Heat and acid induced autolytic digestion of Pacific hake (Merluccius productus) Public Deposited

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  • The optimum conditions for the proteolytic solubilization of trash fish and processing carcass waste in a simple shell-in-tube heat exchange system was investigated. The nutritional and chemical effects of processing variables were evaluated. An exogenous source of proteolytic enzymes (albacore tuna viscera, Thunnus alalunga) yielded a linear increase in the rate of the proteolytic hydrolysis of ground whole Pacific hake (Merluccius productus) up to 40% in the reaction mixture at 55°C. At 60°C optimum temperature for proteolytic hydrolysis was established with a 20% viscera 80% hake reaction mixture. Acidification of hake with 85% H₃PO₄ greatly accelerated proteolytic hydrolysis yielding an optimum between pH 3.6 and 3.7. The optimum pH for liquification based upon the viscometric properties of reaction mixtures varied from that for hydrolysis and was between 4.3 and 5.1. The chemical characteristics of the proteolytic solubilization of hake at 55°C was determined. The total nitrogen and free amino and tyrosine equivalent content of trichloroacetic acid (TCA), water and sodium dodecylsulfate (SDS) fractions of reactions containing 0, 2 and 4% 85% H₃PO₄ were determined at various times over a two hr period. With accelerated reactions (2 and 4% 85% H₃PO₄) the rate of hydrolysis was greatly reduced after 30 min. Under less than optimum pH conditions (0% 85% H₃PO₄) proteolytic hydrolysis proceeded in a linear fashion throughout the two hr period. The relationship between the quantities of chemical indices in the TCA and water fractions suggested that the majority of the proteolytic action yielded amino acids and short peptides. The total nitrogen content of the SDS soluble fraction inferred that very little of the protein that was not SDS soluble was being solubilized. The protein efficiency ratio of samples of hake reacted at 55°C in the presence of 0, 2 and 4% (wt/wt) 85% H₃PO₄ at 30, 60, and 120 min was determined to assess the nutritional effects of proteolytic solubilization. Proteolytic action reduced protein quality to a degree related directly to the level of acid accelerated proteolytic action. Full reduction in quality was accomplished at the end of 30 min for reaction mixtures containing 0 and 2% acid in comparison to a raw hake control; no reduction was observed between 30 and 120 min. In addition to the initial reduction in quality within the first 30 min of reaction, mixtures containing 4% acid were significantly reduced between 30 and 120 min. Conversion of native protein to nutritionally less utilizable amino acids and peptides probably played the major role in reducing protein quality in reaction mixtures containing 0 and 2% acid. The time dependent reduction in protein quality for the reactions containing 4% supports the destruction of amino acids, probably tryptophan, at this higher level of acidity.
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