Sugar Production from Sugar Cane
The basic process
Sugar cane must be crushed to extract the juice. The crushing process must break up the hard nodes of the cane and flatten the stems. The juice is collected, filtered and sometimes treated and then boiled to drive off the excess water. The dried cane residue (bagasse) is often used as fuel for this process. The remaining liquid is allowed to set into a solid mass known as j aggery, gur, chancaca or panela (Gur is used in the rest of this document).
The following terminology is used in this technical brief: The fibrous residue of sugar cane which remains after the crushing operation. The evaporation of moisture from the juice at temperatures of between 90 and 116ºC. Brix The term 'degrees Brix' (or more usually ºBrix) is the sugar 'technologists' measure of the concentration of dissolved solids in solution. Clarification Removal of impurities from the juice. Extraction The removal of juice from the cane by crushing. Factory This term is used throughout to indicate a sugar processing plant regardless of its type, processing capacity or physical size. Invert sugar High temperatures and acid conditions can cause chemical decomposition of the sucrose resulting in simpler sugars such as glucose and fructose forming. These sugars are known as invert sugars and are not desirable in the final product. Massecuite The concentrated cane juice obtained after boiling, also known as rab or final syrup. Molasses A syrup by-product from the manufacture of sugar, containing sucrose, invert sugars, moisture, ash and other insoluble matter. Open Pan (OP) Describes sugar produced by boiling juice in open pans at atmospheric pressure. OPS Open Pan Sulphitation (OPS) is a method for production of white granular sugar, developed in India. Recovery The proportion of sugar produced by weight of cane processed, usually expressed as a percentage. For example, 10% recovery means that for every 100kg of cane processed 10kg of sugar is produced. Strike The removal of massecuite from the boiling operation at the required concentration. Sucrose An organic chemical of the carbohydrate family, found in the sap of most green plants. Ordinary white crystal sugar is almost (99.9%) pure sucro se while some of the non-crystalline sugars may contain less; for example syrup and jaggery which contain as little as 50 and 80% sucrose respectively.
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Sugar Production from Sugar Cane
Tonnes of Cane per Day refers to the amount of cane a processing plant crushes each day and not the amount of sugar produced. Most sugar processing plants are sized according to this figure which is based on a 24 hour day. However, many small-scale factories, and some large ones, only operate for part of a day and in some cases for only part of the year. Therefore care must be taken when analysing TCD figures as they only represent a factory's capacity and do not necessarily reflect the actual throughput. Vacuum (VP) pan describes a particular type of technology used to boil or evaporate the sugar cane juice. It was developed by the large-scale industry to improve efficiency but some small-scale VP factories are in operation, especially in India.
Yields The yield of gur from sugar cane depends mostly on the quality of the cane and the effi ciency of the extraction of juice. The table below gives some extreme values. High quality cane Juice per 100kg of cane % sugar in juice Gur per 100kg of cane 50kg 22 10kg Poor quality cane 40kg 17 7kg
High quality cane has a good juice content with high sugar levels (20%+). Poor quality cane or cane that has been harvested early may have similar juice content but the sugar levels will be reduced. The efficiency with which juice can be extracted from the cane is limited by the technolo gy used. The simple three roller crushers used by most artisanal producers will never extract more than 50kg of juice from each 100kg of cane. Yields are also improved by careful control of the boiling process. Boiling should be completed as rapidly as possible and the conditions kept as clean as possible.
Weight of gur = Weight of cane x
weight of juice sugar in juice x weight of cane sugar in gur
For the technically minded, the weights of the gur, juice and cane can be related as follows:
10kg gur = 100kg cane x
Scale of production
Scale Small Medium Large
50kg juice 19% sugar in juice x 100kg cane 95% sugar in gur
Cane processed/day up to 50 tonnes 50 to 500 tonnes 500 tonnes upwards
Type of enterprise Cottage and small village industry using traditional OP technology. Small to medium enterprise using modified traditional, OPS or small-scale VP technology. Large industry using modern VP technology.
Sugar Production from Sugar Cane
Types of cane sugar
Processed sugar comes in two forms: non-crystalline and crystalline of which there are two basic types; centrifuged and non-centrifuged. The different forms of sugar are produced in many different countries and often have different names, therefore for clarity the different types are described as follows:
Syrups A non-crystalline liquid of high viscosity (thickness) concentrated from whole cane juice. It can vary from golden brown to dark brown and contains; up to 50% sucrose, high levels (up to 20%) of invert sugars, up to 20% moisture and the remainder is made up of other insoluble matter (ash, proteins, bagasse etc).
These can be divided into two types: non-centrifugal sugars and centrifugal sugars. Noncentrifugal sugars are basic lump sugars where the molasses and crystals have not been separated. Centrifugal sugars are free flowing granular sugars where the molasses and crystals have been separated to some degree.
Lump sugars These sugars are a concentrated product of the cane juice and are produced in many countries for direct consumption. They vary from yellowish brown to dark brown (almost black sometimes) in colour and contain up to 80% sucrose with the remainder made up from moisture, invert sugars and other insoluble matter such as ash, proteins and bagasse fines in varying proportions. Lump sugars are produced in many countries around the world and are known by a range of names: jaggery in Africa, gur in India and Bangladesh, desi in Pakistan, chancaca in Peru; other names include panela, piloncillo, and rapedura.
Khandsari A basic raw granular sugar, developed in India, that has been separated from most of the molasses. Khandsari varies in colour from golden yellow to brown and contains between 94 and 98% sucrose. It is produced by the small to medium-scale sector and has a considerable market in India. At its most basic, khandsari is manufactured using simple animal-drawn crushers, is subjected to simple clarification, boiled to the consistency of a thick syrup, and allowed to stand until sugar crystals are formed. The small crystals are then separated in manually operated centrif uges and sun dried. At the other end of the scale the production plant can use diesel or electrically driven crushers, crystalliser to ensure uniform formation of crystals, power -driven centrifuges, and forced-air driers to dry the product. Factories processing between one and two hundred tonnes of cane per day are common, yielding between 6.5 and 13 tonnes of sugar per day respectively. Since the late 1950s traditional khandsari production has been in decline in favour of a modified process known as open pan sulphitation (OPS) sugar processing. The OPS production method produce an off-white to white crystal sugar which can be of the same quality as that produced by modern large-scale VP sugar factories.
Sugar Production from Sugar Cane
White granular sugars
Free flowing white granular sugars are often referred to as plantation white. These sugars are traditionally produced in large-scale VP factories. It is possible for the medium -scale sector to develop small-scale or mini VP plants for the production of good quality white sugar.
Brown granular sugars
There are two categories of granular brown sugar: those produced directly from the cane juice at the place of origin and those that are produced during the refining of raw sugar. The first type includes demerara, muscovado and turbinado sugars. The second types are coated brown or 'soft' sugars and manufactured demerara. Those produced directly from the cane juice at the place of origin can be made using medium scale open pan production methods. The refined brown sugars, however, ten d to be produced in modern large-scale VP sugar factories. Demerara sugar Named after the area in Guyana where it was first produced, demerara is a centrifuged sugar prepared from the first crystallisation of cane syrup and has large yellow crystals and a slightly sticky texture. Production of this sugar is suited to the medium -scale sector as the juice needs to be carefully clarified to ensure purity and crystalliser are required to ensure uniform grain size. Muscovado Also known as Barbados sugar, muscovado is the product of the third crystallisation. It is dark brown in colour with small grains and sticky texture. A by-product of both the OPS and mini-VP scale of production, muscovado tends to be produced as an alternative to white sugar if the standard is not very high.
The technology used will depend on the type and amount of sugar being produced. Syrup and lump sugars can be produced using relatively basic low-cost equipment consisting of a crusher and a furnace with boiling pan. Granular sugar production also uses crushers and boiling equipment but requires additional equipment such as a clarification plant, crystallisers, driers and packaging equipment. The crushing and boiling technology used by the OP factories in both the small and mediumscale sector are similar. In most cases the technology used by the medium scale sector is a development of that used by the small-scale sector or sometimes vice-versa.
A wide range of crushers are suitable for use by the small to medium-scale sugar processors with capacities ranging from 200kg to over 900kg of cane per hour. There are two basic types: the roll mill, which is the most common, and the screw expeller. It is important to remember that cane must be crushed within 24 hours of being cut. After this time the sugar begins to 'invert' into different sugars that will not set solid.
Roll mills, of various types, are a proven technology which are easy to use and maintain and are extensively used in the large-scale sector as well as in the small. Typically roll mills used in the small-scale sector are either two or threeroll configuration set vertically or horizontally. The rolls, usually made from cast iron or steel, Figure 1: Animal powered crusher.
Photo: Neil Cooper/Practi cal Action.
Sugar Production from Sugar Cane
are located in wooden, steel or cast iron frames. Some of the smaller crushers, with capacities of between 200 and 500kg cane per hour, have vertically set rolls and are animal powered but can be modified to use small engines or electric motors. A crusher driven by a single ox can be expected to process around 50kg of cane per hour. A 5HP diesel set could increase this to around 300kg per hour. Others are horizontal and are usually powered by small engines or electric motors. Some small manually powered horizontal crushers are available but they tend to be used by street vendors, especially in South and South East Asia, to provide cane juice for drinking. For the small-scale producer the main advantage of using automated crushers is that of throughput rather than extraction efficiency. An extraction rate of up to 65% is achievable from both animal driven and small automated roll mills. However, the medium to large -scale producers use much larger roll-mills that are can extract up to 80% of the available juice depending on the fibre content of the cane and if the cane is prepared carefully. If greater extraction is required then it is usual to use two or more automated three -roll mills in tandem, one behind the other. Cane is fed through the first mill and the resulting bagasse is then fed into the second mill and so on with each stage extracting juice. Although some large scale factories use five three-roll mill tandems it is more usual for the larger open -pan factories to use two or three crushers in tandem. The pressure exerted by the mill on the cane is determined by the gap between the top roller and the bottom rollers. If the gap is too wide then poor extraction will result; if the gap is too small then the cane may not pass through it or may cause the rollers to jam. The correct se tting of the rollers has to be determined at the site of crushing as it will be dependent upon the cane variety, size and quality. Improved extraction can be achieved by hydraulically loading the pressure roller and by slicing or shredding the cane along its length before crushing. This type of system can achieve extraction of around 66%. Another important factor for efficient extraction is the operational speed of the crushers. Operating speeds are typically between 5 and 50 r.p.m. The lower speeds are easily achievable when using animal power but the use of engines or motors requires speed reduction. Typically diesel and petrol engines have speeds of 700 Figure 2: A Diesel powered cane to 2000 r.p.m while electric motors are usually rated at crusher with three vertical rollers in 750 or 1500 r.p.m. Speed reduction is usually Bangladesh. Photo: Neil Cooper achieved by coupling the engine to the crusher via flat or vee type belts and pulleys. The use of flat belts is a common and well proven technology while at the same time it is also one of the cheapest and easiest methods. Its major draw back is the distance required between the prime mover and the crusher to ensure maximum efficiency from the drive mechanism. Where space is limited then vee belts and pulleys can be used as they allow for relatively close coupling. However, vee belts and pulleys are more complicated and expensive to manufacture than flat belts and pulleys. Direct coupling between prime mover and crusher is possible but a reduction gear box will be required to obtain the optimum crushing speed. Compared to belt an d pulley systems gear boxes are expensive and complex requiring more maintenance.
By Andrew Russell, Published by Practical Action on 02/02/02
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