Critical parameters in cost-effective alkaline extraction for high protein yield from leaves
Introduction
Leaf protein has been regarded as an additional protein source since 1960s [1], [2], [3]. These proteins can be used in food [3], [4], animal feed [5], [6], or when hydrolyzed to amino acids for other applications, such as bulk chemicals [7]. Tea leaf residue is one example of a potential new protein source. As a major agro product in China, 1.6 Million tonnes (dry weight) of tea leaf products were produced in 2011 [8]. Tea producers estimate that around one fifth (dry weight) of the tea residues are produced centrally and can be collected from instant tea factories [9].Tea residues, which are the waste of tea leaves after hot water extraction, contain 20–30% protein [10], [11]. There are at least seven different types of protein, including Rubisco and glycoproteins [11], [12], [13]. The superior quality of tea leaf protein in terms of amino acid profile compared to soy bean meal has been documented [11].
However, although huge economic potential lies in leaf protein, its applications are severely impeded by low cost-efficient production. Protein extraction yield is relatively low that varies from 15% to 60% of total protein, depending on species and processing methods [6], [14], [15], [16]. Furthermore, protein production yields are reduced during recovery, particularly for those processes, such as alkaline extraction, that generate protein hydrolysates [15], [17]. Conventional alkaline extraction, has already been studied decades ago, but no significant improvement was made in leaf protein extraction. Despite the lower cost, alkaline extraction has the lowest profit among all extraction techniques for leaves primarily due to low protein yield [17]. If protein yield can be increased without increasing cost for extraction, the economic value of leaf protein can be exploited.
The low productivity of alkaline extraction may result from overlooking two points. Firstly, applying high temperature in alkaline extraction has shown to increase protein yield in some cases [18], which is conflicting with the general knowledge that heating results in protein precipitation. Secondly, the influence of solution to raw material ratio (v/w) and alkaline concentration (pH) on protein yield were always studied independently [11], [19], [20], but the influence of alkali amount, which is determined by both v/w and alkaline concentration, was never considered.
In this study, the possibility to increase protein yield at elevated temperature was investigated, followed by an evaluation of the influence of v/w and alkali amount on protein extraction yield. The parameters that involves in alkaline extraction were grouped to protein yield related and cost related, and its economic value was estimated. In addition, the general applicability of new parameter setup was tested by using other materials, like oolong tea residue, grass, and barley straw.
Section snippets
Materials
Green tea residue (GTR) is our main material, which is a gift from Damin Company, Fujian Province, China. This residue from tea lemonade production was collected from Camellia sinensis trees in Zhejiang province, and it was sun-dried after soaking green tea leaves in water at 85 °C for 45 min. The dried residue was then ground into powder. Its protein content is 26.5%, which was determined by the method of Kjeldahl [21].
Oolong tea residue (leaves collected from the C. sinensis trees in Fujian
Protein yield
To test the influence of temperature on protein extraction yields from GTR, 0.1 M NaOH was tested at v/w of 40 ml g−1 and different temperatures. As shown in Fig. 1a, approximately 20% of protein can be easily extracted in 15 min and the increase in protein yield is temperature dependent. At 95 °C, about 95% protein can be extracted in 4 h, which is almost twice the amount using peer technology in green tea leaves or other leaf species [3], [11], [16], [24], [25], [26], [27], [28].
In protein
Conclusion
In this study, we proved that high protein yield can be obtained by alkaline extraction from leaves in an economic way. In the extraction, temperature, alkaline amount, and extraction time were crucial for high extraction yields, while pH and v/w could be varied to limit production cost. Protein product, which is comparable to soy bean meal in protein content and nutritional value, can be then obtained at low cost. This technology can be universally applied to other leaf species, such as grass,
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