Evaluation of harmful heavy metal (Hg, Pb and Cd) reduction using Halomonas elongata and Tetragenococcus halophilus for protein hydrolysate product

Abstract

Background: Many health claims surrounding antioxidative, antihypertensive and anti-inflammatory properties have been addressed in natural protein hydrolysates, including fermented fish. Besides being sold as animal feed, tuna viscera is used for for the production of fermented products like fish sauce and Tai pla, fermented viscera. However, toxic heavy metals including Hg, Pb and Cd have been found in various food items, particularly within the internal organs of tuna. Therefore, the consumption of fermented tuna viscera containing heavy metal involves health risks. Consequently, the detoxification and reduction of these toxic elements are relevant and important issues, particularly with the use of their bacterial cells and metabolic products. Halomonas elongata is a moderately halophilic bacterium which has the ability to remove heavy metal, and is normally found in hypersaline environments. Tetragenococcus halophilus is a moderately halophilic lactic acid bacterium and probiotic which is found in fermented food products, such as fish sauce, shrimp paste, and fermented fish. Some scientific studies have reported using T. halophilus improves amino acid profiles and desirable volatile compounds, in addition to reducing biogenic amine content in fish sauce product. Therefore, it was hypothesized that using H. elongata and T. halophilus could reduce heavy metal content and improve the organoleptic quality of fermented fish viscera product (Tai pla). Objective: This present work attempted to determine the growth characteristic of H. elongata and T. halophilus reared at various NaCl concentrations: 10, 15, 20 and 25%. Consequently, heavy metal reduction using these microorganisms reared at optimum NaCl concentration was evaluated. Methods: H. elongata and T. halophilus were reared in saline nutrient broth (SNB) and de Man, Rogosa and Sharpe (MRS-broth) added with NaCl at concentration 10, 15, 20 and 25%, respectively. Cultures at each NaCl content were added with mercury (Hg), lead (Pb) and cadmium (Cd) at concentration, 0.5, 1, and 3 mg/L, respectively. Subsequently, the supernatant of each condition was incubated at 48h and taken for heavy metal analysis at 96 h. Results: The results showed that higher NaCl content resulted in slower late log and stationary phases, particularly in T. halophilus. This may due to T. halophilus not producing special metabolite such as exopolysaccharide, which was found in H. elongata. Regardless of heavy metal concentration, the results revealed that Cd at 3 mg/kg caused more cell death of H. elongate, but not that of T. halophilus. Furthermore, removal of Hg, Pb and Cd was 12.70, 84.78 and 75.83% respectively, by rearing with H. elongata for 48 h and by rearing with T. halophilus for 96 h was 12.68, 91.27 and 95.12%, respectively. Conclusion: H. elongata and T. halophilus prefered SNB containing NaCl concentration between 10-20%. At higher NaCl concentration, 20-25%, the log phase was extended. Both H. elongata and T. halophilus were able to remove all test heavy metals. However, T. halophilus appeared to have higher Pb and Cd removal capability compared with H. elongata. Therefore, using H. elongata and T. halophilus for fermented tuna viscera is possible.