Ultrasonic Modification of Starch Granules Slurries
For most commercial applications starches must be modified chemically or physically to enhance their positive attributes or to minimize their defects. Ultrasonication is a highly effective means for the physical, chemical, and enzymatic modification of starches. Hielscher ultrasonic devices transfer highly intense ultrasonic waves into starch slurries. The resulting ultrasonic cavitation promotes:
- deagglomeration and dispersion
- mechanical degradation and disruption
- granule penetration and swelling
- mass transfer
- radical formation
- chemical reactivity
Chemical Modification of Starch
Ultrasonic cavitational disruption of the granule associated with higher facility for liquid entrance in the starch granule leads to improvements in the reaction kinetics for esterification, etherification, hydroxypropylation or oxidation and acid modification of starch polymers. Hielscher ultrasonic reactors are designed for a continuous inline processing. Higher reaction rates lead to increased reaction kettle capacity.
Alkaline Starch Modification
For the production of many commercial starch derivatives, reactive, organic reagents are added to aqueous starch slurries while controlling alkalinity and temperature. The esterification of starches is generally performed at pH 7 to 9. A pH of 11 to 12 is commonly used for the etherification of starches. Typical process temperatures are approximately 60°C. Without sonication, the degree of substitution of commercial starches is often less than 0.2. Ultrasonication assists the substitution resulting in a more cold-water-soluble starch.
Acidic Starch Modification
The reaction of a granular starch slurry with dilute hydrochloric or sulfuric acid at 40 to 60°C leads to fluidity starches or thinned starches. These partially depolymerized starches produce products that generate less viscosity. Starch octenylsuccinates are partially depolymerized to allow a higher solids content to be used while spray drying of encapsulated products. Ultrasonication during mild acid hydrolysis can dissociate the nanoparticle aggregates that form during the hydrolysis. This increases the yield of starch nanoparticles.
After the process, the reaction slurry is neutralized, e.g. by adding hydrochloric or sulfuric acid after alkaline processing.
Water washing, such as countercurrent washing in hydrocyclones, follows the neutralization of the modified starch slurries. At this stage, ultrasonication assists the washing and rinsing of the individual starch particles. The ultrasonic cavitation disperses starch granule agglomerates and increases mass transfer at the boundary layer between starch granules and the aqueous phase.
Starch Filtration and Drying
Hielscher ultrasonic devices are used ultra-filtration or nano-filtration processes as well as subsequent spray drying.
Physical Modification of Starch (Mechanical)
Physical modification of starches does not involve the use of chemicals. Yet, ultrasonication results in changes in the starch molecular structure followed by variations in physicochemical properties and functionality. The violent cavitational shear forces distort the crystalline region in starch granules. Polymer chains near the collapsing microbubbles are caught in a high gradient shear field that leads to the breakage of macromolecular C-C bonds, and the formation of long chain radicals. SEM pictures of sonicated starch granules show mechanical damages, such as fissures, depressions, and pitting. This results in a higher water absorption capacity, higher swelling power, and increased solubility. This effect is better for higher sonication amplitudes. Therefore, probe sonication is much more effective for starch modification than bath type sonication. Intense ultrasonic processing shows more disrupted granules when compared with native or heat-treated starch.
Ultrasonication can lower the beginning of the gelatinization temperature significantly. Starch gels prepared from sonicated starch granules present a higher hardness and higher values of adhesiveness and cohesiveness when compared with native starch. Adhesiveness, cohesiveness, springiness, and gumminess increase significantly with ultrasonic modification of starch.
Ultrasonication uses much less energy input and stressful processing conditions than conventional starch modification procedures. Hielscher ultrasonics supplies high power ultrasonic reactors for commercial processing.
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Modified starch is used in wide range of food and non food applications. Starch octenylsuccinates are an important stabilizer of oil-in-water emulsions. In papermaking cationic starches improve wet and dry strength, stabilize emulsions and act as a surface sizing agents. Many wet-end additive systems incorporate inorganic microparticles (colloidal silica, bentonite) and synthetic polymers with modified starch. Other uses include starch latex dispersions or granular starch as a filler for polymers.
Scientific Articles about Ultrasonically Assisted Starch Modification
- S. Manchun, J. Nunthanid, S. Limmatvapirat and P.Sriamornsak (2012): Effect of Ultrasonic Treatment on Physical Properties of Tapioca Starch, in: Advanced Materials Research Vol. 506 (2012) pp 294-297. [PDF]
- Anet Rezek Jambrak, Zoran Herceg, Drago Šubaric, Jurislav Babic, Mladen Brncic, Suzana Rimac Brncic, Tomislav Bosiljkov, Domagoj Cvek, Branko Tripalo, Jurica Gelo (2010): Ultrasound effect on physical properties of corn starch, in: Carbohydrate Polymers 79 (2010) 91–100.
- Herceg I.L., Jambrak A.R., Šubarić D., Brnčić M., Brnčić S.R., Badanjak M., Tripalo B., Ježek D., Novotni D., Herceg Z. (2010): Texture and pasting properties of ultrasonically treated corn starch, in: Czech J. Food Sci., 28: 83–93. [PDF]
- D. Knorr, B. I. O. Ade-Omowaye and V. Heinz (2002): Nutritional improvement of plant foods by non-thermal processing, in: Proceedings of the Nutrition Society (2002), 61, 311–318. [PDF]
Native Starch Sources
Starch comes from various native sources, such as: maize, waxy maize, high-amylose maize, tapioca, potato, wheat, rice, waxy rice, pea (smooth pea, wrinkled pea) sago, oat, barley, rye, amaranth, sweet potato, oat, cereal, cow cockle, quinoa, lentil, navy bean, sorghum, arrowroot or cassava.