Sucralose, also known as sucralose, its chemical name is 4,1',6'-trichloro-4,1',6'-trideoxygalactose.
Sucralose is a sucrose derivative formed by sucrose halogenation, its sweetness is 600~800 times of sucrose, white powdery substance, odorless, non-hygroscopic, its melting point is 125 °C, soluble in water, ethanol and methanol solvents [1-3]. The synthesis methods of sucralose mainly include whole group protection method [4], single ester method [5-6] and enzyme chemical method. Sucralose is synthesized by complex organic reactions, so separation and purification is an important unit of operation.
At present, continuous water crystallization is widely used in industry to purify sucralose, and the method has crystallization time
Long, crystalline mother liquor product residue and difficult recovery and other defects.
Although some literature patents have improved the water crystallization method, it is mainly for the adjustment of the crystallization solvent, and the effect is not obvious.
In recent years, the research of ultrasonic technology and its application has made great progress, and has been widely used in metallurgy, chemical, food, medical and other industries. A. VanHookz (1958) pointed out when discussing the formation of crystal nuclei: acoustic radiation has a strong directional effect, which can supplement and enhance the fluctuation effect required to form critical crystal nuclei, so it can accelerate the crystallization process. In addition, ultrasonic crystallization also has the characteristics of simple equipment, convenient operation, and not easy to introduce other impurities, so that ultrasonic assisted crystallization has received widespread attention.
In this experiment, the effects of ultrasonic waves on the induction period, crystallization rate, yield and product quality of sucralose crystallization process were studied using crude sucralose as raw material.
1 Experimental reagents and instruments
1.1 Main raw materials and reagents Crude sucralose (purity 95%): Ji'an Xinqian Technology Co., Ltd.; Distilled water; Karl-Fischer reagent: commercially available; Water, acetonitrile, methanol: HPLC grade.
1.2 Main instruments SK2210HP ultrasonic cleaner, HH-4 digital display constant temperature water bath, PB602-N electronic precision balance, RE52-98 rotary evaporator, JJ-1 precision booster electric agitator, TDL80-2B benchtop centrifuge, HY-2C melting point instrument, LC-10AT high-efficiency liquid chromatograph.
2 Experimental methods
2.1 Experimental setup The ultrasound-assisted sucralose crystallization device is shown in Figure 1.
2.2 Sucralose solution preparation
Weigh 120 g of sucralose (purity 95%) in a 500 mL beaker, add 200 mL of distilled water to dissolve at 70 °C, filter at a constant temperature for 30 min to remove insoluble impurities, cool to room temperature, and make a supersaturated solution for later use.
2.3 Determination of sucralose crystallization induction period
One of the phase-transition phases is the latent transition phase, when new phases are not directly visible, and this phase is called the latent or induced phase [8]. The common measurement methods of crystallization induction period mainly include visual method, laser method and conductivity method. In this experiment, the induction phase of sucralose was determined by visual method.
Take 100 mL of stock solution, steam out 50 mL of water under reduced pressure and prepare a solution of 0.8 g/mL, and then configure it into three solutions with different saturations, concentrations of 0.4, 0.6 and 0.8 g/mL, respectively, put them in an ultrasonic field of 40 and 59 kHz, set the output power, and start the stirrer until the nucleus is observed with the naked eye, which is the induction period. Three samples of corresponding concentration were taken and allowed to stand as a comparison experiment.
2.4 Determination of sucralose crystallization amount
After the crystallization is completed, the supernatant is removed by centrifugation, and the crystallization amount in this time period can be obtained by vacuum drying. Centrifugal separation: speed 4000 r/min, time 10 min;
Vacuum drying: temperature 40 °C, vacuum degree 0.09 MPa, time 4 h.
2.5 Moisture
The Karl-Fischer method determines its water content. Take 50 mL of methanol in the reactor, titrate 50 mL of methanol trace water with Karl-Fischer reagent, titrate until the pointer is comparable to calibration and remain unchanged for 1 min, open the feeding port, immediately add the weighed sample, plug the epithelial plug and stir, titrate with Karl Fischer reagent to the end point for 1 min, and record the volume consumed.
Moisture content = FV/W
Formula: F is the water equivalent of Karl Fischer reagent, mg/mL; V is the Karl Fischer reagent consumed for titration, mL; W is the sample mass, g.
2.6 Sucralose content determination
Determination of sucralose by reversed-phase high performance liquid chromatography[9].
Column: R a d P a k C - 1 8 reversed-phase spectroscopy column (2 5 0 mm× 4.6 mm id.5 μm); Mobile phase composition: V(acetonitrile): V(water)=15:85;flow rate: 1.0 mL/min; Injection volume: 20 μL; Detector: differential refractometer; Column temperature: 43 °C.
Standard solution preparation: Accurately weigh 0.025 g (accurate to 0.0001 g) sucralose standard, dissolve it in mobile phase, volume it into a 5 mL volumetric flask, and filter it with a 0.45 μm microporous membrane.
Sample solution preparation: Accurately weigh 0.025 g (accurate to 0.0001 g) sample, dissolve it in mobile phase, volume it into a 25 mL volumetric flask, and filter it with a 0.45 μm filter membrane. Manual injection of 20 μL, according to the retention time qualitative, peak area quantification of the principle to obtain sucralose content.
3 Experimental results and discussion
3.1 Effect of ultrasound on sucralose solution
Supersaturation is the driving force behind the crystallization process of the solution. For a solution, the concentration exceeds the equilibrium concentration (solubility), which is a supersaturated solution, but not all supersaturated solutions crystallize. The solution can be in at least 3 states: stable zone, metastable zone, and unstable zone. Crystallization will begin immediately only when the solution is in the unstable zone, and when the solution is in the metastable zone, especially when the supersaturation is not high, the solution will not crystallize, and crystallization is only possible when the seed is added.
As shown in Figure 2, when a saturated sucralose solution at 55 °C is gradually cooled at a freezing rate of 0.5 K/min, if no ultrasonic wave is applied, crystal nuclei appear in the solution when cooling to 15 °C.
If ultrasonic waves are applied, crystals precipitate at 26 °C. That is, the introduction of ultrasonic waves increases the nucleation temperature of the crystal and reduces the width of the metastable zone of the solution.
3.2 Effect of ultrasonic waves on sucralose crystallization induction period
The induction phase is an important parameter for solution crystallization. The length of the induction period depends mainly on temperature, stirring strength, and impurity concentration. All else being equal, the higher the temperature, the shorter the induction period. Similarly, the stirring strength and the increase in insoluble particles in the liquid phase shorten the induction period [8]. The introduction of ultrasonic waves in the crystallization process can effectively increase the stirring strength, reduce the viscosity, and change the crystallization induction period due to the cavitation effect and mechanical effect generated by ultrasonic waves[10-11].
3.2.1 Effect of ultrasonic power on sucralose induction period The solution concentration was 0.8 g/mL, the ultrasonic frequency was 59 kHz, and the influence curve of ultrasonic output power on the crystallization induction period of crude sucralose was shown in Figure 3. It can be seen from Figure 3 that at a certain frequency, the induction period shortens with the increase of output power, so in order to make it easier to determine the crystallization induction period, the output power of 100 W is selected in the following experiment.
3.2.2 Effect of ultrasonic frequency on sucralose induction period
The effect of 40 kHz (100 W) and 59 kHz (100 W) ultrasonic waves applied to three sucralose solutions with different saturations and compared with standing samples is shown in Table 1. Table 1 Effect of ultrasonic frequency on sucralose crystallization induction period
| Sucralose concentration / (g/mL) | Induction period/min | Time-saving ratio | ||
| Frequency 40 kHz | Frequency 59 kHz | No ultrasound | Ultrasound: no ultrasound | |
| 0.8 | 69 | 57 | 245 | 1:4 |
| 0.6 | 116 | 97 | 489 | 1:4 |
| 0.4 | 271 | 235 |
- Note: - Indicates that crystallization does not occur.
From Table 1, it can be concluded that in poorly saturated solutions that are difficult to nucleate, the use of ultrasonic waves can effectively promote nucleation and reduce the time between the establishment of the supersaturated state and the beginning of nucleation and crystallization, that is, the induction period.
The induction period of sucralose by ultrasonic waves of different frequencies is also different, and the higher the frequency, the smaller the induction period. Taking the induction period without ultrasound as a reference, when the concentration was 0.8 g/mL and the ultrasonic intensity was 40 kHz (100 W) and 59 kHz (100 W), the induction period was shortened to 1/4~1/3 of the control sample.
When the concentration was 0.6 g/mL, the induction period was shortened to 1/5~1/4 of the control sample.
At a concentration of 0.4 g/mL, nucleation does not occur in contrast samples, and ultrasonic waves significantly promote nucleation of the solution. Theoretically, the shock wave generated by the cavitation effect generated by ultrasound in a very small space and in a very short time can promote nucleation and make the nucleation process can be carried out at low supersaturation, and the experimental results are consistent with the theoretical analysis.
3.3 Effect of ultrasonic waves on sucralose crystallization rate
The crystallization rate can be expressed by the change of crystal mass with time, and the crystallization rate depends on the supersaturation of the solution, or the degree of supercooling, pressure, stirring strength and characteristics of the liquid phase, including the action of various fields and the presence or absence of impurities.
In Figure 4, sucralose solutions of a, b and c were added (0.07±0.001) g of sucralose seeds, and then placed in ultrasonic fields without ultrasound, 40kHz and 59kHz, respectively, after 3 minutes, it was observed that the crystals in (a) only grew slowly along the container wall and seed, while (b) and (c) rapidly became white turbidized, and a large number of white crystals began to appear in the aqueous solution. After centrifugation and vacuum-drying, the mass of the crystal is called (b)>(c)>(a). The results showed that ultrasound had a significant effect on the crystallization rate of sucralose, and the lower the frequency, the more obvious the effect.
3.4 Effect of ultrasonic waves on the crystallization amount of sucralose
The supersaturated sucralose solution was placed in an ultrasonic field at 59 kHz and compared with a standing sample. The crystallization amounts for different time periods are shown in Table 2.
Table 2 The effect of ultrasonic waves on the amount of crystallization
| Processing time/h | Crystallization amount/g | ||
| Add seed crystals | No seed added | No ultrasound (seeded) | |
| 0.5 | 0.71 | - | 0.05 |
| 1.0 | 1.46 | 0.32 | 0.09 |
| 2.0 | 2.14 | 1.56 | 0.17 |
| 3.0 | 2.47 | 1.88 | 0.53 |
Note: - Indicates that no crystals are produced or are not obvious.
It can be seen from Table 2 that under other conditions being unchanged, the effect of ultrasonic waves on the crystallization yield of sucralose is obvious. Ultrasound can not only increase the crystallization yield, but also obtain the same quality of sucralose crystals, and the time spent on ultrasonic crystallization is shorter.
3.5 Effect of ultrasonic assistance on some indexes of sucralose crystallization products
Some indicators of sucralose crystallization products are shown in Table 3.
Table 3 Some indicators of sucralose crystallization products
| products | Color and form | Melting point/°C | Water content/ (w/w, %) | Sucralose content/% |
| 40kHz ultrasonic-assisted crystallization | White powder | 120~121 | 0.6 | 98.78 |
| 59kHz ultrasonic-assisted crystallization | White powder | 122~124 | 0.4 | 99.34 |
| Unwave-assisted crystallization is not added | Needle-like crystals | 43~45 | 22.1 | 76.78 |
| Sucralose standard | White powder | 125~127 | 0.01 | 99.98 |
It can be seen from Table 3 that the melting point of products without ultrasonic crystallization is much lower than that of ultrasonic crystallization products and standards, and the water content is high. This may be that sucralose pentahydrate is obtained without ultrasonic crystallization at room temperature [12], while ultrasound crystallization obtains anhydrous sucralose. The sucralose content of ultrasonic crystallization products is close to the standard.
4 Conclusion
Ultrasound reduces the metastable zone width of sucralose solution. The sucralose solution was subjected to ultrasonic waves of 40 and 59 kHz, respectively, and the crystallization induction period was shortened, the higher the frequency, the higher the power, and the more obvious the effect.
The crystal seed was added to the sucralose solution, ultrasonic waves with frequencies of 40 and 59 kHz were applied, and the solution was rapidly white and turbidized after 3 minutes, and crystal nuclei were rapidly generated, while the contrast sample was unchanged.
Under the same conditions, the amount of crystallization obtained in the ultrasonic field with a frequency of 59 kHz is significantly greater than that without ultrasonic crystallization, and the time required to obtain the same amount of ultrasonic crystallization is shorter.
At room temperature, sucralose-free products without ultrasonic crystallization contain 20.1% moisture, while anhydrous sucralose products can be obtained by ultrasonic crystallization at 40 kHz (100 W) and 59 kHz (100 W), and the sucralose content meets FAO and WHO requirements.