**6. Conclusions**

income, microbial fatty acids should be fractionated depending on their price; low value fatty acid should be used for biodiesel production, while high value (GLA, DHA and ARA) should be used as food supplement and in production of nutriceuticals [11–18]. Therefore, oleaginous microorganism with high content of unsaturated fatty acids such as fugus *Mortierrella* sp. would be favorable for process of microbial lipids production. Other value-added products such as pigments or sophorolipids could also give additional revenue. Oleaginous yeast such as *R. glutinis*, *R. rubra* and *S. ruberrimus* accumulate valuable pigments, ß-carotene, torulene and astaxanthin [13, 104–107]. Sophorolipids can be used as biosurfactants instead of classical chemical-derived surfactants in cosmetics, food, cleaning and petroleum industry. Unlike chemical surfactants, sophorolipids are biodegradable and also have interesting biological activities including anti-microbial, anti-cancer, anti-HIV, anti-inflammatory and antiviral activities [108–110]. Lignin is by-products generated during pretreatment that should be separated and sold. In biorefinery, lignin can be converted to heat and power for the processing steps. Building blocks derived from lignin can be used for production of vanillin, carbon fiber, bio-oil, resin, adhesives, polymer fillers, coating agents, bioplastics, paints, soil amendment, slow nitrogen release fertilizers, rubbers, elastomers and microbial agents. Proteins from lignocellulosic biomass and microbial biomass after lipid isolation could also be used as animal feed or after acid hydrolysis to amino acids could be used as building blocks for the synthesis of different chemicals [111–114]. Significant influence on production cost of biodiesel has process of lipid recovery from cell biomass. Lipid isolation on laboratory scale is based on laborious and expensive isolation protocols that include cell harvesting by centrifugation, energy-intensive step of biomass drying and lipid extraction using an organic solvent.

**time/note**

batch(28°C,12 days)

(28°C, 12 days/ substrate was added every 3 days)

**Enzyme activity**

11.1 U/g DM

Cellulase: 19.0 U/g DM xylanase: 65.6 U/g DM

Cellulase: 18.4 U/g DM xylanase: 119 U/g DM)<sup>c</sup>

Batch (28°C,5 days) Cellulase:

**YL/S (g/g)a Reference**

0.0799 [99]

0.0719 [99]

0.0919 [99]

**Substrate Microbial strain Pretreatment Fermentation mode/**

Alkali (10% NaOH, 100°C, 15 min)

As above Fed-

YL/S: Lipid yield, g produced lipid/g of dry matter of pretreated lignocellulosic biomass.

Cellulase activity was determined on 6th and lipid content on 10th day of cultivation.

Cellulase activity on 10th and lipid content on 9th day of cultivation.

**Table 4.** Production of lignocellulosic lipids by solid state fermentation.

As above Repeated-batch

Palm empty fruit bunch and palm kern cake

a

b

c

*Aspergillus tubingensis* TSIP9

154 Advances in Biofuels and Bioenergy

*Aspergillus tubingensis* TSIP9

*Aspergillus tubingensis* TSIP9 The current production of microbial lipids from lignocellulose biomass faces a number obstacles associated with low lipid yield of producing strains, low tolerance of microbial strains to lignocellulose-derived inhibitors, insufficiently high substrate concentration in lignocellulose hydrolysate and high costs of product isolation. In order to reduce production cost and improve feasibility of the bioprocess, research efforts must be focused on: (1) optimization of oleaginous microorganism applying genetic engineering methods and adaptive evolution to obtain higher lipid concentrations and tolerance to inhibitors from pretreatment process, (2) new effective method of pretreatment and hydrolysis of lignocellulosic biomass that provide high concentration of fermentable sugars in growth media, (3) novel innovative designs of bioreactor should improve the productivity of the process and reduce the production cost, (4) optimization of lipid isolation from wet cell biomass and (5) generation of valued-added products that could provide additional income and improve economic feasibility of the bioprocess (**Figure 3**) .

**Figure 3.** Strategies for improvement of lipid production process.
