion-controlled process the overall mass transfer coefficient becomes Kct which is a function of parameter, such as diffusibility, viscosity, solution density, relative velocity, etc. The viscosity changes with the nature and the amount of impurities and the crystal growth thus depends upon the impurities present in the massecuite, and these impurities are responsible for the final colour of the sugar crystal5

.

The relationship among the quantities causing adsorption follows Freundlich adsorption whereby, at a particular temperature the quantity adsorbed by the adsorbent increases with the increasing concentration of adsorbate. The adsorption is exothermic, so the quantity adsorbed is in inverse proportion to the temperature.

Mother liql.lor (syrup) embodied in the crystal is another source of crystal coloration. Investigation on syrup occlusion6 is closely governed by the conditions of crystallization, differing markedly in several conditions. It appears usually at higher rates of crystallization but hardly occurs when the rate of crystallization is slow. Clearly occlusion only relates to the physical aspects of crystallization and has nothing to do with colorant itself. If the rate of crystallization is fixed, any colorant would have the same, or similar degree of occlusion with no selectivity as to the colorant type.

Co-crystallization could also colour the crystal however, it is qualified by several limitation, generally hold that co-crystallization takes place only when colourants and sugar crystal have the same

approximate structure, There is lack of litera ture referring to co-crystallization.

Experimental Procedure In the present work, therefore, whether the colour is

adsorbed or occluded or both, three artificial colorants, viz, caramel, iron-phenol complex and reducing sugar amino acid (RS-AA) browning reaction products were employed to examine the role of each on crystal colour. Due to lack of literature on co-crystallization and inadequate experiments, the present investigation has been limited to adsorption and occlusion. It has been 7-

9 shown that the crystallization process involves the induction period which is closely associated with super-saturation coefficients and the grain size of the crystal. Experimental conditions were, therefore, worked out to eliminate the occurrence of induction period. In solution where super-saturation index was in between 1.02-1.15, no induction period was observed. Calculated quantity of sucrose was weighed and dissolved in water. Employing crystallization temperature of 50°C, super-saturation coefficients of 1.15 and crystallization-in-motion during 5 h, crystal colouration was studied. Absorbance of the solution was noted employing a spectrophotometer (UV -260). The relationship between colouration and crystalli­zation conditions were also investigated. The iron­phenol browning reaction products was chosen as colour source, since it has the highest affinit/ 0

-12 for

sugar crystal of the three types of colourants used in this investigation. Rate of crystallization was altered by varying the crystallization temperature (35, 45 and 55°C) and super-saturation index ( 1.02, 1.05 and 1.08) but with a fixed crystallization duration ( 15 h). Crystal colouration is expressed as the relative absorbancy index of unit incremental crystal. i.e. a 2

where a 1=absorbancy index of mother liquor. a2=absorbancy index of incremental crystal. and ~w=the gained weight of the crystal.

Results and Discussion The results of the role of the three types of artifici al

colourants referred on crystal colouration are shown in Tables 1-3. Data shown in Fig. 2 illustrate that under the same crystallization conditions, the degree of colouration caused by three type of colourants increases with the amount of colorant in the mother liquor and rate of colouration varies with different colorants; viz, caramel is the slowest, RS-AA browning products is intermediate and iron-phenol

Articles

Table !-Effect of caramel on crystal colouration

Absorbancy index of Absorbancy index of mother liquor incremental crystal

(a,) (az)

0.0715 0.0118 0.1532 0.0216 0.2166 0.0285 0.3238 0.0283 0.3866 0.0363 0.4811 0.0348

Table 2-Effect of reducing sugar-amino acid (R.S. A.A.) crystal colouration

Absorbancy index of Absorbancy index of

mother liquor incremental crystal

(a,) (az)

0.0766 0.0124 0.1344 0.0192 0.1867 0.0266 0.2656 0.0376 0.3282 0.0424

0.3866 0.0468

Table 3--Effect of iron-phenol complex on crystal colouration

Absorbancy index of Absorbancy index of

mother liquor incremental crystal

(a,) (az)

0.0442 0.0933

0.0522 0.0124

0.0666 0.0139

0.718 0.0138

0.719

0.0861 0.0146

0.0101 0.0155

complex is the fastest. Similar results have also been reported by Chen and Chang6 using RS-AA is the main colour source. The earlier10 investigations give confidence to this finding in the sense that the crystal colouration does not seem to be slow caramelization8

catalyzed by alkali impurities, rather it seems more likely a fast caramelization IO-IJ catalyzed by iron compounds naturally present in sucrose or compounds formed during processing of the sugar. This phenomenon demonstrates that the sugar crystal possesses different affinity for different colorants. Since degree of colouration of the sugar crystal increases with the amount of colorant in mother liquor and also the nature of the colorant, it can be

388

Indian J. Chern. Techno!. , July 2003

C• c. ~

N ~

.... • " ~ , ~ 0

.... RS... A.A browning~· reacti on pro,J uct.

~ c ~ ~ ~ ~ 3 0 Caramel c ·~

... 0

iron-phenol complex

Absorbancy of mother liqueur ( a 1xl 0 )

Fig. 2--Plots showing the effect of artificial colourant on sugar crystal colouration.

6

, <I

' .... "'

a:= 1.02

' 4 N

"' c 0 .,., ... 3 "' ... 0 .... a:~ 1.05 ~ 0 u ... 2 0

... c

" 0 (;

l <

35 45 55

Temperature , cc

Fig. 3--Piots showing the crystal colouration under different experimental conditions during crystallisation of sucrose due to iron-phenol complex.

concluded that under the present experimental conditions.. crystal colouration is due to or at least mainly caused by adsorption, because this fact is consistent with the relevant theory of adsorption.

The influence of crystallization temperature and super-saturation coefficient on crystal colouration are shown in Table 4 . Data shown in Fig. 3 demonstrates

Singh & Mohan: Kinetic study of crystal colouration during crystallization of sucrose Ar ticles

Table 4----Effect of crystallization condition crystal colouration

Super-saturation index

(a) 1.02

35°C 45°C 55°C 35°C

4.46 2.38 1.96 2.68

4.63 2.86 2.48 3.24

4.18 4.88 1.64 3.42

5.65 2.88 2.67 2.68

5.46 3.46 3.11 3.42

Aveg. 4.88 3.29 2.37 3.09

:. <l

::::: .-<

"' ' J" -c 3 0

·--< .., "' ... 0

.-< 0 2 0

"-' 0 .., c

l IV .., X

"'

2 6 8 Rate of crystalliza tion (kXl~mg/m2/hr)

Fig. 4-Piot illustrating the effect of rate of crystallization on crystal colouration due to iron-phenol complex.

that the quantity of colouration decreases with rise in crystallization temperature, the extent of which decrease with the increase in super-saturation. The quantity of colouration decreases with the increase in the super-saturation, the extent of the fall decrease with increase in the temperature. The influence of rate of crystallization on crystal colouration as shown in Fig. 4 illustrates that the quantity of colouration decreases with increase in rate of crystallization, i.e. when the crystallization rate (k) is lower than 3.14x105 mg/m2/h, it drops rapidly; when higher than 3.14x 105 mg/m2/h , it drops slowly (Table 5).

From the above facts it is revealed that under the present experimental conditions, colouration of sugar crystal is due to or mainly caused by adsorption . If the rate of crystallization is slow, colouration is due to

1.05 1.08

45°C 55°C 35°C 45°C 55°C

1.62 0.98 1.36 0.94 0.62

1.40 0.77 1.44 1.05 0.52

1.18 0.58 1.27 1.01 0.48

1.56 0.92 1.33 0.72 0.83

1.31 0.74 1.42 0.58 0.58

1.42 0.80 1.36 0.86 0.61

Table 5--Rate of crystallization under different experimental conditions (kx I 05 mg/m2/h)

Temperature Supersaturation index (a)

1.02 1.05 1.08

35°C 0.86 1.55 3.26

45°C 1.82 3. 14 4.44

55°C 3.02 4.11 5.84

adsorption; however, at higher rates it may he accompanied by other mechanisms of coloration. although adsorption is still in effect. According to the relevant theory of adsorption, the quantity of colouration decreases with the ri se of temperature . There is no such relation in the other colouration mechanism; in the case of occlusion, the quantity of colouration would decrease with the ri se in temperature owing to higher rate of crystallization. If the rise of temperature or super-saturation causes the rate of crystallization to rise, then another mechanism of colouration such as occlusion may accompany adsorption. As the quantity of adsorption caused by the higher temperature is partly offset, the degree of adsorption will drop. Consequently, the degree of adsorption decreases with the rise of temperature of the super-saturation; i.e. at higher rates of crystallization.

Thermodynamic data of crystal colouration The Arrhenius plots for· the rate of crystal

colouration under different experimental conditions has been shown in Fig. 5 and the plots gave reasonable straight lines with activation energies of 52.5, 40.2 and 24.8 k J mor' at super-saturation index of 1.02, 1.05 and 1.08, respectively . The thermodynamic data obtained are given in Table 6. The entropy of activation of system is calculated employing Eyring equation and data shown in Table 6 indicate a loss in the entropy change.

389

Articles Indian J. Chem. Techno!., Jul y 2003

Table 6-Thermodynamic data of crystal colouration

8 . 0

7 .0

-~ .c

' N E 6 . 0 ' "' E

~

"' "' 5 . 0 ~

"' 0 .., 4. 0

Temperature

4

3 . 0 4

308 K

318 K

328 K

3.14

1 0 00 / T ( K)

kx l05

(mg/m2/h)

0.86

1.82

5.48

0 1.02

cr • 1.o5

d 1.08

3. 24

Fig. 5--Arrhenius plots for the crysta l colouration under different experimental conditions during crystallization of sucrose due to iron-phenol complex.

Conclusion Static and dynamic analysis of crystal colouration

during crystallization of sucrose leads to the following important conclusions:

(i) At slow rate of crystallization (k=0.86-6.05 x l05

mg/m2/hour) the colouration is mainly caused by adsorption. However, at higher rates it may be accompanied by other mechanism of coloration, although adsorption is still in effect.

(i i) The amount of colouration increases with increase of rate of crystallization .

(i ii ) Different colourants have different affinities for the crystal and causes different colouration under the same experimental conditions; i.e.

390

llG !lH /lS (kJ mor 1

) (kJ mor' ) (J mor 1K' 1)

12.62 49.97 -6 1.49

13.9 1 37.55 -10.53

14.95 22.27 -3 1.38

caramel is the slowest, RS-AA browning products is intermediate and the iron-pheno l complex is the fastest.

(iv) By reducing the amount of these colourants of higher affinity which are present in mother liquor and employing varying crystallization conditions crystal colouration may be overcome.

(v) Entropy of activation for the crystal colouration indicates a loss in the entropy change of the system.

Acknowledgement The authors are indebted to Professor Mahendra

Prasad, Diirector, National Sugar Institute Kanpur, for his invaluable help. Financial assistance from the Ministry of Food, Government of India is great ly acknowledged.

References I VanHook A. Crystallization Theory and Practice Chapmann

& Hall London) 1961 ; Zuckerind, 11 3 ( 1988) 591: ln t Sugar 1, 82 (1980) 33 1; 91 (1989) 220.

2 Sanyal P, Gupta A K & Shukla N P. fnt Sugar 1, 94 ( 1992) 150.

3 Dutton D, Colloid and Capillary Chemistry (Wiley, ew York) , 1922, 155.

4 Parry C, Chemical Engineering Handbook, 5'h edn . (McGraiHill, Tokyo) 1975, 10, 174.

5 Noyes A, 1 Am Chem Soc, 19 (1987) 930; Z Phys Chem, 23 (1987) 689.

6 Chen W J & Chang W Y , lnt Sugar 1. 94 (1989) 115. 7 Chen C P, Cane Sugar Handbook (Wiley. New York), 1985. 8 Ramaiah N A, 33'd Proc Sugar Assoc. India, 1965. 233. 9 VanHook A, lnt Soc Sugarcane Tech, ( 1959) 306.

I 0 Singh K, 1 Indian Chem Soc, 76 ( 1999) 49, 231 . II Singh K & Shukla A K, Taiwan Sugar, 49 (2) (2002) 8. 12 Prasad M, Jain P K & Nigam G D, Int Sugar 1, 9 1 (1 989)

222. 13 Riffer R, Proc Conf Sugar Processing Res, 50 ( 1984) 23 1.