ABSTRACT
The olive tree, Olea europaea L. ssp. europaea, is among the oldest crops of the Mediterranean basin and one of the best biological indicators of the Mediterranean climate (Blondel et al. 2010). The exact geographic origins, the time and the reasons of its early exploitation are still open to arguments (Breton et al. 2009, Kaniewski et al. 2012, Besnard et al. 2015). The domestication of the olive tree was characterized by selection and propagation of the most valuable trees and establishment of olive orchards (Breton et al. 2009). Based on archaeological remains, it is believed that the olive tree was first domesticated during the early Neolithic period in the Near East (Galili et al. 1989) and it was later introduced into the West of the Mediterranean region via human migrants. Cultivation of the olive tree has been practiced from antiquity to modern times for its wood (utensils, furnishing, manufacturing solid fuel) and for its fruit which is used in olive oil and table olive production (Kaniewski et al. 2012, Riley 2002). The fruit of the olive tree is considered as one of the most extensively cultivated fruit crop in the world. The annual world consumption of table olives has steadily increased during the last two decades, reaching approximately 2,398,300 tons from 2008-9 to 2013-14 crop seasons (IOC 2014). The world’s crop season of table olives production of 2014-15 is estimated at 2,554,500 tonnes, the majority of which comes from countries in the European Union (IOC 2014). Spain is the leader among producer countries followed by Greece and Italy while Turkey, Egypt, Algeria, Argentina, Syria, Morocco, USA and Peru are major non-European producers. Olives are botanically classified as drupes and anatomically consist of three component tissues, namely, the epicarp or skin, the mesocarp or flesh
and the endocarp or pit which encloses one or, rarely, two seeds (GarridoFernández et al. 1997). The epicarp comprises a layer of epidermal cells rich in chloroplasts and is covered by a thin cuticle. The mesocarp, rich in protoplasm, surrounds the endocarp which progressively sclerifies during fruit development (Connor and Fereres 2005). Fruit development, which starts approximately 30 days after fertilization and fruit set, is completed within four to five months and generally involves cell division, cell expansion and storage of metabolites. The primary stage of fruit development is characterized by intense cell division, resulting in rapid growth of the endocarp with little mesocarp. During the middle stage, the drupes are covered by an epicuticular wax layer, the mesocarp cells develop vacuole cells while the endocarp gets completely sclerified with a stop to its enlargement. Then a period of marked fruit growth follows due to expansion of the pre-existing mesocarp cells. At the same time, intense oil synthesis and accumulation in the mesocarp is observed which continues at a slower rate until the maturation/ripening phase. Upon maturation, the color of the drupes changes from lime green to purple-black and the texture of the flesh becomes softer and easier to squash until some juice is released. (Connor and Fereres 2005, Conde et al. 2008). Depending on the color of the fruit upon harvest, they are categorized as (i) green olives, harvested at the early stage of maturity prior to coloring and on having obtained the appropriate size, (ii) black olives, harvested at the full stage of maturity or slightly earlier on having attained deep violet black color, and (iii) turning color olives, harvested between the two stages presenting a wine-rose color (Garrido-Fernández et al. 1997, IOC 2004). Nutrient components are present at the highest percentage in the mesocarp. They are represented by a high level of water and lipids and a low level of sugars and protein. The value of each nutrient may significantly vary depending on the cultivar, degree of maturation and post-harvest treatment (Wodner et al. 1988, Nergiz and Engez 2000, Marsilio et al. 2001a, b, Sakouhi et al. 2008, López-López et al. 2009, Lanza et al. 2013). Upon harvest, the lipid content is dominated by oleic acid followed by palmitic acid, linoleic acid and stearic acid. Carbohydrates in olive drupes are represented mainly by soluble-reducing sugars, such as glucose and fructose and non-reducing sugars, such as mannitol. Their concentration is lower in comparison to any other drupes since they act as precursors for fatty acids synthesis during fruit growth (Wodner et al. 1988). The sugar content that remains serves as a carbon source for development of the desired microorganisms during table olive production (Garrido-Fernández et al. 1997). Complex sugars, such as lignin, hemicellulose, cellulose and pectin are distributed in the olive fruit. Lignin is present in the stone while the rest of the polymers are present in the mesocarp, playing a substantial role in the structural characteristics of the olive flesh (Kailis and Harris 2007, Lanza et al. 2010). The protein content
is low but of high quality due to the presence of essential amino acids for adults (threonine, valine, leucine, isoleucine, phenylalanine and lysine), and (arginine, histidine and tyrosine) for children (Lanza et al. 2010, 2013). Phenolics are also present with oleuropein, a secoiridoid glucoside, being the representative phenolic compound responsible for the bitter taste of the fruit. Its concentration decreases during ripening (Amiot et al. 1986), giving rise primarly to hydroxytyrosol and other simple phenolics (Bianchi 2003), like tyrosol, homovanillic alcohol, caffeic acid, coumaric acid, phloretic acid and vanillic acid (Boskou et al. 2006) contributing also to sensory and aromatic characteristics of the olive while imparting pharmaceutical and physiological benefits (Tassou 1993, Kountouri et al. 2007, Omar 2010, Ghanbari et al. 2012).
