Study On Preparing Seafood Flavor Condiment Base From Decapterus Maruadsi And Sarotherodon Nilotica

In commercial fishing industry, the average of by-catch can reach to as high as 30-40%, only a small portion of which was used as feeding stuff, while most of them were discarded directly. This is obviously a huge waste of the valuable fish resource. However, there is no low-cost processing technique for the utilization of these low commercial value fishes. In this study, strategy and techniques were developed to process these fishes to produce some valuable flavor products. These products can then be used for the liquefied or powdered seafood flavor condiment productions.The study used two fish species, Decapterus maruadsi (marine fish) and Sarotherodon nilotica (freshwater fish), representing the majority fish types of the by-catch. A protocol including first a two-stage enzyme-based hydrolysis and then further process by using post-fermentation treatment and Maillard reaction, was established to produce the flavor fish hydrolysates. Both the two-stage enzyme hydrolysis reaction and the Maillard reaction were optimized regarding to the flavor and the free amino acids constitution of the hydrolysate products.To optimize the reaction condition for enzymatic hydrolysis, a series of experiments were carried out which varied either each single factor a time, or a combination of several key factors. The optimum reaction condition for treating the Decapterus maruadsi can be described at two stages. In the first stage, the protein substrate with a final concentration of 1.56%(m/m) was mixed with protamex protease with a final concentration of 2700U per gram of protein at pH 7.5. After incubated at 60℃for 8h, the mixture was boiled to denature the enzyme. In the second stage, flavor proteinase was added to the mixture with a final concentration of 2400U per gram of protein at pH 6.5, and then the mixture was incubated at 55℃for another 8h. After the two stages of enzymatic hydrolysis, the content of amino-acid nitrogen was measured as 1.48mg/mL, DH% was 58.72%, and the recovery of total nitrogen was 95.68%. The optimum condition for Sarotherodon nilotica was different to the Decapterus maruadsi. In the first stage, the protein substrate with a final concentration of 2.50%(m/m) was mixed with protamex protease with a final concentration of 17660U per gram of protein at pH 7.5. The mixture was incubated at 55℃for lOh and then boiled to denature the enzyme. The procedures of second stage were same as that of Decapterus maruadsi. The results showed that the final Sarotherodon nilotica hydrolysate contained 1.93mg/mL of amino-acid nitrogen,58.25% of DH, and the recovery of total nitrogen was 76.82%.To further improvements for the flavor and nutrition of obtained hydrolysates, the variety of treatments such as the addition of supplementary materials and post fermentations were performed. The results showed that the total content of free amino acids was increased at least 49.7% in Decapterus maruadsi and 277.1% in Sarotherodon nilotica using the improved mimic fish sauce processing, and increased at least 463.2% and 1095.0%, respectively using the improved mimic soy sauce processing. Meanwhile, the proportion of different amino acids in the hydrolysates were more harmonious and different volatile components were generated after the corresponding post treatments.Maillard reaction was then applied to improve the flavor of enzymatic hydrolysates. The conditions of reaction were established on the basis of arbitrary flavor score and the loss rate of amino-acid nitrogen. The optimum Maillard reaction conditions for Decapterus maruadsi hydrolysate was at carbonyl/amino ratio of 2:1 by the addition of glucose with an initial pH of 8.5, and incubating at 110℃for 60 min; the optimum reaction conditions for Sarotherodon nilotica hydrolysate were almost same as that of Decapterus maruadsi, except the ratio of carbonyl/amino was 1:1. The results demonstrated that the products had a higher flavor score and the total proportion of free amino acids was further balanced after Maillard reaction. Compared to the original hydrolysates of Decapterus maruadsi and Sarotherodon nilotica, the total numbers of free amino acids were increased up to 2.4 fold and 1.6 fold, separately.Techniques regarding the separation and stabilization of the fat components in the fish hydrolysates have been developed in the study. Centrifugation is adopted to remove the fat from the hydrolysate. The factors that affect the separation efficiency were determined as the temperature, pH, motor frequency and the volume discovery. The optimal processing condition of producing the liquid flavoring base for Decapterus maruadsi was as follow. After adjusted the pH to 3.0, the solution was extracted by continual centrifugation at 50℃, 70Hz frequency. By controlling the volume discovery about 62%, the heavy phase was collected and emulsified at 30-40℃for lmin, then the emulsion was bottled to sterilize at 80℃for 30 min. For Sarotherodon nilotica, on the other hand, the pH was adjusted to 3.0, and the samples were continually extracted by centrifugation at 40-50℃,70Hz frequency under the control of the volume discovery about 65%. Then the collected heavy phase was emulsified at 50℃for 3min by adding 0.02% emulsifying agent (HLB=12) and finally bottled to sterilize at 80℃for 30 min.Under the optimum conditions, enzymatic hydrolysates of Decapterus maruadsi and Sarotherodon nilotica were processed in the form of liquid flavoring bases. The amino-acid nitrogen of them was 4.9mg/mL,2.7mg/mL, respectively. Moreover, these samples have been proven to be able to create distinctive seafood flavor and the quality was met the national hygienic standard for aquatic flavoring. Thus, the processed fish hydrolysates have the potential commercial value as food additives or flavor base.