Drying Starch
Starch drying is a process of fundamental importance before placing the product on the market. The purpose of drying starch and food, in general, is crucial as it reduces the amount of water inside. In fact, without this process, the foods would develop pathogens, fungi, and spores harmful to the organism. This is the primary purpose of dry food, to reduce the possibility of pathogens developing. For this reason, the optimal value so that there is no uncontrolled growth of bacteria turns out to be around 20%. Drying starch also has other advantages. The weight reduction due to water evaporation reduces the final weight, reducing transport costs.
Even though potatoes only contribute a few percent to the global starch production, they are still a rich source, which makes it an excellent feedstock depending on the local climate.
Even though potatoes only contribute a few percent to the global starch production, they are still a rich source, which makes it an excellent feedstock depending on the local climate.
Saving energy with scCO2
Although drying starch with high-temperature ovens is a highly efficient method and, as a result, the most used process in the industry, it suffers from many disadvantages. One of the most significant disadvantages is using very high-temperature ovens (160 °C), which involves using a substantial amount of energy, particularly methane gas, to power the ranges. Therefore, the massive use of methane gas affects the emission of CO2 into the atmosphere and pollutants.
Last but not least, it is necessary to consider the effect on the final structure of the starch, which is altered by the temperature, modifying the final properties of this crucial bio-based polymer. Therefore, the need arose to find alternative methods to dry that are less energy-intensive while maintaining the final properties of the starch intact. A valid alternative can be the use of CO2 in the supercritical state. This requires lower operating temperatures, approximately 40 °C, and it is also essential to compress it to 70 bar.
This project aims to evaluate the use of CO2 in a supercritical state for drying food and, particularly, of starch, thanks to continuous extractors able to recycle CO2 continuously. It can also recover the water extracted from the starch during the process, constituting an excellent example of a circular economy.The use of supercritical CO2 in dry foods has already been studied for other foods, including carrots [3], coriander [4], and chicken [5].
Starch is used and is widely used in the food, paper, textile, chemical and pharmaceutical industries as a thickener, stabilizer, adhesive, gelling agent, water retention agent and filler agent.
Although potato starch production is lower than world production, but this tuber plays a significant role in starch production, especially in Europe. In modern factories, the tuber is suitably chosen so that it has the highest yield of medium possible; for this reason, these potatoes are not suitable for food use.
The potatoes are harvested and processed in Europe between August and April; this period is referred to as the starch campaign. The process currently used for starch drying requires energy as very high-temperature ovens are used [1].
Starch
Current drying methods
In addition to starch, potatoes also contain a lot of water. To remove the water, the starch is dried in a sequence of two steps:
-
The goal of the first process unit is to remove as much water from the starch as possible. The more water mechanically removed in this step, the less that will have to be removed by evaporation in a dryer. The water from the concentrate from the starch refinery is removed on rotating vacuum drum filters. A starch cake with 40% moisture is produced [2].
-
The dryer has four distinct parts. The first part involves heating the air to 150°C. Generally, this is done in a heat-exchanger using steam. The second part, the heart of the dryer, is a tube where the starch is dried utilizing hot air. The residence time (and thus the drying time) is ~2 seconds. The third part is the separation section, where the dried starch is separated from the air. This is done in dust cyclones. The fourth part of the dryer is the transportation element .
The figure below illustrates both steps
[1] J. J. M. Swinkels, “Industrial starch chemistry: Properties, modifications and applications of starches,” AVEBE U.A. , P.O. Box 15, 9640 AA, Veendam, Netherlands, p. 48, 1996.
[2] H. E. G. and D. A. van der Krogt, “Potato starch,” AVEBE U.A. , P.O. Box 15, 9640 AA, Veendam, Netherlands, vol. 1, no. 5, pp. 16–17, 2009, doi: 10.1007/BF02894904.
[3] Z. K. Brown, P. J. Fryer, I. T. Norton, S. Bakalis, and R. H. Bridson, “Drying of foods using supercritical carbon dioxide - Investigations with carrot,” Innov. Food Sci. Emerg. Technol., vol. 9, no. 3, pp. 280–289, 2008, doi: 10.1016/j.ifset.2007.07.003.
[4] S. Bourdoux et al., “Inactivation of Salmonella, Listeria monocytogenes and Escherichia coli O157:H7 inoculated on coriander by freeze-drying and supercritical CO2 drying,” Innov. Food Sci. Emerg. Technol., vol. 47, no. September 2017, pp. 180–186, 2018, doi: 10.1016/j.ifset.2018.02.007.
[5] G. Morbiato et al., “Supercritical carbon dioxide combined with high power ultrasound as innovate drying process for chicken breast,” J. Supercrit. Fluids, vol. 147, no. February, pp. 24–32, 2019, doi: 10.1016/j.supflu.2019.02.004.