Introduction
How to feed 10 billion people worldwide by 2050 is one of the primary challenges that is being faced by countries globally. The food consumption pattern has changed and concern for a healthy diet has seen a dramatic rise post COVID-19. There has also been a significant hike observed in the price of healthy and nutrient-rich food, ever since the COVID-19 epidemic (Ghazani & Marangoni, 2022). The Food and Agricultural Organization of the United Nations (FAO) has revealed that the global food prices had risen sharply ever since May 2021. Food prices had recorded their sharpest growth, growing at its fastest rate month on month in over a decade. Although, there is now enough food available for the roughly seven billion people on Earth, poverty and unequal wealth distribution has caused nearly a billion people to go without food or suffer from malnutrition.
Animals and plants are the traditional sources of lipids for humans and other animals. Along with nutrients like carbohydrates, protein, vitamins and minerals, lipids are also essential for retaining the health and nutrition of human beings, (Ji & Ledesma-Amaro, 2020). Lipids are concentrated sources of energy and are rich in PUFA (Poly Unsaturated Fatty Acids). Moreover, they carry fat-soluble vitamins like A, D, E and K and they include monoglycerides, diglycerides, triglycerides, phospholipids, glycerophospholipids and compounds that resemble hydrocarbons (such as sterols, terpenes and waxes). Lipids are the carriers of different hydrophobic and amphiphilic chemicals. Lipids are crucial for various biological processes, such as cell signalling, energy storage and membrane structure. Food, biofuels, oleochemicals, medicines and cosmetics are a few of the uses for lipids. Triacylglycerols are the lipids of interest in terms of food (TAGs). TAG oils are a part of edible fats and oils, providing both necessary fatty acids and the nutrient with the highest calorie density (9 kcal/g). The primary source of energy in the human diet is TAG oils. (Chen et al., 2021)
Several species of microorganisms, specifically oleaginous microorganisms can produce and accumulate 20% lipids, including yeasts, fungi, algae, bacteria and marine-derived thraustochytrids. Microorganisms are usually synthesizing these lipids to regulate their metabolic flux. These microbial lipids are not only sources of essential fatty acids, but are also capable of regulating fuel requirements and the future economy, (Caporusso et al., 2021). The industrial production and purification of microbial lipids are more expensive than conventional products; however, it is crucial to introduce an economical and cost-effective methodology to avail these products globally, (Ochsenreither et al., 2016).
1. Microbial lipids
Lipids make up most of the microbe’s cell wall. Nevertheless, “oleaginous” bacteria have only a lipid content exceeding 20%. Polyunsaturated fatty acid (PUFA) derivatives and phospholipids are known to be produced by prokaryotic microorganisms like bacteria and cyanobacteria (blue-green algae), as well as eukaryotic microorganisms like yeast and fungi, (Ghazani & Marangoni, 2022). The main distinction between these two groups of organisms is that prokaryotes lack nuclear membranes and cellular compartments (organelles) (Ji & Ledesma-Amaro, 2020). It is possible for both groups of species to produce the polyunsaturated PUFAs omega-3 and omega-6, which are said to be beneficial for human health. To address the demand for these lipids worldwide, microorganisms manufacture these advantageous fatty acids in circumstances that are relevant to industry, (Leong et al., 2018).
1.1. Lipids from Bacteria
The tubercle bacillus (Mycobacterium tuberculosis) was extracted in alcohol and ether to the tune of 26-28% of its dry weight, which is possibly the oldest account of significant lipid accumulation in prokaryotes. The results of a subsequent study conducted revealed the presence of stearic (C18:0, nomenclature indicates the numbers of carbon atoms and double bonds) and palmitic (C18:0) acids, as well as additional compounds designated as tuberculostearic (10-methyloctadecanoic) and phthisic (3,13,19-trimethyltricosanoic) acids (Ghazaei, 2018). A substantial buildup of TAGs has also been seen in bacterial isolates from the genera Mycobacterium, Rhodococcus and Streptomyces.
Arthrobacter sp. AK 19, Gordonia sp. DG, Rhodococcus opacus DSM 1069, and R. opacus PD630 are a few of the promising gram-positive bacteria utilized to make lipids. Gram-negative bacteria that show promise include Serratia sp. ISTD04 and Synechococcus sp. HS01. Although, the storage of intracellular TAG has not yet been confirmed in cyanobacteria, they may constitute another unique case of prokaryotes. Certain strains of Synechococcus sp. or Cyanobacterium aponinum create complex lipids that must be digested to liberate fatty acids. Fatty acids are primarily stored as esters, (such as triacylglycerols) because most free fatty acids harm almost all organisms if their levels grow. Under straightforward cultivation conditions, bacteria exhibit significant cell growth rates, as compared to other oleaginous organisms.
1.2. Yeast lipid
Lipomyces starkeyi (ATCC 12659), Apiotrichum curvatum (ATCC 10567), Cryptococcus albidus (ATCC 56297) and Rhodosporidium toruloides (ATCC 204091) are a few of the most researched oleaginous yeast strains. In order to grow oleaginous yeast under nutrient-restrictive circumstances, various carbon sources, including starch hydrolysate (glucose), biomass hydrolysate (glucose, xylose), glycerol and municipal organic waste have been used. Protein and nucleic acid production are halted in nitrogen-deficient environments and carbon substrates are transformed into store lipids (Ji & Ledesma-Amaro, 2020). Different quantities of unsaturated lipid accumulation occur depending on the kind of available carbon source.
1.3. Mold lipid
Aspergillus terreus, Trichosporon fermentans, Claviceps purpurea, Tolyposporium (genus of smut fungi), Aspergillus oryzae (A-4), Mortierella alpina, Mortierella isabellina and Mucor rouxii are some of the more well-known oleaginous fungi. Since the fungi produce unique lipids containing docosahexaenoic acid (DHA), gamma-linolenic acid (GLA), eicosapentaenoic acid (EPA) and arachidonic acid (ARA), they are investigated. In fungi, lipid accumulation mechanisms resemble those in yeast. Due to variations in their metabolism, carbon sources have an impact on the generation and make-up of fatty acid esters in lipids. Age of the mycelia was associated with levels of polyunsaturated fatty acids in fungal cells. Fungal lipids are employed to produce cocoa butter substitutes, due to the high saturated fatty acid content (up to 60%) in oleaginous fungi.
2. Cocoa butter equivalent – Microbial Lipids
Due to the increased consumption of cocoa butter in confectionery and cosmetic industries, the current production rate has not been sufficient for meeting future requirements. Hence, scientists found that certain oleaginous microorganisms, including bacteria and fungi can produce saturated fatty acids (Cocoa Butter Equivalent-CBE), including stearic acids (Ghazani & Marangoni, 2022). Scientists have found that different subspecies of Cryptococcus curvatus (30% fatty acids) are highly efficient in producing CBE. Oleaginous yeast like Yarrowia lipolytica synthesize CBE by cultivating in glycerol, glucose and stearin media, (Papanikolaou et al., 2003). Compared to animal sources and natural cocoa butter, the total number of saturated fatty acids is less in microbial CBE, which enhances the nutritional quality of the fatty acids. Along with CBE, microbes produce PUFA to improve their nutritional value.
3. PUFA
Along with other microbial lipids, PUFA plays a vital role in human nutrition and metabolisms. PUFA are essential fatty acids for humans and they are usually obtained from animal and plant sources, which are essential for controlling diseases like cardiovascular diseases, inflammatory diseases, cancer and tumours. Arachidonic acid, dihomo-γ-linolenic acid, docosahexaenoic acid, eicosapentaenoic acid and γ-linolenic acid are the commonly extracted microbial lipids, (Bellou et al., 2016). The fungi commonly synthesize dihomo-γ-linolenic acid, including Cunninghamella blakesleeana-JSK2, Cunninghamella echinulata and Mucor circinelloides strains. Arachidonic acid and eicosapentaenoic acid are commonly synthesized by Halophytophthora spinosa var. spinosa IMB162, Mortierella alpina spp. and microalgae (Makri et al., 2011). A very high amount of lipid accumulation was observed in most of the fungal and algal biomass; thus, high-value fatty acid production from microorganisms can have a market value (for examp le, GLA, DHA and ARA). However, due to low yield, bad quality and other factors, generating microbial lipids is far more expensive than producing them in plants.
4. Advantages and Limitations of Producing Microbial Lipids
Eukaryotic bacteria create a variety of polyunsaturated fatty acids, which are highly accepted by the general public for moral and ethical reasons. These bacteria’s genetic machinery can be easily modified to produce a single chosen fatty acid with significant economic value. According to calculations based on prior work in New Zealand on the production of Cocoa Butter equivalent from lactose using Candida curvata, which is not viable in the current market where vegetable oil prices are sold for around US$ 800-900 per tonne, the price for the microbial oil has been calculated to be US$ 3000 per tonne (excluding the cost of feedstock).
Low tolerance for pre-treatment degradation products, low lipid production and low titer are the main disadvantages of microbial lipids, as compared to plant lipids. There are three main expenses involved in manufacturing microbial lipids: (1) carbon and nutrient source; (2) fermentation operation costs; and (3) post-processing expenses involved in isolating lipids from bacteria. Microbial lipids are produced by moulds, yeast and bacteria using feedstocks, such as carbon resources (such as glucose, xylose, glycerol, starch and cellulose hydrolysates) and nitrogen sources (such as peptone, yeast extract or maize steep liquor). The production price is influenced by the cost of producing carbon and nitrogen resources.
5. Conclusion
Although, traditionally employed for a variety of purposes, plant-derived lipids are being increasingly sought after as a result of the rising population and shrinking arable land. The production of microbial lipids for high-value applications has recently been made possible by several promising technologies and research is currently being done to genetically modify microorganisms to produce desired lipids in high titers. Although, plant-derived lipids are often used for various purposes, the rising population and dwindling arable land are pushing a quest for alternative resources to fulfill rising lipid demand.
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1. Aysha C.H.
2. Subith C.