Starch gelatinisation and retrogradation
What starch is, how it gelatinises and pastes when heated, why bread stales and rice hardens (retrogradation), modified starch, and pasting sums.
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Starch supplies most of the calories in the human diet, from rice, wheat, maize, potatoes, cassava and other staples. Raw starch is hard to digest and has little thickening power; cooking transforms it. When starch is heated in water it gelatinises, absorbing water and swelling, which is why rice softens, sauces thicken and bread crumb forms. When cooked starch cools and ages, it partly retrogrades, which is why bread stales, leftover rice hardens and some puddings weep. These two processes shape the texture, shelf life and nutritional value of countless foods.
Starch structure, gelatinisation and retrogradation are core topics in food chemistry and cereal technology exams.
Starch structure
Starch is made of glucose units in two polymers:
- Amylose: mostly linear chains of glucose linked α-1,4; typically about 20 to 30 per cent of normal starches.
- Amylopectin: large, highly branched molecules with α-1,4 chains and α-1,6 branch points; the majority of most starches.
Starch is stored in granules whose size and shape differ by plant: large oval potato granules, small polygonal rice granules, lenticular wheat granules. Inside, amylopectin forms alternating crystalline and amorphous layers, so raw granules show birefringence (a "Maltese cross") under polarised light.
Waxy starches (waxy maize, glutinous rice) contain almost no amylose; high-amylose starches contain much more. Amylose content strongly affects texture: high-amylose rice cooks fluffy and firm, while glutinous rice is sticky.
Gelatinisation
When starch is heated in excess water:
- Water enters the amorphous regions and the granules swell.
- Above the gelatinisation temperature, typically about 55 to 75 °C depending on the starch, the crystalline order melts and birefringence is lost.
- Amylose leaches out of the granules.
- With continued heating and stirring, swollen granules become fragile and may break down.
Gelatinisation needs water: in low-moisture foods such as biscuits much starch stays ungelatinised. Sugar and fat raise the gelatinisation temperature and slow swelling, which is important in cakes, where starch must set only after the batter has risen.
Pasting
Pasting describes the changes in viscosity during heating, holding and cooling, measured with instruments such as the Rapid Visco Analyser (RVA) or Brabender Amylograph.
- Pasting temperature: where viscosity begins to rise.
- Peak viscosity: maximum swelling.
- Breakdown = peak − trough: how easily swollen granules break down.
- Setback = final − trough: re-association of amylose on cooling, linked to gel firmness and retrogradation.
Retrogradation
As gelatinised starch cools and is stored, starch chains re-associate into ordered, partly crystalline structures:
- Amylose retrogrades quickly, within hours, giving gel firmness on cooling.
- Amylopectin retrogrades slowly, over days, and is the main cause of bread staling: the crumb becomes firm and crumbly even though it has not dried out.
Retrogradation also causes syneresis (water separating from gels), hardening of cooked rice and noodles, and the firming of potato and cassava products. It is fastest at refrigeration temperatures, which is why bread stales faster in the refrigerator than at room temperature, while freezing slows it greatly. Reheating partly reverses staling.
Retrograded starch forms part of resistant starch (type 3), which escapes digestion in the small intestine and behaves like dietary fibre, lowering the glycaemic response of foods such as cooled potatoes and rice.
Starch in everyday foods
- Rice and noodles: gelatinisation during cooking softens them; amylose content and retrogradation decide whether leftovers harden.
- Bread: starch gelatinises partly during baking, setting the crumb, then retrogrades during storage.
- Sauces, soups and custards: starch thickens on heating; overheating and stirring can thin them as granules break down.
- Parboiled rice: soaking, steaming and drying gelatinises starch inside the grain, hardening it so it breaks less in milling.
- Cassava products such as gari and tapioca pearls depend on partial gelatinisation during roasting or steaming.
Modified starches
To suit industrial uses, starches are modified:
- Pregelatinised starches thicken in cold water (instant puddings).
- Cross-linked starches resist breakdown in heat, acid and shear (canned foods).
- Substituted (acetylated, hydroxypropylated) starches resist retrogradation and freeze–thaw damage.
- Acid-thinned and oxidised starches give low viscosity for gums and coatings.
- Enzymatic hydrolysis gives maltodextrins and glucose syrups.
Reading a pasting curve
An RVA test on a rice flour gives a peak viscosity of 3,000 cP, a trough of 1,800 cP and a final viscosity of 3,200 cP.
- Breakdown = 3,000 − 1,800 = 1,200 cP.
- Setback = 3,200 − 1,800 = 1,400 cP.
A second rice flour gives peak 2,600, trough 2,000 and final 3,800 cP: breakdown 600 and setback 1,800. The second rice has more stable granules and a higher setback, typical of higher-amylose rice, which cooks firmer and hardens more on cooling. Breeders and millers use such profiles to predict cooking and eating quality.
Key points
- Starch = amylose (linear, α-1,4) + amylopectin (branched, α-1,6); about 20–30 per cent amylose in normal starches; waxy starches lack amylose.
- Gelatinisation: water uptake, swelling and loss of crystallinity and birefringence, typically at about 55–75 °C; needs water.
- Pasting: peak viscosity, breakdown (peak − trough), setback (final − trough).
- Retrogradation: amylose (fast, gel firmness) and amylopectin (slow, bread staling); fastest at refrigeration temperatures; forms resistant starch.
- Modified starches are pregelatinised, cross-linked, substituted, acid-thinned or hydrolysed.
What the exams ask
- School-leaving and entrance papers: what happens when starch is cooked, and why bread goes stale.
- Degree and research-fellowship entrances: starch structure, gelatinisation mechanism, pasting curves, retrogradation and resistant starch, and modified starches.
- Food technology examinations: starch functionality in products, cereal quality evaluation, anti-staling strategies and starch processing.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does bread go stale faster in the refrigerator?
Retrogradation of amylopectin is fastest at low temperatures just above freezing. Bread keeps its softness longer at room temperature or frozen.
Why is cooled potato or rice said to be healthier?
Cooling causes some retrogradation, forming resistant starch that is digested more slowly, lowering the blood glucose response somewhat.
What makes glutinous rice sticky?
It contains almost no amylose; its amylopectin swells and becomes sticky when cooked and retrogrades only slowly.
Sources
- BeMiller, J. N. and Whistler, R. L. (eds) Starch: Chemistry and Technology, 3rd edition. Academic Press, 2009.
- Damodaran, S. and Parkin, K. L. (eds) Fennema's Food Chemistry, 5th edition. CRC Press, 2017. Chapter on carbohydrates.
- Delcour, J. A. and Hoseney, R. C. Principles of Cereal Science and Technology, 3rd edition. AACC International, 2010.
- Wang, S., Li, C., Copeland, L., Niu, Q. and Wang, S. "Starch retrogradation: a comprehensive review." Comprehensive Reviews in Food Science and Food Safety 14, 568–585 (2015).
- Englyst, H. N., Kingman, S. M. and Cummings, J. H. "Classification and measurement of nutritionally important starch fractions." European Journal of Clinical Nutrition 46, S33–S50 (1992).
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