Phenolic Compounds

Phenolic compounds occurring in foods are classified as simple phenols and phenolic acids, hydroxycinnamic acid derivatives, and the flavonoids (93). The various classes of phenolics and individual compounds within each class have various antimicrobial activities.

Smoking of foods, such as meats, cheeses, fish, and poultry, has a preservative effect through drying and the deposition of chemicals. The major antimicrobial components of wood smoke are phenol and cresol. Smoke distillates and liquid smoke are inhibitory to Staphylococcus aureus, E. coli, Listeria monocytogenes, and Saccharomycescerevisiae (43). Consistent with results using other phenolic compounds as antimicrobials, smoke is more effective against Gram-positive than Gram-negative bacteria. Isoeugenol is one of the most effective antimicrobial compounds in liquid smoke, followed by m-cresol and p-cresol (94).

The phenolic acids, including derivatives ofp-hydroxy-benzoic acid (protocatechuic, vanillic, gallic, syringic, el-lagic) and o-hydroxybenzoic acid (salicylic) may be found in plants and foods. Esters of gallic acid including butyl, isobutyl, isoamyl, p-octyl, and p-dodecyl have been shown to be effective against Clostridium botulinum in microbiological media at 37°C (95). Another ester of gallic acid, propyl gallate, inhibited growth of Alcaligenes faecalis,En-terobacter aerogenes, E. coli, Klebsiella pneumoniae, L. monocytogenes, Proteus vulgaris, Salmonella enteritidis, Salmonella paratyphi, and Shigella flexneri (96). Tannic acid has antimicrobial activity against Aeromonas hydro-phila, E. coli, L. monocytogenes, S. enteritidis, S. aureus, and Streptococcus faecalis (43). Stead (97) demonstrated stimulation of growth of the lactic acid spoilage bacteria, Lactobacillus collinoides and Lactobacillus brevis, by gallic, quinic, and chlorogenic acids. It was suggested that the compounds were being metabolized by the microorganisms and that presence of these compounds in beverages may potentially result in antagonistic reactions with other food-grade antimicrobials.

Resin from the flowers of the hop vine (Humulus lupu-lus L.) used in the brewing industry for imparting a desirable bitter flavor to beer contain a-bitter acids (humulone, cohumulone, adhumulone) and ^-bitter acids (lupulone, colupulone, xanthohumol, adlupulone). Both types of acids possess antimicrobial activity primarily against Grampositive bacteria and fungi at low water activity (98). A compound isolated from green olives and identified as the phenolic glycoside oleuropein, or its aglycone, inhibits Lactobacillus plantarum, Leuconostoc mesenteroides, Pseudomonas fluorescens, Bacillus subtilis, Rhizopus sp., and Geo-trichum candidum (43).

Hydroxycinnamic acids, including caffeic, p-coumaric, ferulic, and sinapic acids, are found in plants and occur as esters and less often as glucosides (93). Hydroxycinnamic acids have been shown to inhibit bacteria, including B. ce-

reus, Erwinia carotovora, Lactobacillus collinoides, Lactobacillus brevis, S. aureus, and fungi, including Aspergillus flavus, A. parasiticus, and Saccharomyces cerevisiae (43). Cinnamic acid was found to be inhibitory to L. monocytogenes Scott A growth and listeriolysin O activity at pH 5.5 (99). The degree of inhibition is generally inversely related to the polarity of the compounds.

The furocoumarins are related to the hydroxycinna-mates. These compounds, including psoralen and its derivatives, are phytoalexins in citrus fruits, parsley, carrots, celery, and parsnips at levels of 2-6 /¿g/mL. They inhibit many Gram-positive bacteria including, L. monocytogenes, E. coli 0157:H7 and Micrococcus luteus, following irradiation with long wave ultraviolet light (100). These compounds inhibit growth by interfering with DNA replication.

The flavonoids consist of catechins, proanthocyanidins, anthocyanidins and flavons, flavonols, and their glycosides (93). Proanthocyanidins or condensed tannins are polymers of favan-3-ol and are found in apples, grapes, strawberries, plums, sorghum, and barley (93). The tannins have been tested for their antimicrobial effectiveness against molds, yeasts, bacteria, and viruses. For example, proanthocyanidins and flavonols accounted for the majority of inhibition of Saccharomyces bayanus by cranberries (101). Tannins have been reported to be inhibitory to Aeromonas, Bacillus, C. botulinum, C. perfringens, Enterobac-ter, Klebsiella, Proteus, Pseudomonas, Shigella, S. aureus, Streptococcus, and Vibrio (102). Extracts of blueberries, crabapples, strawberries, red wines, grape juice, apple juice, and tea inactivated poliovirus, coxsackievirus, echovirus, reovirus, and herpes simplex virus (103,104). It was concluded that the primary viral inhibitors in these products are condensed or hydrolyzable tannins.

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