Essential oils are aromatic and volatile liquids extracted from plants. The chemicals in essential oils are secondary metabolites, which play an important role in plant defense as they often possess antimicrobial properties. The interest in essential oils and their application in food preservation has been amplified in recent years by an increasingly negative consumer perception of synthetic preservatives. Furthermore, food-borne diseases are a growing public health problem worldwide, calling for more effective preservation strategies. The antibacterial properties of essential oils and their constituents have been documented extensively. Pioneering work has also elucidated the mode of action of a few essential oil constituents, but detailed knowledge about most of the compounds’ mode of action is still lacking. This knowledge is particularly important to predict their effect on different microorganisms, how they interact with food matrix components, and how they work in combination with other antimicrobial compounds. The main obstacle for using essential oil constituents as food preservatives is that they are most often not potent enough as single components, and they cause negative organoleptic effects when added in sufficient amounts to provide an antimicrobial effect. Exploiting synergies between several compounds has been suggested as a solution to this problem. However, little is known about which interactions lead to synergistic, additive, or antagonistic effects. Such knowledge could contribute to design of new and more potent antimicrobial blends, and to understand the interplay between the constituents of crude essential oils. The purpose of this review is to provide an overview of current knowledge about the antibacterial properties and antibacterial mode of action of essential oils and their constituents, and to identify research avenues that can facilitate implementation of essential oils as natural preservatives in foods.

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Essential oils in food preservation: mode of action, synergies, and interactions with food matrix components.

Antimicrobial properties of allicin from garlic.

A Cook's tour is presented of the organosulfur chemistry of the genus Allium, as represented, inter alia, by garlic (Allium sativum L.) and onion (Allium cepa L.). We report on the biosynthesis of the S-alk(en)yl-L-cysteine S-oxides (aroma and flavor precursors) in intact plants and on how upon cutting or crushing the plants these precursors are cleaved by allinase enzymes, giving sulfenic acids—highly reactive organosulfur intermediates. In garlic, 2-propenesulfenic acid gives allicin, a thiosulfinate with antibiotic properties, while in onion 1-propenesulfenic acid rearranges to the sulfine (Z)-propanethial S-oxide, the lachrymatory factor (LF) of onion. Highlights of onion chemistry include the assignment of stereochemistry to the LF and determination of the mechanism of its dimerization; the isolation, characterization, and synthesis of thiosulfinates which most closely duplicate the taste and aroma of the freshly cut bulb, and additional unusual compounds such as zwiebelanes (dithiabicyclo[2.1.1]hexanes), a bis-sulfine (a 1,4-butanedithial S,S′-dioxide), antithrombotic and antiasthmatic cepaenes (α-sulfinyl disulfides), and vic-disulfoxides. Especially noteworthy in the chemistry of garlic are the discovery of ajoene, a potent antithrombotic agent from garlic, and the elucidation of the unique sequence of reactions that occur when diallyl disulfide, which is present in steam-distilled garlic oil, is heated. Reaction mechanisms under discussion include [3, 3]- and [2, 3]-sigma-tropic rearrangements involving sulfur (e.g. sulfoxide-accelerated thio- and dithio-Claisen rearrangements) and cycloadditions involving thiocarbonyl systems. In view of the culinary importance of alliaceous plants as well as the unique history of their use in folk medicine, this survey concludes with a discussion of the physiological activity of the components of these plants: cancer prevention, antimicrobial activity, insect and animal attractive/repulsive activity, olfactory–gustatory–lachrymatory properties, effect on lipid metabolism, platelet aggregation inhibitory activity and properties associated with ajoene. And naturally, comments about onion and garlic induced bad breath and heartburn may not be overlooked.

The Organosulfur Chemistry of the Genus Allium – Implications for the Organic Chemistry of Sulfur

The health benefits of garlic likely arise from a wide variety of components, possibly working synergistically. The complex chemistry of garlic makes it plausible that variations in processing can yield quite different preparations. Highly unstable thiosulfinates, such as allicin, disappear during processing and are quickly transformed into a variety of organosulfur components. The efficacy and safety of these preparations in preparing dietary supplements based on garlic are also contingent on the processing methods employed. Although there are many garlic supplements commercially available, they fall into one of four categories, i.e., dehydrated garlic powder, garlic oil, garlic oil macerate and aged garlic extract (AGE). Garlic and garlic supplements are consumed in many cultures for their hypolipidemic, antiplatelet and procirculatory effects. In addition to these proclaimed beneficial effects, some garlic preparations also appear to possess hepatoprotective, immune-enhancing, anticancer and chemopreventive activities. Some preparations appear to be antioxidative, whereas others may stimulate oxidation. These additional biological effects attributed to AGE may be due to compounds, such as S-allylcysteine, S-allylmercaptocysteine, N(alpha)-fructosyl arginine and others, formed during the extraction process. Although not all of the active ingredients are known, ample research suggests that several bioavailable components likely contribute to the observed beneficial effects of garlic.

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Intake of Garlic and Its Bioactive Components

Throughout the years numerous investigations concerning the inhibition of microorganisms by spices, herbs, their extracts, essential oils and various constituents have been reported Many of these materials possess significant antimicrobial activity, which in many cases is due primarily to a particular constituent Interpretation and comparison of results of various studies is complicated by variations in the methodology used for the determination of antimicrobial activity The antimicrobial activity varies depending on the microorganism, the spice or herb and the test medium These and other factors are examined in the light of their effect on the outcome of the test method

Spices and herbs: their antimicrobial activity and its determination

The observation by two of us (Cavallito and Bailey, 1944) that a variety of antibiotic substances of heterogeneous chemical nature are inactivated by cysteine and by certain other compounds containing very reactive sulfhydryl groups has led to further investigation of this reaction and to the formulation of a theory of the mode of action of these antibacterial agent& Several specific instances of sulfhydryl inactivation of antimicrobial agents are already in the literature. Eagle (1939) has shown that sulfhydryl compounds inactivated the arsphenamine type derivatives; Fildes (1940) described the inactivating action of thioglycollate, cysteine, and glutathione on the antibacterial activity of mercury; andAtkinson and Stanley (1943) have shown a similar inactivation for penicidine. More recently, Geiger and Conn (1945) have extended this observation to inelude clavacin, penicillic acid, and some a-,B unsaturated ketones. That so many antibacterial agents are inactivated by highly reactive thiol compounds indicates that this is more than just a fortuitous action and leads to a working hypothe for the mode of action of a large class of antibacterial agents. Tests for antibacterial action and for inactivation of this activity were conducted by two methods, the Oxford cup test and titration in broth. Preliminary inactivation tests were conducted by treating relatively concentrated solutions of the antibacterial agent with the inactivators for various time intervals, followed by dilution of the mixture and testing by the cup method. Inactivation could be shown readily in this manner. Titration tests were conducted in two ways. In one group the reaction mixture of inactivator and antibacterial agent prepared as for the cup tests was diluted in broth so as to obtain the inhibiting concentration in broth cultures. In another group the inactivator was added to the antibacterial agent in broth after the final dilution. The tubes were incubated for 18 hours. The antibacterial agents tested quantitatively for inactivation included mercuric chloride, pyocyanine, penicillin, the antibacterial principle of Arctium minus, C15H2006 (Cavalito, Bailey, and Kirchner, 1945), and the antibiotic from AUium sativum, CsH-SO--S-C&H5 (Cavallito, Bailey, Buck, and Suter, 1944).

The Inactivation of Antibacterial Agents and Their Mechanism of Action.

Antibacterial Substances from Asarum canadense. I. Isolation, Physical Properties and Antibacterial Action.

The killing of bacterial cells by chemical agents may be considered as the resultant of a reaction between two systems, the cells and their suspending medium and the germicide and its solvent. The method for testing sanitizing agents proposed by Weber and Black (1948) allows evaluation, on the basis of the two systems, of factors such as ions present in the suspending or diluting medium, organic matter, time, temperature and the bacterial cell itself, that may influence the killing of the test organism. This method, therefore, was used in the experiments here reported.

Factors influencing the rate of killing of Escherichia coli exposed to benzalkonium chloride.

Cantaurea maculosa, the spotted knapweed, is a common wild plant, aqueous extracts of which demonstrate antibacterial activity against gram-positive and gram-negative bacteria. Of 13 species of Centaurea screened for antibacterial activity by Osborn (1943), only three yielded extracts that were inhibitory to the growth of Staphylococcus aureus and none was active against Escherichia coli. Our C. maculosa showed, by cup-plate tests, even greater activity against Salmonella paratyphi A than against S. aureus. Several horticultural varieties of Centaurea cyanus showed no activity. The substance responsible for the activity has been isolated as a neutral, colorless, crystalline, bitter compound of tentative molecular formula, C20H2607. Chemically the substance belongs to the group of unsaturated lactone antibiotics. It is in general slightly more active against gram-negative than grampositive bacteria. Neutral solutions of cysteine or thioglycolate rapidly inactivate the antibiotic and the compound belongs to the class of antimicrobial agents that may act by binding essential sulfhydryl groups in the microorganism (Cavallito et al., 1945). The reaction with thioglycolate -SH places it in the category of nonspecific -SH inhibitors (Cavallito, 1946), differing from penicillin

John Hays Bailey

Papers

The Inactivation of Antibacterial Agents and Their Mechanism of Action.

Antibacterial Substances from Asarum canadense. I. Isolation, Physical Properties and Antibacterial Action.

Factors influencing the rate of killing of Escherichia coli exposed to benzalkonium chloride.

An antibacterial principle from centaurea maculosa.

The Reversal of Antibiotic Action.