Improving quality maintenance of fresh-cut watermelons

Fresh-cut fruit products value-add convenience for consumers. This research was to identify significant factors that impact the quality maintenance of fresh-cut watermelon and suggest processes that can be implemented by the industry to address them. The effects of both processing and storage factors on fresh-cut watermelon quality and shelf-life were therefore investigated in both laboratory and industrial conditions in collaboration with industrial partners. Experiments were conducted on harvest factors that determine whole watermelon fitness for fresh-cuts, effects of commercial operating cutting conditions on quality maintenance responses and impacts of ethylene from harvest through to storage. The significance of processing factors on fresh-cut watermelon quality maintenance was evaluated with two screening trials with four processing factors in each. Trial 1 utilised Plackett-Burman Screening Design and Trial 2 used a Definitive Screening Design. Better fresh-cut quality over storage was associated with 0.5 hr holding time, 200 µL/L free chlorine pre-cut sanitation of whole watermelons, 0 µL/L free chlorine post-cut sanitation (no sanitiser application), 3 cm cut size, 0.3 M calcium ascorbate and misting of both post-cut sanitiser and antioxidants. Further trials indicated the importance of pre- and post-cut storage temperatures and methods of sanitiser application. The exposure of watermelons to ethylene was examined by assessing the concentration of ambient ethylene at various points along the supply chain. These were from the air at the farm, processing area, distribution centres, and supermarket stores as well as from the headspaces above packaged fresh-cut product. Ethylene concentrations in distribution centres and in the fresh-cut headspace were at levels (4 µL/L to 10 µL/L) associated with accelerated degradation of whole watermelon quality such as firmness, appearance, and development of watersoaking symptoms. The exposure of fresh-cut watermelon to ethylene was investigated in the laboratory. The effects of four ethylene concentrations (< 0.05) in only two batches. Similarly, respiration, ethylene production rates, ion leakage and watersoaking were significantly affected by ethylene concentration in two of the six batches. There was no evidence for firmness, and soluble solid content being affected by ethylene concentration. In contrast, combined data from the six batches identified undesirable changes in hue angle and lightness in fresh-cut watermelons flushed with the lowest ethylene concentration (<0.001 µL/L) (p=0.025 for hue and p=0.32 for lightness). The potential effects of controlling ethylene exposure from the point of watermelon harvest were tested. Treatment with ethylene removal sachets was applied at harvest into the bulk packs of commercially grown whole watermelons along with a no-treatment control. The watermelons were then transported 3,200 kilometres by a chain of different transport companies from Ali Curung, Northern Territory to Western Junction, Tasmania, Australia via Adelaide to Melbourne. In the journey from Adelaide to Melbourne Markets, involving 700 km and 8 hours, two transport temperature treatments (refrigerated and non-refrigerated) were used. Ethylene removal sachets did not significantly affect appearance, aroma, and development of watersoaking of whole watermelons. However, the mean values for watersoaking index were lower for whole watermelons with the sachets than untreated controls after being stored for 6 days in the distribution centre (13 °C, ethylene ~4 µL/L). Lowering transport temperatures earlier in the supply chain better maintained whole watermelons appearance and slowed watersoaking development. New post processing means to preserve watermelon quality were trialled. The potential of four novel gas treatments (carbon dioxide, hydrogen, potassium permanganate-based ethylene removal sachets, hydrogen sulfide) were evaluated on fresh-cut watermelon commercial quality parameters under different treatment times (5-hour shock and continuous) and concentrations (varied to be appropriate to gas type). The 5-hour shock treatments of permanganate-based ethylene removal sachets enabled better maintenance of fresh-cut watermelon appearance and reduced watersoaking symptoms. The 4.2 µL/L hydrogen gas substantially improved maintenance of firmness, appearance, watersoaking symptoms, SSC and respiration rates but not juice leakage. With oxygen kept above 13 % v/v over storage, modified atmosphere with 7 % v/v CO\\(_2\\) improved maintenance of juice leakage, appearance, aroma, ion leakage, respiration rates. Firmness, pH, SSC, and conductivity were unaffected by 7 % v/v CO\\(_2\\). The research has therefore indicated that development of better fresh-cut watermelon quality systems will rely on managing multiple factors along the supply chain. This includes managing ethylene exposure and temperature from harvest, as well as improvement of processing and storage factors. Novel gas treatments, beyond existing MAP, also showed potential for enabling better fresh-cut watermelon quality maintenance.