During chemical modification of starches, the ______________ groups can be replaced with bulkier groups, which in turn will reduce the formation of gels, disallowing the chains to come together, if gel formation is undesirable for certain products.

Question

BlackTom AI · Accepted Answer

Hydroxyl groups on the glucose units of starch molecules are the reactive sites that can be substituted to modify functional behavior. Starch is composed of amylose and amylopectin, polymers of α‑D‑glucose linked primarily by α‑1,4 and α‑1,6 bonds. Each anhydroglucose residue presents three hydroxyl (–OH) groups (at C2, C3, and C6). These –OH groups engage in intra‑ and intermolecular hydrogen bonding, which promotes association of chains and formation of gels and crystalline regions.

Step‑by‑step reasoning:
1. Identify the functional group responsible for chain association: the hydroxyl (–OH) groups form hydrogen bonds between starch chains, enabling gel formation and retrogradation.
2. Chemical modification targets those hydroxyls: reactions such as acetylation (introducing acetyl groups) or etherification (introducing hydroxypropyl groups) substitute the –OH hydrogens with bulkier, less polar substituents.
3. Effect of substitution: replacing –OH with bulky moieties reduces the number and strength of hydrogen bonds and increases steric hindrance, preventing close chain packing and chain–chain association. As a result, gel formation is reduced or prevented when this is desirable in a product formulation.
4. Molecular locations and examples: common substitutions occur at C2, C3, and especially the primary C6 hydroxyl; commercially, starches are modified to acetylated or hydroxypropyl derivatives to lower gel strength and retrogradation.

Why other options are incorrect:
- Carbon: carbon atoms form the glucose backbone; you do not simply “replace” backbone carbons to alter gelation—such changes would destroy the polymer.
- Hydrogen: single hydrogen atoms are not the functional groups mediating intermolecular bonding; the hydrogen in an –OH participates in bonding, but the functional group is the hydroxyl as a whole.
- Oxygen: oxygen atoms are parts of the ring and glycosidic linkages and are not the typical substitutable functional group responsible for hydrogen bonding.

Therefore, the correct answer is hydroxyl, which matches the provided answer.

During chemical modification of starches, the ______________ groups can be replaced with bulkier groups, which in turn will reduce the formation of gels, disallowing the chains to come together, if gel formation is undesirable for certain products.单项选择题

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