Principles of Baking
What happens in a baking oven
Baking is a very important process but is very complex and somewhat difficult to understand and describe. The design of an oven is principally a matter of heat transfer, and its control, but for the baker what happens is a matter of temperatures and turbulence at specific stages.
Heat and temperature are not the same and should not be confused. It is relatively easy to measure temperatures in an oven but much more difficult to measure heat, or heat flux, which is the rate at which heat is being transferred. Heat is transferred much more effectively if the air is moving near the dough piece at a given temperature.
Nearly all biscuits are now baked in band or travelling ovens with several independently controlled zones. This means that oven conditions such as temperature, movement and humidity of the atmosphere may be altered during the course of the baking period. Baking times for biscuits are quite short, ranging from 2.5-15 minutes. It is not normally possible to change, quickly, the temperature of a static or reel oven so the results of baking in these ovens compared with that in travelling ovens are often very different.
The conditions needed for different types of biscuit are are not the same because the way in which the structure is developed and the amount of moisture that must be removed depends on the richness (level of fat and sugar) of the recipe. The baking requirements for different types of biscuit will be considered later.
There are four major changes to the dough piece which can be seen as it is baked,
1. A large reduction in product density (the dough gets thicker) associated with the development of an open porous or flaky structure.
2. A change of shape associated with shrinkage or spread and increase of thickness
3. A reduction of moisture level, to between 1-4%.
4. A change in surface colouration (reflectance).
Although these changes are thought of as being distinct and sequential, broadly in the above order, as the product passes through the oven, it will be shown that there is considerable overlap and coincidence of these physico-chemical changes. It is, however, convenient to firstly consider them separately. The chart below summarises the changes that occur and relates them to dough temperature.
Development of biscuit structure and thickness
This takes place mainly in the first quarter or third of the baking period. The changes are all temperature related and involve several aspects of the recipe and formed dough piece. Bubbles of gas or water vapour are formed which expand and result in a large reduction in the density of the dough. It is the open porous structure that gives a biscuit a pleasant eating texture. The development of the structure is often known as oven spring as it relates to the thickness of the baked biscuit. The conditions for giving maximum spring which is sustained through the remainder of the bake, are imperfectly understood but the changes to the dough piece that are involved include:
- Heating the starch and proteins to levels where swelling, gelatinisation, denaturation and setting occur.
- Liberation of gases from leavening chemicals.
- Expansion of these bubbles of gases as a result of increasing temperature which also increases the water vapour pressure within them.
- Rupture and coalescence of some of these bubbles.
- Loss of moisture from the product surface by evaporation followed by migration of moisture to the surface and continued loss to the oven atmosphere.
- Increase of sugar solution concentration as the temperature rises.
- Reduction in consistency of sugar solutions and fat with temperature rise.
It will be appreciated that the most important changes centre around the formation of gas bubbles and their expansion in a medium that at first becomes softer and more flexible followed, in low sugar and fat types, by tightening and hardening.
It has been shown by many experimenters that the increase in volume associated with the mass of gas that is liberated from leavening agents is inadequate to explain the magnitude of the oven spring in biscuits. Steam production is responsible for the extra increase that occurs because there is a dramatic increase in the volume of water as it changes to vapour. Although the expansion of the dough must, in fact be due to water vapour, the term "steam" is misleading because this word should be reserved for water vapour above 100°C. Consideration of the physics of water vapour pressure provides the answer. Figure 6 shows the increase in volume of air (or other gases like carbon dioxide) when dry and when saturated with water vapour. It is should now be clear how the water vapour contributes to the expansion.
Reduction of moisture content
It would be ideal if it could be arranged that moisture loss occurs only after the structure has set. Clearly this is impossible as the dough piece starts to dry as soon as it enters the hot oven atmosphere. During baking, moisture can only be lost from the dough piece surface so migration of water to the surface by capillary action and diffusion must occur to enable the centre of the piece to dry. Both of these phenomena are accelerated by temperature gradients. The centre of the dough piece is heated principally by conduction of heat from the surfaces but as the crust dries it acts as an insulator and it becomes progressively more difficult to heat the centre. If the surface is heated too quickly, not only does it make it more difficult to dry the centre but colour changes occur prematurely. If a high heat continues colouration will be excessive before the centre is dry. Thus baking is a matter of finding the best conditions of heat and time to allow structure development, surface colouration and drying of the centre. Thicker biscuits need longer bake times at lower temperatures to achieve this drying. Thin and small biscuits can be baked faster at higher temperatures.
The loss of moisture from the biscuit surface is related to the temperature, heat flux and water vapour pressure (humidity) at the surface. The idea of "humidity" in the oven atmosphere can lead to a misconception in terms of baking conditions. No matter how much water vapour is present in an oven atmosphere which is at a temperature of more than 100°C, moisture will always be lost from the dough piece surface. The only conditions where moisture loss will be retarded is where the surface of the dough piece is at less than 100Â°C and the microclimate at the surface is saturated with water vapour.
The desired moisture level of biscuits is determined by two main factors. Too low a moisture level and the biscuit will have a burnt taste and may be a brown internal colour. Too high a moisture and the structure will not be crisp, the centre will be wetter than the edges and flavour changes associated with staling will be more rapid.
Although there is usually a change to a yellow brownish hue during baking, the term colour here is used to imply merely a darkening, reduction in reflectance, of the biscuit surface. The colour changes are due to a number of reasons. The main one is the Maillard reaction, non-enzymic browning, which involves the chemical reaction between reducing sugars in the dough with proteins and produces attractive reddish brown hues. This occurs around 150-160°C and will occur faster in a mildly alkaline situation and only in a moist situation. It is not possible to reheat baked biscuits to make it happen. The protein in the dough is from the flour and ingredients like milk powders and cheese. The reducing sugars are not from sucrose but from ingredients like glucose syrup, invert syrup and milk powders. The alkaline reaction is mostly from sodium bicarbonate. If this chemical is omitted from the recipe colouration will be much less.
Colour also develops associated with dextrinisation of starch and caramelisation of sugars. At even higher temperatures the biscuit structure chars or burns.
It will be appreciated that if the biscuit structure is very open, migration of the moisture to the surface is slower so a local increase in surface temperature and therefore colouration can more easily be achieved. Thus well sprung puff dough will colour more easily than a dense puff structure.
An excess of alkali, usually resulting from too much sodium bicarbonate in the recipe, will cause a general yellowish colour throughout the biscuit structure and this will be unattractive in products where there is no other colouration present.
As drying continues, the colouration due to the changes already described will develop in the thinner or more exposed areas of the biscuit. This change is accompanied by the development of a bitterness of flavour. A condition known as "perishing" will occur if this continues throughout the biscuit structure. Perished biscuits are most unpleasant to eat!
The aim is to produce as even a surface colouration as possible except in crackers which have blisters. In order to prevent parts of the edges of the dough pieces becoming over coloured it is important to arrange that they are placed close together on the oven band. Try to arrange that the spaces sideways are the same as those in line. Some adjustment of the spacing can be achieved when the dough pieces are panned onto the oven band. Where dough pieces are missing it is common for edges of the surrounding biscuits to be coloured too much and these have to be rejected and not packed.
Oven temperatures and heat transfer
An oven is a hot box or tunnel which is designed to provide the desired conditions of heat and temperature to the dough pieces and to allow removal of moisture. The heat is provided by burning a fuel such as gas, oil or electricity and this heat is transferred by the three modes known as radiation, conduction and convection. All three modes are always involved though engineering techniques are used to enhance the effects of each separately. Oven design is determined by the constraints to provide efficient heat transfer, allow rapid and precise control of temperatures under varying load conditions and to provide heat in predominately one of the three modes required. All ovens fall short in one respect or another so an understanding of what is happening should help in setting an oven to provide the best possible conditions.
Dough pieces are supported on a baking surface which is usually a sheet of steel or a metal wire mesh. On entering the oven, heat is applied to the dough piece by a combination of conduction (through the baking surface), convection (from the hot air moving in the oven) and radiation (from hot surfaces of the structure of the oven). Radiant heat, at the wave lengths involved, does not penetrate the dough piece significantly. The most effective form of heat transfer is by convection but moving hot air also sweeps away moisture and dries the dough surface very rapidly.
As the moisture evaporates from the product and as cool air, brought into the oven with the product, expands, there is a rise in pressure in the oven. In the case of direct fired ovens where gas is burnt in the baking chambers, the increase in pressure is even greater. This pressure must be relieved so flues are provided which exhaust to the atmosphere. If the pressure rise is exactly matched by the extraction rate through the flues, it will be seen that the oven atmosphere will soon become very rich in water vapour. This is often referred to as humidity but because the temperatures are above 100°C, the connotation of relative humidity used to measure the moisture content in meteorological terms is not applicable. The amount of moisture present has to be defined in terms of relative mass to the air present, for example, grams of water in a given weight of air, or as dew point (the temperature to which the air must be cooled before condensation occurs). To measure the moisture content of the atmosphere in an oven is not easy so it is rarely done!
The gases which are extracted are hot and represent a waste of heat so great efforts have been made to devise means of limiting the extraction rates to a minimum. Although it would be possible to bake and dry a biscuit in an oven with a very "damp" atmosphere the effect at the mouth of the oven should be understood. As a cold dough piece enters the warm humid atmosphere moisture will condense on the surface. This condensed water will release a lot of heat as a result of the latent heat of evaporation and this heat will be conducted into the dough piece causing it to warm up quite rapidly. Later, as the dough temperature rises, this water will again evaporate and retard the rate at which the surface temperature rises. The wetting of the dough surface at the oven entrance if the oven atmosphere is high in water vapour, aids the general rise in temperature of the dough piece and maintains the flexibility of the "crust" longer than if the oven atmosphere is dry. However excessive condensation on the dough surface can cause uneven colouration and spotting where sugar is locally dissolved. Provided that the moisture film is not excessive, solution of surface sugars may give the biscuit an attractive "bloom" later in baking. The humidity of the oven can be deliberately increased by injecting live steam. Drying of the dough piece surface may also be retarded by spraying with water before placing in the oven.
Apart from this effect of a damp oven atmosphere it has been difficult to be sure about the benefits or otherwise to baking performance of moisture vapour contents in oven atmospheres. Reducing the extraction rates of flue gases to increase oven humidities may also affect the convection currents and thus heat transfer rates. It would seem, theoretically, that the moisture level of the oven atmosphere is irrelevant but many critical tests have suggested that oven spring is reduced at high moisture levels.
It would be ideal if the dough piece could be heated uniformly and rapidly before much moisture is lost. This means that air movement at the dough surface should be minimum. In conventional ovens it is not possible to apply heat fast enough to the surface (and at a low enough temperature to prevent colouration) merely by conduction and radiation. Some preheating of the oven band before the dough pieces are placed upon it is very desirable to promote heat transfer by conduction.
After the dough has expanded and the structure has set, it is necessary to concentrate on moisture removal. Air movement will keep the temperature and humidity at the dough piece surface favourable for this to occur. However, as the biscuit dries conduction of heat from the surface to the wetter centre becomes progressively more difficult and a large moisture gradient will develop.
During this drying phase and before the surface becomes too dry the temperature may be adjusted to give a desired level of colouration to the biscuit.
It is important throughout the bake that oven atmosphere conditions are kept even across the width of the oven otherwise dough pieces which are similar at the entrance to the oven will appear as biscuits with different thickness, shape or colour after baking. In many ovens the movement of gases in the oven is not ideal. A common problem is that air to replace that pulled out through the flues enters too much at each end of the oven. There is thus considerable cooling due to this ingress of air and the effective length of the oven, and therefore the baking time, is thereby reduced. Also it is usual to find inspection doors down one side of the oven only. If air enters through these as a result of bad seals etc., this side of the oven will be cooler.
Baking of laminated crackers
"Laminated" means that the dough has been reduced to a thin sheet and this sheet has then been piled up into a number of layers, with or without the introduction of another material between the layers, and the pile has been further gauged to a final dough sheet for cutting. The term "gauging" is used to mean the reduction of sheet thickness by rolling between a pair of rollers.
There are two basic forms of biscuit structure, those where a more or less even bubble formation is required and those where some large cavities are formed. Water biscuits, cream crackers, soda crackers and puff biscuits are examples of the latter. The baking conditions required for the two types of structure are very different and are determined by the formation of a different number and type of gas nuclei which are subsequently expanded by water vapour. The large nuclei in the cracker and puff types are derived from discontinuities in the dough produced by layers of fat or skins of drier dough resulting from lamination. Rapid heat input causes great expansion of these long flat nuclei (lenticels) which result in the blistered and flaky structures.
Thus efforts are made to raise the temperature of the dough pieces quickly as they travel into the oven by using high temperatures followed by lower temperatures to effect the drying out. Considerable power is needed to raise the dough piece temperature and this is aided by using very light wire bands often combined with band preheating. In the case of soda crackers typically, a very heavy woven wire is used which has been heated before the dough pieces are placed on it. In this way conducted heat to the base of the dough pieces is at a maximum.
Laminated cracker dough is one of the wettest biscuit doughs and weight losses of around 26%, dough piece to biscuit, occur in the oven. This means that provision must be made for adequate extraction during baking. The optimum "humidity" level within the oven is still uncertain but there is evidence that high humidities impair biscuit development.
During baking, the laminations lift apart irregularly and this is how the blisters may form on the biscuit surfaces. The mechanism of this lift is principally as a result of the discontinuity formed between thin sheets of dough (the laminations) by the cracker dust filling or by skinning caused by flour or the dough drying out. The lenticular cavities formed, initially expand as the air warms but the greatest expansion results as the water vapour pressure rises when the dough piece temperature reaches 60°C and more.
Eventually the gas bubbles burst but hopefully the gluten and starch structures in the dough have been coagulated and gelled by the heat to form a structure that does not collapse completely before the biscuit has dried out.
Some of the carbon dioxide from the yeast (in a fermented dough) or by decomposition of the aerating chemicals will form minute bubbles which will swell the structure of the dough that forms the laminations. This mechanism contributes to a soft texture in the baked biscuit.
High temperatures at high speed give the best cracker structures.
The desired background colour of baked laminated crackers is fairly pale and relief is given because the blisters colour preferentially. If the spring of the dough is not good during baking the colour will seem pale because of lack of this relief.
Three minutes is about the fastest that cream crackers can be baked. More normally the baking time is between 4.5 and 5 minutes. Water biscuits and soda crackers are often baked faster.
Typical temperature profiles are given in below,
|Baking time||Zone 1||Zone 2||Zone 3||Zone 4|
The faster the bake the higher must be the temperatures at the front of the oven.
The appearance of the crackers as they come from the oven will be determined by the design of the cutter used. In all cases some shrinkage will have occurred in the direction of travel. The amount of this shrinkage and thus the length of the crackers can be controlled to a certain extent by the relaxation of the dough before cutting. Crackers that were cut with a cutter which had scrap all round each piece will have a uniform colouration at each edge. Those from a cutter that did not have scrap at the sides of each piece and were panned onto the band as wide strips,will have darker front and back edges than at the sides. Due to lateral dough shrinkage these strips of dough pieces will have broken and now not be complete across the band. There will typically be one or two places where the biscuits have separated laterally but unless special provision has been made on the cutter, these positions of separation will be more or less at random. Typically the breaks will be at least a third way in from the edge of the band. Where the strips have separated the biscuits will have a little more colour round the edges, and this is always the case on the pieces at the ends of each row. It will be appreciated that those crackers with three edges, as compared to only two, exposed to hot oven gases will have different edge to centre moisture distributions. Furthermore where a separation occurs the relative position of pieces to the corresponding ones before and after them on the oven band will not be in line. The biscuits in a strip are separated, cracked apart, by a system of discs used to flex the baked strips on the lines demarcated by the cutter, first one way and then the other. Significant misalignment caused by separation during baking may result in damage to some crackers by these discs. It is therefore common that arrangements are made on the cutter for slightly greater imprints to be made at, say, one-third and two-thirds across the band to encourage regular separation within the strips during baking.
Baking of semisweet and non-laminated cracker biscuits
These biscuits are made from developed and extensible, doughs. To obtain an even round cell internal structure it is necessary to allow expansion before significant setting of the structure precludes further expansion. As has been stated, setting of the structure is a combination of gelatinisation of the starch/protein matrix and hardening due to moisture loss. There is a requirement in the first part of an oven, where structure development is occurring, to conduct heat to the dough piece as quickly as possible with a minimum of moisture loss at the surface. Thus high temperatures and much air turbulence are not desirable. A typical baking profile is
|Baking time||Zone 1||Zone 2||Zone 3|
There is a considerable amount of moisture to remove from the dough pieces during baking and this is aided if baking is on a wire mesh or perforated steel band. However some hard dough biscuits are baked on a steel band and it is said that the biscuit is more tender eating and less hard. The bake time is always longer.
A prerequisite for these types of biscuits is a smooth surface of even fairly pale colour with a sheen.
The importance of dough piece dockering should be mentioned. By creating air passages right through the dough piece crust formation is encouraged and this reduces the chance of big blisters. The greater the aeration and the faster the temperature rise in the dough piece, the more important are the docker points. All but the smallest cracker and semisweet hard doughs require the dough piece to be dockered.
As for laminated crackers the biscuits shrink during baking and some control of the biscuit length can be achieved by attention to the amount of dough relaxation allowed before cutting.
Control of the thickness of these biscuits is generally by the amount of aerating chemical used in the formulation. Some additional control is available in the front temperatures of the oven, particularly the top heat in the first zone of the oven.
Baking of short dough biscuits
Products which are rich in fat and sugar have less water in the dough. This means that the protein is imperfectly hydrated to form gluten and when the dough is heated there is insufficient water present to gelatinise much of the starch. The structure relies more on a sugary or toffee-like matrix which becomes softer rather than sets as the temperature rises. Thus during the baking of sugar rich short doughs an expansion, in all directions, is observed followed by some collapse in the thickness. The spread of the dough piece and collapse are responsible for the cracked surface of biscuits like ginger nuts and crunch types.
Doughs which spread appreciably during baking cannot be baked on open wires as they would sink deeply into them. Although a flat steel band is best, some may be baked on woven wires.
Many short doughs with low fat do not spread during baking and can be baked on wires. These types of short dough biscuits may require docker holes to achieve a flat baked biscuit and to allow good moisture removal from the centre parts. However checking in all but the largest short dough biscuits is unusual.
Baking times are generally longer than for hard dough biscuits because lower temperatures are required to prevent excessive colouration. The baking times are related to biscuit thickness and short dough biscuits can be very thick and are generally thicker than hard dough types. Baking profiles are usually flat at about 180°C for all the oven zones.
For sugar rich types the surface cracking can be greatly increased by spraying water on the dough pieces before the oven or by injecting steam into the mouth of the oven. This ensures that the dough surface remains soft and flexible for longer during the bake allowing greater development of thickness and spread before collapse which gives the cracks.
*Mowbray, W.R., (1981) Technology of the "hot box". Food Manufacture, October