As we all know, as a raw material for papermaking, starch is the most used component besides fibers and fillers. It includes native starch, cationic starch, anionic starch, amphoteric starch, etc. And it is widely used to achieve reinforcement, retention, and surface sizing, emulsion stability and other purposes.
Under normal circumstances, starch needs to be cooked and pasted or gelatinized before use. There are two types of starch cooking: batch type and continuous type. Among them, intermittent equipment is more difficult to control, and is generally suitable for occasions where starch consumption is small. Continuous cooking often uses a jet cooker, which has the advantages of more uniform products after cooking, more thorough hydration of starch molecules, easy automatic control, and good operating economy. That’s why it is suitable for occasions where starch consumption is large. To
The step of cooking and pasting seems like simple and easy, however, there may be some things you don’t know. Now I will briefly explain to you the following three points:
- When diluting dry starch into a slurry suspension, the temperature of the dilution water should be controlled between 30-38℃to avoid the adhesion of starch granules, otherwise it may cause some troubles in the subsequent dispersion and cooking process;
- In theprocessof cooking and pasting, as the temperature rises, the starch particles continue to absorb water, the bonds between starch molecules are dissociated or destroyed, and the volume of the starch particles continues to expand; the temperature at which the degree of swelling is greatest is called gelatinization or pasting temperature , this temperature is generally 55-80℃; continue to heat the starch for further hydration, while the viscosity of the starch solution gradually reaches the maximum; then the edges of the starch particles begin to break, and the starch molecules are small aggregates The form migrates to the water phase, and the energy provided by heating continues to make the water molecules enter between the starch molecules, and the starch molecules get the best dispersion.
- The significant difference between continuous and intermittentcookingand pasting is that the former uses pressure cooking (intermittent cooking and pasting is generally not pressurized), that is, the starch is heated for a few minutes at a temperature exceeding the normal boiling point of water, and then the starch suspension is passed through a back pressure Valve, when the pressure is suddenly released to normal pressure, extremely high turbulence and shear are generated. Starch particles quickly rupture under the action of the internal and external pressure difference, and their molecules are completely dispersed and dissolved. This process is generally referred to as a thermomechanical conversion process. If an acid or oxidant is used during cooking, the process is called a thermochemical conversion process, which generally results in a lower viscosity. For continuous cooking, starch suppliers generally recommend that the cooking temperature be as low as possible to avoid degradation of starch molecules. Cooking at a temperature higher than actually required is called overcooking. Generally speaking, the molecular weight of starch at cooking temperature below 140℃ will not be significantly reduced.
Now let’s talk about the factors that need to be controlled during the starch cooking process.
Pressure cooking and pasting is generally not used for batch cooking, and the starch liquid should be avoided to boil. It is generally recommended that the cooking temperature be controlled at 88-95 degrees Celsius and maintained for at least 20 minutes; continuous cooking generally only takes a few minutes, and the minimum cooking temperature of waxy corn starch is controlled at 104-116 degrees Celsius; if it is starch containing amylose, the minimum cooking temperature should be controlled between 116-127 degrees Celsius.
- Shear force
Shearing is very important for intermittent cooking and pasting. The mixing mixer provides the lowest shearing force to maintain uniform heat transfer and prevent over-cooking or under-cooking in local areas, and at the same time completely disperse starch molecules; the shear of continuous jet cooking is provided by the flow of the starch suspension in the pipeline (mixed with heating steam uniformly) and the back pressure valve (to obtain the best heating efficiency and uniform temperature).
The steam used in intermittent cooking and pasting is generally only at normal pressure or slightly higher than normal pressure. The steam used in continuous cooking and pasting is generally only normal pressure or slightly higher than normal pressure. The pressure used in continuous jet cooking is between 103-138KPa or even higher, and the steam pressure has a great influence on its cooking results. The fluctuation of steam pressure leads to changes in cooking pressure and temperature, and ultimately affects the quality of gelatinized starch liquid.
- Solid content
Due to the limitation of heating capacity, heating efficiency and high peak viscosity, the solid content of intermittent cooking and pasting is generally lower than that of continuous cooking. The former is generally 4%-6%, and the latter is 8%-15% or even higher.
- Apparent viscosity
Different cooking and pasting methods have a greater impact on apparent viscosity. Generally, the more heat input during the cooking process, the better the starch molecules will be dispersed. Under the same solid content condition, the viscosity of the gelatinized starch obtained by continuous jet cooking is lower than that obtained by batch cooking, but the reason is not because the starch molecules are degraded, but the entanglement of the starch molecules after gelatinization. Better to be dissociated.
- Quality inspection of gelatinized starch
A microscope can be used to observe whether the gelatinized starch is uniform and whether there are starch granules; the appearance of a good gelatinized starch is translucent (the uncooked starch suspension is milky white). The solid content can be measured with a refractometer (fast) or by the drying method (accurate).