Cooking sugar syrups for hard candy

The most commonly used ingredients in sugar confectionery are water, sugar and glucose.  Their behaviour is the basis of most confectionery processes.

When water, sugar - usually delivered as small crystals - and viscous glucose are mixed, they form a gritty slurry.  As most sweets have a smooth mouth texture, this gritty slurry will not usually be acceptable, so it is necessary to dissolve the sugar.

Sugar dissolves in water but the addition of glucose to the water helps to dissolve the sugar; however, the moisture content of the resulting solution is too high for finished confections.

The typical moisture content of fondant crème is 10 to 12% (dissolved solids content 88-90%); of toffee and caramel 8-12% (88-92% solids) and of boiled sweets/hard candy 1-4% (96-99% solids).

The solution of ingredients with all the sugar dissolved has a typical moisture content of 20% (80% dissolved solids), so it is necessary to boil the syrup and drive off moisture to produce the final product.

The most common way to monitor and control the moisture content of a sugar syrup is to measure the boiling temperature of the syrup.  As the sugar content increases, the boiling point rises.  At moisture contents below 20% (i.e. solids content over 80%) the gradient of the boiling temperature/moisture graph is sufficiently steep to use the boiling temperature of the syrup to give a measure of its moisture content
  
For hard candy and boiled sweets, the sugar syrup is cooked to a low moisture level so that when the syrup cools it forms a hard glassy product.  There are three potential faults that can make the product unacceptable to the consumer:

Discolouration
Stickiness
Graining

Discolouration is usually obvious during the manufacturing process, but the other two only become apparent during storage.

Discolouration

Discolouration can be caused in two ways - by inversion of the sugar, or by caramelisation of protein, probably from milk.

Inversion

Inversion occurs when molecules of standard sugar (sucrose) are split into molecules of dextrose and fructose (two other sugars).  These new sugars are called invert or reducing sugars and give the syrup a yellow or brown colour.  They are also hygroscopic which can lead to stickiness (which is dealt with later).

Inversion takes place when sugar molecules are in an acidic solution at high temperatures.  The acidity can come from acid added in the recipe, or from residual acid from an ingredient - for example, acid-converted glucose.  The rate of inversion increases with acidity and temperature, so these two factors and the time spent in those conditions all increase the final degree of inversion.

The following steps can be taken to minimise inversion:

Use high-Maltose glucose (enzyme converted)
Add the acid at the latest point in the process
Use buffered acid to give better control of the pH of the acid
Reduce the cooking temperatures - for example by using vacuum cooking
Reduce the cooking time
Do not store the cooked syrup at high temperatures

Caramelisation

Caramelisation occurs when protein and sugar are kept together in a hot solution.  The syrup becomes brown, and develops a toffee flavour.  The process is also called the Maillard reaction.

For sweets like butterscotch, caramelisation is essential to give the sweet its characteristic flavour; in contrast, cream sweets are often made with a white segment and any browning is undesirable. These steps can be taken to minimise caramelisation:

Keep the milk protein level to a minimum (reduce the milk/cream content of the recipe, or use double cream to reduce the protein content)
Reduce cooking temperatures, e.g. by using vacuum cooking
Reduce cooking time
Do not store the cooked syrup at high temperature

To increase the degree of caramelisation, in butterscotch for example, the opposite requirements apply:

A good milk protein level
High cooking temperatures (atmospheric cooking)
Increase the residence time of hot syrup in the process

Stickiness

Sweets may start to become sticky during storage due to moisture absorption, and this is a result of several factors:

The quantity of reducing sugars in the sweet.  Reducing sugars are hygroscopic and draw in moisture from the atmosphere; they come from:

a) inversion in the cooking process, a problem explained above; and

b) the dextrose equivalent (DE) part of the glucose.  The DE component contributed by the glucose increases with the use of higher DE glucose and by using a larger proportion of glucose in the recipe.
Wrap quality, and how well it seals the sweet from the atmosphere.  In hot and humid climates, pillow packs are increasingly being preferred because they give a better seal than twist wrapping; sealed bags are also often used for secondary packaging.  Longer shelf life is an additional benefit.
The humidity of the storage location.  The answer is simple - adequate humidity control.
One aid for prevention of stickiness involves calculating the Equilibrium Relative Humidity (ERH) of the sweets.  This is the humidity at which the sweet will neither absorb moisture nor dry out.  It can be calculated using nomograms and graphs.  If the Water Activity of the sweet is known, the value of the ERH can be calculated by multiplying the Water Activity by 100.

Calculating the ERH will reveal the level of humidity that can be tolerated, and therefore the level of air conditioning required.  Another suggestion for reducing stickiness is based on the fact that ERH varies with temperature, so moisture absorption between de-moulding and wrapping can be kept to a minimum if the sweets are wrapped when still warm.

Graining

The symptoms of graining are that the sweet becomes cloudy and, when eaten, has a rough texture instead of the usual smooth glassy texture.  Graining is the formation of minute sugar crystals in the sweet.

Because of the low moisture content of cooked syrup, the sugar solution is super-saturated and the sugar would happily crystallise and come out of the solution during cooling.  The reducing sugar content in the syrup increases the solubility of sugars, reducing the degree of super-saturation and risk of graining.

The solution is to minimise the degree of shearing that the syrup experiences while it is being handled after cooking, particularly if it has partially cooled, for example on a cooling band for die forming. The secret is to transfer the syrup with a slow-moving (low shear), well heated pump, and to pump the syrup at as high a temperature as feasible.

This is not a problem in depositing plants, when the syrup is pumped immediately after cooking.

Another cause of graining can be condensation.  If condensation forms on a sweet, it will dissolve some of the sugar from the surface.  As the sweet dries out again, sugar crystals may be precipitated on the surface, and these can be a source of crystal nuclei which allow graining to spread.  The solution is to ensure the sweet is not left in a humid atmosphere: this is a storage issue rather than a production problem.