As explained earlier, the cost of lipase production is the main hurdle to the commercialization of the enzymatic process. Therefore, the reuse of lipase is essential from the economic point of view, which can be achieved by using the lipase in immobilized form. The operational stability of the catalyst in a continuous process plays a vital role. Further details about stability and possible ways of enhancing it is found in Sections 6.6 and 6.8. Shimada et al. (2002) achieved 93% conversion of SO in absence of organic solvents in a series of three continuous packed-bed bioreactors at a rate of 6.0 ml h-1. The productivity relative to the total mass of enzyme used was however lower than when t-butanol was added to the continuous reactor (Royon et al., 2007). The necessity of solvent recovery can be a drawback to such a process. However, the relatively low optimum t-butanol concentration, and low boiling point, allows easy separation, and hence the energy expense required for its recovery is usually acceptable. Chen and Wu (2003) achieved 70% conversion in continuous packed-bed bioreactor in the absence of organic solvent, but with periodical regeneration of the immobilized lipase with t-butanol washing. Nie et al. (2006) used lipase immobilized on cheap cotton fibers in a series of three packed-bed bioreactors with stepwise addition of methanol to produce biodiesel from SO and WO and achieved 93% and 92% conversions, respectively. A hydrocyclone was used on-line to separate glycerol. The operational stability of the immobilized lipase was more than 20 days at input flow rate of 15 L h-1 of substrate and ether solvent in a volume ratio of 2:3.

On the other hand, the use of membrane bioreactors for the enzymatic processing of fats and oils is increasingly becoming more attractive to substitute conventional stirred tanks or packed-bed reactors (Basheer et al., 1994). As the reaction proceeds, glycerol is generated and physically mixes with the alcohol to form a second liquid phase that is not completely miscible with the oil. This second polar organic phase serves to extract alcohol from the oil phase, thereby decreasing the concentration of this substrate in the reaction medium and causing a concomitant decrease in the conversion achieved in a fixed amount of time. In addition, glycerol is adsorbed on the surface of the immobilized lipase, and blocks the substrate from reaching the active sites. Consequently, conversions will be enhanced if glycerol is removed from the substrate mixture as the reaction proceeds. To achieve this, membrane reactors with immobilized lipase are proposed, which may take either a flat sheet (Isono et al., 1998) or hollow fiber form (Hilal et al., 2004; Shamel et al., 2005). Membrane reactors enhance efficiencies by combining in one unit a reactor that generates a biodiesel and a separator that separates it from the other products. Removal of a product drives equilibrium-limited reactions towards completion and prevents product inhibition.

152 Handbook of biofuels production

There is no doubt that combustion of fossil fuel in motor vehicles releases huge amounts of gases that can have a negative impact on human health and will change global climate drastically. Bioethanol as a fuel has the potential to lower emissions of harmful substances. The CO2 emissions from the combustion of bioethanol from biomass will be consumed by plants during photosynthesis and the net introduction of C O2 to atmosphere will be zero in the long term, while fossil fuels gives a net increase. Life cycle analyses (LCA) of bioethanol as a fuel have shown that emissions of CO2 and other greenhouse gases are lower than when just using gasoline as a fuel in transport systems (Niven, 2005). Especially ethanol produced from lignocellulosic feedstock is reducing the emissions of fossil CO2 by up to 90%.

9.7 Flow chart of different steps in bioethanol pilot plant in O-Vik, Sweden.

(1) Intake feedstock, (2) steaming step, (3) pre-treatment step,

(4) reactor steps, (5) membrane filter press to remove lignin,

(6) detoxification step, (7) fermentation step, (8) yeast separator stage, (9) distillation system, (10) evaporation step, and (11) storage tank (with permission from Swedish Energy Agency and SEKAB for reproduction).

G. ANTONOPOULOU, I. NTAIKOU, K. STAMATELATOU and G. LYBERATOS, University of Patras, Greece

Abstract: This chapter discusses all the biological hydrogen production processes such as indirect and direct water biophotolysis, biological water gas shift, photo and dark fermentation and hydrogen production through microbial electrolysis cells. Dark fermentation or fermentative hydrogen production is focused on this chapter, since it is considered as the most promising compared to all biological hydrogen production methods. However, there are significant remaining barriers to practical application. The chapter includes the limitations of each process and suggests several methods that are aimed at overcoming these barriers.

Key words: biohydrogen, biological hydrogen production processes, fermentative hydrogen production, advantages and limitations.

13.1 Hydrogen

Hydrogen is a colorless, odorless gas that accounts for 75% of the universe mass. It is also the simplest element in the periodic table, since its atom consists of only one proton and one electron. Despite its simplicity and abundance, hydrogen does not exist naturally as a gas, but is found in water, biomass and fossil fuels (gasoline and natural gas), where it is always combined with chemical bonds with other elements such as oxygen, carbon and nitrogen. In order to get hydrogen into a useful form, it must be extracted and separated from these substances. These ‘extraction’ processes are often quite energy intensive. For this reason, many efforts have been invested on the exploration and development of cost-effective and efficient methods of hydrogen production.

Apart from being a very useful reagent for the production of many chemicals, hydrogen is also the most clean and environmentally friendly fuel, which produces water instead of greenhouse gases when burned and possesses a high energy yield of 122 kJ/g, which is 2.75 times greater than that of hydrocarbon fuels. Hydrogen is indeed considered a viable alternative fuel and the ‘energy carrier’ of the future.

Today, hydrogen finds a wide range of industrial applications being a widely used feedstock for the production of chemicals, hydrogenation of fats and oils in food industry, production of electronic devices, processing steel and also for desulfurization and re-formulation of gasoline in refineries. Furthermore it is used in NASA’s space programme as fuel for the space shuttles and in fuel cells for heat and electricity generation. Proton exchange membrane fuel cells (PEMFC) fed with hydrogen are

believed to be the best type of fuel cell that could be used as power sources in vehicles and have the potential to replace the gasoline and diesel in internal combustion engines (http://www. fctec. com). Beyond its use in fuel cells, hydrogen could be directly burned in a fossil internal combustion engine (very similar to petrol or gas-fired engines) to produce mechanical energy without producing CO2 at the point of use. According to the National Hydrogen Program of the United States, the contribution of hydrogen to the total energy market is projected to be 8-10% by 2025 (Armor, 1999). In Fig. 13.1, a hydrogen station in Tsurumi of Japan is depicted.

S. PINZI and M. P. DORADO, University of Cordoba, Spain

Abstract: This chapter presents the most frequent vegetable-based feedstocks to biodiesel and bioethanol production. The chapter focuses on first — and second-generation biofuels with special emphasis on low-cost feedstocks. Finally, raw materials for developing technologies, including anaerobic digestion to produce biogas, Fischer-Tropsch from biomass, pyrolysis and biological production of bio-hydrogen are discussed.

Key words: first-generation biofuels, second-generation biofuels, third-generation biofuels, low-cost biofuels, biomass.

4.1 Introduction

Main differences between generations of biofuels lie in both conversion technology and raw materials. First-generation biofuels are made using conventional chemical technology to convert mainly oilseeds and grains into biodiesel and bioalcohol, respectively. In many cases, same feedstocks could be used for animal or human feeding purposes, thus suffering criticism from organisations that point at biofuels as the leading factor of food price rises and even deforestation in the Amazon or Indonesia. Although arguments against these assumptions are exposed, second-generation biofuels are based on non-food crops (i. e. Miscanthus) and biomass residues (from crops and forests), thus providing a socially accepted alternative. However, conversion technologies to produce biohydrogen, biodimethyether (Bio-DME), Fischer-Tropsch (FT) diesel, etc., are still under development.

There is also a third-generation emerging consisting of biofuels from algae and even an incipient fourth-generation based on the conversion of biodiesel into gasoline or on the recycling of carbon dioxide back into gasoline. Some companies claim that they can produce economically-sounded petroleum from microorganisms having the ability to efficiently convert renewable feedstocks into hydrocarbon-based fuels (Du, Li, et al, 2008). Although there is a wide variety of feedstocks and biofuels, this chapter is mainly focused on the most frequent vegetable-origin feedstocks to biodiesel and bioethanol production.

Acid catalysis is simultaneously performing esterification of FFAs and TAGs. In this way, it is more economical to use low-quality feedstocks and lower processing costs.

The reaction mechanism using solid Brpnsted acids catalyzed esterifications is similar to that of the homogeneously catalyzed process. The reaction involves a nucleophilic attack of the adsorbed carboxylic acid with the free alcohol in the rate-determining step. The formation of a more elecrophilic intermediate is also occurring with solid Lewis acids. The rate-determining step is dependent on acid strength. If the strength of the acid sites is too high, the desorption of the ester is decreased. This mechanism is valid for both homogeneous and heterogeneous catalyst (Bonelli et al., 2007).

Many studies for the heterogeneous acid esterification have been carried out, mainly using acid resins (Lotero et al., 2006).

Lopez (2006) has tested the activity of various acid catalysts in the transesterification of triacetin at 60°C. The flowing order of activity was observed: H2SO4 > Amberlyst-15 (polystryrenesulphonic acid resin) > sulfated zirconia > Nafion NR-50 (perfluorinated alkanes resin sulfonic acid) > tungstated zirconia B > supported phosphoric acid > zeolite B.

The remarkable low activity is due to diffusion limitations in the zeolite pores of the bulky TAGs. At low temperatures the transesterification activity is slow and in order to obtain high reaction rates the temperature has to be increased above 170°C. However, many sulphonic acid catalysts are unstable at these high temperatures and therefore lower temperatures have to be used (120°C). Esterification is an equilibrium reaction and a nearly complete ester formation can only be reached after stripping off the water and adding additional methanol (Pasias et al., 2006).

An industrial process for the conversion of FFAs into FAMEs using heterogeneous catalyst (e. g. acid Amberlyst™ BD20), called FACT (Fatty Acid Conversion Technology) has been described by Soragna (2008).

The process involves a continuous, counter-current, multiple-step esterification using a solid catalyst in fix bed reactors at 90°C and 3.5 bar with intermediate methanol recovery. Production of biodiesel is performed by direct conversion as ‘stand-alone process’ where the quality of the FAMEs are increased by distillation or by an ‘integrated process’ where the ester content is increased by transesterifica­tion of the residual acylglycerols. A schematic representation of these two processes has been discussed later in this book (Chapter 22). High acidity feedstocks such as animal fats, used cooking oils, fatty acid distillates and high acidity vegetable oils can be used.

Esterification of FFAs in waste cooking oils was studied by Ozbay et al. (2008). The highest FFA conversion (46%) was obtained over a strong acidic macroreticular ion-exchange resin A-15 at 60°C with two per cent catalyst. Conversion of FFAs increased with increasing temperature and catalyst amount.

A comparative study of different heterogeneous catalyst (Dowex Monosphere 550A and zeolites NaY, VOx over USY) and different alcohols with oleic acids show FFA conversion of 51%. Enzymatic esterification is looking more promising (Marchetti and Errazu, 2008).

Superiority of physical properties of resins may be a dominant factor for high activity. Other acid catalyst A-16, A-35 and Dower HCR-W2 are less active.

Similar results have been obtained by Marchetti et al. (2007), showing that reuse of the catalyst results in low conversion rates. A general overview of the production from acidulated soapstock (acid oil) has been described by Luxem and Mirous (2008) emphasizing various processes using homogeneous and heterogeneous catalyst, mainly converting FFAs to FAME (87-92%) with 20% catalyst, a ratio of methanol to FFA of 3.8:1 and 3.5 hours.

Tin (Sn2+) complexes using the ligand 3-hydroxy-2-methyl-4-pyronate (maltolate) have been used to convert various vegetable oils into FAME at 80°C using a molar ratio 400:100:1 of methanol:oil:catalyst.

Yields up to 90% can be obtained but methanolysis is dependent on the nature of acid chain favoring the presence of unsaturation and chain length. Technological potential is rather low as the complexes remain dissolved in the reaction medium. Attempts have been made to immobilize the complex (Suarez et al., 2008).

A combined acid esterification and alkaline transesterification using a base and acid functionalized mesoporous silica nanoparticles has been proposed by Huang et al. (2008). These nanoparticles contain base (primary amines) and sulfonic acids inside the porous channels and are employed for one-pot reaction cascades.

An alternative to transesterification of TGs contained in vegetable oils to obtain biofuels is to transform these renewable sources via different chemical processes in conventional petroleum refineries.

The production of high-quality diesel fuel from vegetable oils can be obtained by hydrocracking of TGs treated with high-molecular weight hydrocarbons in conventional oil refineries, as described by Huber et al.64 In this way, renewable liquid alkanes can be produced by treatment of mixtures of vegetable oils and fractions of heavy oil vacuum (HVO), under hydrogen flows and conventional catalysts (sulphured NiMo/Al2O3) at standard temperature conditions (300- 450°C). The reaction involves the hydrogenolysis of C=C bonds in vegetable oils, which leads to a mixture of lower molecular weight alkanes by three different

routes: decarbonylation, decarboxylation and hydrodeoxygenation (Fig. 7.5). Waxes can be formed. Straight-chain alkanes can be isomerized and cracked. The organic acids formed by hydrotreating could catalyze the isomerization and cracking reactions.

The yield of straight-chain alkanes C15-C18 obtained by hydrotreating of pure vegetable oil is about 71% (for sunflower oil), with a theoretical maximum yield of 75%. These yields can be increased by diluting pure vegetable oils with petroleum feedstocks such as HVO. The straight-chain C15-C18 yield of a 5% sunflower oil-95% HVO mixture has been reported to be 87%, higher than that obtained using pure sunflower oil (75%).64

In conclusion, the hydrotreating of vegetable oils also seems to be a promising alternative to produce biofuels from renewable sources, especially because it has the advantage of using existing petroleum refineries without the need to purchase additional capital equipment.

Problems with bacteriophage infections emerged in almost all historical industrial ABE fermentation processes, no matter how good hygiene and plant practices were applied (Hastings, 1971; Jones et al., 1986). While most bacteriophage infections manifest in similar symptoms such as decreased growth rates and poor solvent production, bacteriophages seem to be very strain specific. The most successful method to overcome the effects of bacteriophage infections proved to be strain immunization (Jones et al., 1986).

10.5 Future trends

As already stated in the introduction, a problem that every biofuel candidate must face is the question of substrate. Competition between nutritional and transportational needs represents a major ethical problem. Although new plants have been built and existing ones reopened in Brazil and China (see Section 10.3), these are still based on sugar cane or starchy materials. In future, the possibility of converting other compounds into butanol will be becoming more and more important. One interesting resource is lignocellulosic hydrolysates. Processes using such substrates are already used for ethanol formation (Durre, 2007). Currently, a large number of research projects focus on this topic. Another exciting possibility is the use of synthesis gas. This mixture of mostly carbon monoxide and hydrogen can easily be obtained from biomass, thus avoiding costly pretreatments. As syngas is a common bulk material in the chemical industry, technical experience in operation of the respective equipment already exists. The proof of principle of biological butanol formation from syngas has been demonstrated (Kopke, 2009). An additional argument in favor of such a process is the direct consumption of gaseous CO and CO2, thus helping to reduce the global greenhouse effect. It is envisaged that therefore such processes will become important industrial applications in future.

Apart from the type of microbial inoculum and feedstock, which is used for fermentative hydrogen production, many other factors such as pH, temperature, hydraulic retention time (HRT), nutrients concentration, hydrogen partial pressure, the presence of inhibitors and hydrogen-consuming microorganisms and the reactor configuration, influence the process. Although the role of each parameter in fermentative hydrogen production is well defined, the optimum conditions of a given factor are not clear, so far. For example, it is well known that the pH influences the activities of hydrogen producing microorganisms, since it directly affects the hydrogenase activity (Dabrock et al., 1992) as well as the metabolic pathway followed. However, there is a wide range of pH values, which have been proposed as optimum for fermentative hydrogen production from different feedstocks. The pH range of 5-7.5 (Fang et al., 2002a; Calli et al., 2008) is usually reported as optimum, even though lower or higher pH values such as pH of 4.5 (Ren et al., 1997) and 9.0 (Lee et al., 2002) have also been proposed that are supposed to give the maximum hydrogen yield.

The operational temperature is another important factor affecting the metabolic pathways involved and influencing the whole process. Up to now, most studies on hydrogen production have been carried out under mesophilic conditions, even though it is well known that hydrogen fermentation at high temperatures (thermophilic conditions) has higher hydrogen yield than the mesophilic equivalent, owing to higher suppression of hydrogen-consuming bacteria. Nevertheless, mesophilic biohydrogen production is preferred for preventing the need for external heating, improving the economics of the process.

Regarding the HRT, for pure substrates such as glucose and sucrose, the widely used values are in the range of 3-8 hours, with the lowest being 1 hour (Chang et al., 2002) and the highest 13.7 hours (Fang and Liu, 2004), while for more complex substrates such as starch, an HRT of 15 or 17 hours is suggested to be necessary due to the slow initial step of hydrolysis (Hussy et al., 2003; Lay, 2000).

From this discussion, it becomes clear that the optimum value for each aforementioned key factor depends on the feedstock, the inoculum used and the prevailing conditions under which the experiments are carried out. Thus, predictions of the reactor performance in terms of hydrogen yields and rates as well as carbohydrate conversion efficiency under different conditions are not accurate. So, the selection of the operational conditions of a real scale hydrogen — producing bioreactor at this stage may be safely predicted only on the basis of lab-scale and pilot-scale experiments.

Lignocellulosics and algae have been recently considered the most promising alternatives for the production of later-generation biofuels. A full account of the production of a wide range of second-generation biofuels from lignocellulosic biomass (e. g. wood, grasses, agricultural and forestry waste) is given in Parts III and IV of this monograph. The process is identical to that described in the production of first-generation bioethanol: decomposition of the material into fermentable sugars (hydrolysis) and transformation of the sugars into bioethanol (fermentation). The main changes are in the processing technologies and the feedstocks that usually account for the majority of the plant cost. Lignocellulosic biomass comprises three main components: cellulose and hemicellulose (complex carbohydrate polymers), accounting roughly for about a 70-75 wt% of the lignocellulose, and lignin (Fig. 1.2).

A mixture of enzymes (cellulases and hemicellases) different from those used in the first-generation bioethanol production are employed in the hydrolysis step. Lignin is obtained as a by-product of the process that can be burned to produce heat and power for the processing plant and potentially for surrounding homes and businesses. It has also a great potential as it is hoped to become a future source of aromatic chemicals and materials. Alternative organisms also need to be employed due to the impossibility of traditional yeast and bacteria to process the pentose (C5) sugars derived from hemicellulose.20 We refer the readers to

Chapters 8 and 14-18 for more detailed information on advanced technologies for the processing of lignocellulosic feedstocks.

Algae is the second relevant feedstock with a great potential for future development. It has not been included as such in the monograph, but we believe that Chapters 4 and 8 will give some details about these microorganisms for the production of biofuels.

Microalgae are sunlight-driven cell organisms that convert atmospheric CO2 (via photosynthesis) into a plethora of chemicals, including methane, hydrogen, polysaccharides and oil.21-23 Interestingly, the production of algal oil is remarkably more efficient compared with conventional oil crops, providing higher oil yields (up to a 75% dry weight) and lower land area utilisation (Tables 1.1 and 1.2).

The process involves the extraction of the oil from microalgae and subsequent transesterification with alcohols using homogeneous or heterogeneous catalysts (in a similar way to that of biodiesel obtained from (non)edible feedstocks) to give biodiesel.

Despite significant advances in the field, which have been recently reported in the area of biofuels produced from algal oil, there are several drawbacks that

currently limit its widespread utilisation, primarily the economic feasibility of the technology.24

The recovery of such bio-oil from algae is a very challenging task. The algal broth produced in the biomass production generally needs to be further processed to recover the biomass24,25 and then the concentrated biomass paste is extracted with an organic solvent (e. g. hexane) to recover the algal oil that can be transesterified into biodiesel. Furthermore, the valorisation of the dry residue of the algae is not normally taken into account in current processes, and this largely implies a significant increase in costs as these algal residues need to be disposed of/removed upon extraction.

On the other hand, algal oil is rich in long-chain polyunsaturated acids, including eicosapentaenoic (EPA; 20:5 n-3) and docosahexaenoic acids (DHA; 22:6 ro-3), which are generally undesirable in conventional biodiesel due to the negative impact of the polyunsaturation on the oxidation stability. The presence of EPA and DHA is not contemplated in the EU (EN 14214 and EN 14213, biodiesel for transport and heating) and US (ASTM D6751) quality biodiesel standards that specify a limit of 130 g (EN 14213) and 120 g (EN 14214) iodine/100 g of biodiesel (iodine value). Storage issues arising from the oxidation instability may be overcome through either chemical transformations (e. g partial catalytic hydrogenations of the polyunsaturated compounds in the oil)26 or genetic modification of certain species.7,24 It is yet unclear as to how the presence of much more saturated FAM/ EE will affect the cold performance (CFPP) of the biodiesel.

These main drawbacks remarkably influence the economics of the process, in which problems related to capital infrastructure costs, contamination through open — pond systems and costs associated with harvesting and drying of the algae may also have a major contribution. A full and precise estimation of the economics of the process is therefore needed in order to demonstrate its feasibility,23-25 in which the valorisation of the algal residue (potentially via gasification to syngas and/or other biofuels) is believed to be critical to improve the economics of the process.

The potential for biofuels has been recognised throughout the twentieth century, but the new century has brought with it a widespread realisation that the petroleum age is coming to an end. The use of petrol fuel replacements has generated a lot of controversy; ideally, they should contribute to global sustainability, ensuring the energy supply and meeting the GHG targets (as well as being profitable and cost — competitive as much as possible) without compromising the economies, culture, societies and the environment of our future.

There are in our views exaggerated expectations from second-generation technologies which probably will take a long time to materialise, with topics such as fuel ‘versus’ food and the consequence of land use changes on GHG emissions being ‘politicised’.

These important issues should however not let us get distracted from the potential benefits of biofuels and, more widely, of biomass exploitation. It should rather encourage us to redouble our efforts to research low-carbon technologies

for the production of later-generation biofuels (and biochemicals) from low-value waste biomass, with properly measured and reported environmental impacts. A combined effort from politicians, economists, environmentalists and scientists is needed now, more than ever, to address the issues of the progressive incorporation of biofuels in our society and to come up with alternatives, policies and choices to advance the key technologies for a more sustainable future.

1.7 Acknowledgements

R. L. gratefully acknowledges Ministerio de Ciencia e Innovation, Gobierno de Espana, for the provision of a Ramon y Cajal contract (RYC-2009-04199).

1.8 Sources of further information

www. rsc. org; Royal Society of Chemistry, keyword: biofuels http://www. refuel. eu/ and links therein. http://www. biofuelstp. eu/

Ethanol (i. e. ethyl alcohol, bioethanol) is a liquid oxygenated biofuel employed either as a fuel or as an additive. When it is used as the latter, due to its high oxygen content, a less amount of additive is required. The increased percentage of oxygen allows a better oxidation of the gasoline hydrocarbons with the consequent reduction in the emissions of CO and aromatic compounds (Sanchez and Cardona, 2008).

Bioethanol is the most widely used biofuel for transportation applications, specially in the Western hemisphere, where it surpasses biodiesel in importance. One major problem related to bioethanol production is the availability of the raw materials that varies considerably from season to season and depends on geographic location (UNCTAD, 2006). The major global producer of bioethanol is Brazil, which produces 50% of the world fuel ethanol, using sugar cane juice (Brazil is responsible for 25% of all sugar cane production worldwide) and molasses (Murphy, 2004). In the USA, 95% of the fuel ethanol produced comes from corn, while more temperate countries like Canada uses other less efficient starchy crops like wheat, corn and barley (Murphy, 2004). Table 4.3 depicts ethanol yield from the most common crops.

Biological feedstocks that contain appreciable amounts of sugar — or materials that can be converted into sugar, such as starch or cellulose — can be fermented to produce bioethanol (Kim and Dale, 2004). Bioethanol feedstocks can be classified

into three types: (1) sucrose-containing feedstocks (e. g. sugar beet, sweet sorghum and sugar cane); (2) starchy materials (e. g. wheat, corn and barley); (3) lignocellulosic biomass (e. g. wood, straw and grasses).

The price of the raw materials can highly affect the production costs of bioethanol because feedstocks typically account for more than one-third of the production costs, and maximising bioethanol yield is imperative (Murphy, 2004). In this sense, sugar cane and sugar beet present an alcohol yield of around 3 and 1 hl/t respectively, while cereals such as wheat and corn present higher alcohol yields (3.6 and 4 hl/t, respectively). However, processing costs to produce ethanol from sugar cane and sugar beet (where sugars are easily accessible since disaccharide can be broken down by the yeast cells) are lower compared to cereals and most of all compared to starchy materials and lignocellulosic biomass (Cardona and Sanchez, 2007; Prasad et al., 2007). Starchy, lignocellulosic, urban and industrial wastes need costly pretreatment to convert into fermentable substrates.