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The Biology of the Saccharum spp. (Sugarcane)

Section 5 Biochemistry

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5.1 Toxins

Sugarcane is a well-established agricultural crop with a long history of safe use; it has been cultivated in Australia for over 100 years. Commercial sugarcane is grown as a source of sugar (sucrose) for human food. By-products from processing sugarcane into sugar such as molasses and bagasse have been mainly used as additives in stockfeed.

Sucrose is the primary product of plant photosynthesis and, therefore, common in food crops consumed regularly by humans and animals. Sucrose has an exceedingly long history of human dietary exposure. It has been classified as a non-toxic substance to humans (MSDS 2004). The oral LD50 of sucrose for rats is 30–35 g kg-1 body weight (Boyd et al. 1965). Consuming sucrose in extremely large oral dosages may produce gastrointestinal disturbances. Although there is no direct evidence that links sucrose consumption with toxicity, there are several studies indicating that sucrose intake should be limited because it may be associated with health problems (Howard & Wylie-Rosett 2002). Studies found that a high intake of sucrose was associated with cardiovascular diseases, development of type II diabetes, obesity and hypertension (Howard & Wylie-Rosett 2002). In addition, it is well established that sucrose consumption is a risk factor for dental caries (Sreebny 1982; Rugg-Gunn & Murray 1983).

Sugarcane also contains cyanogenic glucosides. These can be cleaved to produce hydrocyanic acid, a poison that acts by inhibiting cytochrome oxidase, thus preventing transfer of oxygen from the blood to the tissues. It is present in many plants, including sugarcane, where it is not at a dangerous level for humans (Rossoff 2002).

A mixture of bagasse and molasses can be used as a food source for cattle. Molasses can be added to cereals at up to 15% in the final mix to improve palatability (Perez 2004). In Cuba, molasses is often used as a much higher proportion of the diet, mixed with urea and fed as 70% of the total diet (Perez 2004). Experiments on cattle in Australia have shown that feeding a diet containing up to 50% molasses may be useful for the beef feedlot industry, especially in northern Australia in areas close to sugar mills (Tomkins et al. 2004). However, feeding of molasses has to be carefully managed as it may be toxic when fed incorrectly or in large quantities. The symptoms of molasses toxicity include reduced body temperature, weakness and rapid breathing and the animal may have difficulty standing (Perez 2004). Molasses toxicity often affects eye-sight and the animal may become blind due to brain damage thought to be cerebro-cortical necrosis (CCN). Studies of an affected ox suggested that the encephalopathy was indistinguishable from CCN (Edwin et al. 1979). The necrosis is likely to be caused by a decrease in energy supply to the brain because of either thiamine or glucose deficiencies (Preston 1988). The glucose deficiency is thought to result from a reduction in propionate, required for gluconeogenesis, due to a complete digestion of molasses in the rumen and therefore a reduction of glucose in tissues and ultimately the brain (Edwin et al 1979 and references therein).

Bagasse, like many other agricultural by-products such as cereal straws, is high in ligno-cellulose and may have a depressing effect on feed intake. The digestibility of bagasse is very poor because of the presence of lignin which protects carbohydrates from being digested by the rumen microbes (de la Cruz 1990; Leng 1991). To improve the nutritive value of ligno-cellulose materials for livestock, physical or chemical pre-treatments are required (Playne 1984; de la Cruz 1990).

5.2 Allergens

Sugarcane pollen is transported by wind and therefore has the potential to act as an airborne allergen. The allergenicity of sugarcane pollen was evaluated in India where 70% of field workers with respiratory disorders showed positive reactions to sugarcane pollen in skin tests (2001). The authors also tested rice and several other plant species and concluded that sugarcane pollen was the most significant allergenic type. However, there are no reports of any major allergic responses to the commercial hybrid cultivars of sugarcane in Australia.

Exposure to organic dusts, such as those present in mouldy sugarcane, can cause bagassosis. Bagassosis is an occupational lung disease of the extrinsic allergic alveolitis type and is caused by breathing dusts containing fungal spores, and/or thermophilic actinomycetes which grow in stored, mouldy bagasse (Lacey & Crook 1988). In Australia, bagasse may be stored covered with tarpaulins at the end of the crushing season to be used to fuel the boilers at the beginning of the next season before fresh bagasse is available (Dawson et al. 1996). The stored sugarcane bagasse contains approximately 50% water and 5% sucrose, so is colonised by bacteria, causing it to heat up and create ideal conditions for fungi and thermophilic bacteria such as Aspergillus fumigatus, Thermoactinomyces vulgaris and Thermoactinomyces sacchari (Lacey & Crook 1988). In India, it is thought that T. sacchari and Saccharopolyspora rectivirgula are the most likely cause of bagassosis (Khan et al. 1995). Prolonged, repeated exposures can lead to permanent lung damage and scarring, and significant disability (Phoolchund 1991; Hur et al. 1994). In Puerto Rico, a study showed a four-fold increase in risk of cancer of the oral cavity amongst sugarcane farmers and farm workers, which may be due to exposure to actinomycetes (Coble et al. 2003). However, a study at two Australian sugar mills did not identify very high levels of airborne bacterial spores and none of the 271 mill workers surveyed showed any symptoms of bagassosis (Dawson et al. 1996).

5.3 Beneficial phytochemicals

There have been some reports that very long chain fatty acids/alcohols (policosanols) from sugarcane wax lower cholesterol in humans (reviewed in Hargrove et al. 2004). However, other studies reported no effects on cholesterol (Kassis et al. 2009). Policosanols have also been reported to decrease risk of cardiovascular disease (Janikula 2002) and may have anti-inflammatory effects (Ledón et al. 2007).

Other beneficial phytochemicals from sugarcane include glycolic acid, which can be used in cosmetics, primarily for skin rejuvenation (reviewed in Allen et al. 1997).

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