Value-added functional protein products and endogenous antioxidants from aquatic species

Onodenalore, Akhile Collins (1998) Value-added functional protein products and endogenous antioxidants from aquatic species. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Protein dispersions were prepared from male and spent female capelins and shark after aqueous washings of their meats using 0.5% NaCl, 0.5% NaHCO₃ and water, consecutively. Extraction of pigments, odorous matter and free amino acids occurred during washings. The unacidified dispersions were highly viscous and stable to heat and centrifugal force. Over 90% of the protein in the dispersions remained in solution after heating at 70 or 100 °C. However, on addition of acetic acid, viscosity of shark protein dispersion was lowered. Total amino acid composition of the dispersion virtually remained unchanged due to washing. The protein efficiency ratio (PER) values of washed meats, calculated by an amino acid scoring methodology, were comparable to those of unwashed meats. However, over 60% of the free amino acids were extracted due to washing, thus producing a bland product. -- Enzyme-assisted hydrolysis of shrimp heads and female lumpfish after roe extraction was carried out. Of the enzymes considered (Alcalase, Neutrase and Trypsin), Alcalase was the most efficient in producing protein hydrolysates with desired degree of hydrolysis (DH) and highest nitrogen recovery (NR). Response surface methodology (RSM) was employed to optimize the hydrolysis conditions using Alcalase for shrimp heads and lumpfish proteins. A polynomial regression model was used to predict the optimum conditions for the process, these were 58 °C at pH 8.1 and enzyme/substrate concentration ratio of 33.1 AU/kg crude protein for shrimp protein hydrolysis for a predicted DH of 17.8%. The optimum hydrolysis conditions for lumpfish protein, as predicted by the regression model, were temperature, 63 °C; pH. 8.1: and enzyme/substrate concentration ratio. 28.5 AU/kg crude protein for a DH of 18.4%. Nitrogen recovery in shrimp and lumpfish protein hydrolysates prepared under optimum hydrolysis conditions were 72.2 and 79.9%, respectively. The regression coefficient (R²) values which were >0.95 and the close values of experimental hydrolysis results to those predicted, indicated the adequacy of the models employed. The total amino acid composition of the prepared shrimp and lumpfish protein hydrolysates were similar to those of their starting materials. However, over a 14-fold increase in the content of free amino acids was noted in these protein hydrolysates. Furthermore unique functional characteristics of the hydrolysates with respect to moisture and fat adsorption, emulsifying capacity, emulsion stability, among others, were noted. Addition of 3% (w/w) shrimp protein hydrolysate to mechanically deboned chicken meat (MDCM) resulted in cooking yield and drip volume of 118% and 5.3 mL/100 g, respectively, as compared with 76% and 10.6 mL/100 g for the control. These effects of the addition of shrimp protein hydrolysates compare favourably with those obtained for commonly used phosphates (sodium tripolyphosphate, trisodium pyrophosphate and sodium hexametaphosphate) in food. The protein hydrolysates also displayed antioxidant activity in a meat model system by inhibiting the formation of 2-thiobarbituric acid reactive substances (TBARS) and in a β-carotene/linoleate emulsion system by delaying the bleaching of β-carotene. Sephadex G-15 column separation of the peptides of shrimp protein hydrolysate afforded 5 fractions based on their absorbance at 220 and 280 nm, with fraction IV being most antioxidatively active. All fractions were further separated using a reversed-phase high performance liquid chromatographic system and afforded multiple peaks with different retention times based on the hydrophobic nature of the peptides. The eluates also had antioxidative activity when tested in a β-carotene/linoleate system. -- The high level of free amino acids and peptides in the shrimp protein hydrolysates was exploited in the generation of volatile flavour compounds from the reaction involving shrimp protein hydrolysate and glucose via Maillard reaction. The compounds obtained included pyridines, pyrazines, aldehydes, furan derivatives, sulphur-containing compounds, among others. The number and concentration of volatile compounds from hydrolysate 1 (DH, 11.2%) and hydrolysate 2 (DH, 17.1%), only differed marginally. The major compounds formed were Strecker aldehydes and their corresponding substituted pyrazines. The most abundant volatile compound formed was 3-methylbutanal which accounted for 28% of the total amount of flavour compounds identified in hydrolysate 2. -- Extraction of antioxidant components of shrimp was performed by employing various solvents which differed in their polarity. Ethanol was the most suitable extraction medium and afforded extracts which exhibited antioxidant activity in a β-carotene-linoleate emulsion system. The components of the extract were separated using a silica gel column and further isolation was performed by thin layer chromatography which afforded four components. Two low polarity compounds as well as two high polarity compounds were separated using n-hexane-acetone (3:1. v/v) and n-butanol-water-acetic acid (3:1:1, v/v/v), respectively, as developing solvents. Reversed-phase HPLC was used to purify the compounds followed by their structural elucidation using electrospray mass spectrometry. The compounds are 3-membered ring phenolics and were identified as 7-(3-butenyl)-2-hydroxy-6-(1.2.5.6-tetrahydro-pyridin-2-yl) quinoline and 7-(3-butenyl)-2-hydroxy-6-(1.2.5.6-tertahydropyridin-2-yl)-7.8-dihydroquinoline and their isomers.

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