Flower Extracts and Their Essential Oils as Potential Antimicrobial Agents for Food Uses and Pharmaceutical Applications Han Ching Voon, Rajeev Bhat, and Gulam Rusul Abstract: Plants with potential therapeutic value have been used from time immemorial to cure various ailments and infectious diseases. Secondary metabolites or the bioactive compounds (phytochemicals) present in plants have been reported to be accountable for various observed biological activities. Consumer awareness of the possible side effects of using chemical-based antimicrobial agents has forced researchers to identify and explore natural plant-based antimicrobial agents (or preservatives) that are toxicologically safe, especially when used in food applications. Of late, scientific evidence has been provided on the potential antimicrobial activities exhibited by certain traditionally used flower extracts or their essential oils (edible and wild). This review focuses on providing and updating available information on the antimicrobial activities exhibited by flowers, which are envisaged to find potential applications as natural preservatives for foods or applications in the pharmaceutical industries to develop new and economical herbal-based products for treating various diseases. Introduction procedures has been attributed mainly to the presence of active Infectious diseases and foodborne illnesses can cause severe phytochemicals or bioactive compounds in plants (Quarenghi and health effects and can even lead to death among the residing others 2000; Ye and others 2004; Zhang and Zhang 2007; Dung population, especially in the developing regions of the world. and others 2008; Zhao and others 2009). The continual emergence of antibiotic-resistant microorganisms Given the scope of searching new antimicrobial agents, antimi- has prompted researchers’ world over to search for new antimicro- crobials derived from plant materials are often regarded as natural bial agents that are more effective against the resistant microbial and safe compared to industrial chemicals. Of late, plant-based pathogens (Nascimento and others 2000; Thaller and others 2010). medicine has become more popular due to the increasing concern Structural modification of the antimicrobials (against which mi- of consumers with regard to the use of synthetic chemical prepa- crobial resistance has been developed) is reported to improve the rations and use of artificial antimicrobial preservatives, especially effectiveness of antimicrobial agents against bacteria, fungi, and in modern food protection practices (Marino and others 2001; viruses (De Clercq 2001; Poole 2001; Jeu and others 2003; Zhang Hamedo and Abdelmigid 2009). and others 2010). However, of late, research efforts have been Some of the hoped-for advantages of using natural antimicro- put forth to improve the effectiveness of antimicrobial drugs by bials include: reducing total dependence on antibiotics, reducing developing novel and a new class of antimicrobial drugs that can development of antibiotic resistance by pathogenic microorgan- effectively work on multitargeted sites or organisms (Esterhuizen isms, controlling cross-contaminations by foodborne pathogens, and others 2006; Alka and others 2010). improvizing food preservation technology, and strengthening im- Traditionally, plants with potential therapeutic or medicinal val- mune system in humans (Abou-taleb and Kawai 2008; Fisher ues have been successfully utilized for preventing and treating var- and Phillips 2008; Tajkarimi and others 2010). Today, growing ious ailments and foodborne illnesses. Since time immemorial, market trends indicate a rapid increase in the number of natu- various plants and their products have been used in traditional ral plant-derived products (such as green tea, herbal decoctions, medicine to cure some of the common disorders and degenerative or herbal medicines) that may include aerial parts, seeds, fruits, diseases in humans as well as in animals (such as Ayurvedic and tra- roots, rhizomes, and flowers. Among these, flowers have attained ditional Chinese medicinal practices). The effectiveness of these high priority and found various applications. Floral extracts and their isolated essential oils are traditionally believed to be rich in phytochemicals exhibiting rich bioactivity. These compounds are MS 20110898 Submitted 7/26/2011, Accepted 9/26/2011. Authors are of interest to the local industry as well as to the general pop- with Food Technology Div., School of Industrial Technology, Univ. Sains Malaysia, Penang 11800, Malaysia. Direct inquiries to author Bhat (E-mail: ulation and are actively being explored for various commercial [email protected] and [email protected]). applications (such as tea, bakery products, and more). Floral ex- tracts and essential oils are also considered to be potential natural c 2011 Institute of Food Technologists r 34 ComprehensiveReviewsinFoodScienceandFoodSafety Vol.11,2012 doi: 10.1111/j.1541-4337.2011.00169.x Flowers as potential antimicrobial agents . antimicrobial agents. Available reports indicate their efficacy and at reduced pressure (temperature preferably ≤ 40 ◦C) in a rotary to possess a broad spectrum of antimicrobial activity against vari- evaporator to prevent degradation of heat-sensitive compounds. ous spoilage and pathogenic microorganisms, which is attributed Solvent extractions are classified into 2 methods: continuous to their bioactive constituents (Quarenghi and others 2000; and noncontinuous. In continuous extraction method (such as Ye and others 2004; Zhang and Zhang 2007; Dung and others percolation, soxhlet extraction), solvent flow through the sample 2008; Zhao and others 2009). Based on these facts, the present re- continuously and the saturated solvent is constantly replaced with view focuses mainly on providing baseline information on explor- a less saturated solvent. In noncontinuous method (such as mac- ing some of the common and wild (edible and nonedible) flowers eration, infusion, decoction), the extraction is stopped when a possessing potential antimicrobial activities. The details on these suitable equilibrium is reached between the solute concentration aspects are hopefully expected to be useful for the commercial ex- (inside the flowers and the solvent), unless the solvent needs to be ploitation of flowers to develop natural preservative preparations replaced with a new batch of solvent (Jones and Kinghorn 2005). with applicability in the food and pharmaceutical industries. Percolation. This is an efficient method wherein a percolator is used for extraction. Percolator is comprised of a wide opening Extraction Method (at the top) to accommodate addition or removal of a sample Solvent extraction along with a valve at the bottom, designed to allow outflow of Solvent extraction is one of the most widely employed methods the solvent. With the valve held at a closed position, samples in for preparation of flower extracts. Solvent extraction (solid-liquid powdered form are added and packed into the percolator leaving extraction) involves the process of leaching (simple physical so- sufficient space to allow expansion. Then the samples are covered lution or dissolution process). Leaching is a separation technique by addition of a suitable solvent, and are allowed to soak for few that involves removal of soluble solids from a solid mixture by hours or overnight. Further, the solvent is allowed to flow out at employing a suitable solvent or solvent mixture. Various factors a controlled flow rate from the bottom of the percolator through influence the solvent extraction procedure, which includes: the the valve. Fresh solvent is added at the top to replace the saturated rate of transport of solvent into the material, rate of solubilization solvent “flow-out” from the percolator (Jones and Kinghorn 2005; of soluble constituents in the solvent, and the rate of transport Singh 2008). of solution (extract) out of the insoluble matter. Solvent polarity, Soxhlet extraction. Soxhlet extraction is a common conven- vapor pressure, and viscosity are also of importance for effective tional method used for extracting heat-stable compounds. The extraction. In case of plant materials, adequate time is required for Soxhlet extractor consists of a distillation flask, an extractor, and diffusion of solvent via plant cell walls for dissolution of soluble a condenser. The solvent in the distillation flask is heated and the constituents and for diffusion of the solution (extract) out to the resulting vapor is condensed in the condenser. The condensed sol- surface of the cell wall (Houghton and Raman 1998; Singh 2008; vent from the condenser fills into the thimble-holder containing Wijekoon and others 2011). the sample that needs to be extracted. When the solution in the Flower extracts can be prepared either from fresh or dried sam- extractor reaches the overflow level, a siphon aspirates the solution ples. Prior to extraction, flower samples are subjected to air-drying of the thimble-holder and unloads it back into the distillation flask, or freeze-drying, followed by grinding, milling, or homogeniza- carrying dissolved solute into the bulk liquid. The solute is left in tion to reduce sample particle size. These procedures are followed the distillation flask while the solvent is evaporated, condensed, in order to enhance the efficiency of extraction process and yield of and passed back into the sample solid bed. This process is repeated the resulting extract. Various solvents, such as methanol, ethanol, 3 to 5 times or until