The waste-derived extracts from olive oil and wine production, performed better against pathogens such as E. coli and Salmonella than three standard antioxidants (quercetin, hydroxytyrosol and oleuropein), reports the study in the journal Innovative Food Science and Emerging Technologies .
"From results presented, we can assume that the natural extracts grape extract and olive extract are promising natural preservatives, with application in food industry," wrote lead author Ana Teresa Serra from Portugal's Instituto de Biologia Experimental e Tecnológica (IBET/ITQB-UNL).
The research taps into an ever-growing body of study exploring novel source of natural alternatives to synthetic preservatives, such as like BHA and butylhydroxytoluene (BHT), to slow down the oxidative deterioration of food is gaining interest.
At present, 'natural' is a powerful force in the food industry, and there is increasing resistance at regulatory and consumer level - as well as from food retailers and manufacturers aiming to meet their demands - to synthetic preservatives.
Serra and co-workers tested the olive and grape residue extracts against five microbial species ( E.coli, S.poona, B.cereus, S. cerevisiae and C.albicans ).
They report that the grape extract contained high levels of phenolic compounds, with 3400 milligram of gallic acid equivalents per litre (GAE/L), compared to 400mgGAE/L in the olive extract.
In addition, the grape extract was found to be the most effective antibacterial agent, inhibiting the growth of the three bacteria strains in a dose dependent-manner.
The researchers note that the extract also exhibited a higher antimicrobial activity than pure, synthetic quercetin at the same concentrations.
Olive extracts, and synthetic hydroxytyrosol and oleuropein, performed less effectively than the grape extract, and found that Gram(-) bacteria were more resistant to olive phenolics than Gram(+) strains. Gram(+) and Gram(-) bacteria differ in the composition of their cell wall, and the classification is based on whether the cell retains a violet dye during the Gram stain process.
Against two yeasts strains of S.cerevisiae and C.albicans , both extracts showed significant inhibition in a dose-dependent manner.
On the other hand, similar concentrations of quercetin and hydroxytyrosol did not inhibit the yeast to the same extent.
"The action of the synthetic antioxidants alone was much less effective than natural extracts," wrote Serra, "suggesting i) a positive synergetic effect within the constituents of grape extract, that reinforce the response and/or, ii) the presence of other components in the matrices with antimicrobial activity."
The researchers note that, in addition to the 'natural' tag, the extracts can be sources cheaply since they come from olive oil and wine production wastes.
According to a 2003 report by Frost and Sullivan, the synthetic antioxidant market is in decline, while natural antioxidants, such as herb extracts, tocopherols (vitamin E) and ascorbates (vitamin C) are growing, pushed by easier consumer acceptance and legal requirements for market access.
Source: Innovative Food Science and Emerging Technologies (Elsevier) Published on-line ahead of print, doi: 10.1016/j.ifset.2007.07.011 "In vitro evaluation of olive- and grape-based natural extracts as potential preservatives for food" Authors: A.T. Serra, A.A. Matias, A.V.M. Nunes, M.C. Leitao, D. Brito, R. Bronze, S. Silva, A. Pires, M.T. Crespo, M.V. San Romao and C.M. Duarte