If you like gravy, risotto, sushi and French fries, you owe a lot to gelatinisation.

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About 3.5 million years ago our ancestors mixed up their diet. Prior to this our ancestors ate fruits and leaves, just like the other apes. But for whatever reason, at this time, our ancestors also started to eat grasses.

Once we popped though, we couldn’t stop. By about 100,000 years ago humans were processing starchy grains as a major part of their diet.

This made a lot of sense. There is a lot of energy in starch. That’s why plants pack it into their seeds as food for developing seedlings. There is a problem for humans though, we suck at digesting starch in its raw form.

Cows, and other ruminants, need multiple stomachs and some helpful bacteria to break down starch. With only the one stomach humans struggle. We need other ways to process starch before we eat it.

There are multiple ways to do this but, as the masters of fire, we learned how to cook our starch. Cooking starch breaks it down into a digestible form.

And there was a bonus. We learned that something magical happens when you cook starch in the presence of water. Something that we now call gelatinisation.

Gelatinisation turned out to be very useful.

Using gelatinisation we could thickens stews, sauces, and desserts. Without it there would be no gravy. Without gelatinisation mashed potato would be a lot less smooth, risotto less creamy and stir fries less glossy. We have gelatinisation to thank for french fries.

If you want to improve your cooking it pays to understand gelatinisation. So, let’s have a look at the science of starches and the miracle that is gelatinisation.

What is Starch and Why Do Plants Make It?

To understand gelatinisation we need to understand starch. Conceptually you can think of starch as plant fat. Plants use it to store excess energy, just like we store excess energy as fat.

Starch is a different kind of molecule from fat though. Starch is a carbohydrate, or complex sugar, that plants produce from glucose generated during photosynthesis (see here for refresher on sugars and carbohydrates if you need it).

Plants also use starch as an energy reserve for developing offspring. Potatoes, for example, have a lot of starch because they are a means of asexual reproduction for the potato plant. Similarly, wheat and other grains have a high starch content to provide energy to budding offspring when the seeds germinate.

The Chemistry of Starch: Amylose vs. Amylopectin

Chemically starch is a mixture of two different types of carbohydrate called amylopectin and amylose.

Amylose is a straight chain polysaccharide made up of between 500 and 5,000 glucose molecules linked together by glycosidic bonds (in the business known as alpha-1,4-glycosidic bonds).

Amylopectin is also a polysaccharide made up of between 500 and 20,000 glucose molecules. Unlike amylose, the glucose molecules in amylopectin are occasionally bound by an alpha-1,6-glycosidic bond which gives amylopectin a branched structure. A branch typically occurs every 25-30 units. This makes amylopectin a large and complicated molecule.

Natural starches typically consist of 10-30% amylose and 70-90% amylopectin. Though some cultivated plant varieties can have starches composed of almost 100% amylopectin. Glutinous rice, for example, that is used to make Thai sticky rice has a starch almost completely made from amylopectin.

Starch is Stored in Amyloplasts as Starch Granules

In plant cells starch is made and stored in specialised cellular compartments called amyloplasts. In the amyloplast starch is stored as insoluble particles called starch granules.

Starch granules are made of layers of crystallised amylopectin interspersed with less ordered regions composed of amylose and some of the amylopectin branches (if you want to look into this further you can start here).

Gelatinisation is simply the process in which these starch granules are broken down into a matrix of carbohydrates that trap water and form a gel. A gel that makes the carbohydrates available for digestion but which also thickens our food.

How Gelatinisation Works Step-by-Step

The starch granule is a robust structure which is why we can’t digest starch in a granule. But in cooking we break it down using heat.

All the amylopectin and amylose molecules in the starch granule are held together by chemical interactions (a lot of these interactions are hydrogen bonds). When you heat a granule the heat energy causes the carbohydrate chains to vibrate which disrupts the chemical interactions and the molecules begin to move apart.

In an aqueous environment water will take advantage of the extra space and start moving into the granule, forming it’s own hydrogen bonds with the carbohydrates.

The influx of water causes the granule to start swelling and as heating continues the granule will grow even more and amylose will start leaking from the granule.

Finally after more heating the granule will burst apart as the amylopectin melts, releasing all the starch molecules. Once burst the amylopectin and amylose form a large complex that traps water and forms a gel.

Comparing Common Culinary Starches

If all starch granules were created equal we’d only need one type of starch in the kitchen, but they are not.

Starch granules from different species of plant, or even different cultivars of the same species, can have different sizes, they can be made of different ratios of amylose to amylopectin and the amylose and amylopectin molecules can differ in size and branching complexity.

This means that different starches have different properties that make them suited to different applications in the kitchen.

We’ve already come across one example of this, the difference between the starch granules in waxy and starchy potatoes makes a big difference in the quality of a french fry.

Starch Type	Amylose Content	Gelatinisation Temp	Gel Behavior & Thermal Stability	Best Culinary Uses
Wheat Flour	Moderate (~25%)	51°C – 60°C	Heat-stable; moderate thickening power; browns via Maillard reaction (protein + starch)	Roux, gravies, long-simmered stews, baked goods
Cornstarch	Moderate to High (~25-28%)	62°C – 72°C (Full at 95°C)	Requires high heat; high clarity/gloss; can break down if boiled too long	Asian stir-fry sauces, fruit pie fillings, puddings
Potato Starch	Low to Moderate (~20%)	58°C – 65°C	Thickens rapidly at lower heat; breaks down quickly with prolonged cooking	Quick-thickening sauces, gluten-free baking, potato dishes
Waxy Rice / Glutinous Rice	Very Low (< 1-2%)	60°C – 70°C	Highly stable; soft, sticky texture; resists firming when cooled (low retrogradation)	Sticky rice, frozen food stabilization, gluten-free thickening

Gelationisation Temperatures

One of the ways in which starches differ is in the temperature at which they gelatinise.

Corn starch, for example, needs to be cooked at higher temperatures to gelatinise (from 62C to 72C with full gelatinisation at 95C), while flour has a lower gelatinisation temperature (51C to 60C). This means that flour will thicken your food faster at a lower temperature.

A lot of Asian cooking takes advantage of the high gelatinisation temperature of corn starch. In some Asian cooking food is cooked quickly at very high temperatures in a wok. The high gelatinisation temperature of corn starch makes it a good starch to use in these conditions.

Thermal Stability

Another consideration is the thermal stability of the starch gel. If you are making something that is going to simmer for a while and you add the starch early in the cooking process it is a good idea to select a starch that will not start breaking down during cooking.

Potato starch, for example, thickens quickly and strongly but it will break down if kept at a high temperature for too long and you’ll end up with a thin sauce at the end.

Flour is relatively heat stable and rice starches are very heat stable, which is why rice starches are often used for gluten free thickening. Corn starch, even though it has a high gelatinisation temperature, can break down if cooked too long.

If you are using a starch in a long cook and you are worried about its stability either monitor the temperature so it doesn’t get too high, decrease the cooking time or add the starch towards the end.

Thickening Power

The workhorses when it comes to thickening are the amylopectin molecules. Because of the size and ‘branchiness’ of amylopectin it does a good job of increasing the viscosity of a sauce.

What amylopectin doesn’t do well is impart strength to a sauce because the branches prevent the molecules from packing together too tightly. It is similar to what we saw with saturated and unsaturated fatty acids.

Amylose, a much smaller compact molecule without extensive branching, is able to form a much tighter association with other amylose molecules and this can impart more strength to a starch gel.

When serving a hot sauce or mashed potatoes, for example, this is not much of a consideration as you are aiming for viscosity not a strengthened gel. But it is a factor when we consider retrogradation.

Retrogradation: What Happens When Gelatinised Starch Cools?

Gelatinisation is not reversible, that is once you cool down the mixture the starch granules will not reform. What does happen is that the carbohydrates start to associate with each other again instead of with water.

This contraction in the carbohydrate matrix reduces the space available and forces some water out of the gel. This process is called retrogradation.

Because amylose packs together more tightly than amylopectin, a cooled starch gel that is low in amylose, made primarily of amylopectin, will be loose and runny. A gel with a lot of amylose will give you a much stronger gel.

You can see retrogradation in action in leftover food. Leftover mashed potato is stiffer and grainier than when first served because of retrograded amylose structures. Similarly, long-grained rice becomes hard when refrigerated over-night because of amylose.

Retrogradation is also important when the food will be cooled before eating. If you are making sushi, for example, the very low amylose content of short-grained rice means that they will retain some softness and stickiness when cooled. A cold long-grained rice sushi roll could be a little tough on the teeth with all the retrograded amylose.

Bread also retrogrades after baking and this is how, over time, it goes stale. The starch molecules in the bread contract, force out water and the amylose molecules form their tight association; the net effect of this is dry, hard stale bread.

Putting bread into the fridge only accelerates this process as the colder temperatures accelerate the re-association of the starch molecules.

Can Retrogradation Make Your Food Healthier?

Sometimes the new structures formed during retrogradation are more stable than the original starch granule and this can be used to reduce the amount of digestible starch in food.

If you cook and then cool things like pasta, potatoes and rice the retrograded amylose will be resistant to digestive enzymes in the gut.

Essentially you are turning starch into fibre and so you are getting less calories and potentially reducing the glycemic response to the starch content of the food (if your interested here is one paper on the topic).

The Roux: Starch, Protein, and the Maillard Reaction

You can also use starches to bring flavour to a dish. The classic example of this is a roux.

Traditionally a roux is made by cooking equal amounts of fat and flour over medium heat. It can form the base of a dish or be added later in the cooking process.

A roux can be cooked for a minute or two, known as a blonde roux, and it is used primarily to thicken a sauce. But, because flour has proteins as well as starch, if you continue to cook the roux you will start getting our old friends the Maillard reactions and the mixture will brown and develop nutty flavours.

The longer you cook a roux the less thickening power it will have, because the starch will breakdown, but the more flavour it will impart to the dish because of all the Maillard reactions.

The classic example of using a brown roux is gumbo. In gumbo the flavour of the roux is more important than it’s thickening power, file powder and okra will help thicken the sauce towards the end anyway.

Because the Maillard reactions are between a sugar and a protein, if you try this with pure starches, like corn starch, you’ll still get a thickening agent but no browning as they don’t have the protein content that flour does.

Conclusion

Carbohydrates get a bit of a bad rap these days. Mostly because, like most things, we have them in massive abundance. Historically, though, an awful lot of humans would have gone very hungry without starches.

In moderation starches are an essential part of our diet. Without the starches that we harvest from our domesticated grasses, like wheat and rice, our diet would not only be much poorer but also less interesting.

Without gelatinisation some of my favourite foods wouldn’t exist. From mac and cheese to risotto, we owe a lot to starches, and gelatinisation.