Onna Colour and Related Factors in the Raw Sugar Crysta

COLOUR AND RELATED FACTORS I N THE RAW SUGAR CRYSTAL*
C. C. Tu, KENNETH
ONNA,RICHARD
OKAMOTO
and
JOHN
H. PAYNE
Experiment Station, Hawaiian Sugar Planters' Association, Honolzrlu, Hawaii, U.S.A.
(Presented by Dr. J. H. Payne)
INTRODUCTION
A major refining quality factor for raw sugar is the colour of the crystal from which
the molasses layer has been removed. In the Hawaiian sugar industry the standard
analytical procedure for Crystal Colour involves measurement in a spectrophotometer of the transmittancy of a solution of the crystal to light of wavelength 420 mp.
The value thus obtained is a measure not only of light absorbancy by coloured compounds but also of light scattering by substances which may not be coloured.
A study was made of the relative effects of absorbancy and light scattering. The
effects of pH and ultracentrifugation on the crystal-colour measurements were also
investigated. The relationship between the internal moisture content of the crystal
and Crystal Colour is discussed on the basis of the inclusion theory. Finally, a correlation between light scattering and filterability was established.
EXPERIMENTAL
Determi~tatioaof Crystal Coloztr
The molasses layer was removed from the crystal by washing successively with
a saturated granulated sugar syrup, methanol, and iso-propanol. After drying under
vacuum, a 50 per cent solution of the crystal was made using distilled water. The
solution was filtered (stantlard diatomaceous earth), adjusted to pH 7.0, and Crystal
* Published as Paper No.
Sugar Planters' Association.
102
in the Journal Series of the Experiment Station, Hawaiian
I
C. C.
TU,
KENNETH
ONNA, RICHARD OKAMOTO, JOHN
H. PAYNE
9 s
Colour values determined using a 'Beckman DK-2 Ratio Spectrophotometer'. Crystal
Colour is defined as a* x 10 where a*2 is the Attenuation Index at 4zompwavelength defined as:
n*
I
- log T (420 mp)
bc
where T is the transinittancy,
b is cell length, and
c is concentration in grams per nlillilitre.
Effect of @H on Crystal Colour
Samples of sugar crystal solutions prepared for Crystal Colour determination
were adjusted to a pH of 9.0 using 0.05 N sodium hydro$ide solution and 4.0 using
0.05 N sulphuric acid solution. Crystal Colour values were determined as in the standard procedure.
Determination of Light Scattering
The same solutions on which Crystal Colour analyses were made were used for
light-scattering measurements. An 'American Instrument Company Light Scattering
Photometer' was used.
have been
The definitions and terminology used by RIEGERand CARPENTERS
adopted. The Scattering Index s is defined as:
s =
z
2.303 X
C
where z is turbidity and
c is solids concentration in grams per millilitre.
Turbidity is defined as a measure of all light scattered in all directions as:
z = zn
S:
Re sin 0 dB
where 0 is the angle of observations, and
R, is the Rayleigh ratio
i,~%
-
v
10
I
where r is the distance between the scattering volume V and the observer, i, is the
intensity of scattered light, and I, is the intensity of the incident light.
The Rayleigh ratio is the ratio of scattered light to incident light at the angle of
observation and is the value measured in the light-scattering instrument.
Since the instrument was equipped with a 436 mp filter, all measurements were
.made at this wavelength and at go0.
Attenuation, Absorbancy, and Light Scattering
The relationship between Attenuation Index, Absorption Index, and Scattering
Index is assumed6 to be represented by:
a* = a + s
where a* = Attenuation Index, a = Absorption Index, and s = Scattering Index.
By subtracting light scattering from attenuation, n value for absorption results.
This is an approach to a measure of the light absorption by the coloured compounds
present. Light scattering, however, is dependent upon the angle of observation and,
commercial-sugar solutions show a
as pointed out by RIEGERand CARPENTER&,
'predominantly forward scattering envelope' which is not Rayleigh scattering. The
sizes, colours, and refractive indices of the particles all play a role in determining
the scattering pattern. Nevertheless measurements of scattering under standard
conditions and definite angles are useful in characterizing the nature of attenuation
values.
Ultracentrifugation
Samples of sugar crystal solutions on which Crystal Colour,had been measured
were subjected to an average force of 104,000 times gravity in an ultracentrifuge.
The machine used was a 'Spinco Model L' which is capable of speeds up to 40,000
r.p.m. Time of centrifuging was two hours.
After spinning, Crystal Colour measurements were again made.
I
Internal Crystal Moistztre
Moisture within the sugar crystal was determined by the method of HILLand
DOBBS~.
This method is based upon measurement of the vapour pressure of the water
released by grinding the sample in a vacuum in a s y ~ t e mof known volunle.
Recovery of known weights of water liberated when capillary vials were broken
in the system proved the procedure reliable as shown below:
Water
introduced
(mg)
4.0
4.2
8.1
24.4
Water
recovered
(%I
107
104
99
99
I
Four refined sugars of different grain sizes were analyzed by this method without
any pre-treatment : 35-80 mesh, 20-28 mesh, 10-20 mesh, and retained on 10 mesh.
Raw sugars were washed first with granulated-sugar syrup, then with sugarsaturated dimethylformamide, followed by methanol and ether before making the
determination.
Laboratory Crysta1Zizatio.n
Factory syrup was centrifuged and granulated sugar was dissolved in the supernatant liquor at 80-85" C without addition of water. The solution was cooled to 60" C
(refractometer solids of supersaturated solution/refractometer solids of saturated
solution, 1.05 rjc 0.02) and seeded. Crystallization was conducted in a closed-system
apparatus by reducing the temperature to ambient in 48 hours. Internal moisture,
corrected for seed moisture, was determined and Attenuation Index measured.
A weight of molasses was taken so as to contain 1.000 g water; 49 g of distilled
water were added. Granulated sugar was added to bring the solution to 50 refractometer solids. Attenuation Index corrected for the granulated background was
measured.
To study the colour change of the mother liquor during closed-system crystalliza-
C. C. TU, K E N N E T H ONNA, RICHARD OKAMOTO, JOHN H. PAYNE
953
tion, moisture and Attenuation analyses were made before and after crystallization
in one test.
Recrystallization from a solution by dissolving raw sugar was conducted in a
manner similar to that used for factory syrups: 205 g of washed raw crystals were
dissolved in 120 g water; 185 g granulated sugar were dissolved in the solution a t
elevated temperatures without further addition of water. Attenuation Index of the
original crystal and the recrystallized crystals was measured. The internal moisture
of the latter corrected for seed moisture was also determined.
TABLE I
EFFECT
Sugar
pH
OF
pH
ON CRYSTAL COLOUR
Crystal colour
(a* 420 x 10)
S~gar
--
Kelraha
Hutchinson
Ololrele
Haw. 4g.
McBryde
Puna
Grove Farm
Onomca
Lihue
Kilauea
9.0
4.0
7.0
9.0
20.7
4.9
6.6
15.4
4.0
7.0
9.0
4.0
1.9
3.3
9.8
2.6
7.0
9.0
4.0
7.0
9.0
4.1
13.1
3.3
5 .o
16.1
4.0
7.O
9.0
4.0
7.O
9.0
4.0
7.0
9.0
4.0
7.0
9.0
2.1
3.8
16.0
1.8
3.3
11.8
2.7
4.5
12.6
Pepeekeo
Halralau
Oahu
Ewa
Papaaloa
Waialua
Kahuku
Puunene
Paia
Wailulru
Pioneer
.
2.0
3.7
13.1
Hamalrua
Paauhau
Honolcaa
Icohala
fiH
Crystal colour
(a* 420 x 10)
C . C. TU, K E N N E T H ONNA, RICHARD OICAMOTO, JOHN H. P A Y N E
955
RESULTS
Effect of $H
0% Cvystal
CO~OUY
Colour measurements at pH 4.0, 7.0, and 9.0 on samples of sugars from all Hawaiian factories are shown in Table I. In order to establish the form of the curve,
measurements were made at several other points for two sugars. The resulting curves
are plotted in Fig. I.
Colour and Light Scatte~ing
I
I
Colour and light-scattering measurements at go0 on sugar crystal from several
factories are given in Table 11.
Effect of Ultracentrifugation o n Crystal Co1o.u~
Results of Crystal Colour measurements before and after ultracentrifugation
for samples from all Hawaiian factories are presented in Table 111.
TABLE I11
EFFECT
OF ULTRACENTRIFUGATION O N
Sugar
Icekaha
Olokele
Mc Bryde
Grove Farm
Lihue
Kilauea
Oahu
Ewa
Waialua
Kahulru
Puunene
Paia
Wailulru
Pioneer
Hutchinson
Haw. Ag.
Puna
Hilo
Onomea
Pepeelreo
Hakalau
Papaaloa
Oolcala
Hamakua
Honokaa
Kohala
CRYSTAL COLOUR
Crystal colour (a* 420
Before
Afier
4.5
4.3
4.9
4.7
5.9
6.0
2.8
3.8
3.8
5.1
3.6
3.9
5.6
3.5
7.4
5.4
6.0
5.4
3.9
4.8
4.8
5.8
7.4
6.9
4.8
4.7
4.5
4.0
4.8
4.5
5.6
5.7
2.7
3.6
3.6
4.8
3.3
3.8
5.4
3.4
7.0
5.0
5.6
5.0
3.5
4.6
4.6
5.4
7.0
6.3
4.6
3.9
x
10)
Difference
0.0
0.3
0.1
0.2
0.3
0.3
0.1
0.2
0.2
0.3
0.3
0.1
0.2
0.1
0.4
0.4
0.4
0.4
0.4
0.2
0.2
0.4
0.4
0.6
0.2
0.2
.
TABLE I V
MOISTURE I N REFINED SUGARS
Sugar
35-80 mesh
Average
20-28 mesL
Average
~
10-20
mesh
Average
retained on
10
mesh
Average
Total
External
(%)
(%I
Internal
(%)
0.061
0.060
0.060
0.060
0.013
0.014
0.013
0.013
0.048
0.046
0.047
0.047
0.105
0.104
0.105
0.007
0.009
0.008
0.120
0.114
0.117
0.004
0.003
0.004
0.116
0.111
0.113
0.158
0.162
0.165
0.162
0.005
0.005
0.005
0.005
0.153
0,157
0.160
0.157
0.098
0.095
0.097
a
TABLE V
COLOUR AND INTERNAL MOISTURE RELATIONSHIP I N COMMERCIAL RAW SUGAR CRYSTALS
.-
i
Screen
size
(mesh)
Total
moisture
Internal
moisture
(%)
(%)
Pepeelreo
on 16
Through 16 on 20
Through 20 on 28
0.085
0.079
0.073
0.066
0.065
0.064
4.1
3.7
2.9
62
57
45
Lihue
on 16
Through 28 on 35
o.ogr
0.086
0.064
0.062
9.1
8.1
142
131
Oolrala
Through 14 on 16
Through 28 on 35
0.114
0.111
0.082
0.086
14.3
14.0
I74
163
Through 14 on 16
Through 28 on 35
0.102
0.070
0.073
8.9
8.5
127
116
Sugar
I
Pioneer
I
0.102
Crystal
Ratio
colour
crystal colour/
(a* 420 x 10) internal moisture
'
TABLE V I
MOISTURE RELATIONSHIP I N
I
COLOUR AND INTERNAL
LABORATORY CRYSTALLIZATION
Crystallization
I
2
I
t
Colour (a*420 x 10) per gram moisture
Syrup
Molasses
Crystal t
10.2
9.9
12.4
73
91
Internal moisture
Internal Cry.stnl Moist26re and Colozw
The internal moisture content of refined sugar crystal of various mesh size is
shown in Table IV.
Similar figures for commercial raw sugar crystals as well as Crystal Colour data
are given in Table V.
C. C . TU, K E N N E T H ONNA, RICHARD OKAMOTO, JOHN H. P A Y N E
957
Table VI shows the relationship between internal moisture and colour in laboratory crystallization tests.
Light Scattering and Filtevability
In Table VII are shown the filterability and light-scattering data for 16 commercial raw sugars.
TABLE VII
LIGHT SCATTERING AND FILTERABILITY
Sugar
Oahu
Ewa
Kilauea
Kekeha
Mc Bryde
H.C. & S.
Wailulcu
Hutchinson
Puna
Onomea
Papaaloa
Pepeekeo
Halcalau
Paauhau
Hamalrua
Kohala
Fitterabzlzty
(Elliott)
Scatterzng index at go"
(s 436 X 10)
104
96
85
105
92
106
89
8I
63
73
O7
61
70
91
99
0.10
0.12
0.34
IIO
0.11
0.18
0.14
0.10
0.24
0.32
0.19
0.37
0.35
0.29
0.18
0.15
0.15
Correlation coefficient -0.83
DISCUSSION
Effect of $H on Crystal Colour
The data in Table I show an increase of measured colour with pH for all sugar
solutions. The curves in Fig. I indicate that this increase becomes rapid above pH 7.
I t is apparent also that different sugars differ appreciably in relative pH effect.
Two principal classes of colour compounds in the sugar crystal have been shown
to be: (I) substances produced in processing by the thermal degradation of carbohydrates; and (2) reaction products of amino acids and reducing sugars (melanoidins)=.
These vary strikingly in pH effects. The first type of substances shows a marked
increase in colour intensity with higher pH whereas the second remains essentially
unchanged in the range 4-II~.
Thus, it may be concluded that these sugars, showing more rapid increase in
colour with pH, contain relatively higher percentages of thermal-degradation prod. ucts.
Colour and Light Scattering
In Table 11, it is noted that light scattering a t go0 is a relatively small percentage
of the attenuation. Subtraction of the Scattering Index from Attenuation Index gives
the values for Absorption Index shown in the last column. As pointed out previously
this is only an indication of the relative values of absorption and scattering for the
various sugars. There is no simple way of obtaining true values of these effects.
Scattering, nevertheless, is due primarily to suspensoids and polymeric substances which are not removed by filtration. Scattering, therefore, is a measure of
these; and it is evident that they may have a significant effect on colour measurements
in some sugars.
Effect of Ultracentrifugation on Crystal Colozcr
Suspensoids and high molecular weight polymers not affected by filtration may
be sedimented by high relative centrifugal force. Table I11 shows that in general the
sugars with higher Crystal Colour have a higher amount of these substances present.
The effect on the Crystal Colour measurement is probably mostly that of light scattering.
Internal Crystal Moisture and Colour
The results in Table IV point out that the larger the crystal the greater the
percentage of internal moisture. This would appear to be due to greater imperfection
in the larger crystals with greater inclusion of mother liquor as a result.
If the presence of other non-sucrose materials within the crystal is due to the
simple inclusion of mother liquor, then they should show the same pattern as moisture.
The figures in Table V on commercial raw sugars show that colour changes more
significantly than internal moisture as crystal size increases.
That the presence of colour within the crystal is not due essentially to simple
inclusion of mother liquor is indicated by the results shown in Table VI. Although
the apparent concentration of the mother liquor within the crystal is unknown, so
that the values are not strictly comparable, it is apparent that colour is trapped
preferentially relative to moisture. This would be expected as a result of the effect
of forces working at the crystal-liquid interface. The fact that the molasses had about
the same colour per unit moisture as the syrup indicates that the high concentration
of colour in the crystal was not the l;esult of colour development in the mother liquor.
Light Scattering and Filterability
The data in Table VII show a relationship between light scattering and filterability. The correlation coefficient of -0.83 is highly significant. I t may be concluded
that the substances contributing to light scattering, namely suspensoids and polymers
not removed by filtration, are responsible also for a lowering in the rate of filtration.
SUMMARY
The effect of various factors on the colour of the commercial raw sugar crystal was investigated.
I t was shown that although all sugars in solution show an increase in colour with increase
in pH, the extent varies. Since, of the two principal types of colour substances present, the thermal
degradation products of carbohydrates and melanoidins, only the former show this pH effect,
pH-colour measurements are of value in relating the quantities of the two present.
Light scattering was shown t o be a major factor in measured colour values for some sugars.
Substances responsible for attenuation, probably by light scattering, were shown to be partially
removed by ultracentrifugation. A relationship between light scattering and filterability was
established.
'
Measurements of internal moisture in the sugar crystal showed that the quantity increases
with increase in crystal size. Crystal Colour also increases with crystal size. That the presence of
colour within the crystal involves more than simple inclusion of mother liquor was demonstrated
by the internal moisture-colour relationship.
REFERENCES
Oflicial Methods of the Hawaiian Sugar Technologzsts. Hawaiian Sugar Technologists, Honolulu,
1955, P. 77.
a DEITZ, V. R., 1956. Color evaluation in the cane sugar industry. J . Res. nut. Bur. Stand., 57 : 159.
s HILL,S., and DOBBS,A. G. R., 1958. Determination of water in granulated sugar. Analyst, 83 :
1
143.
PERSSON,
S., 1959. Types of color. Socker, 15 : 5.
6 RIEGER,C. J., and CARPENTER,
F. G., 1959. Light scattering by commerical sugar solutions.
J . Res, nut. Bur. Stand., 63A : 205.
6 TU, C. C., and OKAMOTO,
R. H., 1960. Non-sucrose constituents in raw sugar crystal. Part 11:
Classification of color substances, and Part 111: Color formation from reducing sugars. Hawazi.
I
Plant. Rec., 56 : 161, 169.
4

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