Plant Extracts
Plant extracts usually contain phenolic compounds such as phenols and flavo- noids (Cowan, 1999), which are potential antimicrobial agents. A lot of extracts from different plants have been investigated with their antimicrobial activity against bacteria. The extracts of several edible Asian plants were screened for their antibacterial activity against Bacillus cereus, Staphylococcus aureus, L. monocytogenes, E. coli, and S. infantis (Alzoreky and Nakahara, 2003). About 16 out of 26 extracts were found to be effective against these bacteria, with MIC ranging from 0. 165 to 2.64mg/mL. The antimicrobial activities of rosemary extracts with different solvent were compared. Methanol extract with higher polyphenol content showed lower MICs against bacteria than water extract (Moreno et al., 2006). Antimicrobial activity of water-soluble arrowroot tea extract was evaluated against E. coli O157:H7, S. enterica Serotype Enteritidis, L. monocytogenes, and S. aureus in ground beef and mushroom soup. At 6% concentration, the extract could inhibit 1.5log for pathogens inoculated on beef after 7 days storage. The extract was more effective in liquid food. At 1%, it could reduce the pathogen population by at least 2.3 log CFU/ml in mushroom soup after storage (Kim and Fung, 2004). The antimicrobial efficacy of six grapefruit seed extracts was tested, and five out of six extracts showed a high growth inhibiting activity against the test germs including Bacillus subtilis, Micrococcus flavus, Staphylococcus aureus, Serratia marcescens, E. coli, Proteus mirabilis, and Candida maltose (Von Woedtke et al., 1999). The methanol extract of the leaves of Calpurnia aurea exhibited broad spectrum of antibacterial activities, with MIC ranging from 31.25 to 125 mg/mL against seven tested pathogens (Umer et al., 2013). Olive leaves extracts have been found to be effective against a wide spectrum of bacteria and fungi, such as B. cereus, L. monocytogenes, E. coli, and C. oleophi- la (Rahmanian et al., 2015). The antimicrobial activities are related to their ter- penes content and phenolic compounds that are well reported to possess a good antimicrobial activity. Even though many plant extracts have shown antimicrobial activities, the effective concentration is usually high and the dark colors of raw extract is one big challenge of applying them directly in foods.
