|Environmental Handbook Volume II: Agriculture, Mining/Energy, Trade/Industry (GTZ, 1995, 736 p.)|
|Trade and industry|
Sugar is the only food extracted from two different plants - the sugar beet (beta vulgaris) and the sugar cane (saccharum officinarum) - which grow in different regions. Competition between these two international cash crops only arises in small border areas where both are considerably below their physiological optimum, usually latitude 25 to 38 degrees north. The main sugar beet growing areas are located in the temperate climate of Europe and North America in regions with average mid-summer temperatures of 16° to 25°C and an annual rainfall of at least 600 mm, but sugar beet is also grown in the sub-tropics in the winter months. Irrigation is essential where the rainfall is less than 500 mm. Beet thrives best on deep loamy soils with a neutral to weakly alkaline reaction, and in intensive farming requires adequate mineral compound fertilisation. Since beet can only be grown in the same field every fourth year to ensure a healthy crop (avoiding, for example, beet nematode, the main cause of the disease known as beet sickness), the catchment area of a sugar beet factory is very large. The vegetation period is generally five to six months, with yields in a temperate climate ranging from 40 to 60 t/ha, and in the sub-tropics averaging 30 to 40 t/ha. The sugar content ranges from 16% to 18%. Sugar cane grows in tropical lowland climates, and is farmed almost exclusively between latitude 30° south and 30° north, and particularly between the north and south 20°C isotherms. Besides intensive sunlight, a rainfall of at least 1,650 mm or irrigation is essential. Heavy, nutrient-rich soils with a high water capacity are preferred; pH values in the weakly acidic to neutral range are best. Nutrient requirements are high due to the huge mass production. Pest and disease attack have been reduced by resistance breeding and plant development, with biological pest control playing an increasingly important role. Sugar cane is suitable for monoculture and is indeed mainly grown as such. Plant cane is generally harvested after 14 to 18 months, and the new growth (ratoon) after 12 to 14 months. Yields are from 60 to 120 t/ha; the sugar content is on average 12.5%. Harvest quantity and sugar content decline as stocks age, with the result that the total useful life does not normally exceed 4 to 5 harvests.
Sugar factories are agro-industrial centres which contract out the cultivation of their raw material or, alternatively, grow it themselves, have their own energy and water supplies and large, varied workshop complexes. The plant installed is designed to handle a single natural raw material. Where used for direct processing of the harvest, the seasonal processing period coincides with the period of use of the sugar production plant. New plants process between 5,000 and 20,000 t daily (24 hr), although in order to handle 10,000 t/day, sugar production plants must have an appropriate infrastructure. The production plant should be situated wherever possible in the centre of the cane or beet growing area, it should be close to water and should be connected to the public railway and road networks. The by-products arising during sugar manufacture - molasses, sliced beet and cane bagasse - are either used or processed further in the plant, or alternatively form the raw materials for other industries.
· Harvesting, storage and cleaning of the raw material
Sugar beet is harvested almost exclusively mechanically while sugar cane in contrast is harvested largely manually (cutting of stalks). The raw material is then transported to the factory by rail or road. Exceptionally, sugar cane is transported by water. Sugar beet can be stored for one to three days, depending on the temperature and method of storage, whereas sugar cane cannot be stored and must be processed immediately after harvesting or in any case no more than 12 hours later; sugar losses of up to 2%/24 hr are possible. Sugar beet is always washed before processing, but sugar cane is usually only washed if it has been harvested by machine.
· Cutting, crushing and extract purification
Sugar beet is chopped in slicing machines, and sugar is extracted from the slices in the countercurrent with water at 60 - 70°C in an extraction plant; the water is then removed mechanically and before drying the extracted beet is usually mixed with up to 30% molasses, normally made into pellets and used as animal feed. Because of their residual sugar content (approx. 0.8%), the slices - after drying - can be used as silage (preserved by fermentation) and as an agricultural feedstuff.
Sugar cane is prepared by revolving knives, crushers and/or shredders and then the juice is extracted in four to seven rollers in line or is extracted like sugar beet in a diffuser. A fibrous residue (bagasse) with a low sucrose content is produced at a rate of 25 to 30 kg/100 kg cane. The fibre content is approx. 50%/bagasse.
· Extract purification
Beet and cane are processed in a very similar way after the raw juice has been extracted. The raw juice is purified mechanically and chemically. First fibre and cell particles are removed mechanically, then the juice is purified chemically by precipitation of some of the nonsucrose substances dissolved or dispersed in the juice, and the precipitate is then filtered off. In the beet sugar industry, repeated precipitation with calcium carbonate has proved successful, an operation in juice purification where lime and carbon dioxide are introduced into the juice at the same time. Synthetic flocculants, in particular polyelectrolytes, improve particle agglomeration and reduce sedimentation times in the decanter from the normal 40 - 60 minutes to 15 - 20 minutes. In the cane sugar industry, simple liming (defecation) is usually employed as the extract purification process, lime/sulphur dioxide treatment (sulpho-defecation) being less common and lime/carbon dioxide treatment rare. The decantate is then finely filtered for a second time and goes directly to the evaporation station. The sediment or sludge concentrate (approx. 25 to 30 kg/100 kg raw material) is usually separated in rotary vacuum filters into filtrate and filter sludge/cake (approx. 3 to 6 kg/100 kg raw material), the filtrate returned to the process and the filter sludge separated.
· Evaporation and crystallisation
The clear juice (from 12 to 15% dry matter/dry sugar content) is continuously concentrated by multiple stage evaporation until it has a dry content of 60 to 70%, each stage of this process being heated with the steam (steam-saturated air released when the clear juice is concentrated) from the previous stage. In the boiling process, more water is removed from the concentrated juice (syrup) in boilers operating at an approx. 80% vacuum. The juice is boiled at a lower temperature than normal because of the low pressure in the equipment, thus preventing any discoloration due to caramelisation. When a certain ratio of sugar to water (supersaturation) is reached, crystals form. By adding more syrup and evaporating more water, crystallisation continues under controlled conditions until the required crystal size and quantity are obtained. The boiling process is then complete and the resulting boiled mass, now called massecuite, is drained from the vacuum pans into crystallisers. As the massecuite is constantly cooled, the supersaturation changes, causing the sugar crystals to grow once again. The massecuite is then transferred from the crystallisers into centrifuges, in which the crystallisate is separated from the syrup, leaving behind the yellowy brown raw sugar. The centrifuged syrup is boiled to form a massecuite once again and the crystallisate obtained from it is centrifuged. The syrup produced from the centrifuging is called molasses. If, when the massecuite is centrifuged, the crystallisate is cleaned of the residual syrup still attached to it by a water and/or steam jet (affination), white sugar is extracted in just one process from beet or cane. In refining (recrystallisation), a plant-intensive technology, raw sugar and poor quality white sugar are dissolved, and then decoloured and filtered by the addition of activated carbon or bone char, or ion exchange resins. Refined sugar, which meets the most exacting requirements in the sugar processing industry, is extracted from the subsequent crystallisation process. The quantity of molasses produced ranges from 3 to 6% of the raw material fed in, depending on the technological quality of the raw material and the end product. The sugar content of the blackstrap molasses is around 50%.
The sugar extracted is cooled and dried before storage or packaging. It can be stored loose, packed (1 kg) or bagged (50 or 100 kg). The essential factor for proper storage is a relative humidity of around 65% in the store. This is approximately the point of equilibrium between the absorption and the release of moisture from the sugar crystals.