# Food biopolymers, starches & polysaccharides Starch granule architecture, gelatinisation and retrogradation, why staling is amylopectin recrystallisation, the five classes of resistant starch, and enzymatic versus chemical modification. Bread goes stale because a polymer slowly recrystallises — not because it dries out. Source: https://en.bioecon.ru/docs/agri-food/food-alt-protein/food-biopolymers-starches-polysaccharides/ Updated: 2026-08-25 Starch is the most abundant food polymer, and almost everything it does in food is explained by one structure and two transitions. ## The granule Starch is stored in plants as semicrystalline granules built from two glucose polymers. Amylose is essentially linear. Amylopectin is very large and heavily branched, and its outer branches pair into double helices that pack into crystalline lamellae, alternating with amorphous regions to give the granule its layered organisation. That organisation is why raw starch is insoluble, resists enzymes and holds its shape. Everything useful is done by destroying it and then watching it partly rebuild. ## Gelatinisation, then retrogradation Heated in water, the granule absorbs water and swells, the crystalline order melts over a temperature range characteristic of the botanical source, and amylose leaches out. Viscosity rises steeply as swollen granules crowd; pushed further, granules fragment and viscosity falls again. This is gelatinisation — the thickening of a sauce, the setting of a custard, the transformation of dough into crumb. On cooling and storage the dispersed polymers begin to re-associate into ordered structures. This is retrogradation, and it happens on two timescales. Amylose recrystallises within hours, and it is what sets a cooled starch gel firm. Amylopectin recrystallises over days, its short outer branches slowly re-forming double helices. **Bread staling is that second process.** It is a polymer phase transition, not simple moisture loss — which is why staling proceeds fastest at refrigeration temperatures, well above freezing but below the range where recrystallisation is reversed, and why briefly reheating stale bread restores much of its softness by melting those crystallites again. Understanding this is what makes maltogenic amylases effective anti-staling agents: by trimming amylopectin's outer branches they leave chains too short to recrystallise readily. ## Resistant starch is a physical category Starch that escapes digestion in the small intestine reaches the colon and is fermented, behaving as dietary fibre. Resistance arises in five distinct ways, conventionally numbered: starch physically enclosed in intact cell walls or whole grains; native granules whose crystalline form resists amylase, as in raw potato and green banana; **retrograded starch**, where recrystallisation after cooking and cooling has made the polymer inaccessible; chemically modified starch; and starch complexed with lipid. The third of these is the mechanistic basis of the observation that cooking and then cooling starchy food raises its resistant-starch content. The effect is real; its metabolic significance at ordinary portion sizes is modest and frequently overstated in popular accounts. ## Modification, chemical and enzymatic Native starches are often unsuitable industrially — too fragile under shear, acid or freeze–thaw. Chemical modification substitutes or cross-links hydroxyl groups: cross-linking reinforces the granule against shear and acid, while substitution obstructs retrogradation and improves freeze–thaw stability. Both are regulated and must be declared. The enzymatic route uses branching enzymes, amylases and transglucosidases to rearrange the polymer directly, and is the basis of most clean-label alternatives. It gives less precise control over the outcome than chemistry does, which is the trade being made.