California’s table olive industry is built upon the cultivar Manzanillo that is harvested green, before full maturity. It becomes the California black ripe table olive through oxidation from green to black in processing. Increasingly, California table olive production, with its current combination of rising hand harvest costs and stagnant per ton prices, is not economically sustainable (Ferguson et al., 2010). The industry is currently slightly under 22,000 acres and declining (Warnert, 2011). Hand harvest costs are volatile, often exceeding 50% to 75% of gross return, crop value has not increased in tandem with harvest labor costs, and competition for the aging and less-skilled hand labor pool has increased (California Farm Bureau Federation, 2012). As with most of California’s horticultural crops, if the table olive industry continues to rely on hand harvesting, it is only a matter of time before table olives are no longer produced in California (Sarig, 2012; Sarig et al., 2000). The only realistic solution is to develop economically feasible mechanical harvesting. Mechanical harvesting has three components. The first is a harvesting technology able to remove and catch the fruit without economic damage to the crop or tree. The second is a grove trained and pruned to facilitate harvesting. The third is a “fruit loosener,” a chemical that can facilitate harvesting by accelerating the normal ripening process of fruit abscission.
‘Manzanillo’, the major California black ripe processed table olive cultivar, poses specific impediments to mechanical harvesting. Most traditional California table olive trees are large, generally over 18 ft tall with 12- to 18-ft-wide canopies (Fig. 1). The fruit is borne on the flexible pendulous shoots of the previous season’s growth of in a 2- to 3-ft shell over the entire canopy. ‘Manzanillo’ olives destined for black ripe processing are harvested physiologically immature, before the abscission zone has initiated the decrease in removal force that accompanies fruit maturity. The immature olives seldom have a detachment force less than 2 N and usually as high as 10 N (Ferguson et al., 2010). Additionally, the olives are small, weigh less than 0.5 oz, and bruise easily. However, ‘Manzanillo’ olives used for processing as California black ripe table olives have one advantage for mechanical harvesting. In postharvest processing, they are oxidized from green to black, which masks superficial bruises. As Zipori et al. (2014) recently demonstrated, mechanically harvested ‘Manzanillo’ olives could not be successfully processed as marketable green olives.
Developing mechanical harvesting for any tree crop follows the same experimental sequence. First a successful fruit removal method is identified and developed, or more usually adapted from other crops. Second, the harvesting technology is tested to ensure it produces fruit that can be successfully marketed and does not inflict lasting damage to the tree. These first two steps can be done within university research programs. The third step consists of simultaneously modifying the prototype harvester and the existing groves, then evaluating the combination for efficiency and operating parameters of ground speed, tons, and acres per hour. University research programs seldom produce more than a prototype, often without a mobile platform and catch frame. Incorporation of these harvester components, and subsequent testing, is generally done in cooperation with commercial fabricators. In this review, the summarized data presented for the trunk-shaking harvester was generated in a commercial trial, with a commercially available harvester and the research is now completed. The data summarized for the canopy contact harvester was generated with a prototype in randomized, replicated trials and is continuing.
Initially, we simultaneously investigated all three components of mechanical harvesting, harvest method, grove adaptation, and identifying a loosening agent. Good progress has been made with the first two components, the harvester and the groves, but investigations of abscission compounds have ceased. Two harvesting technologies and their weak and strong points have been identified and modified. A traditional grove adapted for mechanical harvesting with mechanical pruning and a newly established mechanically harvestable hedgerow grove have been evaluated. The initial experiments on abscission compounds, consistent with the later more focused work of Zipori et al. (2014), demonstrated the best potential candidates, ethylene-generating compounds, produced inconsistent results (Burns et al., 2008). Therefore, all subsequent research has focused on improving the mechanical harvester through engineering and on adapting current groves with mechanical pruning and developing new groves.
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