ReviewSweet sorghum as a bioenergy crop: Literature review
Introduction
Sweet sorghum is a widely adapted sugar crop with potential for bioenergy production. Sweet sorghum was introduced to the United States in the 1850s for syrup production [1]. Production peaked at about 136 million L yr−1 of syrup in 1946, substituting for sugar during World War II [2]. Sweet sorghum production declined steadily after the war because of low sugar prices coupled with inadequate advancement in harvesting and processing technology. Sweet sorghum is now of interest for ethanol production to be mixed with gasoline or to produce ethyl tert-butyl ether (ETBE), an octane additive to gasoline.
Sweet sorghum is often competitive with corn (Zea mays L.) and grain sorghum for ethanol yield when grain yield is less than 9 Mg ha−1, and for water and nitrogen use efficiency [3]. Sweet sorghum can easily substitute for corn or grain sorghum in many annual cropping systems while adding to crop diversity.
Currently most ethanol produced in the US is from corn grain with federal subsidies. Energy gains with production of ethanol from grain are modest typically ranging from 30% to 80%, depending partly on the efficiency of use of the distillers grain co-product. Sweet sorghum can be produced at less cost than corn with higher energy gains [3]. Rather than producing starch, sweet sorghum carbohydrates are stored in the stalk as sugar with sugar concentrations of 12–20% [4]. Sweet sorghum has the potential to produce up to 6000 L ha−1 of ethanol [2], equivalent to corn grain yields of approximately 20 Mg ha−1. Sweet sorghum can yield more ethanol per unit area of land than many other common energy crops including corn (Table 1). Seasonal availability, the need to transport and maybe store much mass, and storability of sweet sorghum constrain sweet sorghum as a bioenergy crop.
Section snippets
Sweet sorghum production
The agronomic principles and production practices for sweet sorghum and grain sorghum are similar [2]. Sweet sorghum can produce grain as well as sugar. Reddy et al. [5] reported much diversity among sweet sorghum genotypes with ranges in India of 13–24% for Brix (a measure of sugar and soluble starch in plant sap based on light refraction; a typical Brix measure for sweet sorghum sap is 85% sugar and 15% soluble starch), 7.2–15.5% for sucrose concentration in juice, 24–120 Mg ha−1 for fresh
Harvesting and stalk handling
Harvesting can be done any time from early dough stage to physiological maturity [27] without much effect on sugar yield. Using modeling, Shih et al. [28] found that biomass yield of sweet sorghum can be estimated from leaf area index and stalk length for scheduling of harvest.
Traditionally, harvesting of sweet sorghum for syrup included topping the plants and stripping the leaves before crushing the stalks for sugar extraction [1]. Two to three plantings with intervals of about two weeks
Limitations of sweet sorghum bioenergy
The primary limitations of sweet sorghum for bioenergy include seasonality of harvest and large masses to be transported and stored. Fermentation of the expressed juice must be initiated quickly after harvest to avoid sugar loss. Wu et al. [56] reported as much as 20% loss of fermentable sugars from storing fresh juice at room temperature for about 3 days and up to 50% loss during one week of juice storage. In temperate climates, the harvest window for sweet sorghum is limited by length of the
Potential areas for research
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Evaluate ethanol yield with a single harvest compared to the first harvest at flowering followed by harvest of the ratoon crop where the growing season is longer than four to five months.
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Determine dry matter and sucrose assimilation and partitioning throughout the reproductive stages.
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Understand the effect of management and growing conditions at anthesis on sugar and juice content and determine growth stages at which both juice and sugar yield will peak.
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Evaluate the effect of management on sugar
Acknowledgment
This research was supported by the Cooperative State Research, Education and Extension Services, U.S. Department of Agriculture, under agreement no. 2004-51130-02249.
References (58)
- et al.
Temperate zone sweet sorghum ethanol production potential
Bioresour Technol
(1993) The economics of energy crop production
Biomass Bioenergy
(1994)- et al.
Sweet and fiber sorghum (Sorghum bicolor (L) Moench), energy crops in the frame of environmental protection from excessive nitrogen loads
Eur J Agron
(2006) - et al.
Comparison of the energy performance of fiber sorghum, sweet sorghum and wheat monocultures in northern Italy
Eur J Agron
(2003) The sorghum syrup industry 1854–1975
Agric Hist
(1984)- et al.
Sweet sorghum
Hortic Rev
(1997) - et al.
Development of a whole-stalk sweet sorghum harvester
Trans ASABE
(1990) - et al.
Sweet sorghum – a potential alternative raw material for bio-ethanol and bio-energy
Int Sorghum Millets Newsl
(2005) - et al.
Culture of sweet sorghum for syrup production
(1973) - et al.
A comparative analysis of the growth of sweet and forage sorghum crops. II. Accumulation of soluble carbohydrates and nitrogen
Aust J Agric Res
(1986)
Agronomic practices for production of ethanol from sweet sorghum
J Prod Agr
Effect of planting date and maturity on juice quality of Rio sweet sorghum
Agron J
Sugar production from sweet sorghum as affected by planting date, after-ripe harvesting, and storage
Agron J
Early assessment of sweet sorghum as an agro-industrial crop. I. Varietal evaluation
Aust J Exp Agric Anim Husb
Potential yields and on-farm ethanol cost of corn, sweet sorghum, fodderbeet and sugarbeet
J Agron Crop Sci
Influence of solar radiation and date of planting on yield of sweet sorghum
Crop Sci
Production of sweet sorghum (Sorghum bicolor L. Moench) increased with increased plant density and nitrogen fertilizer levels
Acta Agric Scand
Stalk and sugar yield of sweet sorghum as affected by spacing
Agron J
Sweet sorghum: production and processing
Effects of row spacing and plant population on sweet sorghum yield
Aust J Exp Agric Anim Husb
Sweet sorghum: a water saving bio-energy crop
Nutrient requirement and use efficiency by sweet sorghum
Energ Agr
Intercropping sorghum into alfalfa and reed to increase biomass yield
J Prod Agr
Natural abundance of 15N in sweet potato, pumpkin, sorghum, and castor bean: possible input of N2-derived nitrogen in sweet potato
Biol Fertil Soils
Biomass and sugar yield response of sweet sorghum to lime and fertilizer
Agron J
Effects of planting date and time of nitrogen application on yield and sugar content of sweet sorghum
J. Environmental Biology
Modeling biomass production of sweet sorghum
Agron J
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