Optimizing Pre Liming pH for Efficient Juice Clarification Process in Sri Lankan Sugar Factories
Turbidity, reducing sugar, Brix, Pol, Purity, Mud Volume, calcium oxide and color were measured to study the individual performances of the each treatments (T1(initial pH), T2(pH = 6.5), T3(pH = 7.5), T4(pH = 8.5)). Lime was added to increase the pH to 6.5, 7.5, and 8.5 during clarification. However, the pH values were slightly dropped after heating to 6.25, 7.04, and 8.30, respectively.
The response of each treatment conditions to select main quality parameters are described separately as below.
Table 2: Average Physical and Chemical characteristics data obtained from clarification test (n=5)
Treatment No.
|
pH
|
Turbidity (IU)
|
Pol %
|
Brix %
|
Purity %
|
TSS (mg/L)
|
TDS (mg/L)
|
RS %
|
CaO (ppm)
|
Mud Volume (ml)
|
UMJ (Untreated mixed juice)
|
|
218.76a
|
15.07b
|
17.56b
|
85.78ab
|
11750a
|
158360b
|
0.399c
|
|
|
T1
|
5.44 (Initial pH)
|
153.66b
|
17.13a
|
20.14a
|
84.98b
|
12140a
|
192700a
|
0.584a
|
0 c
|
0 d
|
T2
|
6.5
|
64.42c
|
17.08a
|
19.44a
|
87.9a
|
10430a
|
186710a
|
0.514b
|
2445b
|
60c
|
T3
|
7.5
|
3.64d
|
17.27a
|
19.66a
|
87.83a
|
10470a
|
188940a
|
0.559a
|
2715ab
|
270b
|
T4
|
8.5
|
6.12d
|
17.34a
|
19.66a
|
88.19a
|
10590a
|
190970a
|
0.540ab
|
2945a
|
364a
|
TSS = Total Suspended Solids, TDS = Total Dissolved Solids, RS = Reducing Sugar
*For each clarification parameter and CJ composition, figures with the same letters are not significantly (P > 0.05) different.
Average physical and chemical characteristics data obtained from clarification test are shown in table 2
Turbidity
Turbidity value of juice samples were decreased as the increase of pH values from initial pH 5.44 (T1) to pH 7.5 (T3), but turbidity value was begun to increase again when the pH increases to 8.5. Therefore pH 7.5 shows the lowest turbidity value.
Juice turbidity mainly caused by suspended impurities. Thus, turbidity removal is considered as primary objective and therefore turbidity measurement can be used as a high degree of confidence measurement to measure the efficiency of clarification (Mkhize, 2003). Lime was added to neutralize the juice and form insoluble lime salts such as Calcium Phosphates. Colloidal matters such as pectins, hemicelluloses, proteins and coloured compounds are absorbed by the precipitated ions and some colloids are flocculated by heat. Therefore turbidity of treated juice samples decreased compare to UMJ, due to removal of impurities. And also T1 and T2 were shown significantly high values of turbidity compared to T3 and T4. Since low liming pH in the process leads to limitation of reaction with existing phosphate content and reduction in catching impurities. But excess lime to 8.5 pH gave adverse effect on turbidity. Therefore, liming to a certain pH is necessary to achieve lower turbidity. Among the tested treatments liming to pH 7.5 (T3) with heating is suggested in cold liming process to achieve lower turbidity in clarified juice. Further, regression analysis revealed that the quadratic model (R2 = 99.2 %, p <0.001) best fitted to explain the effect of pH on turbidity of juice (Equation 01). The fitted line plot and the 95% confidence Interval (95% CI) for turbidity and pH is depicted in Fig. 1 and it revealed that the data are randomly spread about the regression line and majority of data points are within the confidence limit.
Turbidity = 1721 - 425.6 pH + 26.32 pH2 (1)
Reducing sugar
The reducing sugar percentage was also significantly affected by the pre liming pH value. The pH adjustment of sugar cane juice via the addition of MOL is a critical step during the clarification process to avoid sucrose inversion by hydrolysis in acidic conditions (≤ pH 4) and alkaline degradation (≥ pH 8) (Clarke, 1993). Sucrose mostly hydrolyses to make reducing sugars, glucose and fructose, which are available in the form of fructosyl oxocarbenium cation and D–glucose at the extreme acidic and alkaline juice conditions.
According to Table 2, highest value of reducing sugar was recorded in the T1 (pH = 5.44) due to hydrolysis of available sucrose in to reducing sugars in acidic conditions. And also reducing sugar value was begun to reduce again when the pH rises to 8.5 (T4) due to alkaline degradation. Therefore, liming to a certain pH is necessary to maintain optimum level of reducing sugar in the given sugarcane juice sample.
Pol/Brix/Purity
Sucrose is most stable ~ pH 8.3, whereas glucose and fructose (invert sugars) are most stable at severe acid conditions such as pH 3–4, thus balancing both sucrose and invert sugars at optimum level is a real challenges to sugar industries (Eggleston and Amorim, 2006). In the above paragraph, the variation in Glucose and Fructose (Reducing Sugar) was analysed at different pH levels, whereas the variation in Sucrose (Pol %) at different pH levels is analysed in this paragraph.
Since some of the dissolved non sugars removed from the mixed juice (MJ), the pol of the Clarified Juice (CJ) samples (T1, T2, T3 and T4) significantly exceeds that of the UMJ sample. And also brix value of Clear Juice (CJ) samples (T1, T2, T3 and T4) significantly exceeds that of the UMJ due to increase of dissolved ions with introducing liming. However, the ratio of pol to Brix (Purity %) of the treatments T1 to T4 is not significantly different from that of the UMJ.
However there are no significant difference between treatments (T1 to T4), for pol % and Brix %, while purity of T1 significantly lower compared to other treatments.
Mud Volume
After treating with different pH values, Precipitation of various calcium phosphates forms are occurred in sugarcane juice samples. Equation 2 is shown the dicalcium phosphate form of precipitation. Secondary reaction takes place to form intermediate calcium phosphate phases due to the creation of unstable and insoluble dicalcium phosphate in water.Therefore the most stable compound of the calcium phosphate phases are shown in Equations 3, 4, 5 and 6 (Thai, C. C. D., 2013)
00Ca2+ (aq) + HPO42-(aq) → CaHPO4(s) (dicalcium phosphate) (2)
00Ca2+ (aq) +2H2PO4-(aq) → Ca(H2PO4)2(s) (monocalcium phosphate) (3)
003Ca2+(aq) + 2PO43-(aq) → Ca3(PO4)2(s) (tricalcium phosphate) (4)
002CaHPO4(aq) + 2Ca3(PO4)2(aq) → Ca8H2(PO4)4(s) (octacalcium phosphate) (5)
00Ca3(PO4)2 + 2Ca2+ + HPO42-+ H2O → Ca5(PO4)3OH(s) + 2H+(aq) (hydroxyapatite) (6)
Type of calcium phosphate phase which is formed during clarification process is depend on the concentration of calcium and phosphate, pH and the nature of the particle interface (Thai, 2013). Therefore settling rate and final mud volume are differed with pH values of the treatments. Final mud volume was increased with the increase of pH liming and it was highest in treatment 4 (pH = 8.5). That leads to highest juice clarity.
Regression analysis revealed that the cubic model (R2 = 99.4 %, p <0.001) best fitted to explain the effect of pH on deposited mud volume (ml) (Equation 7). The fitted line plot and the 95% confidence Interval (95% CI) for mud volume and pH is depicted in Fig. 4 and it revealed that the data are randomly spread about the regression line and majority of data points are within the confidence limit
Mud volume = 13805 - 6240 pH + 916.0 pH2 - 43.29 pH3 (7)
CaO
When the pH liming increases, the mud volume increases but it was observed that the residual concentration of calcium also increased with the pH. Introduced Ca2+ ions in to juice samples react with P2O5 to form a calcium phosphate precipitation, but some amount of Ca2+ ions are remained in the clarified juice due to limited amount of natural occurring P2O5 in the sugarcane juice. Therefore calcium oxide content of clear juice increased with increased levels of liming pH according to the above Table 2. Therefore higher calcium level in the clarified juice obtained with higher pH (pH ~ 8.5) result in an increase in scale formation in the evaporators.
The regression analysis revealed that the cubic model (R2 = 93.9 %, p <0.001) best fitted to explain the effect of pH on CaO (ppm) of clarified juice (Equation 8). The fitted line plot and the 95% confidence Interval (95% CI) for CaO and pH is depicted in Fig. 5.
CaO = - 131455 + 53651 pH - 7142 pH2 + 316.5 pH3 (8)
Color
The formation of colourants produced during factory processing is mainly due to sugar degradation reactions. Reducing sugars, such as glucose and fructose, formed by the inversion of sucrose, play an important role in the formation of colour. These sugars degrade due to changes in operating conditions such as pH and temperature to form highly reactive intermediates, which undergo condensation and polymerisation reactions to form highly coloured polymers (M. T. Nguyen, 2013). According to the Fig. 3, clarified juice color was darkened with the increase of pH liming from T1 to T4.
Colourants such as caramels and melanoidins are pH insensitive; therefore their colour does not change across pH 5.44–8.5. But flavonoids and phenolic compounds (i.e., colour precursors) are highly pH sensitive. Therefore, these types of compounds are lightly coloured at pH 5.44 (lower liming pH) and darken greatly at pH 8.5 (highly alkaline conditions) (M. T. Nguyen, 2013). This is because at pH 8.5, the ionisation of these compounds is almost complete. Hence, these compounds are more highly coloured in their anionic form than in their neutral form. That’s the reason behind the color variation of clarified juice which is mentioned in Fig. 3.
Main juice quality parameters and mud volume were separately analysed to identify the individual performances of the each treatments (T1 (5.44), T2 (pH = 6.5), T3 (pH = 7.5), T4 (pH = 8.5))., However, turbidity measurement can be used as a high degree of confidence measurement to measure the efficiency of clarification since turbidity removal is the primary objective of the juice clarification process. Therefore, the variation of other key parameters with turbidity is further elaborated using the contour graphs.
Effect of pH on turbidity and pol in juice
In order to achieve high clarification efficiency, the turbidity value should be low. Thus according to table 2, the lower turbidity values are recorded at 7.5 pH (3.64 IU) and 8.5 pH (6.12 IU). When analysing this lower turbidity with pol in juice parameter, pol in juice value should be a higher one. The darkest green color represents the highest pol in juice according to the contour graph mentioned in Fig. 6. By considering these two phenomena’s, the most suitable region with best pH range is between 8.0 to 8.5 (pol % > 17.3%). But the pH range from 7 to 8.0 can also be taken in to consideration since it shows less variation in pol in juice > 17.25 %.
Effect of pH on turbidity and reducing sugar in juice
In order to achieve high clarification efficiency, the turbidity value should be low. According to table 2, the lower turbidity values are recorded at 7.5 pH (3.64 IU) and 8.5 pH (6.12 IU). When analysing this lower turbidity with reducing sugar parameter, reducing sugar value should be low. The darkest blue color represents the lowest reducing sugar according to the contour graph mentioned in figure 7. By considering these two phenomena’s, the most suitable region with best pH range is between 7.0 to 8.5. Even though the pH 8.0 to 8.5 shows much better reducing sugar value than the value showed in pH range of 7.0 to 8.0, this cannot be considered since the reducing sugar started to invert at high pH. As a conclusion, the best pH range is 7.0 to 8.0 by considering both Fig. 6 & Fig. 7.
Effect of pH on turbidity and purity in juice
By considering the purity of the juice, the best juice purity gives when the pH is between 8.0 to 8.5, according to figure 8. But when compare with the reducing sugar and pol in juice parameters, the optimum pH range is 7.0 to 8.0 with respect to Fig. 6, 7 & 8.
Turbidity, reducing sugar, calcium oxide, Mud Volume, Brix, Pol, Purity and color were measured to study the individual performances of the each treatments (T1(initial pH), T2(pH = 6.5), T3(pH = 7.5), T4(pH = 8.5)). However, turbidity, reducing sugar, calcium oxide and purity are more influential in indicating the juice clarification efficiency than the rest.
Over liming to pH ~ 8.5 can result in highly alkaline conditions and it was recorded separation of highest mud volume from the mixed juice. Although an alkaline environment can reduce sucrose losses due to inversion, but it would exacerbate scaling in the evaporators due to increase of residual Ca2+ ions in the clarified juice. As well as it promotes the formation of colourants (dark brown) and initiates to decrease reducing sugar due to the alkaline degradation of glucose and fructose. On the other hand deficit liming can cause the acidic conditions (pH = (5.44 - 6.5)) and it was recorded lowest level of residual Ca2+ in the clarified juice. So it is a good sign for the retardation of scale formation in the evaporators. But under deficit liming, no clear mud separation was observed with each treatment (T1 & T2). Furthermore, each treatments showed higher turbidity values and lower deposited mud volumes of the juice under the acidic conditions (pH = 5.44 & pH = 6.5).