Introduction

The final step in bread making is the baking process in which the dough piece is transformed into a light readily digestible and flavourful product under the influence of heat. Within this baking process the natural structures of the major dough constituents are altered irreversibly by a series of physical, chemical and biochemical interactions. Several apparent phenomena are caused by oven heat:

Meanwhile, the formation of new flavour substances, such as caramelised sugars, pyrodextrins and a broad range of aromatic compounds also accompanies this process. Even tough many of the chemical and physical reactions happening at this stage are only partially understood, no doubt the quality of baked products is influenced by the rate of heat application and the amount of heat supplied, the humidity in the baking chamber and the baking time.

The following table and pictures show what is happening at various temperatures to the dough during the baking process.

temperature

phenomena

30C

because of the rising temperature, gasses present in the dough expand
enzymatic production of sugars
solubility of CO2 decreases

45 - 50C

yeast dies

50 - 60C

intensive enzymatic activity
starch starts to gelatinise

60 - 80C

end of the gelatinisation of starch
enzymatic activity ceases because of the denaturation of the enzymes
crumb starts to form
interaction between gluten and starch

100C

water starts to boil
formation of water vapour
first signs of crust formation

110 - 120C

formation of pale yellow dextrins in the crust

130 - 140C

formation of brownish dextrins in the crust

140 - 150C

start of caramelisation process

150 - 200C

formation of the "crustiness" of the bread and aromatic compounds

> 200C

carbonisation of the crust
formation of a porous black mass

The next pictures show the changes in the dough during the fermentation and baking process

Heat transfer in the oven and in the bread

Heat is transmitted to the dough in three different ways:

All three heat transmission modes play important roles in baking. Their relative importance depends on the type and the design of the oven.

During the baking process, the heated internal surface of the oven emanates invisible infrared rays, which are called radiant heat. This heat is absorbed by the exposed surface of the products thus increasing their temperature. Thermal radiation is a process in which energy is emitted by a heated body in the form of electromagnetic radiation. Infrared rays travel at the speed of light directly to the point of absorption. This kind of thermal radiation represents the most complex mode of heat transmission.

Convected heat is distributed through the baking chamber by the turbulence of the atmosphere and is transferred by conduction to the products when the hot air contacts their surfaces. In general, the more rapid the air movement, the more rapid and efficient the heat diffusion will be. Based on this concept, fans or blowers are used to enhance the efficiency of the heat transfer by convection. However the downside of this technique is that the crust can become dry, a characteristic that is not wanted in for instance hamburger buns.

During the baking process, the side and bottom crusts of the products absorb the heat that is transmitted by the pan walls or the stone hearth and this gradual heating of the interior of the bread is called conduction. Conduction heat and radiant heat raise the temperature of the bottom and sidewalls (in case the product is baked in a pan) and then the heat is transferred into the interior of the products

The surface of the dough will get warm quite rapidly whereas the interior of the dough is warmed progressively more slowly as the distance from the surface increases. Within the first third of the baking period the temperature of the dough surface will reach about 150°C, and it will then in crease slowly to 180°C or higher at the end of the baking. The temperature of the crumb never exceeds the boiling point of water (i.e. 100°C) and the centre of the loaf does not reach the maximum temperature until the end of the bake. It is my experience, in order to get a well baked loaf with a nice soft chewy and stabilised crumb (i.e. the loaf does not shrink on cooling), one has to maintain the boiling point of water in the crumb for at least about 1/4th of the total baking time.

The following graph shows a typical baking curve.

This curve was made with a Datapac logger: 3 probes were measuring the oven temperature and 3 probes measured the crumb temperature (the black, purple and yellow line). All three probes clearly reach 100°C. The fact that one line lags a bit behind another line is due to the fact that the probes are not all sitting at exactly the same debt in the dough. The 3 other probes (blue, green and red line) measure the oven temperature and it can be clearly seen that the temperature in the oven is quite uniform as the probes were placed in such a way that the temperature was recorded to the left, centre and right of the oven. There is also a 7th line which represents the humidity in the oven. It can be seen that when the bread is steamed as it enters the oven, the humidity shoots up immediately to 100 %, but as soon as the bread past the "humidity zone" in the oven, the humidity drops rapidly to about 5 %.

So when is a bread baked? This is one of my favourite questions when teaching or giving professional presentations. Most people really don't know the answer to that question. To my mind it is baked when the water has boiled sufficiently long enough to stabilise the crumb and this in my experience takes about one quarter of the total baking time.

From the graph it can be seen that the pattern of temperature rise in the dough follows a three-phase sequence. The rates of these different reactions and the order in which they take place depend to a large extent upon the rate of heat transfer through the batter or dough. If the crust forms before the mass centre is baked the centre of the baked item nay remain soggy or late escaping gas may crack the crust due to the top heat being to high as compared with the bottom heat.

There is another important aspect to all this. The temperature inside the dough does not rise above 100°C and the dough in fact is heated by something we call "the principle of Watt". As the dough heats up, water starts to boil thus creating water vapour. This vapour diffuses in all directions: towards the surface of the dough and it escapes (as a result the dough loses weight during baking) but also towards the centre of the dough piece. As it encounters a cooler layer in the dough the water vapour will condense, release the heat of condensation and in that why the cooler layer will warm up and eventually the water will start to boil in that layer too. Important to understand is that the oven temperature doesn't really matter, inside the dough the temperature is 100°C and will stay 100°C as long as there is water present.

The following table gives an idea of the moisture content of the dough or bread after various baking times at various debts from the crust. Zero mm means the crust of course. It also can be seen from this table that a few cm under the crust, the moisture content in the dough doesn't really change any more. It stays around 44 - 45 %. Interesting to note is also the fact that a freshly made dough has about the same moisture content. So the weight loss during baking is due to the formation of the crust and not because the crumb loses moisture.

Moisture of the crumb during baking

baking time

0 mm

15 mm

30 mm

45 mm

60 mm

70 mm

22'

6,0 %

18,7 %

44,1 %

44,9 %

44,8 %

45,0 %

30'

5,4 %

14,3 %

43,5 %

44,8 %

45,1 %

44,6 %

38'

3,7 %

15,0 %

41,7 %

45,5 %

45,6 %

45,0 %

That's also why the crust can get darker, because it gets hotter because it dries out and there is no (or very little) water left in the crust. And that is what the oven temperature will influence: the speed at which the crust loses its water and hence it will influence the colour of the crust. Also important to note that the baking time does not really depend on the oven temperature but on the physical shape of the dough i.e. the thickness of the dough because it influences the distance over which the heat transfer has to travel within the dough.

Have a good look at the follozing tables as well:

Baking time and moisture content of the bread

baking time

water absorption of the floru

moisture content of the bread

20'

56 %

35,9 %

22'

56 %

35,3 %

24'

56 %

34,6 %

Baking time and moisture content of the bread

baking time

water absorption of the flour

moisture content of the bread

20'

60 %

36,3 %

22'

60 %

36,6 %

24'

60 %

35,6 %

Baking time and moisture content of a bread

baking time

water absorption of the flour

moisture content of the bread

24'

56 %

34,6 %

24'

58 %

34,9 %

24'

60 %

35,6 %

From this tables we can draw the following conclusions

  1. As the baking time increases the moisture content decreases. This seems obvious but sometimes it is good to put the obvious things in writing. But is not as much as one might expect: 4 minutes more on 20 minutes means 20 % longer baking time but the moisture difference between the two is only about 1,3 %. This is further proof that the moisture loss is due to the crust formation but if the crust is already dry it can hardly lose more moisture.
  2. Increasing the water content in the dough does not mean that the bread will contain more moisture or that it will be softer. Increasing the water content with 4 % only marginally increases the moisture content of the final loaf.

Oven spring

The volume increase during the initial stage of baking is called the "oven spring". The phenomenon of oven spring involves three effects:

The overall contribution of the CO2 to the oven spring accounts for about one half of all the expansion reactions and the evaporation of ethanol due to temperature increase is responsible for the remaining expansion. During the oven spring stage, small gas cells require more pressure to expand then large cells. Hence once the pressure in the gas cells of the dough exceeds the critical limit so the cell walls suddenly give way, the cells expand dramatically. At this stage the crust begins to form and the oven spring will cause the breaking and shredding of the upper crust.

Reactions during baking

Caramelisation is the process by which colourless sweet substances, under the influence of heat, are transformed into compounds varying in colour from light yellow to dark brown and producing a mild and pleasant caramel flavour. If the caramelisation process goes on too far the result will be a dark brown or black, bitter or acrid tasting crust. All the compounds that form during caramelisation have not yet been identified. However the end products of these reactions are mostly unsaturated complex polymers.

Another major thermal chemical reaction during baking is the Maillard reaction. The Maillard reaction is a chemical reaction between an amino acid and a reducing sugar, usually requiring heat. It is vitally important in the preparation or presentation of many types of food, and, like caramelisation, is a form of non-enzymatic browning. The reaction is named after the chemist Louis-Camille Maillard who first described it in the 1910s while attempting to reproduce biological protein synthesis, although it has been used in practical cooking since prehistoric times.

The reactive carbonyl group of the sugar reacts with the nucleophilic amino group of the amino acid, and forms a complex mixture of poorly characterized molecules responsible for a range of odours and flavours. This process is accelerated in an alkaline environment as the amino groups are deprotonated and hence have an increased nucleophilicity. The type of the amino acid determines the resulting flavour. This reaction is the basis of the flavouring industry. At high temperatures, however, the harmful acrylamide can be formed.

In the process, hundreds of different flavour compounds are created. These compounds in turn break down to form yet more new flavour compounds, and so on. Each type of food has a very distinctive set of flavour compounds that are formed during the Maillard reaction

It is easy to get confused between caramelisation and the Maillard reaction. However there are some basic differences that can be used in identifying these two reactions. In general caramelisation needs higher temperatures to initiate the reaction than does the Maillard reaction. Also the flavour and aromatic compounds formed by these reactions are totally different. A lot of research has been done into the flavour and aroma of bread. Lane and Nursten studied over 400 model systems involving mixtures of 21 amino acids and eight sugars that were heated under different conditions of temperature and humidity. They found that the odours of bread crust, biscuits, cakes and so on were produced by heating the carbohydrates with the amino acids arginine, glutamine, histidine, lysine, proline, serine, threonine and tyrosine at temperatures of 100 to 140°C for 0,5 to 4 minutes.

During the baking process the starch granules begin to swell at a temperature of about 40°C. The viscoelastic properties of dough are replaced by fluidity when the temperature reaches the range of about 50 - 65°C. In the initial stage of gelatinisation, the starch granules absorb both free water and water held by the proteins of the dough. However a large proportion of the granules remain intact until the end of gelatinisation due to the fact that there is not enough water present to gelatinise all of the starch.

The extent of starch gelatinisation is influenced by water availability, temperature and the duration of its action on the starch. In general there is a higher degree of starch gelatinisation between the crumb and the crust than in the centre of the bread due to longer exposure at higher temperatures.

Another reaction taking place during baking is the denaturation of the proteins. The gluten forming proteins binding approximately 31 % of the total water absorbed by the dough are present in the dough in a hydrated state. They contribute to the formation of the dough structure by providing the matrix in which small starch granules are embedded. The proteins begin to undergo thermal denaturation when the temperature of the crumb reaches about 60 tot 70°C. The denatured proteins start losing their water-binding ability and release the water from the protein to the starch which at that point is in the full process of gelatinisation. On the other hand, when the temperature of the dough rises above 75°C, the gluten films surrounding the individual gas vacuoles are denatured by heat and transformed into a semi-rigid structure by interaction with the swollen starch.

Of course also enzymes are proteins. As the starch gelatinises the amylases accelerate the hydrolysis of the starch. However these enzymes are inactivated quite rapidly due to the high baking temperatures. The starch amylolysis is stopped when all amylases are inactivated. In the early stage of the process, the amylase contributes to:

Inadequate amylase reactions will cause product defects. The bread will have a small volume is the amylase activity is too low. Conversely, excessive amylase activity produces overexpansion of the loaf and may cause the loaf to collapse. Such a phenomenon can be observed for instance in flour that has been milled from sprouted wheat.

Finally there is one more important aspect and although it is not a chemical reaction, this aspect is nonetheless an important one. I'm referring to the water movement within the dough. During the first few minutes of baking, the oven atmosphere reaches a quasi moisture saturated condition. Even a slight water uptake of the dough takes place at this time as some of the steam will condense on the colder dough surface. As the surface temperature rises above the dew point of the baking chamber atmosphere, crust formation begins and the moisture in the outer layers of the bread turns into steam. Most of the steam evaporates into the air and a small amount migrates into the interior of the dough. As baking proceeds, the moisture content of the crust and outer layer is reduced to about 5 %, whereas in the interior the moisture of the crumb remains relatively constant (see table above). Once the bread is removed from the oven right away the moisture of the crumb migrates into the dry crust. During cooling the moisture evaporation of the crust continues until the water content of the loaf is about 38 %.

From all this it is readily understood that it is important to evacuate the excess moisture out of the baking chamber. That is why dampers should be open during the baking process (except maybe at the end of the process where a little humidity will help the Maillard reaction and thus help the colouring of the crust). If the dampers remain closed the humidity stays in the baking chamber and the dry crust will absorb the moisture resulting in a soggy loaf with a tough crust. The bread will lack that beautiful crusty, crunchy sensation on eating freshly baked bread.

Steaming

Steaming at the beginning of the baking process provides a favourable baking environment for the creation of a smooth glossy and crispy crust on hearth breads and crusty rolls. During the initial baking stages oven steam performs several necessary functions:

Under dry oven conditions, the dough will have rapid evaporation of the water from the exposed surface. This will cause premature formation of a dry inelastic outer shell on the bread surface. it restricts the oven spring and increases the change of tears in the finished crust. The surface layer of starch undergoes pyrolysis rather than partial gelatinisation and the crust will not reach the desired gloss. Finally the evaporative process absorbs a considerable amount of heat from the surface, thereby slowing the rate of heat penetration into the loaf interior.



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