An Objective Method to Measure the Peelability of Citrus Fruit

Ease of peeling is an important trait to consider in the development of new citrus cultivars. To objectively measure this trait, we used a Texture Analyzer (Stable Micro Systems, model TA-XT2, Godalming, England) equipped with a TA-265A platform and pulley system, and attached the cable to a cut section of the fruit peel. The analyzer was set to measure tension. A cutting system was developed by bonding together two adjustable utility knives (Husky SP304-HD) so that the hook blades were 16 mm apart. The cutting depth of the blades was adjusted depending on peel thicknesses so that the blades cut mostly the peel. The fruit peel was cut into strips of varying length depending on the part of the fruit being evaluated. Early experiments evaluated strips of peel from the equator to the stem-end or blossom-end of the fruit, or around the equator of the fruit. No consistent differences were found between these three regions. Therefore, subsequent tests measured peelability around the fruit equator. For this, a continuous strip was cut around the equator, with cross cuts on opposite sides of the fruit, and up to 30 mm of the peel pulled away from the segments to allow attachment of the cable clamp. The fruit itself was held using an adjustable clamp taken from a Homeland Goods Orange Citrus Peeler that allowed the fruit to rotate as the peel was pulled. The Texture Analyzer was set to begin data collection after cable slack was taken up (100 g trigger force) and then tension force measured for the next 75 mm, or until the peel broke. A computer macro was used to report the length of peel before breakage (if it occurred), average tension force (strength of peel adherence to the segments), peak force, and area under the curve. The macro detected if and when the peel broke and automatically excluded data after peel breakage. Tests comparing grapefruit and orange found, as expected, that ‘Valencia’ oranges required greater force (both peak and average) for peel removal, and that the peel would break after shorter distances than the grapefruit. Additional tests with ‘Marsh’ grapefruit, ‘Murcott’ tangerines, and navel oranges found that colder (5 °C) fruit required greater force for peel removal and resulted in easier peel breakage than warmer (22 or 35 °C) fruit. In these tests, ‘Murcott’ tangerines peeled easiest, followed by navel oranges, and then ‘Marsh’ grapefruit.