Evaluation of Antioxidant and Antibacterial Activities of Methanolic Extracts of Freeze Dried and Oven Dried Spearmint (Mentha Spicata) Leaves and Stems

Chong, Christine Yi Ting (2018) Evaluation of Antioxidant and Antibacterial Activities of Methanolic Extracts of Freeze Dried and Oven Dried Spearmint (Mentha Spicata) Leaves and Stems. Final Year Project (Bachelor), Tunku Abdul Rahman University College.

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Abstract

Among the family of Lamiaceae (Labiatae), mint is one of the most popular and cultivated officinal and aromatic plants. Although the antimicrobial and antioxidant properties of Labiatae herbs are well studied, the effects of different drying methods for extraction preparation have yet been explored. In this study, spearmint (Mentha spicata) leaves and stems were subjected to freeze dryer and convection oven. The samples were then extracted in 70 % methanol and further analysed for their antimicrobial, as well as antioxidant properties. Antioxidant analyses of various extracts showed that freeze dried leaves possessed the highest total phenolic (650.11 ± 13.16 mg/g) and total flavonoids (483.71 ± 21.71 mg/g) content, while the oven dried stems produced the lowest total phenolic (8.96 ± 0.36 mg/g) and total flavonoids (20.53 ± 0.26 mg/g) content. Apart from that, the greatest percentage of scavenging activity was found in freeze dried leaves (45.00 ± 0.00 %), whereas the lowest scavenging activity was determined in freeze dried stems (6.67 ± 1.44 %). In order to study the effect of chlorophyll content on total phenolic, total flavonoid contents and antioxidant activity of spearmint, chlorophyll analysis was conducted for all the freeze and oven dried samples. The analysis of chlorophyll content of mint samples revealed that mint leaves possessed higher chlorophyll content compared to mint stems, and the chlorophyll content obtained from freeze dried samples was slightly higher than oven dried samples. For antimicrobial test, Kirby-Bauer disk diffusion assay was performed using two Gram-positive bacteria (Staphylococcus sp. and Listeria sp.), and two Gram-negative bacteria (Salmonella sp., and Pseudomonas sp.) with concentration of extracts tested at 10 mg/ml, 50 mg/ml, and 100 mg/ml. The results indicated that 50 mg/ml of freeze dried mint leaves extract can inhibit the growths of both Listeria sp. and Pseudomonas sp., whereas 50 mg/ml of oven dried mint leaves extract was sufficient to inhibit the growths of both Staphylococcus sp. and Listeria sp.. In addition, 100 mg/ml of the both freeze and oven dried mint leaves and stems extracts inhibited not only Staphylococcus sp., and Listeria sp. but also Pseudomonas sp.. On the contrary, 50 mg/ml of the extracts from freeze and oven dried stems produced inhibition zones when tested on Listeria sp.. Moreover, 100 mg/ml of freeze dried mint stems extract can inhibit the growths of both Listeria sp. and Pseudomonas sp., while 100 mg/ml of oven dried mint stems extract only able to inhibit Listeria sp.. In short, freeze dried samples possessed greater antioxidant properties, higher chlorophyll content and slightly higher antimicrobial activity compare to oven dried sample. Keywords: Mentha spicata, leaves and stems, freeze-drying, oven-drying, antioxidant properties, antibacterial activities, chlorophyll content