There are three main changes which we will see as all biscuits are baked. They are the development of the biscuit structure and texture, the reduction in the moisture content, and the development of the colour. These changes overlap during the baking process, but it is useful to note that the formation of the structure and texture of the biscuit will take place in the first half of the biscuit baking oven, the reduction in moisture mainly in the middle of the oven and the colour in the final third of the oven.
Our aim is to bake a high quality biscuit. The following characteristics are important:
Evenness of the moisture content from the centre to the outside of the biscuit requires penetrative heat and adequate time for baking and cooling to avoid “checking” (cracks in the biscuits after packing).
The structure and texture of the biscuit is determined by the ingredients, mixing and forming and the baking process. Here we will introduce briefly the main ingredients and process requirements for the baking of biscuits. Our aim is to indicate the complex chemical and physical changes which take place with temperature and particularly during baking. This will inform how we design and operate the baking oven.
The principle ingredient of biscuits is wheat flour. The grain consists of bran (12%), which is the outer husk, endosperm, which is the white centre (85.5%) and the tiny germ (2.5%). Typical biscuit flour is milled to a yield or extraction of 70-75%. Wholemeal flour is of 100% extraction and wheat meal flours in between these extraction rates, normally around 84% extraction. The flour will also contain moisture of between 13 – 15%.
The wheat flour is composed of carbohydrate (as starch), protein and fat, together with some fibre, ash and trace minerals and vitamins. The protein is mainly gluten, composed of gliadin and glutenin. The percentage of protein determines the flour strength. A dough made from strong flour with a high protein content, is extensible and can be machined into a continuous sheet for crackers and hard biscuits. A weak flour with a low protein content produces a soft dough which may be moulded or deposited on the baking band and when baked, gives a short texture.
Moisture content: 14.0 – 15.0%
|Property||Soft flour %||Medium flour %||Strong flour %|
The formation of the gluten, its strength and elasticity are largely determined by the flour specification, recipe and the mixing and forming processes. Wheat flour contains proteins including gliadin and glutenin. In the presence of water these proteins combine to form gluten. As the dough is mixed the protein molecules form long strands of gluten, which have strength and elasticity. The gluten forms an elastic web, which gives the dough strength and allows it to be machined into a thin sheet for crackers and hard sweet biscuits. These biscuits are made with “strong” flour, which has a high protein content, typically 10-12%.
The gluten web is also important in trapping air and gas bubbles formed by yeast fermentation or by leavening agents such as sodium bicarbonate (“soda”) or ammonium carbonate (“vol”). This leavening process, combined with the laminating of the dough, gives the characteristic open, flaky texture of crackers during baking
Soft or short biscuits are generally made with low protein flour (7-9%). A low protein flour makes a dough with a much weaker gluten web. In addition these doughs have higher fat contents. The fat coats the flour particles and this inhibits the hydration of the proteins and the formation of the gluten web. Shorter mixing times also result in less development of the gluten strands and hence the biscuits have a short texture.
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Starch is the main component of wheat flour. It represents almost all of the carbohydrate content and around 80% of the total energy content of wheat flour. Starch is a polysaccharide (many sugars) made up of glucose units linked together to form long chains. The principle starch molecules in wheat flour are amylose, which typically comprises 28% of the total amount of starch. Amylose molecules contribute to gel formation. Their linear chains of molecules line up together and are able to bond to make a viscous gel.
Starch is insoluble in water, however the starch granules do absorb a limited amount of water in the dough and swell. Above temperatures of 60o-70°C the swelling is irreversible and gelatinisation begins. The gelatinisation may continue until the starch granules are fully swollen, but it is normal in baked products that only partial gelatinisation occurs. The gelatinisation of the starch contributes to the rigidity and texture of the biscuit.
As the starch gel is heated further, dextrinisation occurs. This contributes to the colouring of the biscuit.
In soft dough products, the high sugar and fat content of the dough inhibits starch gelatinisation. The presence of sugars delays the gelatinisation of the starch, which may be due to the competition for water. The fat, composed of triglycerides and surfectants, also tends to inhibit gelatinisation. With high sugar and fat recipes, the dough has a low gel viscosity and strength and produces short and soft biscuits and cookies.
Common sugar (sucrose) is a carbohydrate derived from sugar cane or sugar beet. It is a disaccharide composed of two monosaccharides, a molecule of glucose joined to a molecule of fructose.
Sugar gives sweetness, but it is also important in developing the texture of the biscuit. Dissolved sugar tends to inhibit starch gelatinisation and gluten formation and creates a biscuit with a more tender texture. Undissolved sugar crystals give a crunchy, crisp texture. Sugar crystals, which melt during baking, cool to a non-crystalline glass-like state which gives a crispy, crunchy texture, particularly on sugar topped biscuits.
Dry sucrose melts at 160°C – 186°C. Biscuits with sugar toppings which are melted to a smooth, shiny surface require high intensity flash heat at the end of the oven to fully melt the sugar.
Invert sugar syrup is a mixture of glucose and fructose. The sucrose is split into its component monosaccharides by hydrolysis. The sucrose in solution is heated with a small quantity of acid such as citric acid. After inversion, the solution is neutralised by the addition of soda. The invert syrup is sweeter than sugar and it contributes to a moist, tender texture in the biscuit.
Yeast is normally used in the production of cream crackers. The yeast is most active at temperatures of 30° – 35°C during dough fermentation. At temperatures above 40°C, the yeast activity stops and is therefore inactive during the baking process.
Sodium bicarbonate (“soda”). Soda is readily soluble and it reacts with acidulants in the dough in the presence of water, producing carbon dioxide and decomposing to salt and water. The speed of the reaction may be controlled by the type of acidulant used. The leavening of the dough takes place during mixing and fermentation of the dough.
Ammonium bicarbonate (“vol”). This leavening agent decomposes completely when heated, producing carbon dioxide, ammonia and water. The reaction is rapid at around 60°C and therefore the expansion of the dough takes place during the initial stages of baking.
Fats are a vitally important ingredient in achieving the texture, mouth feel, and the bite of the biscuit. Crackers and hard biscuits have relatively low percentages of fats in the recipes, while soft cookies have high amounts of fat.
Recipes with high fat contents require little water for producing a cohesive dough and produce soft, short doughs. During mixing, the fat coats the flour particles and this inhibits hydration and interrupts the formation of the gluten. Fats also tend to inhibit the leavening action of the carbon dioxide diffusion in the dough during baking and this produces a softer, finer texture. Where both fat and sugar amounts in the recipe are high, they combine to make a soft, syrupy, chewy texture.
Typical blended vegetable dough fats are solid at ambient temperature and melt over a wide temperature range. Most fats used in biscuit making are melted below blood temperature (36.9°C), and this avoids a waxy mouth feel. Fats are specified with a Solid Fat Index (SFI), which indicates the percentage of solid fat present in the total fat. A dough fat typically has an SFI of around 18 at 25°C and 12 at 30°C.
In baking, our main concern with high fat recipes will be the spread of soft cookies on the steel baking band, which is mainly due to the melting of the fat. This occurs very quickly as the dough pieces enter the oven and the temperature of the dough pieces increases above 35°C.
It will be seen from this brief consideration of biscuit ingredients and that there are complex chemical and physical changes taking place in the biscuit doughs and some of these are heat dependent. The changes that are temperature dependent mainly take place during fermentation and later during baking. These changes are also highly dependent on the moisture content of the dough and the humidity of the baking chamber.
As we have seen, the water in the dough plays a vital role in achieving the biscuit texture and structure. It hydrates the protein allowing the gluten to form and develop and it hydrates the starch granules which swell and gelatinise.
The gluten can absorb up to 200% of its weight in water. As the dough temperature rises, the gluten web swells and becomes strengthened and the structure of gas and air bubbles in the dough forms, causing an increase in volume of the dough pieces. The swelling of the proteins increases from 30°C to around 50°C. However, denaturation of the proteins takes place at temperatures over 50°C, when the long chains of molecules are broken. As more heat is applied, gluten coagulation occurs above 70°C. At this temperature, some of the moisture is released from the gluten and contributes to the starch hydration and gelatinisation.
The air bubbles in the dough are saturated with water and these expand rapidly as the temperature increases. The increase in volume is 3% at 50°C up to 50% at 95°C. This expansion creates a significant increase in volume of the dough piece during baking.
The hydrated starch molecules begin to gelatinise at temperatures of 50°C – 60°C. In biscuits, this process is partial as there is seldom enough water to fully gelatinise the starch. In short doughs with very little water, the starch gelatinisation is very limited. When the dough pieces have reached temperatures over 70°C, the structure is well formed and becomes stable although starch gelatinisation may continue until the dough reaches a temperature of 95°C.
In order for the biscuit to reach an optimum volume, it is essential that the surface of the dough piece is not dried too quickly, making it rigid and preventing the expansion of the dough piece. The dough piece surface must remain moist and flexible for as long as possible. As the dough pieces, at ambient temperature, enter the oven, some moisture will condense on their surfaces. This not only keeps the surface of the dough pieces moist, but the condensation releases latent heat, which assists in raising the temperature of the dough. It is important to maintain a humid atmosphere in the first zone(s) of the oven and in some cases injecting steam into the baking chamber is also beneficial.
The physical and chemical changes noted above which form the texture and structure of the biscuit take place in the first half of the oven. They require not only temperature, but time as well. In some trials it has been shown that there is a limit to the speed of the temperature increase, which if exceeded will result in a decline in quality of the biscuit.
When the gluten and starch have been sufficiently hydrated and the structure of the biscuit is formed, the remaining free water must be evaporated. The water is evaporated from the surface of the dough pieces. This will occur principally at 100°C for pure water, but at higher temperatures (up to 130°C) when the water is held in solution, for example in a sugar solution. At temperatures over 100°C, the application of heat will always result in moisture loss from the surface of the dough pieces, even in an oven atmosphere which is saturated with water vapour. This loss of moisture from the dough piece is dependent on the temperature, method of heat transfer and the humidity of the oven.
Cracker doughs have a large quantity of added water, typically around 15 – 25% of the total recipe. The final product will have a moisture content of around 2.5% and this will require the removal and evaporation of 300 – 440 grams of water for every kilogram of baked cracker. The evaporation of this water requires heat transfer (the latent heat of evaporation), which is of 539 calories/gram of water.
As an example, the latent heat of evaporation required for baking 1200 kg/hour of snack cracker will be around 224,000 kcal/hour. This is clearly a significant energy requirement for cracker baking. It is, however, of much less significance for the baking of soft doughs and cookies. The equivalent energy requirement to provide the latent heat of evaporation for a soft dough biscuit would be between 87,000 and 45,000 kcal/hour.
|Final moisture %||2.5||2.5||2.5||2.5||3.0||3.0||3.0||3.0|
|Moisture to be removed per kg of biscuits (kgs)||0.442||0.439||0.347||0.291||0.230||0.135||0.070||0.104|
There are several chemical and physical changes which contribute to the colouring of the biscuit surface. After the moisture has been mainly evaporated from the dough pieces, the temperature of the surface rises quickly and the colour will change from around 150°C. There are three processes which contribute to the browning of the biscuits.
Caramelisation is a non-enzymatic browning reaction, which is caused by the breakdown of sugars at high temperatures. The caramelisation of different sugars occurs at different temperatures: fructose at 110°C, glucose 160°C and sucrose at 160°C. Caramelisation results in both colour and flavour development.
A second browning process, dextrinisation is the breaking down of starch molecules by heating. This produces pyrodextrins which are brown in colour and have a distinctive flavour. Dextrinisation of the starch occurs at temperatures of 100-200°C.
The third browning process is known as the Maillard reaction. This is a complex chemistry in which many compounds are formed at high temperatures by the reaction of reducing sugars and amino acids. Since milk has a high content of proteins and amino acids, the Maillard reaction will also contribute to the colour of biscuits which have been brushed with milk before baking giving a darker rich brown colour.
These browning processes all require high temperatures and occur when the biscuit surface is already dry. The colouring takes place in the final stage of the baking process.
We have seen that there are many complex physical and chemical changes from the dough piece to the biscuit during baking. These changes are mainly temperature and time dependent and occur at different stages during baking. They may overlap and interact.
It is a useful simplification to say that the structure and texture of the biscuit is formed in the first half of the oven, that the moisture removal is in the middle and the colouring occurs at the final stage.
The oven design should therefore provide a rapid heat transfer at the start of the bake and maintain a flexible, moist outer skin of the dough piece to allow expansion and lift. In the middle of the oven, the moisture should be removed efficiently from the dough pieces and then extracted from the baking chamber. In the final zone(s) the surface of the biscuit will be dry and it will colour. Good lateral heat transfer control is required to maintain an even colour and moisture content of the baked biscuits.
The specification of the baking system should be based primarily on the product(s) to be made and their requirements in terms of structure, texture, density, bite, flavour and colour, very bland even colour or contrasted background with highlights.
The characteristics of the biscuits will determine the type of heat transfer (radiation, conduction and convection) which is appropriate at each stage of the baking process. This will define the oven specification, the appropriate heat ratings and the zone lengths. The diagram below shows the heat ratings (heat energy input per m2 of oven band) for several multi-purpose biscuit tunnel ovens.
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