Impact of a brief postharvest hot water drench treatment on decay, fruit appearance, and microbe populations of California lemons and oranges

A brief (15 or 30 seconds) high-volume, low-pressure, hot-water drench at 68, 120, 130, 140,or 145 degreesF (20.0, 48.9, 54.4, 60.0, or 62.8 degreesC) was applied over rotating brushes to 'Eureka' lemons (Citrus limon) and 'Valencia' oranges (Citrus sinensis). The impact of this treatment on populations of surface microbes, injury to the fruit, the incidence of green mold (Penicillium digitatum)or sour rot (Geotrichum citri-aurantii), when inoculated into wounds one day prior to treatment, and temperatures required to kill the spores of these fungi and P. italicum suspended in hot water were determined. Fruit microbial populations were determined immediately after treatment. Decay and injuries were assessed after storage for 3 weeks at 55 degreesF (12.8 degreesC). The efficacy of the hot water treatments was compared to immersion of fruit in 3% wt/vol sodium carbonate at 95 degreesF (35.0 degreesC) for 30 seconds, a common commercial practice in California. Initial yeast and mold populations, initially log(10) 6.0 per fruit, were reduced to log(10) 3.3 on lemons and log(10) 4.2 on oranges by a 15-second treatment at 145 degreesF. Green mold control improved with increasing temperature and treatment duration. Green mold incidence was reduced from 97.9% and 98.0% on untreated lemons and oranges, respectively, to 14.5% and 9.4% by 30 seconds treatment with 145 degreesF water. However, immersion of lemons or oranges in 3% wt/vol sodium carbonate was superior and reduced green mold to 8.0% and 8.9%, respectively. Sour rot incidence on lemons averaged 84.3% after all water treatments, and was not significantly reduced, although arthrospores of G. citri-aurantii died at lower water temperatures than spores of P. digitatum and P. italicum in in vitro tests. Sodium carbonate treatment for 30 seconds at 95 degreesF reduced sour rot to 36.7%. None of the treatments caused visible injuries to the fruit.