The functional properties of seafood proteins are an important parameter that directly influences the food product developed utilizing the protein source and the sensory and other consumer acceptability attributes. The seafood protein functionalities are influenced by the total physicochemical properties exhibited by the proteins, while being subjected to processing, consuming and storage. The peptide and amino acid sequences associated with the protein determine the nature and intensity of the functionality parameters, (Chalamaiah et al., 2012). The complex molecules such as proteins exhibit an array of functionalities like water holding capability, gelation strength, emulsifying properties, foaming capacity and furthermore, including intra-food interactions with other components.

Water Holding Capacity (WHC)

The native structural protein in seafood exhibits superior water holding capacity, which assists in enhancing textural properties like juiciness and tenderness, influencing the mouthfeel of the products prepared out of it, (Mitchell, 1998). Both WHC and drip loss (DL), which indicates a poor WHC, are representative parameters of freshness, considering the association with water and fish muscle, (Warner, 2014). Unlike terrestrial meat, seafood contains a lower concentration of connective tissue proteins (Listrat et al., 2016) resulting in a lower WHC property. The processing stages like washing in the case of surimi processing have been reported to enhance WHC, as it concentrates the myofibrillar protein intensities, (Park, 2013). Other than inherent properties like structure and type of amino acids, processing conditions like temperature, pH and presence of ingredients such as NaCl also significantly influence the WHC of seafood proteins, as they affect or accelerate the denaturation of structural proteins. The protein extraction methodologies adopted also seems to significantly affect the WHC of seafood proteins, as they determine the amino size and amino acid profile of the peptides. Carp proteins extracted through acid and alkaline process reported WHC values of 66.7% and 62.1%, in comparison with the water extracted carp surimi with 73.2% WHC, (Tian et al., 2017). Presence of considerable amounts of COOH and NH2 polar groups among the peptides has been reported to be significantly affecting the WHC of seafood proteins. Taheri et al. (2013) observed that the occurrence of amino acids like glutamic and aspartic acids in rainbow trout protein hydrolysate with polar side chains significantly increased the WHC properties. They also reported that small fragments of peptides with low molecular weight displayed higher WHC, when compared to larger sized peptides because of the hydrophilic nature of the lower sized peptide chains.

Solubility

The solubility of seafood protein could be a thermodynamic property, represented as the amount of protein in a saturated solution which is in solid phase equilibrium in a crystalline or amorphous form under certain reference conditions, (Kramer et al., 2012). This property could be influenced by extrinsic factors like pH, strength of the ionic solution, temperature and solvents and intrinsic factors such as structure and type of amino acids. The isoelectric properties of the peptides are observed to affect the protein solubility to a great extent, as the net charge of peptides are going to be influenced by the pH of the solution, especially for the side chain groups which belongs to weak acid or base category, Taheri et al. (2013).

Low molecular weight peptides tend to demonstrate solubility over a wide pH range, as they have higher polarity than heavier counterparts and form a greater number of hydrogen bonds with aqueous solution, resulting in heightened solubility as demonstrated by many studies in different seafood varieties, (Foh et al., 2011; Betty et al., 2014). Peptides with larger molecular weights tend to be less soluble, due to their lover solvent-solute affinity resulting in lesser number of hydrogen bonds, (Chi et al., 2014). But extraction techniques like hydrolysis can convert larger hydrophobic peptide chains to shorter hydrophilic peptides with carbonyl and amino side chains, resulting in increased protein solubility. (Betty et al., 2014)

Emulsifying property

Emulsions are complex mixtures of two immiscible components, for example, oil-in-water formed under specific conditions like temperature and presence of elements which support the formation of such complexes called emulsifying agents. These mixtures are highly unstable thermodynamically and have different applications, like providing unique texture and flavour in food and in encapsulating, safeguarding and distribution of functional components into a food medium, (Walker et al., 2015). The presence of additives or emulsifying agents which function as surfactants is important in maintaining the stability of emulsions. A proactive emulsifying agent protects the emulsion stability, in addition to preventing lipid oxidation, which helps in extending the life span of emulsions. (McClements, 2015)

Functional Properties of Seafood proteins
Fig: Functional Properties of Seafood proteins

Proteins are an important group of natural emulsifiers frequently employed by the Industry, (Uluata et al., 2015). Proteins sourced from resources like seafood are amphiphilic, which could be absorbed at the emulsion interface and in turn stabilizes the lipid particles, (Lam and Nickerson, 2015). The emulsifying properties of proteins are determined by the location and quantity of amino acids in the peptide chain, (Damodaran and Parkin, 2017). The seafood proteins are reported to exhibit emulsifying properties, owing to their surface attributes especially created by extraction procedures like hydrolyzation which reduces the Interfacial friction among the hydrophobic and hydrolytic components, (dos Santos et al., 2011). The emulsifying stability of seafood proteins have been observed to be in the range of 0.144 to 130% (Chi et al., 2014; Elavarasan et al., 2014; Nalinanon et al., 2011; Taheri et al., 2013;) and the contributing factors have been attributed as molecular size, solubility and amino acid profile. (Jemil et al., 2014; Taheri et al., 2013; Tanuja et al., 2012)

The extraction conditions like degree of hydrolysis, peptide acetylation, enzyme type and solvent used are also found to influence the property, (Elavarasan et al., 2014; Nalinanon et al., 2011; dos Santos et al., 2011; Tanuja et al., 2012). The pH of the food system has been observed to influence the emulsifying property significantly, as it varies the hydrophobicity of protein surface, (Taheri et al., 2013). It has been demonstrated that the alkaline pH results in higher emulsifying activity index and an acidic pH tends to reduce the same, (Taheri et al., 2013). Similarly, larger molecular weight peptide increases the emulsion stability, while the lower molecular weight peptide chains tend to decrease it. (Tanuja et al., 2012)

Foaming Properties

The foaming capacity of a food constituent like protein finds its application in the Food Industry regarding preparation of texture specific food items, such as whipped cream, ice cream and bakery items which emphasizes on the volume and air occupancy, (Lam et al., 2018). The foaming property of proteins is attributed to its pH affinity, which results in precipitation closer to the isoelectric point, (Yang and Baldwin, 2017). Moreover, the transportation, rearrangement and penetration of protein molecules at the air-water interface determine the foaming capabilities properties, (Elavarasan et al., 2014). Many studies have reported that the foaming capacity of seafood proteins ranged from 23 to 240% and the foaming stability from 20 to 140%, (Chi et al., 2014; Elavarasan et al., 2014; Taheri et al., 2013; Tanuja et al., 2012). Tanuja et al., (2012) observed that dispersing proteins reduced the surface tension at the air-water interface facilitating the formation of foams. The seafood proteins varied their foaming capability according to the pH variation, as at a pH of 4, the foaming activity was low, but at a pH range of 6-10, it was stable, (Taheri et al., 2013). It was also observed by Betty et al., 2014 that the foaming activity of seafood peptides decreased with the decreasing molecular weight.

Gelling Properties

Gelling Property is an important functional attribute which finds application in texture specific food applications. The gelling feature of seafood protein is attributed to the thermal induced partial unfolding of myosin filaments in solution and further irreversible accumulation of unfolded filaments to form a three-dimensional formation trapping water within the matrix. This property is prominent when the seafood proteins are incubated over a period of 12 hours at 0° to 4°C or either when applied with mild thermal process for a period, (Sasidharan and Venugopal, 2020). It has been observed that the disulphide bonds contributed towards the gelling properties of seafood proteins treated at a pH of 11 (Park, 2013). The characteristics of the protein and the associated extraction methods have been found to influence the gelling properties of seafood proteins. The gel matrix stability of the seafood protein gels was also observed to be influenced by the concentrations of proteins within the matrix, (Wang et al., 2015). The functional properties of structural protein derived seafood gelatine include viscosity, gel strength, gelling and melting temperatures, depending on the variety and source of collagen and extraction conditions. (Gómez-Guillén et al., 2011)

Gelatin is a cold-setting, thermally reversible protein, existing as a gel or solution in relation to the temperature allowing it to be used as a gelling, whipping agent, emulsifier, stabilizer and film forming component. Seafood gelatine when compared to mammalian one is observed to possess inferior gelling (4-12°C) and melting (<17°C) thermal properties, which makes them better substitute to mammalian version for a number of industrial food applications, (Sasidharan and Venugopal, 2020). The seafood gelatines have been reported to possess gel strength from 100 to 300 g bloom and viscosity of 2.5 to 13 cp coordinated with the source and analytical conditions. (Gómez-Guillén et al., 2011)

Fat binding capacity

The fat binding capacity indicates the ability of the protein to absorb and retain lipid components within the matrix. This property is primarily linked to emulsifying capacity and enzyme-substrate specificity which are affected by the extraction conditions, (Villamil et al., 2017). The property finds application in meat and confectionery industry, as it regulates the flavour characteristics, (Taheri, 2013). The hydrophobic nature of the seafood proteins and physical factors of the oil are important in generating a fat binding scenario, as the extraction methods like hydrolysis have been reported to break down peptide units developing specific hydrophobicity due to the creation of non-polar peptide chains, (He et al., 2013). The hydrogen, electrostatic and covalent bond formation is also attributed to influence the protein-lipid interactions in seafood proteins, (Mbatia et al., 2014). The fat binding capacity of seafood proteins has been reported to be in range of 1.0 to 10.8 ml/g by many observations, (dos Santos et al., 2011; Tanuja et al., 2012; Taheri et al., 2013; Betty et al., 2014). Fat binding capacity of seafood proteins has been observed to be better than many industrial foods grade fat binders, like soy, milk and casein proteins. (Shaviklo, 2006)

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About the Author:
Dr. Abhilash Sasidharan
Department of Fish Processing Technology,
KUFOS, Kerala, India.
Email ID: drabhilash@kufos.ac.in

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