Antimicrobial activity of raisins and application as a food preservative

Increasing interest in the health benefits and functional properties of natural ingredients has led to the investigation of their antimicrobial activity. Raisins contain phenolic compounds, propionic acid, and Maillard reaction products (MRP) that exert antimicrobial effects. This study evaluated the antimicrobial activity of raisin extracts and their applications as food preservatives. The antimicrobial activities of water and ethanol extracts were evaluated by challenging ropy-bread-causing-Bacillus species and spoilage molds. The extracts exhibited bacteriostatic and fungistatic properties in microbial culture and controlled liquid bread systems. The Bacillus species were more susceptible to the extracts (0.05--0.07 mg extract per mg medium) than fungi. The 7.5% extracts in bread extended the mold-free shelf life one week longer than regular bread. Thus, the extracts have the potential to be used as natural food preservatives. The mold-retarding effect of the 7.5% extracts, containing 45 mug propionic acid per gram of dough, was comparable to the bread containing 0.24% (2400 mug g-1 ) calcium propionate, a preservative, at 28 days of storage. Concentrations of phenolic compounds and MRP positively correlated with antimicrobial activity of the extracts. The concentrations of most phenolic components of the ethanol extract were significantly (P < 0.01 or P < 0.05) higher than those of the water extract. The concentrated forms of the extracts contained significantly (P < 0.01 or P < 0.05) higher concentrations of the antimicrobial active compounds after solid-phase extraction. The antimicrobial efficacy of raisin extracts can be improved by optimizing the extraction and purification of the extracts.