ANTIBACTERIAL ACTIVITY OF PLANT EXTRACTS IN FOOD PRODUCTS Antanas Šarkinas Food institute of Kaunas University of Technology, Taikos pr. 92, LT-51180, Kaunas; direktorius@lmai.lt
Spices Spices have been widely consumed throughout history in the human diet not only as flavouring substances but also as antimicrobial agents. Numerous reports demonstrate an inhibitory effect of essential oils and extracts, as well as purified compounds isolated from various plants on the growth of microorganisms; such substances have been used for preserving foods and drinks.
Effect of spices in food preservation Since the very first scientific experiments on the antimicrobial properties of spices, herbs and their components which were used in the end of the 19th century the interest in this topic has not diminished. Hoffman and Evans (1911) were the first who performed a laboratory study on the effect of spices in food preservation. They found that cinnamon, mustard and clove were useful in preserving apple juice.
Antimicrobial test The assessment of antimicrobial activity was performed on Gram-positive Listeria monocytogenes (ATCC 9117), Bacillus cereus (ATCC 10876), Bacillus subtilis (ATCC 6633), M. luteus (ATCC 9341), S. aureus (ATCC 25923), and Gram-negative E. coli (ATCC25922), Enterobacter aerogenes (ATCC 13048), Salmonella typhimurium (ATCC 14028), and Salmonella agona, Enterococcus faecalis (ATCC 29212), bacterial test cultures.
Strains of yeast In addition, 8 strains of yeast were used: Debaryomyces hansenii, Trichosporon cutaneum, Kluyveromyces marxianus var. lactis, Saccharomyces cerevisiae, Candida parapsilosis, Torulaspora delbrueckii, Pichia kluyveri, and Rhodotorula rubra. They were isolated from dairy products, equipment washing liquid, and the air of industrial premises.
Agar well diffusion method The antimicrobial properties were evaluated by the agar well diffusion method. Bacteria were grown in peptone-soy bouillon (LAB 04, LABM) for 24 h at 37 C. After cultivation, test culture cells were mixed using a minishaker MS 1 (Wilmington, N.C., U.S.A.) and the cell suspensions were adjusted according to McFarland nr 0.5 standard (Hood and others 2003). The suspension of bacteria cells was introduced into the dissolved media and cooled to 47 C; 10 ml was pipetted into a 90-mm diameter Petri plate. Eight-mm diameter wells were punched in the agar and filled with 50 µl of extracts. The plates were incubated overnight at 37 C.
Yeasts Yeasts were grown on a potato dextrose agar slant (LAB 98, LAB M) for 48 h at 25 C. After cultivation, the yeast cells were washed with saline and the cell suspensions were treated as previously mentioned. The media containing the yeast cells and oils was incubated overnight at 25 C. A 50 µl volume of extract was added to the agar wells using an automatic pipettor. After incubation, the inhibition zones were measured with calipers to an accuracy of 0.1 mm and the effect was calculated as a mean of 3 replicate tests.
Press vial into agar
Agar well diffusion method
Microbiological tests Microbiological tests showed that extract from coriander, celery, garlic, marjoram inhibit the growth of test cultures. Effectiveness of celery extract was higher in comparison with other extracts. The results showed that grampositive bacteria were more sensitive to extracts of celery, parsley, coriander, tarragon and lemon balm than gramnegative bacteria.
Preservative effect The possibility to stabilize a number of coliforms in meat mince was also evaluated. The extracts of plants did not have significant influence on the number of coliforms during cold storage. The test with ginger showed that it was contaminated with microorganisms which increased total number of bacteria in mince, and it did not have any preservative effect during storage.
Preservative effect Caraway extract inhibited growth of all test cultures, but its influence was not strong, zones of inhibition were not wide. Differences between sensitivity of salmonella were determined. S. typhimurium and S. agona had smallest sensitivity to caraway extract. Count of bacteria and coliform bacteria had growth in all samples with coriander or caraway extracts. In case with higher amount of extracts growth rate was slower.
Acceptability Consumer focus group had evaluated acceptability of cooked minced meat samples with plant extracts. Samples with 0,2 % of coriander extracts were evaluated as most acceptable but samples with 1 % were not acceptable for consumer at all. Consumer test results showed that increase in concentration of coriander extract in meat gives lower acceptability of samples.
Antimicrobial properties of cranberry extracts Investigation of the antimicrobial properties of cranberry extracts by the diffusion to agar method showed that B. cereus and M. luteus were the most sensitive (average zones of inhibition were 2.28 and 2.24 cm), L. monocytogenes was found to have average resistance, and E. coli was the least sensitive
Different cultivars A comparison of results from the different cultivars showed that the largest zones of inhibition (1.70 to 2.55 cm) weremade by the extract of the cultivar Pilgrim. The cultivar Stevens had the 2nd highest antimicrobial properties. Extracts of less active Ben Lear and Black Veil showed zones of 1.60 to 2.05 cm in diameter. All investigated test cultures were more sensitive to the ceftazidime/clavulanic acid 30/10 µg sensi-disc, except B. cereus, which was more sensitive to the extracts from berry and berry cakes
Natural antimicrobial agents The differences between antimicrobial activity of berry and berry cake extracts were not significant. Antimicrobial berry compounds may have useful applications as natural antimicrobial agents for the food industry as well as for the medical purposes Cranberry extracts did not have any effect on the growth of any of the 8 yeast species studied.
Antimicrobial properties of cranberry extracts Paprastoji spanguolė Stambiauogė spanguolė L. monocytogenes Bakterijų kultūros B. subtilis sporos B. subtilis veg. ląstelės E. faecalis S. typhimurium S. aureus E. coli 0 5 10 15 20 Slopinimo zonos skersmuo, mm