Within this STW Protein project the benefits and limitations to alkaline protein extraction were investigated, and an optimised process design was developed.
In this study, green tea residue (GTR) was used as a starting material. This decision was made mainly for three reasons: it is produced in developing countries with relatively high yields, its components have high potential economic value, and it can be easily recycled from tea factories where it is already gathered and pre-processed for tea production.
Green tea residue contains more than 25% protein. It is a waste stream from tea factories. Our samples were obtained from Damin Company, Fujian Province, China, in 2010 where they amount to 50,000 tonnes/year. Currently, GTR is only used for energy generation through burning.
Alkaline extraction gets you almost all protein
Leaves are potential resources for feed or food, but their applications are limited due to a high proportion of insoluble protein and inefficient processing. To overcome these problems, parameters of alkaline extraction were evaluated. Protein extraction could be maximized to 95% of total protein, and, after precipitation by pH adjustment to 3.5, 85% of extracted protein was recovered with a purity of 52%. Temperature, NaOH amount, and extraction time are the protein yield determining parameters, while pH and volume of extraction liquid are critical parameters for production cost.
This technology was successfully applied to other sources of green biomass and has potential to be used as a part of an integrated bio-refinery process.
Integrated biorefinery to also produce galacturonic acid
Alkaline treatment of leaves disrupts the leaf tissue by dissolving lignin at high pH. At lower pH HG pectin, RGII pectin, and organic acids were extracted before protein extraction, which was followed by the extraction of cellulose and hemi-cellulose. This sequential extraction can be used to design an integrated biorefinery.
Leaf pectin can for instance be used as a feedstock for galacturonic acid (the building block of pectin) production. Leaf pectin extraction was integrated the alkaline protein extraction, focusing on high yield of galacturonic acid without losses of protein. Most galacturonic acid was extracted using Viscozyme pretreatment as a first step, mainly due to its cellulase activity. Extraction yielded more than 95% galacturonic acid with only 5% protein.
Alternatively GA-containing pectin can be extracted in a weak alkaline solution. Here GA yield is dominated by the ratio of extraction volume to biomass weight. The profits of these two integrated processes can be higher than one step protein extraction.
Pre-treatments to protein extraction
As mentioned, Viscozyme pretreatment can be used to obtain galacturonic acid. Alternatively, polyphenols and/or pigments can be obtained by ethanol pre-treatment. Both pre-treatments reduced alkali consumption in the subsequent protein extraction step by 25% and improved protein extraction yield and purity. Additionally, pre-treatment using 50% ethanol reduced browning by 59%.
Increased sustainability by salts recycling
Leaf protein can be extracted cost-efficiently using 0.1M NaOH, but this process is less sustainable due to the generation of large amounts of sodium salts. KOH or Ca(OH)2 were considered to replace NaOH as these salts can be reused. KOH is just as efficient for extraction and the foreseen profits using KOH are comparable to those with NaOH. The environmental sustainability improves and further environmental benefits are obtained when conventional K fertilizer is substituted with the K-rich salty waste water from the extraction process.
The profits of the process using Ca(OH)2 were highly dependent on the extraction yield of the protein product. Protein extraction yields using Ca(OH)2 need to be higher than 70% to be more profitable than the process with NaOH. This is unfortunately not the case for all sources of leafy biomass. The environmental benefits of Ca(OH)2 extraction are in the absence of salty waste water and net production of heat.
Future work on food driven biorefinery
Work on protein and pectin biorefinery on green biomass continues in the future. In 2017, also a new PhD started on polyphenol extraction from tea leaves. When these extraction techniques can be integrated in existing chains, economic feasibility can be obtained more easily. This Food-Driven Biorefinery is a starting point for better use of sides streams.