FOOD CHEMISTRY: CARBOHYDRATES
Diagram
The starch cycle: swell, then set
Starch granule
Cold water: no change. Heat to about 60 to 70 C, water enters.
↓
Granule swells
Amylose leaks out. The paste thickens: GELATINISATION.
↓
Cool and store
Amylose chains line up again and form crystals. Water is squeezed out.
↓
Bread firms, starch sets
RETROGRADATION: the reverse of gelatinisation on cooling and storage.
Figure 1: Gelatinisation and retrogradation of starch, the cycle that thickens a sauce and stales bread. Drawn in our own style from the starch gelatinisation discussion in the FTN Food Chemistry notes, Unit 3.
Introduction
Gelatinisation and retrogradation are the two halves of one cycle in starch. Gelatinisation is what happens when starch meets hot water: the granules swell, amylose leaks out, and the mixture thickens. Retrogradation is the reverse on cooling and storage: the amylose chains line up again and form crystals, squeezing water out, and making bread go firm and stale. One is driven by heat and water, and the other by cooling and time.
Quick comparison
Table 1: Gelatinization compared with retrogradation
| Aspect | Gelatinization | Retrogradation |
|---|---|---|
| When | On heating starch in water | On cooling and storage of cooked starch |
| Temperature | For most food starches, over a range near 60 to 70 C | On cooling to room temperature and below, during storage |
| What happens | Water enters the granule, the granule swells, amylose leaks out, and the mixture thickens | Amylose chains line up again and form crystals, squeezing water out, a change called syneresis |
| Granule order | The granule loses its order and its birefringence, the cross pattern seen under polarised light | Order returns as crystals form again |
| Thickening | The paste thickens as granules swell | The gel firms as crystals form, and water is squeezed out |
| Which chain leads | Amylose leaks out of the swelling granule | Amylose lines up fast; amylopectin retrogrades more slowly, so staling continues over days |
| Reversible | Not reversed by cooling: cooling brings retrogradation, not the original granule | Partly reversed by reheating, but the books used here do not give a reheating temperature, so none is stated |
| Food effect | A sauce thickens, a starch paste sets | Bread goes firm and stale, a starch gel firms and weeps water |
| Useful side | Thickening and gelling in cooking | Resistant starch forms when cooked starch cools and retrogrades, as in cold cooked potato, and it behaves like fibre in the gut |
| What changes it | Sugar, salt, and acid in the mix change the temperature range | Storage time and temperature, and the amylose to amylopectin ratio, change how fast and how far it goes |
Scientific explanation
Definition
Gelatinisation is the swelling of starch granules in hot water: water enters the granule, the granule swells, amylose leaks out, and the mixture thickens. For most food starches this happens over a range near 60 to 70 C, though the exact range depends on the source and on sugar, salt, and acid in the mix. The granule loses its order and its birefringence, the cross pattern seen under polarised light. Retrogradation is the reverse on cooling and storage: the amylose chains line up again and form crystals, squeezing water out, a change called syneresis, and making bread go firm and stale. Amylopectin also retrogrades, more slowly, which is why staling continues over days.
Working principle
Starch is the main thickener and gelling agent in cooking: it swells in hot water, thickens a sauce, and sets a gel on cooling. In gelatinisation, water enters the granule and the granule swells, so the paste thickens. In retrogradation, the chains that were spread apart line up again and form crystals, so the gel firms and water is squeezed out. Modified starches are changed to suit a job: cross-linked starches resist acid and stirring, so they suit a fruit pie filling, and stabilised starches resist freezing and thawing. Resistant starch is starch that escapes digestion in the small intestine and behaves like fibre in the gut; it forms when cooked starch cools and retrogrades, which is why cold cooked potato holds more resistant starch than hot potato.
Apparatus
The books used here name no laboratory apparatus for gelatinisation or retrogradation, so no apparatus list is given. The change is read by eye and by touch: the temperature where a starch paste suddenly thickens, and how firm the paste is after cooling.
Measurement procedure
Make a thin starch paste from cornflour and water, heat it slowly, and note the temperature where it suddenly thickens: that is the gelatinisation range, near 60 to 70 C for most food starches. Let it cool in the fridge overnight and note how firm it is the next day: that firming is retrogradation. The books used here give no standard laboratory procedure with volumes and steps beyond this test, so no procedure is invented here.
Factors affecting readings
The starch source sets the gelatinisation range and the retrogradation rate, because the amylose to amylopectin ratio differs: amylose retrogrades fast, amylopectin slowly. Sugar, salt, and acid in the mix change the gelatinisation range. Storage time and temperature change how far retrogradation has gone: the longer the storage, the firmer the gel and the more water is squeezed out.
Formulas and calculations
This topic has no standard formula in the books used here. Gelatinisation and retrogradation are described by a temperature range, near 60 to 70 C for gelatinisation of most food starches, and by the firming of the gel on cooling, not by a formula, so no formula is given rather than inventing one.
Industrial applications
- Gelatinisation: thickening sauces, soups, and gravies, where starch swells in hot water.
- Gelatinisation: baking, where starch swells and sets the crumb.
- Gelatinisation: pie fillings, where cross-linked starches resist acid and stirring.
- Retrogradation: bread staling, which the baker must manage by storage and by recipe.
- Retrogradation: resistant starch in cold cooked potato and cooled cooked starch, which behaves like fibre in the gut.
- Both: frozen foods, where stabilised starches resist freezing and thawing.
Advantages and limitations
- Gelatinisation, advantage: a strong, simple way to thicken and gel foods, with no added ingredient beyond starch and water.
- Gelatinisation, limitation: it needs heat, and the paste thickens only in a narrow temperature range, so a sauce can go lumpy if it is heated too fast.
- Retrogradation, advantage: it makes resistant starch, which behaves like fibre, and it sets a gel firmly on cooling.
- Retrogradation, limitation: it makes bread go firm and stale, and it squeezes water out of gels, a change called syneresis.
FAQ
What is gelatinisation?
The swelling of starch granules in hot water: water enters the granule, the granule swells, amylose leaks out, and the mixture thickens.
What is retrogradation?
The reverse on cooling and storage: amylose chains line up again and form crystals, squeezing water out, and making bread go firm and stale.
At about what temperature does gelatinisation happen for most food starches?
Over a range near 60 to 70 C, depending on the source and on sugar, salt, and acid present.
Why does bread go stale?
Amylose chains line up again and form crystals, squeezing water out. Amylopectin retrogrades more slowly, so staling continues over days.
Which half of cooked starch holds more resistant starch, hot or cold?
The cold half: resistant starch forms when cooked starch cools and retrogrades, so cold cooked potato holds more resistant starch than hot potato.
References and further reading
- FTN subject: Food Chemistry, Unit 3: Carbohydrates.
- Zeece, Michael. Introduction to the Chemistry of Food. Academic Press, 2020.
- Coultate, Tom. Food: The Chemistry of Its Components. 7th ed., Royal Society of Chemistry, 2023.
- Damodaran, Srinivasan, and Kirk L. Parkin. Fennema’s Food Chemistry. 5th ed., CRC Press, 2017.
- Wang, Dongfeng, Hong Lin, Jianqian Kan et al. Food Chemistry. De Gruyter, 2015.
- Google Books pages read for this article:
- Fennema, Owen R., ed. Food Chemistry, 2nd ed., Marcel Dekker, 1985: carbohydrate chapter on monosaccharides, disaccharides and polysaccharides (Google Books search result, full text read via the search record)
- Provost, Joseph J. et al. The Science of Cooking. John Wiley and Sons, 2016 (Google Books page read)
