What a Waste! Grape Pomace Kills Food-Spoiling Bacteria

In a Turkish study, the grape skins, seeds and stems left over after winemaking proved deadly to E. coli, salmonella and staph.
Sep 23, 2004

A vintner's trash may be a food scientist's treasure. Researchers in Turkey have found that grape pomace, a by-product of winemaking, may be effective in killing harmful bacteria that cause food to spoil.

"The [pomace] extracts can be used in food formulations to protect food against spoilage bacteria," said study coauthor Osman Sagdiç, from Erciyes University in Turkey, in a statement. "People prefer natural preservatives in the place of synthetic counterparts in food."

The study appeared in the online version of the Journal of the Science of Food and Agriculture and will be published in an upcoming issue.

Grape pomace, which is made up of the skins, seeds and stems discarded during winemaking, is filled with polyphenols, just as red wine is. These chemical compounds have been linked to various health benefits, and previous research has shown that polyphenols, as well as wine, can work as antibacterial compounds. For example, in one study, trans-resveratrol increased the shelf life of apples. In another study, wine, particularly white wine, was found to help kill E. coli and salmonella, though this was attributed partly to the wines' high acid levels.

Sagdiç and his team, which included researchers from Süleyman Demirel University, tested the effect pomace had on 14 different bacteria known to cause premature food spoilage, such as strains of E. coli and salmonella, as well as Staphylococcus aureus.

The scientists obtained pomace from local wineries that use the Turkish varietals Emir Karasi, a white grape, and Kalecik Karasi, a red grape. The pomace was dried and ground into a fine powder.

Samples of the 14 bacteria were cultivated in petri dishes and measured. The scientist then added 50 microliters of grape pomace extract mixed with methanol, ethyl acetate or water.

Each strain was tested separately against both the red grape and white grape extracts in concentrations of 0.5, 1, 2.5, 5, 10 and 20 percent by volume, as well as against a control, which was 50 microliters of methanol only. At intervals of 1, 24, 48, 72 and 96 hours, the scientists checked the levels of the bacteria. Each test was repeated three times, and the results were averaged.

The extracts inhibited all 14 strains of bacteria, though the results varied according to the grape variety, extract concentration and method used to apply the extract to the bacteria. The red grape pomace showed a stronger antibacterial effect than the white, and in general, the higher the concentration of the extract, the more effective it was against the bacteria.

For E. coli, even at 0.5 percent concentration, the grape pomace extract stopped the bacteria from multiplying, regardless of which solution it was in. At 1 percent, the E. coli began to die off after just one hour. The higher the concentration, the faster the E. coli died. For the Staphylococcus aureus, the same effect was noted, with 0.5 percent inhibiting growth, and 1 percent or more killing off the bacteria.

The scientists acknowledged that "the antibacterial effect of extracts change according to [the grape], extraction method, concentration of extract and the method used" to apply the extract to the bacteria.

Nonetheless, the scientists concluded that "grape pomace extract has potential use for food preservation purposes," and its economic viability as an antibacterial agent should be evaluated.

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Read more about the antibacterial activities associated with grape compounds :

  • Feb. 4, 2003
    Red-Wine Extract Extends Shelf Life of Fruit

  • Nov. 5, 2002
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