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General Properties of Carbohydrates
Sweetness
All sugars are sweet but do not have the same degree of sweetness. The term sweetness can be defined in subjective terms and the property is measurable only by tasting and not by using any specific instrument. Using the tasting method the sweetness of different sugars can be compared using a point scale in which sucrose is taken as 100.
Examples of the relative sweetness of three different sugars:
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| SUGAR | RELATIVE SWEETNESS
| Fructose
| 170
| | Sucrose | 100 | Lactose
| 15 |
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Starch and other polysaccharides do not have a sweet taste.
Solubility
The solubility of a substance is the degree to which it will dissolve in a given solvent. Sugars (monosaccharides and disaccharides) are very soluble in cold water. Solubility is increased by heating the water. Sugars form what are called supersaturated syrups when their solutions are concentrated by evaporation as a result of boiling. Starches and other polysaccharides are insoluble in cold water.
Gelatinisation (effect of moist heat on starch)
If a suspension of starch in water is heated, the water penetrates the outer layers of the granules and the granules begin to swell. As the size of the granules increase the mixture becomes more viscous i.e. thick and gluey. As the temperature rises this mixture becomes even more viscous, forming what is called a *sol. On cooling this becomes a gel. This entire process is known as gelatinisation of starch and is very important in cooking.
*Sol: A colloidal solution is usually called a sol. It contains particles that do not fully dissolve but that are evenly dispersed throughout the liquid.
Factors that affect the strength of a gel:
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 | The proportion of starch to water: the more starch the stronger the gel. e.g. roux sauce |
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| The presence of sugar: sugar competes with starch for water and therefore reduces gel strength e.g. custard powder |
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| The presence of acid: acid hydrolyses starch and reduces the gel strength e.g. lemon pie filling. |
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| Gel formation also takes place in protein. |
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Dextrinisation (effect of dry heat on starch)
Many foods containing starch also contain small amounts of dextrins. Dextrins are mixtures of soluble compounds formed by partial breakdown (hydrolysis) of starch by heat, acids or enzymes. On heating dextrins polymerise to form longer chains, and become brown coloured substances called pyrodextrins. Pyrodextrins contribute to the brown colour and the characteristic taste and texture of many foods including toast and bread crust.
Hydrolysis
Disaccharides undergo a form of hydrolysis to form monosaccharides. Hydrolysis is the chemical breakdown of a molecule, by combination with water, producing smaller molecules. Hydrolysis is the reverse of the condensation reaction by which disaccharides molecules are formed.
The hydrolysis of sucrose is also known as the inversion of sucrose and the product, a mixture of glucose and fructose is known as 'invert sugar'. Inversion may be brought about either by (a) heating sucrose with an acid or (b) adding the enzyme invertase. Invert sugar is used in the production of jam, boiled sweets and other sugar confections.
Crystallisation
Pure water boils at 100°C and a solution of sugar in water boils at a temperature above 100°C. As the solution boils, water is evaporated from the surface and the sugar concentration will therefore increase. Eventually it will come to a stage where it becomes super saturated. When this saturated solution is cooled, particles of the solute will be deposited from the solution. The particles of the solid solute assume a characteristic geometrical shape and are known as crystals. When sugar solutions are cooled, the size of the crystal depends on the rate of cooling. Slow cooling produces large crystals, which tends to give confectionery a gritty texture. Controlling the size of crystal formation plays an important part in the confectionery industry. Crystallisation is used in the food industry to purify crystalline substances e.g. sugar.
Caramelisation
Caramelisation is a form of non-enzymic browning. When sugars are heated above their melting points they produce a range of brown substances collectively known as caramel. Caramelisation occurs most readily in the absence of water (crème brulee), but sugar solutions (syrups) will caramelise if heated strongly enough. There are ten gradual changes in sugar between melting and caramelisation, which are not easy to distinguish. The first stage occurs at 104°C while caramelisation occurs about 177°C. Too much heat will produce a bitter and very dark caramel. The flavour and colour characteristic of caramel has many uses in cooking e.g. crème caramel.
Pectin Extraction / Gel Formation
Although not strictly carbohydrates, pectic substances have similar structures to polysaccharides. There are many different pectins that are dissimilar in their composition. Chain lengths are variable. It is impossible to find two pectins, which are exactly the same. Pectins are essential in jam making and are used as thickening agents in some foods. They are found naturally between plant cells and also in the cell walls of fruit and some vegetables. Pectin can be isolated from fruits by extraction with hot dilute acids.
There are several different types of pectin:
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| Protopectin is thought to be a parent pectic substance in plants from which other pectic substances are made. It has been referred to as pectose in the past |
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| Pectinic acid is pectin, which forms a gel with sugar and acid in jams and other preserves |
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| Pectic acid is incapable of gelling with sugar and acid. |
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Pectin and Pectic substance are general names for the whole group.
Unripe fruit is rich in protopectin, which is broken down by enzymes to pectinic acids, and then pectic acids. Thus a firm and crisp fruit becomes soft and juicy in its ripe form.
A Firm gel depends on:
1. Amount and quality of pectin:
The quantity and quality of pectin and the amount of acid present in fruit for jam making is of great importance because gel formation only occurs when the concentrations of sugar, pectin and the pH of the mixture lie within certain limits.
Some fruits are rich in pectin and acid and can be easily be made into jam e.g. Currants, Gooseberries, Damsons, Lemons, Bitter Oranges, Apples.
Others are deficient in pectin and require the addition of fruit rich in pectin or the use of commercial pectin e.g. Strawberries, Blackberries, Raspberries.
Heat must also be applied to extract the pectin. The best way to extract pectin from the fruit is to heat the fruit in a small amount of water. Large amounts of water dilute the pectin strength and therefore its ability to form a gel.
2. pH
If the pH of the jam is not around pH 3.5 the pectin chains come apart preventing gel formation.
3. Amount of sugar
A good gel is formed when there is sufficient sugar (65%-68%). The sugar is thought to bring the pectin chains together and binds up water so that a gel structure can develop. When fruit is boiled with sugar a certain amount of inversion occurs. Invert sugar is very sweet and dissolves more readily in water than sucrose. This helps prevent crystallisation and ensures the jam stays as a smooth gel. As the sugar remains dissolved in the jam, mould growth is prevented.
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