Development of a Firmness Tester Designed for Large Watermelon

Watermelon flesh firmness is expected to be a minimum of 12 N at harvest when measured in the locule area in order to withstand shipment and reduce drip loss. Firm fleshed watermelon cultivars are now available and grafting helps further enhance firmness. Measurement of firmness is usually done with a hand held gauge but this requires the same person to take all readings, and even then has limited accuracy. Stationary devices engaging a mechanized lever for readings have had demonstrated improvement in accuracy and better precision among those doing the testing. One of the other challenges in testing watermelon firmness is the size of the fruit. Most commercial firmness testers have plates designed for much smaller fruit. And, since most watermelon testing is done under field or packing shed conditions, a firmness tester has to be robust enough to withstand juice, dirt, and jostling. We designed a firmness tester using a Dremel stand, a Wagner FDIX force gauge, and a thick cutting board (1.6 cm). The force gauge was mounted on the Dremel stand which was set up as a cross bar mounted on aluminum steel tubing, and a slot was cut into the cutting board to allow for balance of the rounded rind. The tubing was set far enough apart to allow the watermelon to slide between tubing for precise alignment with locular or heart tissue and the lever that comes with the Dremel drill stand was used to lower the gauge. The FDIX force gauge, equipped with a 25 lb module and a 11 mm flat compression probe, is digital and results can be downloaded directly to a computer. This system cost about $1,000 in parts to build and proved to be both operator neutral and less tiring than a handheld gauge. It was tested on seedless watermelon (10 to 15 kg size) grafted to ‘Carnivor’ rootstock and held for 0 to 2 weeks in storage at 13 °C. Firmness of locule tissue was slightly firmer in grafted fruit (16.8 N) than not grafted (12.4N) after 2 weeks, and much firmer in heart tissue (32 N vs 18 N) in grafted and not grafted fruit, respectively. This tester provides a means of easily collecting firmness data with more precision and accuracy and may help differentiate the physiological mechanisms that improve watermelon flesh firmness and rootstock and scion influences.