Valorization of blue mussel for the recovery of free amino acids rich products by subcritical water hydrolysis

Highlights

• Subcritical water hydrolysis of M. edulis was performed to recover amino acids.

• Hydrolysis temperature ranging from 120 to 240 °C was applied.

• Subcritical water hydrolysis enhanced Maillard reaction and antioxidant activities.

• Hydrolysis temperature induce release and degradation of free amino acids.

• Hydrolysates at 180 °C showed highest antihypertensive activity.

Abstract

A sequential recovery of free amino acids from blue mussel (Mytilus edulis) by subcritical water after deoiling was performed. Hydrolysis temperatures ranged from 120 °C to 240 °C, pressure and reaction time were fixed at 3 MPa and 30 min, respectively. Results showed that hydrolysis efficiency reached 90.65% at 240 °C. Total amino acids in the lyophilized M. edulis powder were 198.75 mg/g and 381.95 mg/g, for essential and non-essential amino acids respectively, and dominated by glutamic acid, aspartic acid, arginine and glycine. The highest amount of free amino acids in the hydrolysates were 11,718.94 mg/L (w/v) at 120 °C. The highest antioxidant activities were observed at 240 °C owing to Maillard reaction and probably new reaction products. However, the best antihypertensive activity and highest total protein value were detected at 180 °C. M. edulis can be valorized as a source of free amino acids by using subcritical water.

Introduction

Marine resources have attracted the attention of researchers as a source of bioactive compounds with potential economic benefit due to their use as ingredients in functional food, cosmetics, nutraceuticals, and pharmaceutical industries [1], [2], [3]. Marine mussel, which is a shellfish belonging to the mussel family, is one of the important edible bivalves found in coastal reef areas. It is widely distributed throughout the world, including in the coastal areas of Korea, and is known as a nutrient-rich seafood because of its high protein content [4]. The main species of mussel include green-lipped mussel (Perna canaliculus), which is produced in New Zealand; blue mussel (Mytilus edulis); and hard-shelled mussel (M. coruscus). According to the Statistics Korea, 51,560 tons of mussels were farmed and produced domestically in 2019, and the mussels were mainly used as a minor ingredient for soup or boiled foods [5].

Rich in both proteins and functional lipids, the valorization of Mytilus edulis to recover these compounds can benefit from a sequential process in which both polyunsaturated fatty acids (PUFAs) and protein derived peptides and amino acids could all be utilized. The possibilities of utilizing the emerging technologies based on supercritical fluids offer superior advantages due to their adaptation in bioactive compounds isolation [6]. This interest in supercritical fluids utilization for biomass valorization is attributed to the versatility, greenness, usefulness and sustainability of the process [7]. Widely used supercritical carbon dioxide (SC-CO₂) have the potential to extract the non-polar bioactive compounds due to the limitation of CO₂ as a non-polar solvent. A very attractive aspect of this extraction technique is that the easy separation of the solvent and the solid matrix after the extraction produce safe for use biomass rich in polar bioactive compounds. Since the use of SC-CO₂ to recover bioactive compounds from marine resources is limited on the non-polar fractions, this leaves behind an enormous amount of protein rich amino acids and peptides when recovering the PUFAs from M. edulis, and these remaining bioactive ingredients present a huge economic potential in the design of functional food products, nutraceuticals, and cosmetics.

There are reports on the application of different techniques to recover the amino acids from marine products, and the most applied methods rely on enzymatic hydrolysis with the aim of carrying out the proteolytic degradation and produce peptides of different molecular weight distributions, this technique has been used for different marine biomasses like yellowfin tuna wastes [8], shark muscles [9], crustacean species like shrimp, crab and lobster [10], [11], [12]. Although the enzymatic hydrolysis of biomass is a beneficial process, it is very sophisticated, time consuming and costly which limits its economic adaptation. Another promising technology is subcritical water hydrolysis which uses water as a solvent and this technique has garnered huge interests due to its simplicity, versatility, and greenness.

Water is the safest solvent and does not require extra processing steps for removal in the final products, it possesses desirable properties of conventionally used solvents in what is known as subcritical water condition. Under subcritical water thermodynamic conditions, at temperature above the atmospheric boiling point of water, and at a high enough pressure to keep water in the liquid state, the slight change in temperature result in dramatic change in water’s properties. Subcritical water has properties that are similar to conventional solvents such as methanol and hexane, i.e., the dielectric constant of water decreases with increasing temperature resulting in the improvement of dissolution power similar to those organic solvents used conventionally [13]. The ionic product of subcritical water is increased by inducing the formation of H₃O⁺ and OH⁻ ions with the ability to act as an acid or base catalyst [14], [15]. Due to these properties subcritical water has been exploited for the valorization of biomass through hydrolytic conversions of plant bioactive compounds [16].

Protein hydrolysis of mussel has been demonstrated to produce bioactive peptides and amino acids with various biological activities. Fresh water mussel was hydrolyzed by use of ultrasound assisted enzymatic hydrolysis to produce antioxidant peptides [17], and also peptic hydrolysis has been applied in different studies to recover antioxidant peptides [18], [19]. Other biological activities of mussel peptides and amino acids include antithrombotic [20], and antihypertensive activities [21], [22]. The presence of highly valuable bioactive compounds from M. edulis provide an opportunity of using supercritical fluids, particularly subcritical water for the hydrolysis of proteins into functional peptides and amino acids.

This study focuses on the use of subcritical water hydrolysis to treat deoiled M. edulis to recover bioactive peptides and free amino acids. To this end, we applied various temperatures (120–240 °C), and characterized the antioxidant and antihypertensive activities of hydrolysates. The antioxidant and antihypertensive activities were described as a function of amino acids release during hydrolysis and Maillard reaction products. This study recommends the use of subcritical water as a tool for the valorization of marine biomass to recover amino acids.