## 3 Significance of Volatile Organic Compounds in Honey
### Biomedical Activities of Honey and Contribution from Its Volatile Constituents  
Honey has been used in many cultures since ancient times as an effective remedy due to its therapeutic properties including anti-oxidant, anti-inflammatory, antimicrobial and immune modulatory effects [5]. Its antimicrobial activity has been attributed to four salient factors namely, hydrogen peroxide content, high osmotic pressure (low water activity), acidity (pH) and non-peroxide factors (phytochemicals). Other factors include low redox potential, low protein content (a high carbon to nitrogen ratio), bee defensin-1, viscosity (opposes convection currents and limits dissolved oxygen) and induction of increased lymphocyte and phagocytic activity [48,49]. In addition, honey is a saturated or supersaturated solution of carbohydrates of which glucose and fructose are the most abundant (84%) [50], apparently, the osmotic effect of honey is elicited owing to the fact that the strong interaction of the sugars with water molecules leaves very little or no water to support the growth of micro-organisms (bacteria and fungi since they thrive in moist environment); consequently, they become dehydrated and finally die [51]. More to that, honey is characteristically acidic with gluconic acid as the most prevalent acid that emanates from the oxidation of glucose by glucose oxidase in the presence of water and oxygen as well as from bacteria of the genus _Gluconobacter_, occasionally isolated from ripening nectar [52]. Its pH value lies between 3.2 and 4.5 due to the presence of organic acids such as gluconic, formic, acetic, propionic, hexadecanoic acid and is low enough to inhibit the growth of most pathogenic organisms whose optimum growth pH normally falls between 7.2 and 7.4 [53]. Of all the factors, the major contributor to antimicrobial activity is hydrogen peroxide; it is produced from the oxidation of glucose by glucose oxidase upon dilution of honey [54]. The slow release of this compound is mild and does not damage tissues but has antimicrobial activity due to the formation of free radicals such as hydroxyl and superoxide. Its concentration is determined by the relative levels of glucose oxidase synthesized by the bee and catalase originating from flower pollen [55]. More elaborately, major variation observed in the overall antibacterial activity between honey types is due to variation in the level of hydrogen peroxide that arises in honey and, in some cases, to the level of non-peroxide factors. However, hydrogen peroxide can be destroyed by heat or light or by components of honey; it can be degraded by reaction with ascorbic acid and metal ions and the action of enzyme catalase which comes from the pollen and nectar of certain plants [54]. In such circumstances, the antimicrobial activity of honey could be lessened or eliminated. Despite the negative implication of catalase, light and heat on the peroxide generating system, some honeys still retain their antimicrobial activity. They are known as non-peroxide honeys, e.g., Jelly bush (_L. polygalifolium_) and Manuka (_L. scoparium_) possessing non-peroxide antibacterial factors (phytochemicals). Phytochemicals are secondary metabolites that confer color, flavor and defense against infections to plants; they are endowed with antimicrobial and anti-oxidant potential [56]. Plants biosynthesize a great number of various phytochemicals, which may possess health-promoting properties, antioxidants being the major group of bioactive constituents; they are considered as natural agents, which might reduce the risk of oxidative damage in living cells. These bioactive may be transferred from plants to nectar and further to honey [57]. In honey, they can be grouped into carbohydrates, phenolic compounds (flavonoids and non-flavonoid phenolic compounds) and volatiles [42]. Accordingly, raw honey contains copiousamounts of these compounds; however, their contribution in antibacterial activity is small compared to the contribution from hydrogen peroxide [55].