Sweetness, together with salt, is one of the most widely appreciated tastes.
Sweetness is mainly caused by sugars, a class of small, soluble carbohydrates present in fruits, plants and other natural products. Common sugars are fructose (levulose, fruit sugar), maltose (malt sugar), lactose (milk sugar), glucose (dextrose) and especially sucrose (sucrose, table sugar). In processed foods mainly sucrose is used. It can either be obtained from sugarcane or from sugar beets.
Sugar is not only used in foods for its sweetness, but also for its reaction products upon heating; caramel and Maillard products. Caramel is obtained by heating sugar directly without other ingredients or water. It has a brown to black colour and a pleasant taste. Maillard products are formed upon heating of sugars and proteins. This is a very complex reaction, resulting in many pleasant flavours, such as the flavour of bread, cookies, popcorn, fried meat, etc.
Sugars can bind water efficiently. Adding sugar to a product therefore has a preserving effect; the water is no longer available for spoilage organisms. Preserving fruits or other products in sugar (jellies, marmalade) or honey has been practised for over 2000 years.
Finally sugar is an important structural element in many processed foods. Candies without sugar would loose 60% of their volume, many cakes would loose 15-30%.
Annual consumption is now running at about 120 million tons and is expanding at a rate of about 2 million tons per year. The European Union, Brazil and India are the top three producers and together account for some 40% of the annual production. However most sugar is consumed within the country of production and only approximately 25% is traded internationally.
Sugar cane is cultivated in over 100 countries and the amount of sugar from sugar cane is approximately 6 times higher than of beet sugar
Production of beet sugar
White beet sugar is made from the beets in a single process, rather than the two steps involved with cane sugar.
The beets are harvested in the autumn and early winter by digging them out of the ground. They are usually transported to the factory by large trucks because the transport distances involved are greater than in the cane industry. This is a direct result of sugar beet being a rotational crop which requires nearly 4 times the land area of the equivalent cane crop which is grown in mono-culture. Because the beets have come from the ground they are much dirtier than sugar cane and have to be thoroughly washed and separated from any remaining beet leaves, stones and other trash material before processing.
The processing starts by slicing the beets into thin chips. This process increases the surface area of the beet to make it easier to extract the sugar. The extraction takes place in a diffuser where the beet is kept in contact with hot water for about an hour. Diffusion is similar to the process by which the colour and flavour of tea comes out of the tea leaves in a teapot, but a typical sugar-beet-diffuser weighs several hundred tons when full of beet and extraction water. The diffuser is a large horizontal or vertical agitated tank in which the beets slices slowly work their way from one end to the other and the water is moved in the opposite direction. This is called counter-current flow and as the water goes it becomes a stronger and stronger sugar solution usually called juice. Of course it also collects a lot of other substances from the flesh of the sugar beet.
A typical raw juice from diffusion will contain perhaps 14% sugar and the residual pulp will contain 1 to 2% and a total of 8 to 12% solids.
The exhausted beet slices from the diffuser are still very wet and the water in them still holds some useful sugar. They are therefore pressed in screw presses to squeeze as much juice as possible out of them. This juice is used as part of the water in the diffuser and the pressed beet, by now a pulp, is sent to drying plant where it is turned into pellets which form an important constituent of some animal feeds.
The next stage of processing the sugar-liquor is aimed at removing the solids which make the liquor turbid. Coincidentally some of the colour is removed too. One of the two common processing techniques is known as carbonatation.
Carbonatation is achieved by adding milk of lime [calcium hydroxide, Ca(OH)2] to the liquor and bubbling carbon dioxide through the mixture. The gas reacts with the lime to form fine crystalline particles of calcium carbonate which occlude the solids. To obtain a stable flocculation, conditions of the reaction are carefully controlled. The clumps, as they form, collect a lot of the non-sugar substances so that by filtering out the chalk one also takes out these non-sugar substances. Once this is done, the sugar liquor is now ready for decolourisation
Unlike cane sugar production, the alternative process (phosphatation) is not used. Similarly, a separate decolourisation step is normally not used.
For this last stage, the syrup is placed into a very large pan, typically holding 60 tons or more of sugar syrup. In the pan even more water is boiled off until conditions are right for sugar crystals to grow. Some sugar dust is added to the liquor to initiate crystal formation. Once the crystals have grown the resulting mixture of crystals and mother liquor is spun in centrifuges to separate the two, rather like washing is spin dried. The crystals are then given a final dry with hot air before being packed and/or stored ready for despatch.
As in the cane processing, the mother liquor still contains valuable sugar so the crystallisation is repeated several times. However non-sugars inhibit the crystallisation. This is particularly true of other sugars such as glucose and fructose which are the breakdown products of sucrose. Each subsequent step therefore becomes more difficult until one reaches a point where it is no longer viable to continue. This is usually after three steps.
The final sugar is white and ready for use, whether in the kitchen or by an industrial user such as a soft drink manufacturer. As for raw sugar production, because one cannot get all the sugar out of the juice, there is a sweet by-product made: beet molasses. This is usually turned into cattle food or is sent to a fermentation plant such as a distillery where alcohol is made. It does not have the same quality smell and taste as cane molasses so cannot be used for rum production.
Production of cane sugar
Cane grows very tall in good growing regions, certainly up to 3 metres tall. When ripe it still has some green leaves although most leaves have dried off by then. Where possible the cane is fired before harvesting to remove the dead leaf material and some of the waxy coating. The fire burns at quite high temperatures but is over very quickly so that the cane and its sugar content are not harmed.
In some areas burning is not permitted because of the objection by the local communities of the smoke and carbon specs that are released. However there is no environmental impact, the CO2 released being a very small proportion of the CO2 fixed with photosynthesis during growth and the improved sugar extraction meaning that less cane needs to be grown on fewer acres to satisfy the world's sugar demand.
Harvesting is done either by hand or by machine. Hand cut cane -- cane cutting is a hard and dirty job but can employ lots of people in areas where jobs are scarce -- is cut at about ground level, the top green leaves are cropped off and then the stalk is bundled as a whole. Once a complete bundle has been assembled it is removed from the field with a light cart and may then be transferred to a larger vehicle for transport to the mill.
Most machine-cut cane is chopped into short lengths but is otherwise handled in a similar way as hand cut cane. Machines can only be used where land conditions are suitable and the topography is relatively flat. In addition the capital cost of machines and the loss of jobs caused make this solution unsuitable for many sugar estates.
The first stage of processing is the extraction of the cane juice. In many factories the cane is crushed in a series of large roller mills: similar to a mangle [wringer] which was used to squeeze the water out of clean washing a century ago. The sweet juice comes gushing out and the cane fibre is carried away for use in the boilers. In other factories a diffuser is used as is described for beet sugar manufacture. Either way the juice is pretty dirty: the soil from the fields, some small fibres and the green extracts from the plant are all mixed in with the sugar.
A typical mixed juice from extraction will contain perhaps 15% sugar and the residual fibre, called bagasse, will contain 1 to 2% sugar, about 50% moisture and some of the sand and grit from the field as "ash". A typical cane might contain 12 to 14% fibre which, at 50% moisture content gives about 25 to 30 tons of bagasse per 100 tons of cane or 10 tons of sugar.
The factory can clean up the juice quite easily with slaked lime (a kind of chalk) which settles out a lot of the dirt so that it can be sent back to the fields. This process is known as liming.
The mixed juice from extraction is preheated prior to liming so that the clarification is optimal. The milk of lime, calcium hydroxide or Ca(OH)2, is metered into the juice to hold the required ratio and the limed juice enters a gravitational settling tank: a clarifier. The juice travels through the clarifier at a very low superficial velocity so that the solids settle out and clear juice exits.
The mud from the clarifier still contains valuable sugar so it is filtered on rotary vacuum filters where the residual juice is extracted and the mud can be washed before discharge, producing a sweet water. The juice and the sweet water are returned to the process.
After the liming, the juice is thickened up into a syrup by boiling off the water using steam in a process called evaporation. Sometimes the syrup is cleaned up again but more often it just goes on to the crystal-making step without any more cleaning.
The clear juice has probably only 15% sugar content but saturated sugar liquor, required before crystallisation can occur, is close to 80% sugar content. Evaporation in a steam heated multiple effect evaporator is the best way of approaching the saturated condition.
The syrup is placed into a very large pan for boiling, the last stage. In the pan even more water is boiled off until conditions are right for sugar crystals to grow. Crystal formation is initiated by throwing some crystals into the syrup. Once the crystals have grown the resulting mixture of crystals and mother liquor is spun in centrifuges to separate the two, rather like washing is spin dried. The crystals are then given a final dry with hot air before being stored ready for despatch.
The mother liquor still contains valuable sugar of course so the crystallisation is repeated several times. However non-sugars inhibit the crystallisation. This is particularly true of other sugars such as glucose and fructose which are the breakdown products of sucrose. Each subsequent step therefore becomes more difficult until one reaches a point where it is no longer viable to continue.
In a raw sugar factory it is normal to conduct three boilings. The first or "A" boiling produces the best sugar which is sent to store. The "B" boiling takes longer and the retention time in the crystalliser is also longer if a reasonable crystal size is to be achieved. Some factories re-melt the B sugar to provide part of the A boiling feedstock, others use the crystals as seed for the A boilings and others mix the B sugar with the A sugar for sale. The "C" boiling takes proportionally longer than the B boiling and considerably longer to crystallise. The sugar is usually used as seed for B boilings and the rest is re-melted.
Additionally, because one cannot get all the sugar out of the juice, there is a sweet by-product made: molasses. This is usually turned into a cattle feed or is sent to a distillery where alcohol is made. This is why rum factories in the Caribbean are always close to sugar cane factories.
The final raw sugar forms a sticky brown mountain in the store and looks rather like the soft brown sugar found in domestic kitchens. It could be used like that but usually it gets dirty in storage and has a distinctive taste which most people don't want. That is why it is refined when it gets to the country where it will be used.
The first stage of refining the raw sugar is to soften and then remove the layer of mother liquor surrounding the crystals with a process called "affination". The raw sugar is mixed with a warm, concentrated syrup of slightly higher purity than the syrup layer so that it will not dissolve the crystals, but only the surrounding (brown) liquor. The resulting mixture (‘magma') is centrifuged to separate the crystals from the syrup thus removing the greater part of the impurities from the input sugar and leaving the crystals ready for dissolving before further treatment (carbonatation).
The liquor which results from dissolving the washed crystals contains some colour, fine particles, gums and resins and other non-sugar substances. It is discarded from the process.
The first stage of processing the sugar-liquor is aimed at removing the solids which make the liquor turbid. Coincidentally some of the colour is removed too. One of the two common processing techniques is known as carbonatation. Carbonatation is achieved by adding milk of lime [calcium hydroxide, Ca(OH)2] to the liquor and bubbling carbon dioxide through the mixture. The gas reacts with the lime to form fine crystalline particles of calcium carbonate which occlude the solids. To obtain a stable flocculation, conditions of the reaction are carefully controlled. The clumps, as they form, collect a lot of the non-sugar substances so that by filtering out the chalk one also takes out these non-sugar substances. Once this is done, the sugar liquor is now ready for decolourisation. The other technique, phosphatation, is chemically similar but uses phosphate rather than carbonate formation. Phosphatation is a slightly more complex process that is achieved by adding phosphoric acid to the liquor after it has been limed in the same way as above.
There are also two common methods of colour removal from sugar syrup, both relying on absorption techniques with the liquor being pumped through columns of medium. One option open to the refiner is to use granular activated carbon [GAC] which removes most colour but little else. Granular activated carbon is the modern equivalent of "bone char", a carbon granule made from animal bones. Today's carbon is made by specially processing mineral carbon to give a granule which is highly active but also very robust. The carbon is regenerated in a hot kiln where the colour is burnt off from the carbon. The other option is to use an ion exchange resin which removes less colour than GAC but also removes some of the salts present. The resin is regenerated chemically which gives rise to large quantities of unpleasant liquid effluents
The clear, lightly coloured liquor is now ready for crystallisation except that it is a little too dilute for optimum energy consumption in the refinery. It is therefore evaporated prior to going to the crystallisation pan.
In the pan even more water is boiled off until conditions are right for sugar crystals to grow. Some sugar dust is added to the liquor to initiate crystal formation. Once the crystals have grown the resulting mixture of crystals and mother liquor is spun in centrifuges to separate the two, rather like washing is spin dried. The crystals are then given a final dry with hot air before being packed and/or stored ready for despatch.
The liquor left over from the preparation of white sugar and the washings from the affination stage both contain sugar which can be recovered. They are therefore sent to the recovery house which operates rather like a raw sugar factory, aiming to make a sugar with a quality comparable to the washed raws after the affination stage. As with the other sugar processes, one cannot get all of the sugar out of the liquor and therefore there is a sweet by-product made: refiners' molasses. This is usually turned into a cattle food or is sent to a distillery to make alcohol.
Types of sugar and related products