different types of vegetable varies according to the type of vegetable, different vegetables undergo
different texture changes when subjected to the same amount if heat (for example, the difference in
the texture of lettuce before and after boiling for a minute is much more pronounced the texture
change in carrots after boiling for only one minute).
Changes to cellulose: Cellulose is a polymer of glucose arranged in long straight chains. The absence
of side chains allows cellulose molecules to lie closely together and form rigid structures – the
cellulose is responsible for the toughness of the plant cell wall. As the cells are heated up, the
cellulose softens (but is not chemically modified), weakening the cell wall. This reduces the cells ability
to keep in its water, which then leaves, causing the plant to wilt.
Vegetables containing lots of cellulose will remain crisper after cooking, but will need to be cooked for
longer to obtain the desired tenderness. Vegetables low in cellulose will not remain firm after cooking.
Changes to pectin: Pectin acts as a glue to hold plant cells together. During cooking the pectic
polymers are chemically degraded into soluble molecules, which are released from the cell wall
causing the cell wall to weaken. This process is emphasised in the presence of alkali. Alkali causes
the ionisation of the -COOH groups from pectines, forming -COO- groups, which repel each other
favouring their separation and thus the degradation of cell walls.
Acid, however, strengthens pectin. In an acidic environment, pectin molecules are neutral – any
existing -COO- groups will acquire a hydrogen ion, forming neutral -COOH groups. This reduces
repulsion between pectin molecules, which are now all neutrally charged, so they stay associated and
are less likely to dissolve so readily when heated.
Changes to hemicellulose: During cooking the hemicelluloses break down into their constituent
sugars, further reducing cell wall strength.
Overall: ,All the above changes cause the cell wall to soften during cooking and becomes porous, and
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