Starch Cooking - Technical Papermaking

Starch cooking techniques

Starch Addition – Solids content and Viscosity

Starch is mixed up and cooked to specification according to two specific parameters, the solids content of the final starch slurry and the final slurry’s viscosity.

The viscosity of the starch slurry increases exponentially as the cooked solids content increases. Even at low solids content the starch can be too thick to use efficiently. Therefore reagent is added (ammonium persulphate); this chemical reduces the viscosity of the slurry and allows for higher solids content. The reagent works by breaking down the long starch glucose chains into smaller chains this in turn reduces the viscosity. To stop the reduction, the starch is cooked again at a higher temperature to burn off the chemical.

a Pond starch press is designed to work between 40 – 70 cP viscosity (60 Deg C Spindle 62). Too high viscosity can cause a number of issues at the size press. Thicker starch slurry will not penetrate the sheet as efficiently. The starch as a result coats the paper and forces itself between the size press rolls and the sheet causing wear on the rubber rolls. Increasing the moisture of the sheet entering the size press will allow more starch to penetrate but at a cost of web strength through and especially after the press.

Thermoconversion of starch 

An older crude method of converting Starch molecule is Thermo-conversion. Once uncooked starch granules are mixed into a slurry. A reactant is added to the starch slurry to begin the cooking process. Ammonium Persulphate is the reaction chemical added to the starch. The starch and the ammonium persulphate is heated to 145 degrees to start the reaction. The ammonium persulphate breaks down the glucose chains reducing the overall viscosity of the starch.

The positive of thermochemical conversion is the robustness of the process. The system can withstand process variations more so than enzymatic conversion. For example changes in PH value or variations of starches being used. Enzymatic conversion has a tight tolerance on its process, if the pH value goes too acidic or alkali the enzyme will not function. Enzymatic conversion works best with a consistent process and limited variations.

Enzymatic conversion of starch

Enzymatic conversion of starch involves using an enzyme within the starch cooking process to break down the starch glucose chains allows for a reduction in the starch viscosity while being able to increase the starch solids. Without pre-treatment cooking, as the starch solids increase the viscosity increases in an exponential manner. The viscosity of the starch at the desired solid content would be unusable at a size press.

The cost benefits of using an enzyme would be being able to use a cheaper un-modified wheat starch rather than a modified starch. It also allows for more control on the degree of conversion, the starch chain length and the solid content can almost be tailored made depending on the enzyme dosing.

The Enzyme is dosed with the starch slurry and heated up to around 80 (75 for pm6) Deg C to kick-start the reaction. As the starch goes through the reaction tubes the starch chains break down. The mixture is the flash-heated with steam to 130 Deg C to stop the reaction. If this step did not occur the starch would break down to glucose sugars which have no strength additives.

Using the Enzyme cooking technique allows for higher solid content at a lower viscosity when compared with Thermoconversion of starch resulting in a higher strength gain in the sheet. 

Enzymatic conversion has much lower power consumption when compared to the thermochemical cooking process. The temperature needed for the cook is much lower (80 Deg C). The cookers use a flash cooker method to kill the enzyme. A flash cooker allows hot steam to kill of the enzyme without having to heat the entire mixture up.

Starch retrogression (Papermakers amylose)

Starch retrogression or “amylose crystallization” is a result of the re-association (chains link back together) of linear amylose or straight-chain fragments and amylopectin branched fragments. This is an issue that can occur within starch cooking systems. Papermaker’s amylose can lead to a number of paper making problems including hard drying, picking, dusting, low porosity, and a weak sheet.