Sweet cherry (Prunus avium L.) is one of the high-value fruit crops extensively planted in the U.S. Pacific Northwest region. Currently, fresh market sweet cherries are harvested manually by seasonal workforce. Because cherry fruit is small and scattered sparsely in tree canopies, manual harvesting is highly labor intensive and costly. As the availability of skilled labor is declining and the cost of labor is rising, researchers and growers have been seeking mechanical solutions for sweet cherry harvesting.
Vibration or shaking is one of the widely used methods for mechanical harvesting of tree fruit crops. To obtain the fundamental information for mechanical harvesting with vibration, researchers analyzed limb stiffness (Erdoğan et al., 2003; Lenker and Hedden, 1968), and dynamic response of the tree under the vibration (Savary et al., 2010; Upadhyaya et al., 1981). Researchers also worked on identifying key factors for improving fruit removal rate and reducing harvest-induced damage (Halderson, 1966). Mechanical harvesting with vibration has been applied to different kinds of crops including citrus, pistachio nuts, and apricots (Erdoğan et al., 2003; Polat et al., 2006; Whitney, 1997).
Research on mechanical shaking method for harvesting sweet cherries dates back to the 1960s. Norton et al. (1962) used a hydraulic trunk shaker to harvest sweet cherries, and found that the harvesting methods removed 80–90% of the fruit. Markwardt et al. (1964) performed tests using a limb shaker in cherry harvesting, and found that the amount of energy transferred from the excitation point to the tree was dependent on branch diameter, branch length, and the point of shaker attachment. Peterson and Wolford (2001) developed a mechanical harvester using a rapid displacement actuator to apply impact force to the main scaffold to remove sweet cherries from trees. Despite the continuous research and development efforts over the past six decades, no mechanical harvester is commercially available for fresh market sweet cherry harvesting, largely because of excessive fruit damage induced during fruit detachment, falling through the tree canopy and impact on the catching surface (Peterson et al., 2003). Recently, Washington State University (WSU) researchers studied various aspects of the mechanical harvesting technology suitable for harvesting fresh market sweet cherries. These studies have resulted in optimal shaking frequencies and durations for designing an effective mechanical harvester with better fruit quality (Du et al., 2013; He et al., 2013; Zhou et al., 2013).
Fruit maturity is one of the key factors determining overall fruit quality. Generally, fruit maturity is not uniform either within individual trees or across an orchard. As a result, fruit harvested with a single-pass operation may include fruit with different maturity levels ranging from immature to over-mature (Horsfield et al., 1972). The lack of uniformity in fruit maturity levels with single-pass harvesting was identified as one of the most important factors affecting fruit quality (O’Brien et al., 1978). Research has been done to study the possibility of using a mechanical shaker to harvest fruit selectively by the fruit maturity level, and found that more mature fruit would have a lower retention force and could be released relatively easily (Claypool et al., 1968; Richardson et al., 1998). Sweet cherry cultivars exhibit natural variability in the detaching force, also referred to as pedicel–fruit retention force (PFRF), between pedicel and fruit ranging from 4.5 to 9.0 N (Smith and Whiting, 2010). Past studies show that shaking trees for a longer duration to detach immature fruit might induce more fruit damage because of the increased occurrence of fruit-to-fruit and/or fruit-to-tree impacts (Pellerin et al., 1979). Selective harvesting to remove only mature fruit offers a potential to improve the uniformity of fruit maturity level and reduce harvest-induced fruit damage.
In conventional sweet cherry harvesting, generally, pickers go through trees only once to pick all fruit. Therefore, almost all previous mechanical harvesting research was based on single-pass harvesting. As discussed earlier, multipass harvesting methods could offer solutions to improve the quality of harvested fruit through improving uniformity in fruit maturity. In this study, we focused on the evaluation of both the functionalities and performances of a multipass mechanical harvesting system for sweet cherries, and are more focused on functionality. Specific objectives of this study were to 1) evaluate and compare the overall fruit removal efficiencies with multipass harvesting and single-pass harvesting methods; and 2) evaluate fruit maturity level and harvest-induced fruit damage rate with the multipass harvesting method and compare those achieved with single-pass harvesting method. To further validate the system, additional work should be carried out with more expanded experiments including wider variability in plant structures as well as different varieties and geographic locations.
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