Mechanochemical extraction of edible proteins from moor grass

Abstract

Extracting edible nutrient-rich food fractions from unconventional sources, such as grass, could play a pivotal role in ensuring food security, bolstering economic prosperity, combating climate change, and enhancing overall quality of life. Current extraction techniques rely heavily on harsh chemicals, which not only degrade nutrients but can also substantially add to the cost of the process and make downstream separation challenging. In this study, we harnessed a mechanochemical process, liquid-assisted grinding (LAG) with and without Na₂CO₃, termed sodium carbonate assisted grinding (SAG), to extract the protein fraction from moor grass. These techniques were compared to the conventional alkaline extraction (AE) method. Unlike alkaline extraction, which solubilized over 70% of the material, the mechanochemical approach using Na₂CO₃ solubilized only 55% of the grass while still extracting the vast majority of the protein in the original grass feedstock. The protein fractions obtained from the SAG process had a similar amino acid profile to the core feedstock but also contained distinct characteristics over the other methods of extraction. FT-IR analysis, for example, identified the presence of an amide III band in the protein fractions obtained from the SAG process, indicating unique structural features that contribute to improved dispersibility, gelation properties, and water-in-water stability. Furthermore, the extracted moor grass protein contained a higher proportion of glutamic acid in comparison to other amino acids in the protein, which indicates a savoury umami (meaty) characteristic to the protein fraction. The protein extracted via SAG also exhibited good heat stability (139-214 °C), rendering them potentially suitable for baking applications. Additionally, coupling Na₂CO₃ with liquid assisted grinding not only removed the need for organic solvents and conventional heating but also reduced solvent consumption by 83%, compared with the typical alkaline extraction, thus simplifying the downstream processes necessary to produce food fractions. This study demonstrates the potential significance of mechanochemical extraction processes in unlocking nutrients from unconventional resources like grass, to produce the next generation of sustainable food ingredients.

Fig. 1. Significance of parameters on the SAG.

Fig. 2. Amount of moor grass solubilised (wt%) on processing with the various methods where AE is alkaline extraction, AEM is alkaline extraction under mild conditions, LAG, liquid assisted grinding and SAG, sodium carbonate assisted grinding.

Fig. 3. Surface topology of (a) untreated grass; and the residues remaining after processing with (b) AEM (c) LAG (d) SAG.

Fig. 4. FT-IR spectra of the soluble fractions obtained from grass through mild alkaline extraction (AEM), liquid assisted grinding (LAG) and sodium carbonate assisted grinding (SAG).

Fig. 5. Comparative amino acid profile of raw moor grass and the protein obtained from grass through the mild alkaline extraction (AEM), liquid assisted grinding (LAG) and sodium carbonate assisted grinding (SAG).

Fig. 6. Differential scanning calorimetry curves of the soluble fractions obtained from grass through mild alkaline extraction (AEM), liquid assisted grinding (LAG) and sodium carbonate assisted grinding (SAG).