Sucrose, commonly known as sugar is a carbohydrate derived from the sugar cane or sugar beet plants. It is processed in refineries where crystalline refined sugar is obtained. The chemical structure of sucrose is composed of the monosaccharides glucose and fructose joined together to form a disaccharide molecule.
A large variety of sugars are available which vary in flavour, colour, sweetness and particle size. There are various sugar crystal sizes ranging from around 60μm as powder form to 2000μm as coarse granulated crystals. It is also available as a solution, known as liquid sugar. The type of sugar used depends on the function it is required to perform in the recipe.
Sugar is a major ingredient used in various types of biscuits, apart from imparting a sweet taste it also serves various functional properties in the processing of the biscuit dough as well in the structural and organoleptic properties of the baked biscuits. It is used in high amounts in short dough recipes whilst smaller quantities of it are used in semisweet dough recipes. Cookies also have high sugar levels, here glucose or invert syrups are used together with sugar to keep the cookie texture soft and avoid the crisp texture which would be obtained with the use of sucrose alone. Sugar is also used in biscuit cream fillings in the form of icing sugar since the finer particle size gives a smoother mouth feel and a rapid dissolution of the sugar in the mouth.
The sweetening property is the most basic function of sugar which makes it an important ingredient for use in bakery. There are many different types of sweeteners available however sucrose is the most versatile. The crystal size of the sugar used effects the sweetness perception in biscuits. Use of finer sugar particles results in a sweeter taste due to the better distribution of the sugar particles within the biscuit. The sweetness level perceived is also dependent on other factors including the pH of the biscuit and the effect of other ingredients.
Invert sugar is the sucrose molecule split into its basic components, glucose and fructose through hydrolysis. Inverted sugar is sweeter than sucrose whilst glucose syrup is less sweet. In cream fillings a powdered form of glucose, dextrose monohydrate powder may be used to replace part of the icing sugar when a reduction in the overall sweetness level is required. Dextrose also gives a cooling effect as it dissolves in the mouth since this has a negative heat of solution and absorbs heat energy from the mouth as it dissolves.
The use of brown sugar and treacle obtained during the sugar refining process can be sources of flavour in biscuits. The molasses present in the brown sugar and treacle give the baked biscuit a more robust flavour and a darker appearance than with the use of white sugar. Sugar is sometimes used in recipes to enhance the flavours of the biscuit, the interaction of sugar with other ingredients contributes to the overall flavour delivery. It can be used to bring out a particular flavour or mask the perception of other flavours. As sugar caramelises during baking it also adds to the overall flavour of the baked biscuit.
Texture & Biscuit Structure
Short dough biscuits have little water in the recipe. When the sugar dissolves in the water during the first stage of mixing it binds some of the available water preventing starch gelatinisation. It also limits the availability of water to the flour minimising the formation of gluten which results in a soft textured biscuit. The crystal size of the sugar used effects the degree of solubility of the sugar in the available water. The smaller the particle size the easier it is to dissolve and hence binds with water more readily making the dough softer as the water will be less available to the flour. If the sugar is not completely dissolved the sugar crystals will remain evident in the dough and consequently in the baked biscuit. Saturated sugar solutions form a non-crystalline glass-like state on cooling which gives the biscuit a hard crisp texture. Ginger snap biscuits have very high sugar levels which results in its characteristic texture. In semi-sweet biscuits the sugar present in the recipe also gives the product their typical crisp texture. If some glucose syrup is added to the formulation the hardness may be reduced for the same level of sugar present in the biscuit. With the use of invert syrup a more moist and tender biscuit texture can be obtained. Inverted sugar retains more moisture than sucrose and is less prone to crystallization which yields the softer texture. Due to its increased water retention property it may also improve shelf life by delaying staleness.
Appearance/ Surface decoration
Sugar is sometimes sprinkled on to the surface of biscuits before baking as a decoration. The sucrose crystals dusted on to the surface of the dough pieces adhere strongly to the surface of the biscuit and remain visible on the surface of the biscuit after baking. Different sized sugar crystals may be used depending on the desired visual effect. Surface cracking on the biscuit surface is another phenomenon which can be achieved with the use of high sugar levels within the dough.
During the blending stage of short dough the use of granulated sugar incorporates more air into the blend than with the use of finer sugar crystals or liquid sugars. The granulation of the sugar also affects the amount of water bound and consequently the spread of biscuits during baking. Use of smaller sugar particle sizes increase dough piece flow resulting in wider and thinner biscuits. Larger sugar particle sizes decrease dough piece flow causing the biscuits to spread less during baking, consequently thicker biscuits may be obtained. The quantity of sugar used in the recipe also affects the biscuit size. The higher the sugar levels used the softer the dough becomes due to increased binding of available water promoting spread as is observed with cookie recipes. Higher concentrations of sugar also shift the starch gelatinisation point to higher temperatures which results in more spread since higher temperatures would need to be achieved before the biscuit sets.
Sugar binds water molecules within the baked biscuit hindering microbial growth. It also shifts the starch gelatinisation point to higher temperatures reducing the occurrence of staleness even though the low water levels used in short dough biscuits do not promote this. It has also been reported that sucrose prevents the deterioration of flavours due to its antioxidant properties, partially due to its ability to lower water activity. The presence of sugar in biscuits may therefore be beneficial for an improved shelf life.
Sugar is also used as a bulking agent in cream fillings. The use of sugar is ideal since it dissolves readily when eaten and also has a low water activity preventing the migration of moisture from the cream filling to the biscuit. Moisture migration from a phase of high water activity to one of lower water activity may result in textural changes and also spoilage.
When sugar is exposed to temperatures over 160°C the sugar crystals begin to melt, as the temperature continues to increase the molten sugar begins to caramelise. Caramelisation results in non-enzymatic browning of biscuits as the sugar oxidises, different sugar types caramelise at different temperatures. Browning also occurs due to the Maillard Reaction, this is a chemical reaction which takes place between reducing sugars and amino acids from protein molecules at high temperatures. Reducing sugars may be introduced with the use of syrups in the recipe such as glucose syrup, fructose syrup, honey and malt extracts.
There is currently a worldwide drive to reduce sugar levels in a variety of foods in order to provide consumers with healthier options. Due to the various functions which sugar has in the processing of short doughs and the physical properties of the baked biscuits it is not always easy to reduce or completely remove sugar from formulations. In a number of cases this may involve the use of more than one replacement ingredient in order to achieve the multiple functionality achieved with the use of sugar.