Stabilized starches, often referred to as modified starches, have become integral in various industries due to their unique properties and enhanced stability. This article provides a comprehensive exploration of the production techniques employed to create these innovative starch derivatives, shedding light on the intricate processes involved in their synthesis.
Production of Stabilized Starches
The creation of stabilized starches involves the reaction of starches with monofunctional reagents, as outlined by Wurzburg (1986a) and Rapaille and Vanhemelrjck (1997). This transformative process converts hydroxyl groups within starch molecules into larger ester or ether groups, effectively blocking interchain associations. The result is the formation of more stable pastes and gels, exhibiting a reduced tendency to undergo retrogradation. The introduction of certain groups with a negative charge further minimizes interchain associations, enhancing paste stability through like-charge repulsion.
Both esters and ethers are produced through a common method. A gelatinization-inhibiting salt, typically sodium sulfate (10–30% concentration), is introduced to a stirred slurry of starch granules (30–45% solids). The pH is adjusted to 8–12, depending on the desired reaction, and an alkaline pH facilitates nucleophilic substitution reactions by converting hydroxyl groups into alkoxide ions. The temperature is often maintained at around 49ºC. By utilizing a gelatinization-inhibiting salt and maintaining a temperature below the gelatinization threshold of both native starch and the product, pasting is prevented, allowing for the recovery of starch in granular form. After reaction with a monofunctional reagent, the stabilized starch product is recovered through filtration or centrifugation, followed by washing and drying.
Types of Stabilized Starches
Distinguishing between various stabilized starches involves their specific derivatization processes. Notable among these are starch acetates, l-octenylsuccinates, phosphates, succinates, hydroxyethyl starch, hydroxypropyl starch, carboxymethyl starch, and cationic starches.
- Starch Acetates: Prepared using acetic anhydride or, in regions where prohibited, vinyl acetate.
- 1-Octenylsuccinate Ester (OSA Starch): Resulting from the reaction with 1-octenylsuccinic anhydride.
- Phosphate and Succinate Starches: Produced by reaction with sodium dihydrogen phosphate or sodium tripolyphosphate (STTP).
- Hydroxyethyl Starch: Created through the reaction with ethylene oxide.
- Hydroxypropyl Starch: Derived from the reaction with propylene oxide.
- Carboxymethyl Starch (Sodium Starch Glycolate): Formed by the reaction with sodium monochloroacetate.
These derivatized starches exhibit a reduced tendency to undergo retrogradation, resulting in more stable pastes and gels. Additionally, some hydrophobicity is introduced, enhancing their versatility.
Manufactured Monostarch Ethers
- Hydroxyethyl Starch: Produced through the reaction with ethylene oxide.
- Hydroxypropyl Derivatives: Derived from the reaction with propylene oxide.
- Sodium Carboxymethyl Starch (Sodium Starch Glycolate): Formed by the reaction with sodium monochloroacetate.
These ethers share similar properties with monosubstituted starch esters but exhibit stability to acids and bases, setting them apart from esters.
Reactivity and Production Techniques
The reagents employed for monostarch esters, including acetic anhydride, OSA, and succinic anhydride, are relatively reactive. This reactivity suggests that reactions with these anhydrides may occur at or near surfaces before penetrating deep into the granule matrix. In contrast, reagents used for monostarch ethers, such as ethylene oxide and propylene oxide, exhibit lower reactivity, allowing for uniform distribution throughout the granule matrix.
Conclusion
The production of stabilized starches involves a meticulous interplay of chemical reactions and process parameters. The resulting derivatives exhibit unique properties that find applications across diverse industries. As research in this field continues to evolve, further innovations in the production of stabilized starches are anticipated, opening new avenues for their utilization in various industrial applications.