Abstract
Microalgae are considered as excellent candidates for bioactive compounds, yet microalgal residues remaining after the extraction of one or two compounds are usually discarded, which is not economical. This study demonstrates the alkaline extraction of proteins from Chlorella pyrenoidosa residue after lipid and pigment extractions, and their functional properties. Single-factor experiments and response surface methodology were used to obtain the optimal conditions for protein extraction. Based on our results, a maximum protein yield of 722.70 mg/g, was obtained under the following extraction conditions: sodium hydroxide concentration 7.90%, extraction temperature 70.00 °C, extraction time 34.80 min, and microalgal residue concentration 8.20 mg/mL. The molecular weight of microalgal residue protein isolate (MRPI) was mainly distributed at the regions of 0.18–0.50 kDa, 0.50–1.50 kDa, and 1.50–5.00 kDa. The essential amino acid content was greater than the values recommended by FAO/WHO standards; a high essential amino acid index value (1.49) was another good indication that MRPI is suitable for human consumption. Moreover, MRPI exhibited excellent emulsifying properties and antioxidant activity, which suggests it may be useful as an emulsifying agent and antioxidant. These findings could improve the extraction methods of functional protein from microalgal residue and add value to microalgae-based bioactive compound production processes.
Highlights
Microalgae are photosynthetic microorganisms that are rich in high-value compounds, such as lipids, proteins, carbohydrates, pigments, and vitamins [1,2]
This could be due to the high alkali concentration, as this promotes the breakdown of hydrogen bonds and the dissociation of hydrogen from proteins, which can enhance protein solubility [16]
This is consistent with a previous study that showed the protein yield from tea residue was enhanced with an increase in alkali concentration, whereas yields decreased under excessively high alkali concentrations [17]
Summary
Microalgae are photosynthetic microorganisms that are rich in high-value compounds, such as lipids, proteins, carbohydrates, pigments, and vitamins [1,2]. The lipid content of microalgae ranges from 1.5–75% of dry weight, which offers great potential for biodiesel production [3]. Many microalgal lipids contain omega-3 unsaturated fatty acids such as eicosapentaenoic acid (EPA). Docosahexaenoic acid (DHA), which can support cardiovascular, brain, and eye systems, prevent cardiovascular disease, and have anti-inflammatory, antibiotic, and anticancer activities in humans [4,5]. Mar. Drugs 2019, 17, 454; doi:10.3390/md17080454 www.mdpi.com/journal/marinedrugs.
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