The production of crystalline fructose is generally to obtain high-purity fructose syrup first, and then obtain it by controlling crystallization. See GB/T 26762-2011 for the national standard of crystalline fructose. For the production of high-purity fructose syrup, the raw materials mainly include inulin, sucrose and starch. There are also reports about apples as raw materials. Among them, it is difficult to industrialize using inulin as raw material due to the complexity of enzyme and process. Compared with starch, sucrose is widely used in the pharmaceutical industry due to its high purity and no oligosaccharides are produced during the preparation process. In recent years, there are many related reports and applications. However, at present, the main processing technology of crystalline fructose is still to use starch as raw material, through liquefaction, saccharification, isomerization, and fruit glucose separation, to obtain fructose syrup with a purity of more than 90%, and then to obtain it by slow and fine control cooling.
For the production of crystalline fructose, separation and crystallization processes are extremely important. Among them, the crystallization process of fructose is the most important restrictive operation. It is well known that industrial crystallization methods mainly include evaporation, cooling, vacuum cooling, dissolution and reactive crystallization. Fructose is a heat sensitive substance and is not suitable for evaporation; The vacuum cooling method is less used in the production of crystalline fructose due to its high energy consumption and difficult operation control; The characteristic that the solubility of fructose changes greatly with temperature determines that the most ideal production method of crystalline fructose is cooling method; For the low solubility of fructose in organic solvents, the combined dissolution method is also often used.
The main disadvantage of fructose crystallization is the high solubility of fructose. When fructose reaches the supersaturation required for crystallization, the viscosity of the system is extremely high, which is not conducive to crystallization. It is also not feasible to reduce viscosity at elevated temperature, because fructose is a heat sensitive substance, and increasing temperature will easily lead to its degradation, which will deepen the color of fructose solution. The dissolution method combined with the addition of organic solvent is a feasible method for viscosity reduction, but the addition of organic solvent will increase the production cost, and also easily lead to spontaneous nucleation, which will lead to poor particle size characteristics of the product.
The separation of fructose in fructose syrup mainly includes ion exchange resin separation, chromatographic separation, crystallization separation, borate separation, double salt separation, parameter pump separation, etc. Among them, the separation of ion exchange resin and chromatographic separation is effective, and can be operated continuously to achieve industrial scale production, which is an ideal separation method. But at the same time, there are some shortcomings such as complex technology and large investment in equipment.
（1）Ion exchange resin separation
Ion-exchange resin is a kind of macromolecular compound with active group, network structure and insolubility. Ion exchange resin separation method is a method to separate ions in solution by reversible chemical reaction using ion type resin as solid phase exchanger. Fructose can form a complex with the active group of ionic resin and remain on the resin, while the glucose in fructose syrup cannot bind to the resin and most of it is eluted. Therefore, the high-purity high fructose syrup can be obtained by passing the fructose syrup through the column through the ion exchange resin through the processes of exchange, elution, collection, evaporation and concentration. The separation effect depends on the type and cross-linking degree of ion exchange resin. Among several different types of ion exchange resins, such as H+, Ca2+, OH-, etc., the Ca2+ type resin has the best complexing effect with fructose, which improves the purity of fructose syrup.
The chromatographic separation technology can successfully separate the components of fructose syrup, which is the most widely used separation method at present. The separation principle of chromatography is to make use of the difference in physical and chemical properties of different components in the stationary phase and mobile phase, so that each component moves at different rates in the two phases for further separation. In fructose syrup, the affinity of ion exchange resin or calcium salt molecular sieve to fructose component is strong, but the affinity to glucose is weak. Therefore, most of the glucose will not be adsorbed and will flow out from the separation column first. The adsorbent in the separation column will adsorb fructose strongly and flow out finally, achieving the goal of separating fructose and glucose.
Through the development process from fixed bed to simulated moving bed, chromatographic separation has realized the continuous and industrialized separation of fructose syrup. Continuous annular ion exchange chromatographic separation is another chromatographic separation method that can achieve continuous separation. Fructose and glucose migrate in the annular chromatographic bed to form spiral bands, and flow out from different outlet angles to separate.
（3）In the crystallization separation method
The first generation fructose syrup is concentrated to 78°Bé by evaporation, and then cooled to 5℃. Glucose crystal seeds are added to form glucose crystals under stirring, and then the glucose crystals are removed to obtain pure second generation fructose syrup. Finally, the product is separated by centrifugation, and the remaining sugar paste can be used as the seed crystal for the next crystallization to achieve continuous production.
（4）In the double salt separation method
Fructose syrup is first formed into a fructose calcium chloride mixture through the action of calcium chloride. Some studies also use sodium chloride to form a glucose sodium chloride mixture. Organic solvents such as ethanol are added to these mixtures or cooling methods are used to precipitate, remove the precipitation, and concentrate the liquid to obtain a fructose solution with high content, so as to achieve the purpose of separation, but this method is inefficient.
Fructose crystallization method can be divided into intermittent (batch) operation and continuous operation according to the continuity of production process. The advantage of batch crystallization operation is flexible, which can produce crystals with different particle size distribution. The disadvantage is that the yield is low, the consistency of products between batches is difficult to be guaranteed, large storage tanks are required for crystal growth, and separate cooling systems are required for each storage tank. The advantages and disadvantages of continuous crystallization are opposite to batch operation.
At present, the developed countries with mature production technology of crystalline fructose mainly adopt continuous crystallization technology to obtain high yield and large production scale.
The crystallization process can be divided into three categories: dry fructose syrup method, crystallization from water and crystallization from aqueous solution of organic solvent. Among them, US Pat NO.4517021 described the method of drying fructose syrup to produce granular half crystalline fructose, and obtained products with water content less than 2%, crystalline fructose accounting for 60%, and amorphous fructose accounting for 35%. However, the purity of this product does not meet the standards of Food Chemicals Codex, so it cannot be called pure fructose, and it has great water absorption, which is not conducive to storage and transportation. Therefore, the latter two are more operable.