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Sugar Boiling

is a multi-stage process by which many familiar types of sugar confectionery are produced.

The principles of sugar boiling are simple. They depend on the chemical properties of common sugar (sucrose), when it is dissolved in water, to produce a syrup, and heated. Heating drives off some of the water, thus increasing the concentration and the temperature of the solution. Arresting this process at different temperatures and then cooling the mixture produces different textures. Texture and appearance can be further altered by the use of additional ingredients, and by the treatment of the mixture during cooling.

Since the late 19th century, specially designed thermometers have been used for measuring the temperatures of boiling sugar solutions, allowing accurate results. Modern sugar boilers and confectionery manufacturers also have the benefit of chemically pure ingredients. An older system for recognizing different sugar concentrations, relying on skill, observation, and experience, is still frequently quoted in manuals of home confectionery recipes. This gives the temperatures and terms for the stages of sugar boiling shown in the table; the associated confections are given in the third column.

For each stage there is a method of determining the state of the solution by observation. Before testing, the pan of hot sugar solution is taken from the heat and cooled by dipping the base in cold water. This prevents the mixture from boiling to a higher stage.

At the thread stage, a little syrup dropped from a spoon, or stretched between finger and thumb, will form a short, fine thread. The next five stages are all tested for by dropping a little syrup into iced water. At the soft ball stage, the syrup will form a ball under water, but lose shape immediately in the air. At the firm ball stage the syrup will be firm but pliable; it will still lose shape fairly quickly at room temperature. When the syrup has reached the hard ball stage, it will mould easily into a ball; when removed from the water it will hold its shape and feel resistant to pressure. At each of these three stages, the syrup will feel quite sticky.

The next stage is known as soft crack. Some syrup is dropped into iced water, and then stretched gently between the hands. It should separate into hard but elastic strands, and only feel slightly sticky. At the hard crack stage, the syrup is removed from the water and bent. It should snap easily, have a yellowish tinge, and no longer feel sticky.

The final stage is caramel. This is determined by dropping a little syrup onto a white plate and observing the colour. A light caramel is a gold, honey colour; a dark caramel is a reddish amber. If the solution cooks beyond this point, it burns, turns black, and is useless.

Cooks and confectioners have known for centuries that sugar solutions boiled to ‘heights’ or ‘degrees’ or ‘stages’ have variable properties. It is possible that some of this knowledge arrived in Europe with sugar itself, during the Arab invasions of the 7th and 8th centuries ad. How much was known at that time about the properties of sugar is not clear; but the word candy derives from Sanskrit khanda via Arabic qand. By the late 14th century references to penides, a barley-sugar type confection, show that some knowledge of sugar boiling existed in England. By the early 17th century, some of this specialized knowledge was given in cookery books and confectionery manuals.

Common terms from English 17th-century books are thin syrup; thick syrup; manus Christi height; sugar boiled to sugar again; candy height, and casting height.

Manus Christi, as a sugar boiling term, seems to have meant an approximation to the thread stage; and is only used in this sense in books from early in the century. The other ‘heights’ of boiling sugar continued in use in cookery books well into the 18th century, by which time it had been joined by a system of French origin.

The French author Massialot gave six terms which, in the 1702 English translation of his Le Cuisinier royal et bourgeois, appeared as: smooth, pearled, blown, feathered, cracked, and caramel. As Nott (1726) observed, each of the six degrees could be divided into the lesser and the greater, making twelve in all.

These terms, subject to minor variations, gradually came into general use and remained current through the 19th century. They look as though they more or less match modern terms, but the apparent resemblance conceals differences in practice. ‘Caramel’ in 18th- and 19th-century usage corresponded to what would now be considered the hard crack stage; it was then thought that sugar which was cooked until the colour changed to gold or amber was burnt and spoilt. Generally, confectioners of the past seem to have favoured caution, and sugar was not usually boiled to high temperatures. Most of the degrees formerly recognized appear to have been temperatures which would now be classified as thread or ball. Cooking beyond this stage would have been tricky when using heat sources such as open fires, controlled with difficulty. The soft ball stage is also the concentration at which a poorly managed sugar solution will show signs of recrystallizing whilst hot. This must have been a problem when the scientific principles behind sugar boiling were unknown, and may be the origin of the 17th-century expression ‘sugar boiled to sugar again’.

Confectionery was expensive in the 17th and 18th centuries not only because sugar cost a lot but also because of the experience and skill which sugar boilers needed. Many sweetmeats which seem different in appearance and texture begin with the same process of concentrating sugar solutions over heat; but thereafter techniques for adding to and manipulating the sugar become crucial. Recipes which evolved by trial and error can now be shown to have embodied subtle control over the sugar as it cools. This retrospective understanding calls for modern knowledge of the physics and chemistry of sugar.

In chemical terms, common sugar (sucrose) is a substance which is crystalline, with an orderly structure of molecules forming a characteristic shape. This can clearly be seen in granulated sugar; each ‘granule’ takes the form of a cube.

Provided the proportion of sugar to water is suitable, sugar will dissolve completely in water at room temperature; once in solution, the sugar molecules move about freely (as do the molecules in sugar which has been rendered liquid by the application of heat). A solution which holds as much dissolved sugar as it can is said to be saturated.

Applying energy in the form of heat to a sugar solution allows more sugar to be dissolved in it; such a syrup, which is saturated at the higher temperature, is said, when cooled quickly, to be a ‘supersaturated’ solution. The amount of sugar in a solution also affects the boiling point. The more sugar there is in a solution, the higher the boiling point will be.

If a hot saturated sugar solution is allowed to cool slowly, the sugar comes out of solution and crystallizes on the bottom of the container. (This is how ordinary crystalline sugar is solidified at the end of the hot refining process.) If the solution is cooled so quickly that it has no chance to crystallize, it forms a supercooled liquid. There is no fixed proportion of water in such a liquid. If there is only a slight excess of sugar over the normal saturation level, the supercooled liquid is a thin syrup. With increasing sugar concentration the syrup becomes thicker, then a sticky, flexible semi-solid, then a hard glass. Because cooling is fast, the sugar retains the disorderly molecular structure of a liquid.

The formation of a supercooled liquid sugar is encouraged by a process known as ‘inversion’ which takes place when sucrose is heated. As it boils, sugar syrup loses water; it gradually departs from the nature of a sugar in water solution and approaches that of melted sugar. Pure sucrose has a melting point between 160 °C and 186 °C (320 °–368 °F); but, due to the phenomenon of inversion, it is impossible to have pure molten sucrose. This is because during the heating process the sucrose breaks down (‘inverts’) to simple sugars: dextrose (melting point 146 °C, 293 °F) and fructose (melting point 102–104 °C, 216–219 °F). No matter how fast syrup is heated there will be some degree of breakdown. This phenomenon helps to prevent the boiled sugar recrystallizing. This is because the molecules of sucrose, fructose, and dextrose are different sizes, thereby ‘interfering’ with the formation of an orderly crystalline structure.

To encourage the breakdown of sucrose, a proportion of dextrose (glucose) is often included in modern commercial confectionery recipes. Acids, either in the form of tartaric acid (in cream of tartar), or those naturally occurring in fruit are also used to aid inversion. (The interaction between sugar and fruit acids is also integral to the process of jam- and jelly-making).

In their simplest forms, boiled sugar sweets consist of sugar syrup boiled to the hard crack stage with the addition of substances to give flavour and colour, and to encourage inversion. The result is poured onto a cold surface and shaped. Barley sugar and acid drops are examples of this type of sweet.

Similar mixtures are used to make pulled candy. The syrup is boiled to the soft crack stage and poured onto a cold surface. It is worked intensively, first with a scraper whilst still hot, and then by twisting and stretching it by hand or machine as it cools. Pulling is a process which requires some care to prevent the syrup recrystallizing; adding dextrose to the recipe is one precaution. As the syrup is worked, some air is incorporated into the mixture, giving it a distinctive satiny sheen Two colours of mixture may be worked together to give a striped or marbled effect. Favourite sweets of past times, such as humbugs and bullseyes, are produced this way.

A syrup boiled to the hard ball stage, pulled, and left to mature uncovered will start to recrystallize: this phenomenon is exploited in the making of Edinburgh rock, which has a distinctive, friable texture.

Various sweets are made by adding dairy products such as milk, cream, or butter to syrups. These form emulsions with the sugar and help to prevent recrystallization of the mixture when this is not desired. The hardness of the result is dictated by the stage to which the sugar is boiled; this ranges from firm ball to soft crack, giving progressively harder results. Butterscotch is one type; others are known as different forms of toffee. Also in this group are a type of toffee known as caramels. These should not be confused with sugar boiled to 160 °C/320 °F; they gain their distinctive flavour from long cooking, and reactions between proteins and sugars in the mixture.

In some sweets, controlled crystallization is encouraged. The simplest example is fondant: this is a sugar and dextrose syrup boiled to the soft ball stage and poured onto a cold surface. Then the mixture is worked, gently at first, and then more vigorously. The object of this process is to allow the formation of small, even, sugar crystals in a supersaturated syrup. Fudge is based on the same principle; the syrup is enriched with dairy products and flavourings, and beaten either hot or cool depending on the desired texture.

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