Onion
A natural alternative
to artificial food preservatives
JONATHAN SANTAS1*, MARÍA PILAR ALMAJANO2, ROSA CARBÓ3
*Corresponding author
1. University of Barcelona, Department of Nutrition and Food Science, XaRTA-INSA
Av. Joan XXIII, s/n, Barcelona, 08028, Spain
Jonathan Santas
2. The Technical University of Catalonia, Chemistry Engineering Department
Av. Diagonal 649, Barcelona, 08034, Spain
3. The Technical University of Catalonia, Department of Agri-Food Engineering and Biotechnology
Campus Baix Llobregat, C. Esteve Terrades 8, Castelldefels (Barcelona), 08860, Spain
AgroFood industry hi-tech - September/October 2010 - vol 21 n 5
Flavours/preservatives
Keywords: Onion, flavonoids, antimicrobial, antioxidant, food additive, food preservation.
ABSTRACT: Consumer demand of minimally processed foods without synthetic preservatives has led to a growing interest in
their replacement for more natural alternatives. Onion is widely used as a food ingredient and it is known as a good source
of bioactive compounds, such as sulphur-containing compounds and flavonoids with well known health beneficial effects,
antioxidant and antimicrobial capacities. Consequently, onion has been proposed as a promising and safer source of food
preservatives. The overall aim of the present review is to analyze its potential use as a food ingredient by reporting evidence
of onion antioxidant and antimicrobial activities, as well as discussing some concerns in relation to its stability and
effectiveness as a food preservative.
INTRODUCTION
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 (1). 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 (3).
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 (4, 5).
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 (6, 7).
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.
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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 (8). 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 (5, 7). 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
(5, 9). 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 (10). 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 (11). 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 (6, 10, 12). Yeasts, such
as Candida albicans are highly resistant to the inhibitory effect
of onion flavonoids (6, 9, 13). These different resistant patterns
are related to differences in fungus, yeast and bacteria cell
wall, membrane and protein synthesis (14).
ANTIOXIDANT ACTIVITY
A great number of publications have reported the antioxidant
capacity of onion and onion extracts in vitro. In spite of the
fact that the antioxidant capacity of onions can be partly
attributed to the presence of sulphur-containing compounds
(15), flavonoids seem to have a higher contribution on onion
antioxidant properties.
Several studies have reported that onion flavonoids, especially
quercetin and its glycosides, have high radical-scavenging
activity, whereas its reducing activity is lower than that of other
phenolics (3, 16). Furthermore, there is a direct correlation
between the presence of flavonoids in onion and its antioxidant
capacity (3, 16). Subsequently, differences between onion
varieties and cultivation conditions have an important impact
not only on the total phenolic content of onions, but also on
its antioxidant capacity. Therefore, it is generally agreed that
coloured varieties such as red or intense yellow varieties are
richer sources of antioxidant compounds than white onion
varieties (4).
TRENDS IN FOOD CONSERVATION AND TECHNOLOGY
AgroFood industry hi-tech - September/October 2010 - vol 21 n 5
Onions and onion by-products have been proposed as food
ingredients which can be added to food preparations to
enhance their shelf-life preservation. Nevertheless, there are
some concerns regarding its use in food conservation.
Firstly, the stability of onion antimicrobial and antioxidant
compounds during storage until its further use as a food
ingredient is an important issue. Some authors have studied the
effect of different conservation treatments in onion phenolic
content and antioxidant capacity. Roldan et al. (17) reported
that after the pasteurization of onion by-products at 100ºC, their
bioactive compounds and antioxidant capacity remained
mainly intact. Thus, this represents a better option for stabilising
onion by-products than sterilizing at 115ºC due to the possible
formation of caramelized products. Minimal losses of onion
antioxidant capacities have also been reported in frozen and
canned onions during long periods of time (up to 8 months)
especially under the presence of ascorbic acid (18).
Secondly, it is questionable whether onion bioactive compounds
would remain stable after their processing and addition as food
ingredients to processed or cooked foods or preparations. Great
attention has been focused on processing at high temperatures
as it is an important factor which can induce significant changes
in the concentration, antioxidant properties and bioavailability
of onion bioactive compounds (19). It has been described that
peeling of onions significantly decreases their total flavonoid
content and antioxidant capacity (approximately half the
starting level) due to the fact that an important amount of
flavonoids is concentrated in the outer parts of the onion (20).
Nevertheless, further processing by blanching, moderate
microwave heating, frying with either rape-seed oil or butter
has a lower influence on total phenolic or flavonoid content
(21-23). In contrast, it has been described that after treatments
at high temperatures such as roasting (180ºC), quercetin
glycosides (mainly quercetin-3,4’-O-diglucoside and quercetin4’-O-monoglucoside) are strongly degraded leading to their
deglycosilation and the formation of the quercetin aglycon (19,
22). The quercetin aglycon is heat stable, thus explaining the
slight effect of heat treatment on onions’ total phenolic content
(19). Even so, this is not the case of boiling at 100ºC as it can
significantly decrease onion flavonol content and its capacity
to inhibit lipid peroxidation due to the solubilisation of flavonols in
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Flavours/preservatives
AgroFood industry hi-tech - September/October 2010 - vol 21 n 5
boiling water (24, 25). Since the biodisponibility of the quercetin
aglycon is known to be lower than that of its glycosilated forms,
it is questionable whether cooking treatment can negatively
affect the nutriceutical properties of onion in supplemented
foods (26). However, it seems reasonable to think that it may
enhance the preservative effect of onion by-products as it is
well known that quercetin aglycon has higher antioxidant and
antimicrobial capacities due to the free presence of its C-3
hydroxil group (11).
Some recent studies have evaluated the potential effectiveness
of onion or its bioactive compounds as natural preservatives.
It has been described that the addition of onion lyophilized
powder can be effective in delaying the lipid oxidation in oilin-water emulsions which can be considered a good model
of food emulsion, such as mayonnaise and margarine (16).
Special attention has been focused on the utility of onion as a
natural ingredient to delay the deterioration of meat products
as it is well known that the preservation of its palatability, taste,
colour and flavour without adding synthetic preservatives is
highly demanded by customers. Some authors have reported
that the addition of onion powder to pork or beef refrigerated
meat, delayed the formation of primary and secondary
oxidation products (27) and inhibited the growth of food
spoiling microorganisms, such as Enterobacteriaceae (12, 27). In
addition, the effect of juice extracts from onions on lipid oxidation
and sensory quality in refrigerated turkey meat contributed
to improve the sensory quality of the products, and provided
protection against oxidation during cooking and refrigerated
storage (28). Furthermore, onions have been described as
being a good additive to decrease the formation of nitrogen
heterocycles, such as polycyclic aromatic hydrocarbons and
azaarenes, which can be formed during cooking practices and
are especially problematic as it has been suggested that they
can act as human carcinogens (29).
Finally, onion bioactive compounds can also be useful antibrowning agents preventing other vegetables and fruits from the
deterioration associated to the post-harvest process (17). As a
matter of fact, it is possible to incorporate onion oleoresins to edible
films in order to reduce both peroxidise and polyphenoloxydase
activities in minimally processed vegetables (30).
On balance, onion can be considered a good food preservative,
especially in fresh or minimally processed foods. The food
industry has traditionally used physical (i.e. dehydratation or
heat treatment), chemical (i.e. antimicrobials or antioxidants)
and biological (i.e. fermentation) technologies in order to avoid
food deterioration. Nowadays these technologies are still valid,
although the combination of different treatments, rather than
the application of a single intensive treatment, is considered
more appropriate to conserve the nutritional integrity of food
and its organoleptic properties. Consequently, onion can play
a crucial role when conveniently combined with other food
preservation technologies.
CONCLUSION
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.
46
Therefore, although artificial additives can be more effective
as food preservatives, it can be concluded that there is a
real possibility of using onions as more ‘natural’ and safer
preservatives. Onions can be a good ingredient not only
to enhance the organoleptic properties of food but also to
increase its nutriceutical properties, as well as its shelf-life by
reducing its oxidative and microbial deterioration.
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