This review attempts to discuss the potentials, limitations and bottlenecks to be solved in order to optimize sweet sorghum productivity, based on existing literature.

There are four types of sorghum grain, sweet, forage and fiber. Compared to other crops potentially used for energy sweet sorghum shows a much wider adaptability to different environments and soil conditions, is resistant to drought and has a higher water and nutrient use efficiency. A mayor disadvantage, however, is the short harvesting window and the poor storability of the stalks that could critically affect the ethanol production costs. Several studies sawed that managing the plant density and row distance could be an important option for improving productivity and adaptability of sweet sorghum especially in short season areas. Since sorghum can be grown in diversity of ecological zones it is difficult to outline a common cultural method but in general, requires a daily temperature above 10 ^oC. Although that it is possible to cultivate sweet sorghum even under no-tillage farming systems it has been demonstrated that the adequate depth is about 2.5 and 3.5 cm. Moreover, the implementation of good agricultural practices, adequate management of soil fertility and water, crop rotations and the use of high quality seeds contribute to an increased resistance to pests. Sweet sorghum can be considered susceptible to Lepidoptera attacks which damage the stalk level. This has as result up to 30% sugar losses.

Although that N increase biomass yields, is reducing the sugar content in the stalk juice. An interesting way of reducing N needs is to rotate the sorghum with legume crops. It was demonstrated that the fixed N by a preceding soybean crop accounted for 35-40% of the improved yields of sorghum. Sweet sorghum maximizes its productivity under well water conditions nonetheless it is very impressive to see its ability to grow under suboptimal conditions. Irrigations trials indicated that the sugar concentration in sweet sorghum stalks did not change significantly by the stress level or the irrigation frequency but from the irrigation methods.

Generally the sweet sorghum produce a poorer quality juice when harvested too mature, mainly because the starch in the juice increases as the plant become senescent. The maximum sugar extraction, for several varieties, must be done between 20 to 40 days. The limited time constitutes a management problem that have to be sold as the current available harvesting equipment is not appropriate. However, some prototypes that harvest, press and collect the juice in a single pass have been tested with promising results. Moreover, the high moisture content influencing heavily the transportation cost but the main problem is to reduce sugar losses on storage. Some studies indicate that sulfur dioxide could preserve sweet sorghum stalks.

To conclude, according to the paper, sweet sorghum is a very interesting energy crop for bioethanol production. It could be used as a multipurpose crop thanks to its stems rich in structural and non-structural sugars, which can be processed into first and second generation bioethanol. Although, without proven successfully crop-management technologies and established markets, it is unlikely that farmers may be attracted to grow it.

Source: Walter Zegada-Lizarazu, Andrea Monti*

Department of Agroenvironmental Science and Technology, University of Bologna, Viale G. Fanin 44, 40127 Bologna, Italy,

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Biofuels offer the promise of numerous benefits related to energy security, economics, and the environment.

The quantitative and qualitative production of sweet sorghum strongly depends on the use of appropriate and improved agronomic management techniques which is, in some aspects, still largely unknown, since it is, in many aspects, still a wild species. The challenges faced by the producers could be summarizes in the following points:

• Research on the agronomic management of sweet sorghum for bioethanol purposes, however, has faced alternating periods of increasing and decreasing interest according to the fluctuations on international fossil fuels prices and availability.
• Short harvesting window (limited to about 20-40 days) and the poor storability of the stalks that could critically affect the ethanol production costs.
• Even though sweet sorghum benefits from a wide planting density (narrow rows result in higher stalk and sugar yields and improved control of weeds), ranging from 12 to 20 plants m², finding the appropriate density and seeds of improved cultivars for that density could be problematic. Information on seed production and supply chains is not readily available and seems that seed production at commercial level is in its early stages of development.
• In the environments exposed to high evaporation rates and low fertility conditions, low plant densities may reduce the stand water use efficiency as large quantities of water may be lost as direct evaporation.
• However, at closer planting spacing, plants produce thinner stems with slightly more tillers and if the growing areas are dominated by strong winds and thunderstorms, the risk of lodging increases. Proper availability of potassium may be an effective management practice to add strength to the stems and reduce the lodging problems.
• In the temperate regions of the northern hemisphere sweet sorghum is usually sown in springtime and harvested in late summer or early autumn. At these latitudes early spring or late winter sowing is not feasible as sweet sorghum does not tolerate cold stress. Moreover, seedlings sowed too early develop very slowly making weed control more difficult and costly. Delayed harvests leads to reduced oBrix and stem sucrose contents.
• Even though Sorghum is a resilient species and pests do not cause serious damages to the crop, especially in temperate climates, it can be considered susceptible to Lepidoptera attacks. The main damages by Lepidoptera are at the stalk level where the cavities created by the larvae result in a significant yield loss and sometimes cause lodging or even the death of the plant. Juice quality may be also affected by the level of infestation. Different infestation levels can result in as high as 30% sugar losses.
• Another damaging insects for sorghum are the Aphids that feed on the underside of the leaves and inject toxins that destroy leaf tissues, and can be also vectors for diseases. Chemical treatment may be needed, but most varieties are sensitive to organophosphorus pesticides, so appropriate aphid control should be sought.
• Sweet sorghum seeds should be treated with insecticides and fungicides before sowing to prevent seed rots, seedling blights and sucking sap insects. At the leaf level this crop is susceptible to a number of diseases including anthracnose (red stalk rot), fusarium, maize dwarf mosaic and other viral diseases. Since no fungicides are labeled for sweet sorghum, these diseases could be controlled, to a limited degree, by using resistant varieties, planting disease-free seeds, and by crop rotations with sunflower, cowpea, maize or soybean. Anthracnose can be avoided by growing sorghum in dry environments that are unfavorable for the disease.
• Sweet sorghum’s productivity can be seriously affected, if drought stress occurs at a specific growth stage. For example, the yield reduction was only 1% in post-anthesis drought stress, but up to 36% when drought stress occurred during vegetative growth.
• The juice quality, expressed as total dissolved solids, decreases with the highest fertilization level. High levels of N fertilization may enhance the vegetative growth thus reducing the sugar concentration in the storage organs as it generally occurs in other sugar crops.
• The limited storability and fast decaying of the harvested material, and therefore the limited time for its transportation and processing, are factors that severely affect the sweet sorghum production system. Moreover, the high moisture content (approx. 70%) limits the amount of biomass that can be carried out in a truck load, influencing heavily on the transportation cost, especially over long distances.
• Although studies indicated that sulfur dioxide could preserve sweet sorghum chopped stalks for about three to four months without sugar deterioration, a disadvantage of using large amounts of SO2 would be the management and application difficulties due to the toxicity of SO2 which is classified as hazardous material for human health and highly contaminant to the environment.

References

1. Zegada-Lizarazu W, Monti A, Are we ready to cultivate sweet sorghum as a bioenergy feedstock? A review on field management practices, Biomass and Bioenergy (2012), doi:10.1016/j.biombioe.2012.01.048
2. P.S. Nigam, A. Singh / Progress in Energy and Combustion Science 37 (2011) 52e68 doi:10.1016/j.pecs.2010.01.003

Agriculturally significant sorghum varieties can be divided into three broad categories. Grain sorghum varieties grow from three to six feet tall and produce large seedheads that are typically harvested for livestock feed. Sweet sorghum varieties are much larger, typically from eight to twenty feet tall, with thicker and fleshier stems than the grain varieties, but with much smaller seedheads. Forage varieties are similar to sweet varieties, but typically are smaller and lower in water and sugar content. Of less economic importance are the closely related Sudangrass and broomcorn, which are members of S. bicolor ssp. Drummondii. Sweet and forage sorghums may also produce substantial quantities of grains, anywhere from 5 to 25% of total dry weight at maturity, depending upon variety. The grains could be harvested as a separate by-product for their starch content, or as an animal feed or included in the fermentation.

As an energy crop, sweet sorghum has the unusual advantage of being a source of sugar, starch and cellulose. Naturally, the major driver of interest in the plant, both for food and fermentation, has been the large amounts of sugar in its juice, generally comprising anywhere from 20 to 50% of the whole plant’s dry weight. The fermentable sugars are primarily sucrose, glucose and fructose.

The potential fermentation products of sweet sorghum are wide ranging; ethanol, acetone, butanol, various lipids (lipids are most commonly intended for biodiesel production), lactic acid, hydrogen, and methane. Several potential native products of the plant, in addition to cellulose for paper production, are also identified: waxes, proteins, and allelopathic compounds, such as sorgoleone.

The grain of sweet sorghum can be processed according to typical grain fermentation procedures, which generate spent grain and solubles. It was demonstrated that the solubles could be concentrated using ultrafiltration and reverse osmosis to generate, along with the spent grain, a high protein feed. Waxes from sorghum grain may be used for food coatings and have been used in the production of biodegradable and edible films.

It is possible to ferment sweet sorghum sugars, by ubiquitous microbes, to hydrogen, followed by anaerobic digestion of the remaining biomass to biogas. In a study, 90% volatile solids conversion efficiencies were achieved over a 75-day solids retention time. Pretreatment techniques have been also used to increase the saccharides available for hydrogen production. Additionally, some hydrogen-producing microorganisms have been shown to convert both pentose and hexose saccharides, allowing for the conversion of hemicellulose to hydrogen.

Some products may be available for extraction prior to fermentation, or may be generated from the bagasse. The bagasse itself, although not a good soil amendment in its raw state, can be converted into useful compost. The pith from sorghum bagasse has been converted into activated carbon for the manufacture of electrodes in supercapacitors. Sorghum bagasse can also be used to produce syngas and bio-oil, via pyrolysis.

Solvent extracts of sorghum have been shown to inhibit the activity of α-glucosidase and human salivary α-amylase, and so there may be sorghum compounds of use in diabetes treatments.

Lastly, the allelopathic nature of the sorghum plant has been observed and recently, purple nutsedge weed growth suppression has been demonstrated using solvent extracts of sorghum. Some studies have identified potential allelopathic compounds, such as the root exudate sorgoleone and the induced defense compound dhurrin (p-hydroxy-(S)-mandelonitrile-β-D-glucoside). Sorgoleone levels vary widely among varieties, but it is hydrophobic, selective in action, effective at extremely low concentrations and so could have commercial applications.

In the following table, various products and the micro-organisms used, during the fermentation of sweet sorghum, are presented.

References

• M.B. Whitfield et al. / Industrial Crops and Products 37 (2012) 362– 375 doi:10.1016/j.indcrop.2011.12.011

The Sorghum Checkoff is planning its first Sorghum Renewables Summit to be held in April 19-20 at the Holiday Inn in Denver, Colorado. Calling sorghum an adaptable, efficient and sustainable crop, the sorghum boosters hope to bring attention to its potential.

The speakers and topics cover a variety of topics covering sorghum use for renewable energy.  Ed Wolfrum, National Renewable Energy Laboratory, will speak on compositional analysis and Randy Powell, BioDimensions Delta BioRenewables LLC, on first generation processing.

Agronomics and development will be covered with Jeff Dahlberg, University of California Kearney Agriculture Center speaking on agronomics and Bill Rooney, Texas A&M, addressing sorghum breeding for renewables. The logistics of harvesting, hauling and delivering will be covered by Bob Avant, Texas A&M. The state of sorghum technology will be covered by Larry Richardson, Richardson Seeds Ltd.

source: http://www.ethanolproducer.com/articles/8691/