Protection of food from microbial or chemical deterioration has traditionally been an important concern in the food industry. Chemically synthesised preservatives have been classically used to decrease both microbial spoiling and oxidative deterioration of food.
However, in recent years, consumers are demanding partial or complete substitution of chemically synthesised preservatives due to their possible adverse health effects. This fact has lead to an increasing interest in developing more “natural” alternatives in order to enhance food shelf-life and safety (2).
In recent decades, special attention has been focused on spices and aromatic vegetables which are commonly employed as food ingredients. In spite of the fact that they are usually used as flavouring agents to enhance the aroma or the taste of a great variety of foods, it is also well known that spices are a good source of natural antioxidant and antimicrobial compounds.
It is generally agreed that onion (Allium cepa, L.), which is one of the most widely distributed vegetables, is a good source of natural compounds with beneficial health effects. Several reports have ascribed these biological effects to the presence of volatile sulphur-compounds, such as thiosulfinates and phenolic compounds. onions are a rich source of dietary flavonoids with average values ranging from 270 to 1917 mg of flavonoids per Kg of fresh weight, depending on the variety of onion. They are mainly represented by the flavonols quercetin and kaempferol, commonly present as glycosilated forms (4). Onions have been reported to have anti-inflammatory, antiallergenic, vasodilatory and anticarcenogenic properties (5), as well as antimicrobial and antioxidant capacities. Therefore, onions have been proposed as a good source of natural preservatives to enhance not only the stability and preservation of food systems but also to increase their nutriceutical properties.
ANTIMICROBIAL ACTIVITY
Scientific research on the antimicrobial capacity of onion (Allium cepa, L.) has traditionally been focused on the presence of thiosulfinates and other volatile organic compounds.
These molecules originate due to the action of the enzyme alliinase which is released when plant material is disrupted and transforms cysteine sulfoxide precursors into sulfenic acids and thiosulfinates . They are the parts mainly responsible for the onion’s characteristic aroma, taste and lachrymatory effect, but they are also of great interest on account of their antibacterial, antifungal and antiviral properties. Several studies have tested in vitro the effect of organosulfur-containing compounds to inhibit the growth of pathogen or food spoiling microorganism by both minimal inhibitory concentration and well diffusion assay tests.
It has been shown that they exhibit marked inhibition against gram positive bacteria of genera Bacillus, Micrococcus, Staphylococcus, Streptococcus, as well as gram negative bacteria, such as Salmonella enteridis or some strains of Escherichia coli . In addition, onion antiyeast and anti-fungal activity has been mainly attributed to the presence of organosulfur-containing compounds which inhibit the growth of yeast and fungi such as, Candida albicans, Aspergillus niger, Penicillum cyclopium or Fusarium oxysporum .
However, the application of onion volatile compounds seems to be limited as a practical food additive due to their strong flavour, pungent properties and relative biochemical instability. Consequently, there is a growing interest in studying the antimicrobial properties of onion phenolic compounds which are presumably more stable. Flavonols quercetin and kaempferol are commonly present in notable amounts in onions and are the main non-volatile compounds responsible for their antimicrobial properties . Although they are mainly present as 4,3’-O – or 4’-glycosilated forms, it is considered that their respective aglycones are more active because the free presence of their 3-hydroxil group seems to be critical to exert their antimicrobial activity .
It is generally agreed that onion flavonols exert their antibacterial activity by inhibiting dNA, RNA and protein biosynthesis, as well as perturbing the functions of the cytoplasmatic membrane (10). In vitro studies have reported that onion flavonoids can effectively inhibit the growth of gram positive bacteria associated to food spoilage, such as Bacillus cereus, B. subtilis, Staphylococcus aureus, Microcroccus luteus and Listeria monocytogenes, while the gram negative bacteria Escherichia coli and Pseudomonas aeruginosa seem to be more resistant. yeasts, such as Candida albicans are highly resistant to the inhibitory effect of onion flavonoids. These different resistant patterns are related to differences in fungus, yeast and bacteria cell wall, membrane and protein synthesis.
It is well known that onion is a good source of bioactive compounds. Sulfur-containing compounds are the main responsible contributors to its antibacterial and antifungal activity. Nevertheless, its addition to food is limited to its strong odour and flavour, thus increasing interest has been focused on onion flavonoids which are more stable and show both antioxidant and antimicrobial activity. Onion flavonoids are considerably stable in refrigerated storage and are moderately resistant to most home-processing and cooking methods due to the thermostability of quercetin, its most abundant flavonol. Addition of onion or onion extracts to processed foods can reduce their microbial and oxidative deterioration, thus enhancing its preservation.
Credit : JoNATHAN SANTAS1*, MARÍA PILAR ALMAJANo2, RoSA CARBÓ3