Video transcript: sauce making

Flour contains ‘starch’ and starch is a type of carbohydrate made from long chains of simple sugars joint together.

Like sugars, starch absorbs Liquid. When starches are heated with liquid, they swell and will thicken. This is a key process in sauce making, it is known as gelatinisation.

As a white sauce is heated, the starch grains soften, they absorb liquid & swell. The starch grains break open & thicken the liquid by releasing amylose.

The process of gelatinisation starts at 60◦, the sauce begins to thicken at about 85◦ but it’s not fully completed until it reaches 100◦. During heating, the starch grains swell to more than five times their normal size…

…at this point the starch will break open and the amylose will be released into the sauce, it will thicken the sauce.

Making a Roux sauce

A gelatinised sauce making method that makes use of these processes is the Roux sauce.

Roux is a thickening agent for liquids, and it’s a fancy name for flour mixed with fat. Equal parts of butter and flour get cooked over a medium heat, before liquid gets added gradually, stirring after each addition.

The mixture then boils, thickens ‘reduces’, and becomes the base of your sauce.

And here we have it, so we now have our thickened, gelatinised Roux sauce.

Making a Velouté sauce

So we’re now going to do a Velouté sauce, which is started the same way as you would with the roux, we’re going to melt off the butter, once that’s melted we’re going to add the flour & to that I’m going to add a stock.

As it heats up the starch will begin to swell and then we’ll have gelatinisation and a thickened velouté sauce.

So now the sauce is just beginning to bubble and as it bubbles up you can see that the sauce has now gelatinised & formed a lovely, creamy velouté sauce.

Heat transfers

How heat is applied to the sauce to is known as ‘heat transfer’ and there are three methods of heat transfer.

Heat transfer method 1: conduction

So the method of heat transfer used in cooking the roux is primarily through conduction…the heat from the hob passes through the metal pan & heats up the sauce.

Heat transfer method 2: convection – (this can be natural or mechanical)

In natural or mechanical convection heat is transferred through the gases or liquids. ‘Natural’ convection occurs when gas or liquid closest to the heat becomes less dense, rises and is replaced by a cooler denser gas or liquid.

The convection cycle is very similar to what happens inside a lava lamp.

[Joe stood next to lava lamp] – so liquid or gas closer to the heat source becomes less dense, rises, cools down, becomes denser and then falls & this is the convection method going on within the sauce.

Mechanical convection  – so in this roux sauce there is not as much liquid and it’s a very dense consistency so we have to do something called mechanical convection, where we aid the convection ourselves by stirring & also to prevent the lumps from coming together as well.

Heat transfer method 3: radiation

To show how radiation works we are going to add some cheese to our roux sauce and make a cauliflower cheese.

We’re going to pour that over the top of our cauliflower & then add some more grated cheese over the top.

This method of heat transfer is called radiation. Heating by radiation takes place when heat is transferred directly onto food by info red rays from the heat sauce.

Grill for about 5 minutes

And now the finished product; the cauliflower cheese has been browned and crisped by radiation.

Heat transfer in ovens

We’re now going to look at the different heat transfers for different types of oven & how that affects the end product, so we’ve got 2 identical batches of scones & we’ll see how quickly & how evenly they cook in each of the different types of oven.

Batch 1 – gas oven

Batch 2 – electric oven

How heat transfers in a gas oven

In gas ovens the heat is transferred from the gas flames around the oven by ‘convection’.

The heat is also transferred from the metal baking tray to the food by ‘conduction’.

So here are the results from the gas oven, so this is the top level of the oven & the hottest part of the oven, which has resulted in our scones with a slightly darker colour to them, this is the middle of the oven, medium temperature of the oven & this is where we’ve got nicely cooked and finished off scones, and then we have the bottom layer of the oven where the scones are a little undercooked.

Now we have 3 different results for the 3 different heights within the oven which shows the inefficiency of the gas oven, but there are advantages to having 3 different temperatures in the 1 oven……you could have 2 or 3 things cooking at the same time for different lengths of time...

……so you might think it’s strange that the bottom of the oven should have the coolest point when the flames are at the bottom, but hot air rises and as it rises to the top the hottest part of the oven is the top, the air cools and then goes down to the bottom…. And this is the convection currents going round the oven cooking the food.

How heat transfers in an electric oven

Here are the results in from the electric oven: you can see that they are far more even & consistent than the gas oven.

In electric fan ovens there is ‘convection’ heat but it is far more efficient using built in fans to quickly move the hot air around.

There is also still heat transferred from the metal baking tray to the food by ‘conduction’, plus there is radiation (from the walls of the oven) & this all means food cooks far more quickly & evenly.

Cooking by radiation in the microwave

As I said in the introduction to sauce making, starch gelatinisation is when the starch granules break open to release amylose into the liquid.

It begins at 60°C and the liquid will thicken at about 85°C, but it is not fully complete until the sauce has boiled

Now in a microwave there is a variety of temperatures going on so to do a sauce in a microwave we’re looking at a different method, we’re going to use an all in 1 method

All of the ingredients are in the 1 bowl

Microwaves ovens use electromagnetic waves to cook food, microwaves are a form of radiation

We’re going to remove the bowl & stir away, not only are we aiding through mechanical convection but we’re also stopping any lumps forming within the sauce

Repeat the process of cooking for 2 minutes and then whisking until you have a smooth sauce

So this is an all in one white sauce, made with flour, butter & milk in the microwave & the method of heat transfer is radiation.


  • It is the flour in a sauce that causes it to thicken by gelatinisation.
  • For gelatinisation to take place the sauce needs to be heated.
  • During this process, the starch grains swell and grow up to 5 times their normal size.
  • Starch gelatinisation is when the starch granules break open to release the amylose into the liquid.
  • Gelatinisation begins at 60°C and the liquid will thicken at about 85°C, but is not fully complete until the sauce has boiled.
  • You have seen sauces made by the roux method and all-in-one method, on the hob and in the microwave.
  • Conduction, convection and radiation are all methods of heat transfer from the cooker to the food.