Submitted to: Journal of Food Protection
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/14/2000
Publication Date: 9/14/2001
Citation: Unal, R., Fleming, H.P., McFeeters, R.F., Thompson, R.L., Breidt, F., Giesbrecht, F.G. 2001. Novel quantitative assays for estimating the antimicrobial activity of fresh garlic juice. J. Food Prot. 64:189-194. Interpretive Summary: Garlic is one of many plants containing compounds that are inhibitory to certain microorganisms. Some such compounds can contribute to the preservation of foods in which they are compatible. This paper describes methods that can be used to measure the antimicrobial activity of garlic juice against selected species of bacteria, some of which are known to cause food spoilage and/or human pathogenesis. The principles applied in the assay methods for garlic may be applicable to other plant materials. By taking advantage of the preservation action of natural components of foods, the need for synthetic chemical preservatives may be reduced.
Technical Abstract: Novel agar diffusion and broth dilution assays were developed for quantitatively estimating the antimicrobial activity of fresh garlic juice. Bacteria found to be inhibited by garlic juice in agar diffusion assay included two Gram-positive and five Gram-negative species. Leuconostoc mesenteroides was not inhibited. Escherichia coli B-103 (HB101, with pJH101, amp-resistant, 100 microg ml-1) was inhibited and chosen as the standard culture for quantitative assays. The agar diffusion assay was based on the slope ratio method, where the slope of dose/response for garlic juice was divided by the slope of dose/response for methylmethane thiosulfonate (MMTSO2). Juice from fresh garlic varied in activity between 1.76 and 2.31 microg MMTSO2/mg garlic juice. The activity of juice decreased over 11 months of storage of garlic cloves at 5 degrees C from 2.31 to less than 0.1 microg MMTSO2/mg juice. The broth dilution assay also employed the E. coli B-103 culture, which permitted selective enumeration of this bacterium when 100 microg ml-1 of ampicillin was incorporated into the enumerating agar. Selective enumeration was essential since the garlic juice was not sterile and, thus, contained natural flora. Growth of E. coli was unaffected by 0.1%, delayed by 0.25%, and completely inhibited at 0.5 and 2% garlic juice in broth over 24 h of incubation at 37 degrees C.