Cultivation of microalgae influences both biomass growth and lipid productivity. Cultur­ing of algae requires the input of light as an energy source for photosynthesis with a sufficient supply of macronutrients (nitrogen and phosphate) and micronutrients (sulphur, potassium, magnesium) in dissolved form (Mata et al., 2010). The main options for algae cultivation on a commercial scale are open-ponds or closed systems called photobioreactors (Chisti, 2007; Robert et al., 2012). There are also hybrid configurations that include a mix of the two growth options. Innovations in algae production allow it to become more productive while consum­ing resources that would otherwise be considered waste (Campbell, 2008).

і і л

A. Catarina Guedes, Helena M. Amaro ’ ,
Isabel Sousa-Pinto1’3 , F. Xavier Malcata1,4

1CIIMAR/CIMAR — Interdisciplinary Centre of Marine and Environmental Research,
University of Porto, Porto, Portugal
2ICBAS — Institute of Biomedical Sciences Abel Salazar, Porto, Portugal
^Department of Biology, Faculty of Sciences, University of Porto, Porto, Portugal
^Department of Chemical Engineering, University of Porto, Porto, Portugal

10.1 INTRODUCTION

Over the past 50 years, the world population more than doubled. This fact, coupled with an extension of life expectancies and rising standards of living, has led to a dramatic increase in primary energy consumption, chiefly from fossil sources (Jones and Mayfield, 2012).

A suitable alternative is to produce biofuel from photosynthetic organisms, that is, higher plants, algae, and cyanobacteria, which can use sunlight and carbon dioxide to produce a va­riety of organic molecules, namely carbohydrates, proteins, and lipids. These biomolecules can then be used to generate biomass rich in fuel-like metabolites that can then be extracted (Yang, Guo et al., 2011; Jones and Mayfield, 2012). However, the problem remains: What to do with the spent biomass? In particular, third-generation biofuels based on micro — and macroalgae offer an excellent possibility to displace fossil fuels; it is even believed that ances­tors of marine microorganisms were responsible for the formation of petroleum in the first place (Goh and Lee, 2010).

Macroalgae (or seaweeds) are multicellular organisms that take many forms and sizes. They are classified into three broad groups based on their pigmentation: brown algae (Phaeophyceae), red algae (Rhodophyta), and green algae (Chlorophyta). In contrast, microalgae are microscopic organisms, which, beyond Rhodophyta and Chlorophyta, may belong to another three specific groups of unicellular organisms: blue-green algae (Cyanobateria), diatoms (Bacillariophyta), and dinoflagellates (Dinophyceae). These species are commonly referred to as phytoplankton (Garson, 1993; Samarakoon and Jeon, 2012).

Despite looking similar to land plants, microalgae miss the lignin cross-linking in their cellulose structures because their growth in aquatic environments does not require strong supports (John, Anisha et al., 2011). On the other hand, macroalgae contain significant amounts of sugars (at least 50%) suitable for fermentation (Wi, Kim et al., 2009). In certain marine algae (e. g., red algae), the carbohydrate content is strongly influenced by the presence of agar, a polymer of galactose and galactopyranose. Recent research has attempted to develop methods of saccharification to release galactose from agar and to release glucose from cellulose so as to increase fermentation yields in terms of bioethanol (Jones and Mayfield, 2012). Other studies have shown that red algae such as Gelidium amansii and brown algae such as Saccharina japonica are both potential sources of biohydrogen via anaerobic fermentation (Jones and Mayfield, 2012). Unfortunately, harmful algal blooms in lakes, ponds, and oceans may result in drastic effects on those ecosystems, so removal of those algae for biogas production is welcome (Du, Li et al., 2011).

Microalgae are ubiquitous microorganisms that are characterized by a remarkable metabolic plasticity; they may indeed be cultivated in brackish and wastewaters that provide suitable nutrients (e. g., NH4, NO3, and PO4-) at the expense of only sunlight and atmospheric carbon dioxide (CO2). On the other hand, metabolic engineering has been taken advantage of to produce molecular hydrogen or to improve the lipid content as storage products (Amaro et al., 2011).

Overall, economic analyses have consistently indicated that algal-based biofuel feasibility hinges on the possibility of production coproducts with a market value from the spent biomass (Stephens, Ross et al., 2010). A wide range of fine chemicals may indeed be extracted from said biomass, depending on the species at stake (Raja, Hemaiswarya et al., 2008); these hold added value sufficiently high to contribute to the economic feasibility of biofuel manu­facture. Such bioproducts include sugars for production of bioethanol and biomethane, both via fermentation of biomass; intermediate value products, e. g., proteins for animal feedstock; and high-value products such as active principles bearing antimicrobial, antioxidant, antitumoral, and anti-inflammatory features for pharmaceutical purposes. Finally, biomass may be pyrolyzed to produce sequestered carbon in the form of biochar, which holds value as a soil enhancer (Kruse and Hankamer, 2010). A general overview of applications of spent biomass is given in Figure 10.1.

When discussing the upgrade of spent biomass, one should take into account the process that originated it or the target metabolite from which the biofuel is obtained. For example, if the objective is to produce biohydrogen, the spent biomass consists of essentially intact cells, whereas when accumulated lipids are required of biodiesel, the spent biomass takes the form of oilcake. Compounds such as carbohydrates, hydrocarbons, and the biomass itself may still be transformed into secondary biofuels such as ethanol, oil, biochar, and syngas, as shown in Figure 10.2. On the other hand, the spent biomass from production of a biofuel may be used to high value-added products via extraction (see Table 10.1). Therefore, this chapter is organized according to two perspectives: spent biomass used for further biofuel production and spent biomass as a source of value-added products, namely as fine chemicals or feed or even in bioremediation

Today there are three main types of system for the production of microalgae. These systems are open cultivators (flow reactors, raceway ponds), tubular photobioreactors or fermenters (photobioreactors), and vertical reactors (vertical growth reactors). Open cultiva­tors consist of parallel circular tunnels situated on the earth. Microalgae inside them are moved by a wheel mixer (Salis, 2010). It is difficult to control the conditions under which microalgae are developed in these reactors because they can be contaminated by other microorganisms.

11.1.1 Harvesting of Microalgae

Conventional processes used to harvest microalgae include concentration through centri­fugation, foam fractionation (Csordas and Wang, 2004), flocculation (Knuckey et al., 2006; Poelman et al., 1997), membrane filtration (Rossignol et al., 2000), and ultrasonic separation. Harvesting costs may contribute 20-30% to the total cost of algal biomass (Molina Grima et al.,

2003) . The microalgae are typically small, with a diameter of 3-30 pm, and the culture broths may be quite dilute at less than 0.5 gL-1. Thus, large volumes must be handled. The harvesting method depends on the species and cell density and, often, the culture conditions.

Due to the overwhelming response and interest in cultivating algae as a sustainable source of energy, several comprehensive techno-economic assessments have revealed the actual potential of this renewable source on a commercial scale (Amer et al., 2011; Davis et al., 2011; Delrue et al., 2012; Sun et al., 2011). Referring to Figure 12.3, the average biodie­sel selling prices for algae cultivated in an open pond and a closed photobioreactor are $2.97/L and $4.93/L, respectively, or 66% higher for algae cultivated in a closed photobioreactor compared to an open pond. Clearly, although the closed photobioreactor has the advantage of permitting a single strain culture and has high biomass productivity, this cultivation system is still considered expensive from the techno-economic point of view. On the other hand, algal biodiesel produced using the open pond system faces high economic competition from first — and second-generation biodiesel. The estimated biodiesel selling prices for biodiesel produced from soybean oil, jatropha oil, and waste frying oil were $1.35/L (Hu et al., 2008), $1.4/L (Wang et al., 2011), and $0.73/L (Araujo et al.,

2010) , respectively, which are much lower compared to algal biodiesel but very close to the selling price of petro-diesel at $1.2/L (McHenry, 2012). Although the biomass and lipids productivity of algae are superior to terrestrial oil-bearing crops, nevertheless the cultivation and downstream processing stages are much more complicated and consumed significant amounts of energy input. This result also indicates that the positive opportunity for using algal biomass to generate alternative fuel has been overclaimed, and thus ad­vanced improvements are needed to address the feasibility of utilizing this renewable feedstock for commercial use.

The idea of producing biofuels from microalgae comes from the 1960s (Oswald and Golueke, 1960); now, given the high price of petroleum and the global-warming problem, at­tention has refocused on this idea. Microalgae have several advantages over crops in the pro­duction of biofuels: They have high productivity and do not compete for fertile land or water, thus do not affect the food supply nor other crop products. Microalgae have been proposed as the unique third-generation biofuel source (Chisti, 2007). However, for this to become a re­ality, the production of microalgae still has to demonstrate its sustainability, in addition of being produced on a large scale and at a comparably low price, as with traditional crops like soya, corn, or palm. As an example, palm oil is produced at a volume of 40 million t/year and has a market value of €0.5/kg. To replace only 5% of the U. S. demand for transport fuel, it would be necessary to produce more than 66,000 kt/year of oil-rich biomass at production costs below $400/t (Chisti, 2007). Moreover, to replace all transport fuels in Europe with bio­diesel from microalgae, 9.25 million ha (almost the surface area of Portugal) would be needed, assuming a productivity of 40,000 L/ha year (Wijffels and Barbosa, 2010).

To produce microalgae-based biofuels that are able to compete in the worldwide energy markets, it is essential to minimize the energy and nutrient input along with their cost, in addition to optimizing the culture yield and developing adequate transformation routes that allow the valorization of the entire biomass according to the biorefinery concept (see Figure 14.7). For the production of biomass, the use of wastewater is required as the nutrient (nitrogen and phosphorous) source, in addition to free CO2 from flue gases as the carbon source, resulting in purified water and profits obtained from the wastewater treatment pro­cess (Jorquera et al., 2010; Norsker et al., 2010; Acien et al., 2012a). With regard to valorization, the microalgae biomass produced under high-productivity conditions is composed of

FIGURE 14.7 Block diagram of the process for the production of biofuels from microalgae using wastewater and flue gases.

proteins (30-50%), carbohydrates (20-30%), lipids (10-30%), and ash (5-10%) (Vargas et al., 1998; Chisti, 2007). Biodiesel can be obtained from the saponifiable lipids (approximately 50% of total lipids), whereas bioethanol can be produced from fermentable sugars (approximately 30% of total carbohydrates); thus a mere 20-30% of the biomass would be used if only bio­diesel and bioethanol production were carried out. The remaining biomass waste has been suggested as useful in biogas production; however, the economic value of biogas is low due to its low calorific value, CO2 content, and gas nature.

Other than this, biofuel production by hydrothermal liquefaction of the entire biomass has been likewise proposed (Biller and Ross, 2012). In a general scheme, the microalgae could be used to produce biodiesel by extraction/transesterification processes, and waste biomass could be fermented anaerobically to produce biogas, which in the end could be used as both energy and as a CO2 source. Alternatively, amino acids (Romero et al., 2012) and/or bioethanol could be produced from microalgae biomass (John et al., 2011). The biodiesel pro­duction capacity of microalgae is assumed to be up to 35,000 L/ha/year (Rodolfi et al., 2009), whereas the production of bioethanol can reach values up to 38,000 L/ha/year (Harun et al., 2010), although these values have not yet been demonstrated on an industrial scale.

Whatever the transformation route to produce biofuels from microalgae biomass, it is clear that the production step has to be positive in terms of energy balance, in addition to being cheap—a value of $0.5/kg being widely agreed as the upper limit. Recently, several economic analysis approximations of biofuel production from microalgae have been published (Douskova et al., 2009; Norsker et al., 2010; Singh and Gu, 2010; Wijffels et al., 2010; Williams and Laurens, 2010). Due to the lack of both existing facilities and a defined technology, only approximations can be made, all of which include significant uncertainty. Microalgae biomass production costs for different scenarios have recently been analyzed (Acien et al., 2012b). The base scenario considered is the operation of a 100-ha facility consisting of raceway reactors with a V/S ratio depth of 0.2 m3/m2, operated in continuous mode at 0.2 L/day. The power consumption dedicated to mixing is 2 W/m3, while an energy consumption of 0.1 kWh/m3 is assumed for harvesting using a flocculation-sedimentation step, followed by centrifugation. The use of pure raw materials (CO2 and fertilizers) is considered, a biomass productivity of 20 g/m2 day being assumed for the year overall. From these data a production cost of $1.12/kg is reached, a major percentage corresponding to raw material cost due to the use of pure CO2 and fertilizers but especially due to the cost of using pure CO2 (see Figure 14.8).

The second major contribution to overall production is depreciation, especially the cost of harvesting equipment, meaning the sedimenter and centrifugation units, amounting to 59.8% of the total equipment cost. Regarding the utility cost, this mainly corresponds to the power consumption for both operating the photobioreactors and harvesting, water cost being neg­ligible in spite of water evaporation losses of 30,000 m3/ha year. From these data, it is con­cluded that to reduce the biomass production cost and approach the target value of $0.5/kg, it is mandatory to improve CO2 use efficiency or even to replace it using flue gases. Moreover, clean water can be replaced by wastewater, thus avoiding the use of fertilizers. Under these conditions, the production cost reduces to $0.55/kg, which approaches the target value of $0.50/kg.

Considering similar conditions (free flue gases and wastewater), production costs of $0.70/kg have been reported using closed photobioreactors, whereas this value increased up to $1.3/

when using open raceways due to their lower productivity (Norsker et al., 2010). To reduce the production cost below this value, it is necessary to improve the productivity of the system to approximate the maximum theoretical values, which have only been demonstrated under fully controlled conditions at a low scale. Therefore, increasing productivity to 40 g/m2 day, the production cost reduces to $0.21/kg, and considering a maximal productiv­ity of 60 g/m2 day under optimal location and operating conditions, the production cost could be reduced as far as $0.14/kg (Acien et al., 2012a). Recently it has been reported that to be competitive with petroleum at $100/barrel, the biomass with a 40% oil content will need to be produced at $0.16/kg if no credit is allowed for the residual biomass, or at $0.25/kg if a credit is allowed for the nutrients in the residual biomass (Chisti, 2012).

To break the bottleneck for microalgae production used in energy production, it is essential to develop more productive photobioreactor systems while reducing their cost dramatically. The productivity of open raceways varies widely according to the location, strain, and operating conditions; long-term productivity in commercial raceways is lower than 47 t/ha year, although values of up to 91 t/ha year (Borowitzka, 1999) have been reported. Design and operation optimization for open raceways in order to improve their efficiency and productivity is currently performed starting from the basics: fluid dynamics and mass transfer characterizations (Mendoza et al., 2012; Sompech et al., 2012; Chiaramonti et al.,

2013) . Regarding the photobioreactor cost, it has been reported that to guarantee an econom­ical production design for energy products, the investment costs cannot exceed €40/m2
(Hankamer et al., 2007). The cost of open raceways is in the $13/m2 range, which includes the compacted earth, lining, baffles, and paddlewheel, but this cost can be much higher if special designs or plastic-cover structures are used. In addition, this cost does not take into account the harvesting process: The machinery required to collect microalgae biomass from diluted cultures has been demonstrated to be highly expensive. Considering a scaled-up size of 100 ha, the total investment cost has been reported as varying from $48/m2 for open raceways to $66/m2 for tubular photobioreactors (Norsker et al., 2010).

From these data, it can be concluded that although microalgae are not yet produced on a large scale for energy purposes, recent advances allow us to be optimistic and to expect this process to develop in a sustainable and economical way within the next 10 to 15 years (Wijffels and Barbosa, 2010).

Filtration is carried out by forcing algal suspension to flow across a filter medium using a suction pump. The algae biomass is retained and concentrated on the medium and is then harvested. The main advantage of filtration is that it is able to harvest microalgae or algal cells of very low density. A pressure drop must be maintained across the medium in order to force fluid to flow through. Depending on the required pressure drop, various filtration methods have been devised with driving force derived from gravity, vacuum, pressure, or magnetic.

Filtration can be categorized either as surface or deep-bed filtration. In surface filtration, solids are deposited on the filter medium in the form of a paste or cake. Once an initial thin layer of cake is formed on the medium surfaces as precoat, algal cells are deposited on the precoat, serving as a filter medium per se. As algal deposition grows thicker, the resistance to flow across the medium would increase. The filtration flux would decline for a constant pressure-drop operation. In deep-bed filtration, solids are deposited within the filter-bed matrix.

The main problems with using filtration to harvest algae are that the fluid flow is limited to small volumes and by clogging/fouling of the medium by the deposited cells. Several other methods have been devised to avoid filter clogging or membrane fouling. One involves the use of a reverse-flow vacuum in which the pressure operates from above, making the process less vigorous and avoiding algal cell deposition. A second process uses a direct vacuum with a paddle above the filter, providing agitation that prevents the particles from depositing on the medium. Use of a microstrainer as a pretreatment to filtration can reduce clogging and improve algae harvesting. Most filtration operations would include frequent backwashing as a routine maintenance to tackle filter clogging or fouling.

Several filtration methods have been used for algae harvesting with varying degrees of success. The following section discusses various filtration methods that have been used for algae harvesting.

Although heterotrophy of algae shows its potential for oil production, the overall produc­tion cost of heterotrophic oils remains relatively high, restricting the commercialization of heterotrophic algal oils. From an estimation of Yan et al. (2011) using heterotrophic C. protothecoides for oil production, the unit production cost of algal oils was still much higher than that of plant oils. Glucose represents a major share of the cost of heterotrophic oil pro­duction. Using alternative low-cost carbon sources may represent a promising approach to bring down the cost of heterotrophic algal oils. Recently, it has been reported that low-cost sugars were used to grow algae for heterotrophic oil production, e. g., hydrolyzed carbohy­drates (Xu et al., 2006; Cheng et al., 2009; Gao et al., 2010) and waste molasses (Yan et al., 2011; Liu et al., 2012a). All these reports suggested the potential of producing algal oils for less cost, such that the algal oils from C. protothecoides based on waste molasses cost approx­imately half those based on glucose (Yan et al., 2011).

The heterotrophic utilization of sugars for biomass by algae remains at a relatively low level, namely, below 0.5 (Cheng et al., 2009; Liu et al., 2010; Yan et al., 2011), which means that more than 50% of sugars were wasted in the form of CO2. To increase the sugar-to- biomass conversion, a photosynthesis-fermentation mode was proposed and resulted in a high sugar-to-biomass conversion of 0.62 (Xiong et al., 2010b). The increased sugar — conversion efficiency may be attributed to the refixation of partial CO2 released from sugar catabolism by the enzyme RuBisCO, which maintained its carboxylation activity in the fer­mentation stage (Xiong et al., 2010b). In a fermentation system, productivity is greatly related to the medium nutrients as well as fermentation parameters. The manipulation of these factors to achieve a maximized output/input ratio may have great potential for improving production economics of heterotrophic algal oils.

Heterotrophic algal biomass contains not only oils but also substantial amounts of proteins and carbohydrates as well as high-value components such as pigments and vitamins. From a biorefinery’s point of view, the residual biomass after oil extraction can be potentially used as food additives, nutraceuticals, and animal feed (Figure 6.8). Also, carbohydrates may be uti­lized for producing the bio-gas methane by anaerobic digestion. The integrated production of oils and other value-added production, coupled with the possible recycling of water and nu­trients, remains a potential strategy to reduce the production cost of algal oils.

Strain improvement by genetic engineering is another feasible and complementary approach to enhancing algal productivity and improving the economics of algal oil produc­tion. Introduction of a bacterial hemoglobin in various hosts has been shown to contribute to growth improvement in oxygen-limited conditions (Zhang et al., 2007). This strategy is par­ticularly suitable for heterotrophic growth of algae to achieve the ultrahigh cell density that may be restricted by the lowered dissolved oxygen associated with cell mass buildup. The­oretically, enhanced oil content can be achieved by the direct genetic engineering of oil bio­synthetic pathways, e. g., overexpression of the genes involved in fatty acid/lipid synthesis (Madoka et al., 2002; Lardizabal et al 2008); the manipulation of transcriptional factors

related to lipid biosynthesis regulation (Courchesne et al., 2009); or the blocking of compet­ing metabolic pathways that share the common carbon precursors such as starch synthesis (Li et al., 2010). Genetic engineering can also be employed to alter fatty acid compositions of oils for improving biofuel quality, e. g., heterologous expression of thioesterases to accumu­late shorter-chain-length fatty acids (Radakovits et al., 2011) or inactivation of the A12 desaturase gene to produce more oleic acid (Graef et al., 2009). In addition, genetic engineer­ing may confer on algae the possibly improved characteristics of tolerance of temperature, salinity, and pH, which will allow cost reduction in algal biomass production and be ben­eficial for growing selected algae under extreme conditions that limit the proliferation of invasive species. Although genetic engineering of algal oils is currently restricted to certain model algae such as Chlamydomonas, the rapid advances in the development of genetic ma­nipulation tools, plus the better understanding of lipid biosynthesis and regulation, will be extended to industrially important algal species for improving the economics of algal oil production.

6.5 CONCLUSIONS

Heterotrophic production has substantial advantages, including rapid growth, ultrahigh cell density, high oil content, and substantial oil productivity. These merits allow significantly lower downstream process costs, though so far the overall oil production from heterotrophic algae is considered not as economically viable as phototrophic production of algal oils. The relatively high cost of heterotrophic algal oils is mainly attributed to the use of expensive or­ganic carbon—in particular, glucose. Advances in the exploration of using low-cost raw ma­terials such as hydrolyzed carbohydrates and waste sugars have enabled potential cost reductions in heterotrophic production of algal oils. Finding ways to further improve the production economics still remains the major challenge ahead for commercialization of heterotrophic algal oils, which will depend to a large extend on significant advancements in culture systems, biorefinery-based integrated production, and algal strain improvement. Breakthroughs and innovations occurring in these areas will greatly expand production capacity and lower production costs, driving heterotrophic algae from today’s high-value market into the low-cost commodity product pipelines.

Thanks to the increasing interest of using Chlorella biomass as the feedstock for oils, great achievements have been made in heterotrophic culture systems and production models for the algae of this genus, allowing ultrahigh cell density comparable to oleaginous yeasts. To this end, sequencing both the genomes and transcriptomes of several typical Chlo — rella strains is currently underway, which will benefit the development of a new molecular toolbox to successfully manipulate Chlorella for more economically feasible industrial production.

Acknowledgments

This study was partially supported by a grant from the 985 Project of Peking University and by the State Oceanic Administration of China.

The bead-beating method involves the application of beads for the disruption of the algal cell wall. Continuous exposure of biomass to beads leads to cell-wall rupture, resulting in the release of intracellular contents into the solvent medium. Similar to expeller pressing, this method can also be applied for both disruption and extraction. The influence of bead beating on cell-wall disruption was evaluated for the strains Botrycoccus braunii, Chlorella vulgaris, and Scenedesmus sp. using a bead beater (bead diameter of 0.1 mm) (Lee et al., 2010). The method showed a lipid productivity of 28.1%.

Though the disruption of algae cell walls prior to extraction requires an additional step, which is the selection of a cost-effective method, it helps to enhance lipid production efficien­cies. The methods discussed here are economical and applicable to mass cultures compared to few other techniques, such as microwaves, sonication, and autoclaving.

Algal proteins may play both structural and nutritional roles, so their extraction from spent biomass is of potentially commercial interest. One application is for animal feed due their richness in essential amino acids (Williams and Laurens, 2010). The nonprotein nitrogen consists of amino acids, peptides, amines, and nucleotides and accounts for 10-20% of the total nitrogen in algae (Arasaki and Arasaki, 1983).

Recently, a few studies have been reported with respect to the organic solvent extractions due to the experience of remaining toxic residues with the target compounds, so enzyme — assisted extractions have attracted particular interest. Mechanical techniques such as ultra­sound sonication and pulverizing the lyophilized materials by grinding might also be helpful. Namely, bioactive peptides can be obtained in three ways: (1) hydrolysis by digestive enzymes from animals; (2) hydrolysis by proteolytic enzymes, harvested by microorganisms or plants; and (3) hydrolysis by proteolytic microorganisms during fermentation (Samarakoon and Jeon, 2012).

To understand the reactivity of wet algal biomass, it is necessary to understand the reac­tivity of model compounds (components of algal biomass). Experiments with wet algal bio­mass are very useful for understanding how the yields and comparison of different product fractions (e. g., crude bio-oil, aqueous phase products, gaseous products, and solid products) vary with hydrothermal processing conditions. Such data can be used to develop phenome­nal kinetics models that have utility for process design and optimization. Such data provide little insight into the details of the chemistry that occurs. However, to elucidate some of these details, several studies have been carried out with simpler organic molecules (phytol, ethyloleate, phenylalanine, and a model phospholipid) that mimic the structural features and functional groups present in microalgae and/or crude algal bio-oil from hydrothermal liquefaction (Savage et al., 2012a).

Changi et al. examined the behavior of phytol, an acyclic diterpene C20-alcohol and a model compound for algal biomass, in high-temperature water (HTW) at 240°C, 270°C, 300°C, and 350°C. Under these conditions, the major products include neophytadiene, isophytol, and phytone. The minor products include pristene, phytene, phytane, and dihydrophytol. Neophytadiene is likely formed via dehydration of phytol, whereas isophytol can be obtained via an allylic rearrangement. Phytol disappearance follows first-order kinetics with activation energy of 145 ± 20 kJ mol-1 and a pre-exponential factor of 109.94 ±0.12 s-1. Delplot analysis discriminated between primary and nonprimary products and led to a potential set of reaction pathways. A kinetics model based on the proposed path­ways was consistent with the experimental data (Changi et al., 2012).

Formic acid, acetic acid, lactic acid, glycolic acid, 2-hydroxybutyric acid, succinic acid, malic acid, mannuronic acid, and guluronic acid were obtained by the hydrothermal treat­ment of alginate. The total yield of the organic acids was 46% at maximum yield 350°C, 40 MPa, and 0.7 s reaction time (Aida et al., 2012). The formation of organic acids suggests that the carboxyl group structure of the alginate is preserved during the hydrothermal de­composition of the alginate. The formation of dicarboxylic acids is evidence that oxidation reactions occur during the hydrothermal treatment, introducing carboxyl groups into the de­composition products. The product distribution indicates that both acid and base catalyzed reactions occur during the hydrothermal treatment of alginate. Hydrothermal treatment of uronic acid, glucuronic acid, gave the same organic acids as those obtained from hydrother­mal treatment of alginate (Aida et al., 2012).