Sergio O. Serna-Saldivar* Cristina Chuck-Hernandez, Esther Perez-Carrillo and Erick Heredia-Olea

Departamento de Biotecnologia e Ingenieria de Alimentos, Centro de Biotecnologia. Tecnologico de Monterrey, Monterrey, N. L.

Mexico

1. Introduction

Nowadays, there is a growing interest for alternative energy sources because of the reduction of fossil fuel production. Ethanol used as automotive fuel has increased at least six times in the current century. According to the Renewable Fuels Association, in 2010 the USA bio-refineries generated 13 billion gallons of fuel ethanol and the year before worldwide production reached 19 billion. This noteworthy increment is in its majority based on maize and sugar cane as raw materials (Berg, 2004; Renewable Fuels Association, 2010). The use of these feedstocks has triggered concerns related to food security especially today when the world population has reached 7 billion people.

The relatively sudden rise in food prices during 2008, 2010 and 2011 has been attributed mainly to the use of maize for bioethanol even when other factors like droughts or changes in global consumption patterns have also played a major role (World Food Program, 2008). Food price projections indicate that this situation will worsen, breaking the downward trend registered in food prices in the last thirty years (The Economist, 2007).

Even if there was not a food-fuel controversy especially due to the current conversion of millions of tons of maize for bioethanol, the use of only this crop cannot support the ambitious objectives of renewable fuel legislation in countries like the United States of America, where a target of 36 billion gallons of liquid biofuels have been established for 2022. In order to meet this requirement all the 333 million tons of maize yearly produced by USA should be channelled to biorefineries. This production represents 2 and 16 times the maize harvested in countries like China and Mexico respectively, which in turn are two of the five top world producers.

Environmental factors have been also pushing for the quest of new crops dedicated exclusively for liquid automotive fuel in order to reduce the use of prime farming land, irrigation water and other resources. A dedicated energy crop ideally must meet several requirements such as: high biomass yield and growth rate, perennial, with reduced input necessities, fully adapted to the geographic regions where will be planted, easy to manipulate via genetic improvement, non-invasive, tolerant to stress and with a good carbon sequestration rate among others (Jessup, 2009). At the present time, energy crops are

mainly represented by perennial grasses as switchgrass (Panicum virgatum L.), energy cane (Saccharum spp), sweet and forage sorghum (Sorghum bicolor), miscanthus (Miscanthus spp.) as well as other short-rotation forest resources (willow — Salix spp- and poplar — Populus spp) (Jessup, 2009; McCutchen et al., 2008).

The development of new and improved enzymes, bioprocesses and feedstocks could lead to cost reduction from an estimated of 0.69 cents to below 0.51 cents/L that nowadays is the benchmark established for starchy raw materials (Kim & Day, 2011). Besides the development of dedicated crops for energy, one of the best approaches for cost reduction and optimal use of resources is the use of flexible facilities allowing the integration of different streams of same or different feedstocks. Flexibility, balance, diversification and regionalization are indeed keywords in the development of solutions to meet future world energy demands.

In tropical, subtropical, and arid regions from the United States, Mexico, China, India, Southern Africa, and other developing countries, where agronomic harsh conditions prevail, one of the most promising crops for fuel is sorghum (Sorghum bicolor (L.) Moench) (Reddy et al., 2005; Zhang et al., 2010). This is a high efficient photosynthetic crop that reached a worldwide production of 56 million tons of grain in 2009 (FAOSTAT, 2011), just behind maize, wheat, rice and barley. Almost 30% of this production is harvested in North America where sorghum is mainly used for feed. Sorghum is a C4 plant, highly resistant to biotic and abiotic factors as insects, drought, salinity, and soil alkalinity. Furthermore, this crop has one of the best rates of carbon assimilation (50 g/m2/day) which in turn allows a fast growth and a better rate of net CO2 use (Prasad et al., 2007). Sorghum requires one third of the water with respect to sugar cane and 80 to 90% compared to maize (Almodares & Hadi, 2009; Wu et al., 2010b). Thus, sorghum is considered as one of the most drought resistant crops. Furthermore, sorghum requires approximately one third of the fertilizer required by sugar cane (Kim & Day, 2011) and its growth cycle is between 3 to 5 months allowing two or three crops per year instead of one commonly obtained with sugarcane. Besides environmental advantages, sorghum is one of the more acquiescent plants to genetic modification because is highly variable in terms of genetic resources and germplasm. This facilitates plant breeding and development of new cultivars adapted to different regions around the globe (Zhang et al., 2010).

Sorghum can be classified in four broad groups: grain, sweet, forage and high biomass. All belong basically to the same species and virtually there are no biological or taxonomic differences (Wang et al., 2009). Grain sorghum is used mainly as food, feed and for starch production. In the United States only a small percentage of fuel ethanol (around 2-3%) is obtained from grain sorghum (Renewable Fuels Association, 2010; Turhollow et al., 2010; Zhao et al., 2008), but in 2009 about 30% of the U. S. grain sorghum crop was used for ethanol production (Blake, 2010).

On the other hand, forage sorghum is characterized as a high biomass crop. This capacity has been boosted by intensive research programs worldwide, focused in the design of new varieties tailored for ethanol production (Rooney et al., 2007). The main product obtained from sweet sorghums is the fermentable sugar rich juice that is produced and accumulated in the stalks in a similar fashion as sugar cane. The extracted sweet juice is mainly composed of sucrose, glucose, and fructose, and thus can be directly fermented into ethanol with efficiencies of more than 90% (Wu et al., 2010b). According to Almodares & Hadi (2009) sorghum yields a better energy output/input ratio compared to other feedstocks such as sugar cane, sugar beet, maize and wheat. Altogether with the juice, the residue or bagasse can be also converted to ethanol or used for other traditional applications.

In summary, sorghum is a crop well adapted to adverse climatic conditions which at this time is one of the growing concerns in agronomic projections. This is mainly due to the change of rain patterns and climate, greenhouse effect and the steadily rise of world temperature. Given all these advantages of sorghum as a potential source of biofuels, the objective of this chapter is to explore its potential, as an integrated crop for fuel production in terms of yield and technologies available for processing. The chapter especially focuses on optimum technologies to produce bioethanol from sweet sorghums, starchy grains and biomass from dedicated crops.