summary

this paper presents the main results of research work in china on white sugar quality, particularly the effects of various impurities such as phenolics, amino nitrogen and iron. the numerical relationship between impurity contents of the sugar and the syrup is discussed.

a list is given of the measures taken in the factory clarification stations and boiling houses to improve quality.

introduction

since the 1980's, the sugar production in asian has developed rapidly^[1]. at the same time, the proportion of white sugar and refined sugar, has risen^[2], as well as their trade in the international market. this is because the direct production of white sugar is more profitable than that of raw sugar, especially when the product is of high quality and has a favorable price. many sugar factories that originally produced raw sugar have been re-equipped to produce white or refined sugar, and the production of superior sugar at lower cost has been an increasingly important topic.  

china's sugar industry produced about 7 million tons of plantation white sugar in the early nineties, of which about three-fourths were made from sugarcane. they were of two grades, a and b, usually with a color of 50 to 60 and 100 to 130 icumsa respectively, according to buyer's requirement. most of these sugar had a pol over 99.7%, moisture content below 0.05% and ash below 0.07%; the majority of them were of good keeping quality, with a little increase in color on storage.

the majority of our cane sugar factories uses the sulphitation process, mainly producing grade b white sugar, or grade a product by the help of additional measures.  some factories adopt the carbonatation process, often yielding better products.

1. the color of white sugar and the existing colorants

the color of white sugar is the direct mark of its quality. quite much plantation white sugars appear light yellow, or turn yellow during storage. this fact often leads to a decrease in the selling price of the products.

the color of white sugar varies with many factors of the cane quality and the process. but in the last analysis, it depends on the colorants existing in the sugar, especially on the content of phenolics, amino nitrogen and iron.  these substances often combine into complicated compounds. our works revealed following regularities:

1. there exists a significant positive correlation between the sugar color and the content of phenolics.  a yellowish sugar usually contains phenolics more than 50 ppm, and if the content is over 100 ppm, the sugar may have a high color of 250 to 300 icumsa.

2. superior white sugar usually contains less phenolics, say, below 20 ppm. the contents of phenolics and amino nitrogen in sugar both have influence on its color.

3. superior white sugar contains less iron, for example, 0.2 to 0.5 ppm or less. if the iron content were higher, the sugar would turn yellow quickly on storage.

the effects of phenolics and amino nitrogen have been reported in the literature[3,4], but that of iron is still not emphasized enough. our work proved that the iron existing in sugar has direct effect on the change of sugar color during storage.  for example, seven sugar samples taken from different factories were put separately in unclosed bottles at room temperature for two months, their initial and final color values, as well as their iron contents are shown below:  

it can be seen from these figures that these sugars were not far different in their original color, but after two months, the sugars containing more iron turned much yellowed than the others:  three samples with the highest iron content had an increase in color over 100%.

it is common knowledge that the yellowing of white sugar is caused by the oxidation of the existing non-sucrose, including the oxidation of phenolics and amino nitrogen substances, as well as that of ferrous. in most white sugar factories, the evaporator syrup is sulphited, thus containing free sulphurous acid, which makes the coexisting iron compounds turn into the form of ferrous.  while the sugar crystal contacts with air, ferrous in it would be oxidized gradually into ferric compounds that are usually of dark color. this action brings yellowing of sugar color.

in a word, the sugar color is mainly controlled by these three kinds of substances, which should be studied in detail.

phenolics are from sugarcane and have different compositions and properties.  they are removed partly (especially the fraction with high molecular weight) by clarification. its overall removal in the sulphitation or carbonatation process varies from 30 to 50%, depending on the working conditions. when sugar crystal grows up in the pan, a small part of phenolics present in the sugar liquor (especially the high-molecular-weight compounds) is included in it. therefore, it is very important to remove these phenolics as fully as possible in the clarification station.

amino nitrogen substances also have complicated composition, among which the most harmful components to sugar quality are the products of maillard reaction, namely the reducing sugars react with amino acids at alkaline condition and high temperature, producing various harmful substances including color matter. for this reason, cane juice should be kept away from high alkalinity at high temperature.

in the whole course of production, cane juice and syrup contact with iron apparatus constantly, and iron may dissolve in acidic medium. iron existing in the mixed juice is removed largely by clarification, but when the syrup is sulphited to low ph, the boiling materials would increase its   iron content again. the sugar boiled out from low ph syrup may have a white appearance at the beginning because of the low ph effect; however, it often turns yellowed in a few months.

2. relationship between the quality of sugar and syrup

it is well known that the sugar quality is closely related to the quality of syrup from which the sugar is produced. this relation can be expressed with figures. although this numerical relation has not yet been fully studied, it is of significant importance for both theory and practical work.

some researches have noticed the ratio of the color of sugar crystal s₁ to the color of syrup s₂, i.e., (s₁ /s₂) and named it  "color transfer". g.lionnet^[5,6]  has made many experiments and observations on a massecuite boiling, and showed this ratio ranging from 0.01 to 0.016 (the crystal has been washed to remove its outer coating).

in order to make this study more convenient, the author introduces a new term "non-sugar brought-in coefficient", denoted by nsc, to indicate this relation. the nsc value for various non-sugars is different, it is calculated from the contents of a certain constituent present in the sugar product and in the syrup.  let n₁ denotes the content of this component in the sugar, n₂ the content of it in the syrup (on total solids) and p₂ the purity of the syrup, then, assuming the purity of the sugar to be 100:  

 in short, nsc means the percentage of the impurity/sugar ratio in the sugar to the ratio in the syrup. for example, a sugar contains potassium 0.0018% (18 ppm), and the syrup contains 0.378% potassium on solids and is of 88 purity, thus the nsc value of potassium in this example is:             

  0.0018 x 88 / 0.387 = 0.4

as for color matter, its relative total amount is usually expressed by icumsa color (or other unit), let s₁ and s₂ stand for the color of the sugar and syrup respectively, then:    

that is to say, the nsc of color equals the "color transfer" quoted above multiplied by syrup purity, namely, nsc is calculated on sugar basis and is a percentage, so it is more comparable for different purity material and more convenient in use.

the nsc value of some main impurities in white sugar boiling, found by the author, are given below:

(1) inorganic substances  

potassium    0.2 - 0.4   sodium     2 - 2.5  

calcium       5 - 7         silicate     7 - 10  

sulphate       5 - 8  

(2) color value  

the first boiling of evaporator syrup    1.5 - 2  

re-crystallization of remelt liquor       8 - 10  

(3) non-sucrose and reducing sugar  

total non-sucrose in cane syrup       0.7 - 0.9  

reducing sugar                   0.5 - 0.9

the nsc value of each component varies in a certain range and is influenced to some extent by the boiling system and operation, crystal size and equipment conditions, as well as the time of the year. for example, when b magma is used as a footing, nsc would be higher.

the situation of organic substances is quite complicated. calculated on the basis of g.lionnet's figures the nsc for phenolics is 1 to 1.2, and amino nitrogen 0.6 to 0.8. m.clarke^[7] has investigated in detail the organic non-sugars present in the main materials in raw sugar factories, and given figures that revealed the relation between the quality of raw sugar and that of syrup. the proportion of a sugar color to syrup color in two mills was separately 6.5 and 7.7%; while the proportion of high-molecular-weight substances are much higher, e.g., for high-molecular-weight color was 11.2 and 12.4%, for starch 33.6 and 37.7%, for polysaccharide 17.2 and 17.6%. m.godshall et al.^[8] has studied the color matters and polysaccharide in sugar refining, and also found these substances existing in refined sugar crystals with a higher ratio; the proportion of that in sugar to that in syrup reached 20 to 27%  (these figures approximately equal nsc, since the syrup purity was nearly 100).

the nsc values of different impurities vary widely.  this means that different impurities may be brought in the sugar product by different ways, according to their physical-chemical properties. inclusion of mother liquor into crystals may be the main manner for most impurities; whereas for some kinds of impurities, they may be adsorbed (such as surface-active substances or colorants), or co-crystallize with sucrose (as other sugars), and may precipitate and then mix with the crystals (as calcium sulfate and other low-soluble inorganic matters). these effects make nsc value increase many times.  the nsc values listed above clearly show that, as a whole, calcium, sulphate and silicate, as well as organic substances with high molecular weight are the most harmful matters.

3. main measures of improving cane white sugar quality

many innovations have been applied in our sugar factories, and the main measures for improving sugar quality are as follows:

3.1 clarification station

the main task of this section is to eliminate the impurities existing in the mixed juice as fully as possible, especially the most harmful substances such as high-molecular-weight phenolics, amino nitrogen, silicate, iron and calcium etc., and to prevent sucrose inversion and reducing sugar decomposition, so that to obtain good quality juice and syrup. the main measures are:

1. in the sulphitation process, the juice is sulphited to a high so₂ content of 1.2-1.4 g/l, getting higher clarification efficiency.

2. phosphoric acid is added to make the total effective p₂o₅ in raw juice attain 300-400 ppm.

3. before sulphitation, the juice is prelimed to neutral and preheated. this improves the subsequent process.

4. the whole process is performed at a neutral or slightly basic condition, thus the sucrose inversion is reduced to a minimum and the decomposition of reducing sugar is actually negligible.

5. a new-style sulphitator of injecting self-suction and a new-style sulfur furnace of self-melting are used.  they work smoothly, stably and are controlled with ease, giving high absorption efficiency.

6. modified continuous clarifiers are used, which operate at lower juice level, with shorter retention time of juice, keeping the juice quality well.

7. the vacuum filter is of cloth-covered type, yielding a clear filtrate, which feeds to evaporators instead of turning back. this reduces sucrose inversion and eliminates the recycling of impurities.

8. the carbonatation process works with high dosage of lime, low alkalinity and at low temperature. the amount of cao added is 1.3-1.5% juice, alkalinity of the first carbonated juice around 0.05%, obtaining very high clarification efficiency.

9. the processing time is shortened, thus the decomposition of reducing sugar is decreased to a minimum.

10. a new-style fully automatic plate-and-frame filter is applied. clear filtrate is obtained and the process is speeded up.

11. the first carbonated clear juice is further carbonated to a suitable ph and to heat and filter again, getting low color and high purity juice.

12. the syrup is slightly sulphited, but the final ph is not lower than 6.0, for preventing sucrose-inversion and iron-dissolution.

13. a new system of syrup phosflotation is applied for the production of superior white sugar. a new device of syrup aeration makes very minute and uniform air bubbles in a suitable amount, and new-style equipment promotes the phosphate reaction  and flocculation, getting brilliant and low color syrup.

14. a new, high-efficiency and low-cost chemical named "g409" is used when necessary. added to the juice or syrup, it can react and combine with colorant and colloidal materials, coagulate together with other precipitate and then be removed.

3.2 boiling house

the focal point in this section is to get pretty sugar crystals which are of uniform size and tidy shape, with less false grain, conglomerate and other faults of grain, containing less impurities which are brought in through inclusion or other mechanisms, as well as to achieve high sugar recovery. the main measures are:

1. the pan flow diagram is suitably arranged. all materials used for a strike are of high purity and good quality. b magma used for a footing must have tidy grain and low color.

2. a new technique "homogeneous nucleator by organic solvent and ultrasonic wave" is applied, making very tidy sugar seed.

3. a new developed "computer controller for supersaturation in sugar boiling process" serves to indicate directly the supersaturation of the mother liquor in the strike, and to control exactly the boiling process.

4. an advanced style  "middle-installed stirrer" is used to promote effectively the circulation of massecuite in the vacuum pan, helping the crystals grow well.

5. the sugar drier and cooler are improved to keep the sugar quality well.

each of these measures includes plenty of contents that need to be described in detail separately.  

references

1. inter. sugar jour., 1993 (vol.95), 433-434.

2. inter. sugar jour., 1993 (vol.95), 337-338.

3. proc. sugar process. resear. conf., 1988, 75-93.

4. inter. sugar soc. canesugar technol., 17th, 2137-2144.

5. proc. south african sugar tech. assoc., 1987, 70-75.

6. sugar jour., 1989, 4-6.

7. reports in guangzhou seminar, august, 1991.

8. inter. sugar jour. 1989 (vol.91), 145-147.

9. inter. sugar jour. 1990 (vol.92), 97-102.