Types of crystalline sugars

Sucrose, "sugar"

Sucrose is a medium sized molecule, known as a disaccharide (composed of a unit of dextrose plus a unit of fructose). It is derived from sugar cane or sugar beet. The sugar is extracted as a solution in water, this is refined and then crystallised from a concentrated solution. The term sugar is commonly used to refer to sucrose. It is by far the most commonly used type of sugar in the biscuit industry.

White sucrose is available in various particle sizes, for example Granulated, Castor and Icing. The smaller the crystal size the more quickly the sugar dissolves either in a dough mix or in the mouth. Powdered or pulverised sugar is a roughly milled sucrose with a wide spread of particle sizes. Icing sugar is sieved milled sugar so that there are no coarse crystals present. It is very prone to become lumpy in storage so ideally should be used as soon as possible after milling.

Typical specifications for particle size are:

  • Coarse granulated MA = 940-1000µ CV = 20 to 30%
  • Granulated MA = 570-635µ CV = 26 to 30%
  • Caster MA = 276-300µ CV = 16 to 26%

There is always a spread of crystal sizes and the manufacturer's specifications define the average or mean size (MA, Mean Aperture of the sieve that lets the crystals through) and the spread of sizes is indicated by the CV, coefficient of variation. The smaller the CV, the fewer very large or very small crystals present.

White-sugars-thumbWhite sugars

The particle size of sucrose is important for the quality control of short dough biscuits and cookies.

Sucrose is normally purchased as granulated sugar. The crystal size range is quite high but this grade is free flowing and remains so for a long time if correctly stored. Finer particle sized sucrose is often produced by milling in the factory. The product of milling is initially powdered sugar (with a large particle size range) which is often used in doughs because the smaller particles dissolve more rapidly. By sieving powdered sugar, to remove the large particles, a fine powder, icing sugar may be obtained.

If powdered or icing sugar is purchased there is a great potential for it to become lumpy because there is a very large surface area over the small particles and moisture from the atmosphere coats it and crystals may form which causes caking. Caking can be reduced, but not eliminated, by adding small quantities of an anti-caking agent such as tricalcium phosphate or corn starch. Generally it is better to make powder sugar immediately before it is needed.

Caster sugar is not so fine as powdered sugar. It has a narrow particle size range and is stable in terms of caking but is more expensive than granulated sugar because of the extra processing involved.

Sugar, whether in bags or bulk, has a long shelf life and if the bags are kept in a dry place at even temperature the sugar will not become lumpy.

Importance of sucrose particle size

The particle size of sucrose affects:

  • Mouth feel. Important in sandwich creams and short dough biscuits. Small crystals feel smooth and dissolve quickly, larger ones give gritty or crunchy textures.
  • Rate of solution. When mixing doughs, time must be allowed for as much of the sugar as possible to dissolve in the available water. There may be competition for this water between sugar and starchy materials. Fine particles will dissolve more readily than coarse.
  • Spread control. When baking some short dough biscuits they increase in diameter, length and width. This change of size is usually related to the quantity of sugar and its particle size. Sugar with small particle size results in more spread than coarse particles. A variation of sugar particle size, when supplied to the mixer for the dough, can give biscuit size control problems during baking.
  • Appearance when used as surface decoration. Sugar of various sizes, from very large crystals to icing sugar, may be used to dust the surface of dough pieces before backing. In most cases the sugar remains unchanged after baking but in certain biscuit types if the sugar particles are small they melt on the dough surface due to the oven heat and produce an attractive glaze.

Handling granulated sugar in bulk

Sugars delivered in a road tanker are transferred to a silo pneumatically in a similar way to flour. As the sugar passes along the pipe there is a great tendency for the crystals to break up. This problem is reduced if the off-loading is done at a moderate and not too fast a rate and if the pipe has a minimum number of bends between the tanker and the silo top.

Bulk sugar is drawn from the base of the silo through a rotary seal into a pneumatic air line in a similar way to flour. The sugar is delivered to a weigh hopper but excess sugar is never returned to the silo as it will have been broken a little in transit and it is undesirable that dusty sugar is returned to the silo. Any excess sugar in the line after the hopper has made weight should be collected in a special bin sited near the mixing department.

There is a great hazard of dust explosions from sugar dust so much care must be taken to avoid sparks from machinery or static electricity in sugar handling systems.

It is not usual to empty or clean sugar silos except when it is necessary to clear lumps, sugar caked onto the silo walls or collections of dust.

Brown sugars

Brown sugars are either partially refined sucrose, so still have a dark syrup over their crystals, or are manufactured from white sugar by adding syrup to it. Commonly used brown sugars are Demerara (with large crystal size), Muscavado and Soft pieces (various names are used and these sugars usually have small crystal size). London Demerara is an example of a manufactured brown sugar where syrup has been added to white crystals. All brown sugars are sticky and tend to form hard lumps on storage especially after the bag is opened and exposed to the air.

For handling convenience, biscuit manufacturers usually prefer to use white sugar of the desired particle size plus some dark cane syrup in place of brown sugars.

Brown-sugars-thumbBrown sugars