Bioethanol production from mahula (Madhuca latifolia L.) flowers by solid-state fermentation
Introduction
Mahula (Madhuca latifolia L.) is a forest tree found in abundance in the tropical rain forests of Asian and Australian continents. This tree species, however, has been domesticated by tribal people in India and Pakistan for its uses as food (flower), feed (leaves and flower), wood (timber) and alcoholic beverage (fermented flowers) which is locally called ‘mahuli’ in India. The annual production of mahula (also called as ‘mahula’) flowers in India in 2005–2006 was about 48,000 M tonnes [1]. Fresh mahula flowers are a cheap source of fermentable sugars [2].
As demand for the limited global supply of non-renewable energy resources increases, the price of oil and natural gas keep increasing. As a result, production of ethanol by fermentation from renewable carbohydrate materials for use as an alternative liquid fuel has been attracting worldwide interest [3]. There is a growing interest to find alternative bioresources apart from sugarcane/beet molasses and starchy crops like cassava, sweet potato and sweet sorghum for ethanol production. Further, considerable interest has been shown in using these agricultural crops and their products for ethanol production using solid-state fermentation (SSF) [4], [5], [6], [7].
SSF is defined as the cultivation of microorganisms on moist solid support, either on inert carriers or insoluble substrates that can in addition be used as carbon and energy sources. SSF takes place in absence or near absence of free water thus being close to the natural environment to which microorganisms are adapted [8]. Agricultural substrates and crop residues serve as cheap raw materials for use in SSF for production of various bio-products, i.e. organic acids, enzymes, amino acids, and bio-ethanol [9], [10].
Mahula trees have many distinct advantages over traditional crops, such as its flowers have high fermentable sugars, good growth in poor soil under rain fed conditions and high tolerance to various plant diseases [11]. The price of mahula flowers is very cheap, US$35–40 M tonnes; hence, can be economical for ethanol production. Recently, the production of ethanol from mahula flowers by free and immobilized Saccharomyces cerevisiae cells in submerged fermentation has been described [1]. The production of ethanol from mahula flowers by SSF has not been studied. Therefore, the aim of the present investigation was to examine the potential of mahula flowers as a source for ethanol production by S. cerevisiae cells via SSF, as well as to study the effect of various fermentation parameters such as incubation period, moisture, pH and temperature on kinetic parameters of ethanol fermentation.
Section snippets
Mahula flowers
Fresh mahula flowers were collected from the forests of Keonjhar district of Orissa, India, during March–April, 2006. The flowers were brought to the Microbiology Laboratory of CTCRI, washed in tap water to get rid of dust and other debris and sun-dried in open for 7 days to reduce the moisture content to 11.0–12.5%. The sun-dried flowers collected from various locations were mixed thoroughly before being used for ethanol fermentation. The flowers have the following compositions (expressed in
Result and discussion
The production of ethanol from mahula flowers by S. cerevisiae in SSF is shown in Fig. 1. The concentration of ethanol increased with the increase of fermentation time and yeast biomass. The maximum ethanol (195 ± 4 g/kg flowers) concentration (95%) was obtained after 72 h of incubation. In a previous study, maximum ethanol concentration of 193 and 205 g/kg flowers were obtained when free and immobilized yeast cells were grown in mahula flower slurry [mahula flower:water, 1:5 (w/v) ratio],
Conclusion
The results showed that ethanol production from mahula flowers in SSF was as per with the data obtained from submerged fermentation from our previous study. Moreover, the peak ethanol concentration was obtained at 72 h in SSF in comparison to submerged fermentation in which the same concentration was achieved at 96 h. This spares considerable time and energy besides ease in operation and recovery process that are advantageous characteristics of SSF. Mahula flowers are available in plenty in the
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