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UNIVERSIDAD PRIVADA ANTENOR ORREGOFACULTAD DE CIENCIAS AGRARIASESCUELA PROFESIONAL DE INGENIERIA EN INDUSTRIAS ALIMENTARIAS

SOME NOTES ABOUT SUGAR CANE

Fernando Rodrguez Avalos

Trujillo, 26 de agosto, 2015

Sugarcane, with its high fibre and carbohydrate content, constitutes an important renewable source of energy. During its long growth period of 10 to 16 months, this plant converts good amount of solar energy into sugar and cellulose and is considered to be one of the most energy efficient crops, in that the energy provided by the biomass of fully grown cane is four times the energy input during the crop cultivation. Sugarcane sets are planted in soil and the plant, during its life cycle, converts water and CO2 from atmosphere into carbohydrates in the presence of sunshine, a phenomenon termed as photosynthesisIn the growth phase of the plant, sugar accumulation occurs more in the lower portion of the stalk, progressively decreasing from bottom to top joints, but in a fully mature cane, this disparity is practically absent or negligible. Within the stalk, the internodes are richer in sugar while the fibre content is higher in the nodes as shown by the earlier studies on variation in composition of these portions. This difference in composition accounts for two observations:

Cane with short internodes will give high fibre and lower sugar content.The juice expressed in the last mill under heavy pressure is of lower purity than the first or second mill.Sugar cane plant standing in the field consists of three principal portions:1) The leaves, 2) the tops, and 3) the stalk.

The tops and leaves contain very low sugars, but are rich in salts in solution as well as water. The sugarcane stalks delivered to the factories have to be free from the tops and roots and as such during harvesting, care is taken to remove them so that the sugar rich portions of the plant are processed. The principal components of the cane stalks, to be processed for commercial sugar production, are:

1) around 70-75% water2)12 -15% water insoluble fibre and3) sucrose and other carbohydrates.

Besides these the composition of juice extracted from cane is marked by varying amounts of mineral salts and organic compounds. The composition of sugarcane is conditioned by the variety of cane, soils and agricultural factors, in addition to the climate during the different phases of growth of the plant.

At the end of its growth period, the sugarcane crop reaches maturity, during dry weather, marked by highest sugar build up, when it is considered to be suitable for harvesting. If allowed to stand in the field after its maturity phase, it starts deteriorating in the sense that sucrose gets decomposed with the formation of nonsucrose compounds and cellulose. The overmature cane will thus build up higher fibre with reduced sugar content and it is absolutely essential for the processing factories to harvest the cane, after it attains full maturity.Sugars, in general, are known as carbohydrates being formed of carbon, hydrogen, and oxygen. Simple sugars like glucose, fructose, and others, are monosaccharides which cannot be further decomposed into simple carbohydrates by either acids or enzymes. When composed of five carbon atoms these carbohydrates, like arabinose, are termed as pentoses. Likewise sugars like glucose, fructose containing six carbon atoms are known as hexoses. Monosaccharides condense together to form disaccharides or even trisaccharides containing two or three monosaccharides. Sucrose, maltose and lactose belong to the category of disaccharide, which under influence of acid or enzyme form simple monosaccharides. Sucrose is the condensation product of glucose and levulose or fructose, the first being an aldehyde while the latter a ketone. Several polysaccharides produced by condensation of monosaccharides, are present in sugarcane like gums, dextran, starch, cellulose, which when reacted by acids or enzymes are converted to monosaccharides.The major components of sugarcane, besides water are as under:Sugars (a) Sucrose Commonly known as sugar, in the popular parlance. this is the major constituent of sugarcane and belongs to the family of carbohydrates. The chemical formula for sucrose is C12H22O11. Structure. Structurally the sucrose is represented as -D-glucopyranosyl--D-fructofuranoside. The configuration of sucrose and its two main monosaccharides is as under:

Following are some physical and chemical properties of sucrose, which is basically a disaccharide:(i) Molecular weight, 342.3.(ii) Density of 26% solution at 20 C is 1.108175.(iii) Melting point is 188 C, but it decomposes on melting.(iv) Soluble in water and ethanol. The saturated solution in water, at 20 C, contains 67.09% sucrose by weight, and in ethanol, at 20C, 0.9% by weight.(v) Sucrose is optically active and specific rotation of normal solution (26 g/100 mL) is +66.53 at 20 C, but on inversion the solution has optical rotation of -39.7 at 20 C.(vi) Crystals of sucrose are monoclinic prisms with density of 1.588 g/mL).

(a) Chemical(i) When heated to 200 C, the sucrose loses water forming brown coloured compound, known as caramel.(ii) Sucrose on treatment with concentrated nitric acid is transformed into oxalic acid:

In its pure form, this sugar is crystalline in nature. Sucrose content in fully mature cane ranges from 12-15%.Under conditions of low pH or under influence of certain enzymes, like invertase, sucrose is hydrolysed into two monosaccharides: glucose and levulose, in equimolecular proportions according to equation:

(iii) When concentrated sulphuric acid is added to it, sucrose is charred with evolution of C02 and SO2.(iv) Sucrose reacts with hydroxides of calcium, barium and strontium to form the corresponding sucrates:

Sucrates or saccharates are decomposed by passing C02 through the aqueous suspension.

(b) Glucose Also known as dextrose, this monosaccharide has a chemical formula C6H12O6. The two major monosaccharides, glucose or dextrose and fructose or levulose, are present in higher amount in the immature cane but their percentage decreases as cane reaches maturity. In a sugarcane plant the presence of glucose and levulose is more marked in the cane tops than in the middle portion of stalk. Glucose is sensitive to alkaline condition and heat undergoing decomposition under action of heat and alkali to form coloured compounds. It is less soluble in water than sucrose. The major physical properties of glucose are:

(i) Empirical formula, C6H12O6(ii) Molecular weight, 180.2(iii) Anhydrous glucose crystals are rhombic, with density of 1.544 g/cm3, and melting point of 146 C.(iv) Glucose is soluble in water and ethanol. The solubility, at 30 C, in water is 57.6% in a saturated solution.(v) Glucose is dextrorotatory and, in solution, specific rotation changes on standing a phenomenon known as mutarotation. On reaching equilibrium, the glucose solution has a specific rotation of 52.7 at 20 C.

(c) Levulose.Levulose or fructose, a levorotatory monosaccharide, is present along with glucose in the growing portions and tops of cane more than in the main stalks. The empirical formula of fructose is the same as for glucose i.e. C6H12O6. Out of the two monosaccharides, i.e. glucose and fructose, the proportion of fructose is less than that of the former. It is highly soluble in water but less soluble in ethanol and, at 30 C, the saturated solution of fructose contains 81.54% of this sugar. The physical properties are as under:(i) Molecular weight 180.2.(ii) Specific rotation -92.4, at equilibrium.(iii) Crystal structure-orthorhombic with density of 1.598 g/cm3 and melting point of 105 C.

Fructose has a ketonic group in its structure, unlike glucose which has aldehyde group. Being highly susceptible to heat and alkaline conditions it gets decomposed under these conditions. Like glucose fructose reduces cupric salts.

Starch. Starch is formed by condensation of glucose molecules and is present in cane juice in small amounts the percentage depending on the cane varieties as also soil and other natural conditions. Varieties like NCO 310 contain high amount of starch (300 mg/L) in juice and in South Africa sugar cane of any variety is reported to contain high starch. The starches which are polymers of glucose consist of a chain polymer to the extent of 20% known as amylose and remaining about 80% branched polymer, amylopectin. Starch is insoluble in water but its fine granules are extracted in mills and during heating of juice it gets partially dispersed in juice. During clarification, it is only partially removed and if the original starch content in juice is high (250-350 ppm), serious problems are encountered at both clarification and crystallisation, since the sucrose crystallisation is hampered by the presence of starch. The amylose fraction of starch exerts depressing effect on the filtration rate. Presence of starch beyond a certain concentration (50-100 p.p.m) in raw juice creates problems in the process operation and special treatment of juice is resorted to for decomposition of starch. Starch is soluble in hot water and unless its level is significantly brought down in juice the filtration and sucrose crystallisation are adversely affected. Consequently, the factories producing crystal sugar prefer to process varieties low in starch content. In case, however, it is unavoidable to deal with cane of high starch content, it is eliminated by adopting special technique of juice clarification or use of bacterial enzyme.Fibre. This water insoluble portion of the sugar cane plant is present in abundance and forms important component of sugarcane along with sucrose. The principal constituent of the cane fibre is cellulose and the complex fibrous structure of the plant is bound together by lignin, pectin and hemicelluloses. The cellulose is composed of chain of glucose molecules held together closely with varying degrees of polymerisation.

Other organic polymers. Gums in sugarcane composed of six different monosaccharides, are soluble in water but insoluble in acidified ethyl alcohol. Though removed in the process to some extent they persist till the final stage and are present in molasses and to some extent in sugars.

Another polymer of significance is dextran consisting of glucose molecules and is produced by the bacteria Leuconostoc mesenteroides, which is highly dextrorotatory with specific rotation of +199 at 20 C.Frozen cane or harvested cane left over in field or factory without processing for long period are often found to contain high amount of dextran which, unless eliminated in the first stage of processing, creates difficulties at crystallisation, contributing to high loss of sugar in manufacture. In milling, dextran formation is observed in stagnant pockets of cold juice and special bactericides have to be sprayed to check the growth of the Leuconostoc bacteria. In the process, dextran can be decomposed by use of enzyme known as dextranase.

Organic acids and nitrogenous compounds. Cane juice is acidic in nature with pH of 5.0-5.5. Stale, immature or frozen cane yields low pH juice with high amount of free acids. Out of the different organic acids aconitic acid claims the major share in the juice from normal cane. Deterioration of cane after harvest or due to natural causes results in generation of acetic acid, lactic acids etc.

A number of amino acids and amides have been identified in cane juices like aspartic acid, glutamic acid, alanine, valine, etc. Aspargine is the most dominant amino compound. The proteins in cane are extracted to maximum extent in the last one or two mills where cane is squeezed under high pressure with application of hot imbibition water. These are mostly eliminated in clarification. However, the amino acids are unaffected by the treatment in clarification and accumulate in final molasses.Colour forming compounds and pigments. Natural pigments like chlorophyll, xanthophyll and carotene present in the cane plant are extracted in milling but are eliminated in juice clarification and subsequent boiling operations. However organic compounds like polyphenols, flavonoids, which give rise to coloured products in the process of manufacture resist elimination during clarification. Presence of Polyphenols is more marked in the tops and immature portion of sugarcane and they give rise to coloured compounds with ferric ions and are found to create problems in settling of juice when the percentage of phenolic compounds exceeds certain limits. To maintain low level of phenols it is essential to top the canes properly before in the fields at the time of harvesting.Inorganic compounds. Sugar cane plant during the period of growth absorbs various minerals from the soils, which are partly dissolved in water and to some extent are present as organic compounds. Prominent among them are anions phosphates, sulphate, chlorides and cations like silica, Iron aluminium, calcium, magnesium, potassium and sodium. Maximum inorganic matter is found in juices from immature cane tops gradually decreasing to the lower portion of the plant. This makes it all the more necessary to avoid milling of cane tops. These salts are extracted in juice and only partially removed by the normal clarification methods followed in factories as for instance in juice rich in phosphate after clarification the phosphate level is brought down. The potassium which forms major constituent of the mineral matter of the cane juice persists throughout the process getting accumulated in final molasses. The anion chloride also remains unaffected by the clarification or juice concentration and is found in final molasses.The mineral matter content in cane juice depends primarily on the nature of soil, as well as, manurial practices and high concentration of minerals in juice, is responsible for high loss of sugar in molasses.

Lipids. Sugarcane outer surfaces are coated with waxy lipids which are discernible on the rind of the cane stalk as whitish coating. The fatty lipids and wax are extracted in milling to the extent of nearly 40-50% and are eliminated in the filtermud during clarification. These lipids consist of a mixture of alcohols, free acids and esters Crude wax extracted from filtercake contains, resins and soft portion or oils apart from hard wax. The dude wax content of dry filtercake usually varies from 8-12%. Wax extraction from the filtermud separated in clarification is being commercially operated in some countries in view of the potential offered by this product for use in other industries where other vegetable waxes are being used.