Effect of Oven Drying and Freeze Drying on Antimicrobial Activity, Antioxidant Properties, and Allicin Content of Black Garlic (Allium Sativum)

Choo, Zhang Tian (2024) Effect of Oven Drying and Freeze Drying on Antimicrobial Activity, Antioxidant Properties, and Allicin Content of Black Garlic (Allium Sativum). Final Year Project (Bachelor), Tunku Abdul Rahman University of Management and Technology.

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Abstract

Black garlic, produced through thermal processing from garlic, contains higher levels of beneficial compounds such as antioxidants and polyphenols compared to fresh garlic, enhancing its health benefits. However, methods like oven drying and freeze-drying can impact these properties. In this study, the antioxidant activity, antimicrobial activity, total phenolic content, total flavonoid content, and allicin content of dried Black Garlic were investigated. Samples were prepared using oven drying (60°C for 48 hours) and freeze drying (-105°C for 48 hours). The antioxidant properties of Black Garlic was investigated by using FRAP and DPPH assay. The oven-dried sample exhibited significantly higher antioxidant activity (determined by FRAP assay: 59.48 ± 0.11 mg FE/100g; DPPH assay: 59.40% ± 0.23%) compared to the freeze-dried sample (FRAP: 48.40 ± 0.28 mg FE/100g; DPPH: 52.19% ± 0.21%). Similarly, the oven-dried sample had a higher total phenolic content (TPC) and total flavonoid content (TFC), where (TPC: 56.81 ± 0.16 mg GAE/100g; TFC: 97.54 ± 0.12 mg QE/100g) compared to the freeze-dried sample (TPC: 50.52 ± 0.09 mg GAE/100g; TFC: 82.93 ± 0.09 mg QE/100g). Conversely, the freeze-dried sample had a higher allicin content (31.44 ± 0.16 mg AE/100g) due to the non-thermal drying process preserving this heat-sensitive compound (oven-dried: 26.30 ± 0.09 mg AE/100g). The oven-dried sample also showed stronger antimicrobial properties against both gram-positive (Listeria monocytogenes, Staphylococcus aureus) and gram-negative bacteria (Pseudomonas aeruginosa, Salmonella enterica) in the Disk Diffusion and Minimum Inhibitory Concentration tests. The oven-dried sample produced larger inhibition zones (Listeria monocytogenes: 30.5 ± 0.6mm vs. freeze-dried: 17.8 ± 0.6mm) and lower minimum inhibitory concentrations (Salmonella enterica: 10 mg/mL vs. freeze-dried: 20 mg/mL) compared to freeze-dried sample. This study highlights the significant impact of drying methods on the bioactive properties of Black Garlic powder and suggests that oven drying may be more effective in enhancing these properties compared to freeze drying.