Phytochemical properties of some macroalgae species and investigation of their possible uses in the seafood processing technology

Increasing consumer demand for natural foods free from additives and new production techniques resulted in growing interest in natural additives. In this thesis, the possibility of using ethanol extracts obtained from seaweeds Ulva rigida and Gracilaria verrucosa, as a natural antioxidant and antimicrobial agent for minced mackerel (Scomber scombrus) quality preservation during chilled storage was investigated. U. rigida and G. verrucosa were collected from the Coast of Inciraltı (Izmir, Turkey). After collection, the seaweed samples were immediately cleaned and washed with seawater to remove sand, debris, epiphytes and other extraneous matter attached to the thalli and then transported to the laboratory via cold chain. In the laboratory, the samples were cleaned by rinsing with tap and distilled water, and then dried at 40 °C for 24 hours. Dried and pulverized seaweed samples (10 g) were weighed into an amber erlenmeyer flask, and 200 ml of 95% (v/v) ethanol was added. The mixture was shaken and extracted in a water bath shaker at temperatures of 30 °C, 45 °C and 60 °C for 30 min. The extracts were filtered, and ethanol was removed using a rotary evaporator to obtain extracts. In the first part of the thesis, different extracts were obtained from seaweed and total phenolic content, antioxidant and antibacterial activity of these extracts were studied. The total phenolic content (TPC) of all seaweed extracts was found to be significantly different (p<0.01). The highest TPC (0.22 mg gallic acid equivalent/g extract) was obtained at 60 °C from U. rigida. Antioxidant activity was determined using the Trolox Equivalent Antioxidant Capacity (TEAC), 2.2-Diphenyl-1 picrylhydrazil radical scavenging activity (DPPH) and the Oxygen Radical Antioxidant Capacity (ORAC) methods. The U. rigida extract obtained at 60 °C had the highest amount of antioxidant activity with 2.18 µM trolox, 63.89 mg/mg DPPH and 5.95 µM trolox with the TEAC, DPPH and the ORAC methods, respectively. This also confirmed the results of phenolic extraction. This study indicated that ethanolic extracts at 60 °C of U. rigida and G. verrucosa had better antioxidant activity. Ethanolic extracts of U. rigida and G. verrucosa obtained at different temperatures were evaluated in vitro for antibacterial and antifungal activity on six bacteria (Bacillus subtilis, Staphylococcus aureus, Enterococcus faecalis, Listeria monocytogenes, Escherichia coli and Pseudomonas aeruginosa) and two fungi (Aspergillus brasiliensis and Candida albicans) using the paper disk agar diffusion method. All extract groups tested were found to have antimicrobial activity against all bacteria and fungi, except for Aspergillus brasiliensis. In the second part of the thesis, the objective was to preserve the quality of minced mackerel and inhibit lipid oxidation using extracts of U. rigida and G. verrucosa during chilled storage. Based on the results of total phenolic content, antioxidant activity and antibacterial activity, the best extracts were determined to be those obtained at 60 °C, and these were added to the minced mackerel at levels of 0.5%, 1% and 2%. The mince was then stored at 2 °C for 15 days. The application of seaweed extracts to the minced mackerel significantly decreased the TVB-N, TMA-N and pH values during chilled storage. Better results were obtained for the preservation of quality in samples containing seaweed extracts compared to the control group. Oxidative stability of minced mackerel was evaluated by employing peroxide value (PV), 232 and 270 nm UV spectra, and thiobarbutyric acid (TBARS) tests. It was found that all samples with added extracts had a significant pro-oxidant effect compared to controls. The effect of natural antimicrobials from ethanol extracts of seaweeds on microbiological (total mesophilic aerobic bacteria, total psychotropic aerobic bacteria, Pseudomonas, lactic acid bacteria and yeast-mold) changes of minced mackerel stored at 2 °C for 15 days was investigated. The natural compounds from seaweed extracts resulted in a lower bacterial count in minced mackerel during the storage period. Especially, it was found that the group containing 2% extract of U. rigida was more effective in reducing of bacterial growth. According to these results, it was determined that seaweed extracts could have a great potential in preserving the quality and in inhibiting bacterial growth of minced mackerel during chilled storage. However, a pro-oxidative effect on lipids was determined. Therefore, considering all quality parameters, it was determined that these macroalgae extracts should not be used as a natural additive substance in minced mackerel.