Fermentation of Sugar Palm Sap to Form Ethanol
Although the origin of sugar palm (Arenga pinnata, syn. Arenga saccharifera) or Aren is not known with certainty, it seems to originate from North Sulawesi in Indonesia. It was probably a source of plant sugar for human consumption long before sugar cane was cultivated for that purpose. Today the palm grows in Southeast Asia, with its main distribution and best varieties in Indonesia and some presence in Malaysia, Thailand, Cambodia, Laos and Vietnam. Usually it grows close to human settlements where anthropic propagation plays a major role. Otherwise it prefers secondary forest at the border of primary rainforests. Occasionally it is found in virgin forests where its fruits are scattered by wild hogs, fruit bats and civet cats. Optimal growing conditions are determined by temperature (warm climate), water availability (minimum of 1,200 mm/year for good productivity) and soil qualities. The palm is found on a wide variety of soils (Widodo, 2009). The sugar palm is a perennial C4 plant, which means that it has more efficient photosynthesis than other C3 crops (e.g. wheat and barley) and needs less water. In general, C4 crops are more efficient and can work at higher temperatures and light levels than C3 crops, but they need higher temperatures and/or light levels to begin photosynthesis. Other well-known C4 plants are generally limited to high productive annual plants in tropical areas, such as corn and sugar cane.
Sugar palms have a relatively long youth phase before they start producing flowers which can be tapped. The period from seedling to full grown sugar producing palm (when the first flowers appear) varies between 5 – 12 years. The importance of a high temperature shows from the slow growth at higher altitudes. At sea level, flowering begins after 5-7 years and at 900m altitude after 12-15 years (Martin, 1999). Also the amount of exposure to direct sunlight (it is suggested that some shade in early years help the sugar palms to develop) and additional nutrient supplies play a role. Sugar palm thrives best in warm tropical (equatorial) climate with plenty of sunshine and abundant rainfall. Although sugar palm grows best on fertile soils, it grows on various soils from heavy clay to loamy sand and laterite soils, provided they do not regularly flood. The palm can even be discovered on infertile soils and on slopes. Although sugar palm grows best near the equator, it can also be found at higher latitudes (up to 30 ° latitude), characterized by a more intense dry period. Sugar palms can reach heights of up to 24 meters with stems covered in strong fibres. The sugar palm crop is highly resistant against pest and diseases.Pesticide application is not used. Except for minor infestations with insects when the outer bark is damaged, no serious threats are yet known in sugar palm production (van Dam 2007).
The sugar juice from sugar palms is obtained by tapping the male inflorescence (called ‘mayang’, which do not contain any fruits). The male inflorescences start appearing when the palm has reached its full height and stops growing (on average at a height of 15 metres, but this can also be as high as 25 metres tall). Sometimes the juice is obtained simply by tapping the inflorescence, making a cut from which the juice flows, but more often the inflorescence needs to be beaten over a period of time with a wooden stick, and then cut a little each day to keep the juice flowing. The terminal buds and inflorescences are located at the top of the trunk, which is often over 10m high. Climbing the palm and beating the inflorescence in the right way requires considerable skill, and productivity is therefore largely a function of the tapper’s experience. The tool used for beating the inflorescence is a wooden stick and a plastic jerry can for juice collection. The juice that exudes is caught in a hollow joint of bamboo and typically collected in a plastic jerry can (sometimes in bamboo containers), in which it can be transported to a central collecting point.
The juice that is removed contains wild yeasts, which will ferment the sugar rapidly, unless it is inhibited. In order to preserve the sugar until it reaches the conversion plant, several techniques exist. The juice can be boiled to prepare a brown sugar. The fresh juice can be consumed, but cannot be stored for long as it spoils rapidly. For ethanol production, the best option is to increase the concentration of the juice as close to the source as possible. This can be done through evaporation of water, which is traditionally done in open kettles which requires a heat source (firewood). Tapergie International has developed small-scale stoves that make efficient use of available local biomass (branches, wood from other plants etc).
The direct fermentation of the sugar juice into ethanol is a well-established method on small and industrial scale. On a weight basis, the ethanol production from palm sugar has a maximal efficiency of 52% (theoretical). Well established ethanol fermentation systems may achieve more than 90% of the theoretical efficiency, thus yielding 46-49 weight % ethanol. For use of the ethanol as a bio-fuel in combustion engines, distillation is required to remove the water content. The use of ethanol as source for electricity generation is not recommended because of its relative low overall efficiency.
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