The aim of this paper is to discuss on the roles of plant extract in the synthesis of metallic nanoparticles. Synthesis of metallic nanoparticles has started few decades ago through physical and chemical methods. Recently, green technology through biosynthesis method has drawn great attention compared to the physical and chemical method. Biosynthesis was found to be more energy efficient and able to eliminate the use of hazardous chemicals. The biosynthesis studies involved the application of fungi, bacteria, yeast, algae and plant extract. Plant extract has several advantages since the use of microorganism required stringent control on cell culture. Furthermore, the reaction rate is much faster as compared to that of the microorganism methods. The important compounds in the plant extract are hydroxyl and carbonyl groups. Both functional groups allowed plant extract to act as reducing agent as well as stabilizing agent. Several studies have been carried out to optimize the extraction of these compounds such as plant drying technique, extraction temperature and type of extractions solvent. The common method used to quantify the concentration of reducing agents in the extract is through Folin-Ciocalteu method. Utilization of plant extract not only capable of producing well dispersed monometallic nanoparticles, but also bimetallic nanoparticles. Previous studies revealed that concentration of plant extract has significant effect on particle size and shape as well as particle distribution.

Chemical, enzymatic or microbial activities from the surrounding environment and the food itself can cause spoilage to food products. In the meantime, the recent surge in world population, calls forfood products to be stored and delivered from one place to another place. During delivery, food products will start to deteriorate, losetheir appearance and decrease in nutritional values. Thus, the presence of food preservation methods such as heating, pickling, edible coating, drying, freezing and high-pressure processing can solve this problem by extending the food products" shelf life, stabilize their quality, maintaining their appearance and their taste. There are two categories of food preservations, the modern technology preservation method and the conventional preservation method. In the meantime, conventional food preservations usually use natural food preservatives. Meanwhile, the use of the synthetic preservative such as sulphites, benzoates, sorbates etc. for food preservation can cause certain health problems. In this light, replacing these synthetic preservatives with natural preservatives such as salt, vinegar, honey, etc. are much safer for human and environment. Furthermore, natural preservatives are easy to obtain since the sources are from plant, animal and microbes origin. This review paper focuses on preservation methodsand the natural preservatives that are suitable to be used for food preservation.

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