**2. The challenge**

It is clear that growers, technologists and scientists must join in a partnership developing and screening cultivars suitable for mechanical harvesting, either by overhead harvester or trunk shaker. Developing varieties with harder or thicker pills may

help in achieving this goal. Another approach would be to investigate the mechanism of fruit detachment in olives, and develop a selective treatment which is commercially practical, and does not cause leaf abscission. A possible reason for the long survival of traditional harvest methods for table olives may be the low efficiency of the detachment mechanism in olive fruits. Multiple analyses carried out by the authors show that mature olive fruits do not produce or release ethylene to any detectable level. It seems therefore, that the fruit detachment mechanism in olives is not regulated by ethylene release, but by a different mechanism.

The desired characteristics of olives fit for table use include larger fruit size, firmness of the flesh of the fruit, resistance to disease, high flesh-to-seed ratio; skin thickness desirable for eating, resistance of epicarp to cracks and bruising and the ability to maintain intact appearance.

One of the main challenges facing growers of table olives interested in converting to mechanical harvesting, is reducing damage to the fruit caused by the harvesting process. This may be achieved by combining the two parameters: (1) enhancing pill durability by increasing thickness of the pill [19]. As the pill thickens, the fruit is more resistant to damage. However, this must be balanced against sensory sensitivity to the feel of the olive in the mouth (2) Reduction of the attachment force of the fruit to the branch, without increasing leaf abscission.

In developing strategies to reduce bruising of the olives at harvest, it is important to understand major factors influencing bruise susceptibility of fresh fruits. Excessive compression forces and a series of impacts during harvesting can cause severe bruise damage. In addition to mechanical forces applied to the fruit and the tree, the stage of fruit maturity also affects bruising. (The susceptibility to bruising depends partly on physiological and biochemical variables. Environmental factors such as temperature, humidity and post-harvest treatments may play a role in susceptibility to fruit damage [20]. Thus, mechanical harvesting must be performed properly in order to reduce both frequency and severity of bruising and increase the resistance of fresh fruit to bruise damage.

As olive varieties differ in many qualities such as heat and disease resistance, fruit size, yield and many more traits [10, 21–23], they differ also in their resistance to shock and bruising. Jiménez et al. [24] observed significant differences among 14 selected genotypes in sensitivity to bruising. Histological sections of bruised and unaffected fruit tissues revealed a subsurface zone of tissue discoloration but a much greater area in which cell structure was disrupted. To further assess the susceptibility of different table olives varieties to bruising, Jiménez et al. [25] studied damages incurred by two different cultivars–'Manzanillo de Sevilla' and 'Hojiblanca', at 4 and 24 h after impact induced bruising. The predominant post bruising changes they noted in the mesocarp included ruptured cells, cell wall thinning, and discoloration. These changes appeared greater in 'Manzanillo de Sevilla' than 'Hojiblanca' and were more evident 24 h after the impact. This verifies the assumption that different table olive cultivars may demonstrate differential resistance to bruising. Furthermore, the authors noted several factors that may serve as parameters defining the level of damage: total damaged area, the number of tissue ruptures in the mesocarp, and the distance from the fruit exterior to the nearest tissue rupture were different in the two cultivars. These factors were recommended by the authors as useful parameters characterizing susceptibility to bruising among table olives.

Another criterion useful in assessing bruised fruit is the proportional area of brown coloring after injury, compared to the total fruit surface area. Using the knowledge that mesocarp cells are damaged when pressure is applied, Goldental-Cohen

#### *Table Olives: Toward Mechanical Harvesting DOI: http://dx.doi.org/10.5772/intechopen.102700*

et al. [19] screened 106 olive cultivars of the Israeli germplasm collection managed at the Volcani Institute for sensitivity to browning in response to injury. Using the above criterion, they showed that post-bruise browning may vary from 0 to 83.61%. Fourteen genetically different cultivars did not brown 3 h after application of pressure. Among them, some cultivars may be selected as suitable table olives. Cultivars resistant to browning were found to have thicker cuticles than those of susceptible varieties, thus cuticle thickness may very well be an important parameter in selecting table olive cultivars suitable for mechanical harvest. A shift to browning-resistant cultivars in place of the popular cultivars currently in use will encourage mechanical harvest of table olive without affecting fruit quality.

Overcoming the bruising caused by mechanical harvest may be aided by adopting the proper irrigation regime. Casanova et al. [26] showed that fruits under regulated deficit irrigation were less susceptible to bruising than fruits fully irrigated. Fruits of 'Manzanillo de Sevilla' that were subjected to regulated deficit irrigation and full irrigation were bruise-induced by a standardized mechanical blow to evaluate bruising susceptibility. Damage was evaluated 3 h after treatment. Fruits under restricted water regimes in the weeks before harvest were much less susceptive to bruising [27]. Thus, controlling irrigation may overcome bruising effects in existing orchards, even for more susceptible varieties.

The low efficiency of hand harvesting of table olives and the rise in the costs of labor prioritizes the need for mechanical harvesting. To achieve mechanical harvesting, the attachment force of fruit to the branch should be lower than 200 gr. Today, most farmers use a pre-harvest abscission compound in order to decrease the fruit detachment force to a level less than 200 gr before mechanically harvesting their oil producing groves. However, table olives are harvested before ripening. At this early stage, the detachment force of the fruits is still very high and does not differ significantly from the detachment force of the leaves. A selective abscission compound is crucial for adapting table olive groves to mechanical harvesting. Goldental-Cohen et al. [28] studied the anatomical and molecular differences between the fruit and leaf abscission zones in olive. Typically, the abscission zone is characterized by small cells with less pectin compared to neighboring cells. This type of cell is found in the leaf but not in the fruit abscission zone. The fruit abscission zone 3 (FAZ3), located between the fruit and the pedicel, was found to be the active AZ in mature fruits. In an attempt to differentiate between fruit and leaf AZs, olive-bearing trees were treated with ethephon, an ethylene-releasing compound, and the effect of this treatment on the detachment force of fruits and leaves 5 days after its application was determined. Transcriptomic analysis of the various abscission zones revealed induction of genes involved in oxidation stress specifically in the leaf abscission zone. They found that adding antioxidants such as ascorbic acid or butyric acid to the ethephon inhibited leaf abscission but enhanced fruit abscission. Treating olive-bearing trees with a combination of ethephon and antioxidants reduces the detachment force of fruit without weakening that of the leaves. Hence, this selective abscission treatment may be used to promote mechanized harvest of olives [28].

Another way to reduce the consequences of bruising by mechanical harvesting is by application of post-harvest treatments. Zipori et al. [29] studied postharvest field treatments on 'Manzanillo' olives, the main table olive cultivar in Israel. This variety is highly sensitive to bruising and other damage caused by mechanical harvesting. Immersing the fruit in a 1% NaOH solution immediately after harvest seems to be the most effective treatment among those studied. This treatment reduced the percentage of severely bruised fruit to reasonable values. We suggest that this treatment, together with other

advances in the field, such as improved shakers and\or use of abscission agents, will enable cost-effective mechanical harvesting of 'Manzanillo' table olives [29].
