Assessment of ethanol, honey, milk and essential oils as potential postharvest treatments of New Zealand grown fruit.
Brown and Penicillium rot (blue and green mould) are the most common postharvest diseases in New Zealand, causing significant postharvest fruit losses. Current practice uses fungicides to control the postharvest diseases; however there are perceived health risks associated with the use of such chemicals. Recently, there has been substantial interest in chemicals that are considered Generally Regarded as Safe or GRAS and natural products as alternative postharvest treatments to replace currently used fungicides. In this study, ethanol (GRAS chemical) and the natural products honey, milk and essential oils (lemon, lemongrass, manuka and orange) were assessed as potential alternative treatments to replace the currently used fungicides on both peaches and oranges. In pilot studies ethanol was applied to the fruit by either vapour or dipping (30 seconds or 1.5 minutes). Honey, milk and essential oils were applied by dipping at 30 seconds. Essential oils were also tested using a microtiter assay. Exposing fruit to ethanol vapour proved effective at inhibiting fungal growth, but impacted negatively on fruit quality. Peaches that were exposed to 70% to 100% ethanol vapour were protected against fungal infection for up to 30 days when stored at either 4ºC or room temperature. This is compared to two days for untreated peaches and three days for fungicide -treated peaches. However, the ethanol-treated peaches suffered from severe browning. In contrast, 20% ethanol protected peaches for ten days when stored at 4ºC and two days at room temperature. The fruit that were exposed to 20% ethanol did not brown as a result of the treatment. Oranges that were exposed to 20%, 50%, 70% and 100% ethanol vapour were protected from fungal inhibition for 30 days at both 4ºC and room temperature, but they too suffered from severe browning. Dipping was not as effective as vapour at protecting against fungal infection, but had a little effect on fruit quality. Peaches dipped in 20% to 100% ethanol were completely rotten by ten days when stored at room temperature, but the peaches experienced little to no browning. Untreated and fungicide-treated fruit were protected for one day and two days, respectively. Milk and honey do not appear to have potential as postharvest treatments. Peaches that were treated with 20%, 50% and 100% whole milk and 50% manuka honey showed greater degree of fungal infection compared to untreated peaches after both room temperature and 4ºC storage. At room temperature, peaches that were exposed to 20%, 50% and 100% milk were completely rotten at eight days, compared with ten days for untreated peaches. In contrast, at 4ºC, peaches that were treated with 100% milk were completely rotten at 30 days, while only a slight fungal infection observed on untreated fruit. Similar to milk, honey-treated peaches were also completely rotten at 30 days at 4ºC storage.In vitro (microtiter) assay of the essential oils showed that orange and manuka oils appeared to be effective only at high concentrations. In contrast, lemongrass and lemon oils appeared to be effective even at low concentrations. Of the essential oils tested in the in vivo assay, lemongrass and lemon oils have the greatest potential. Oranges that were exposed to 0.05% lemongrass oil, 0.25% and 0.5% lemon oil were protected for 30 days when stored at 4ºC or room temperature. They provided the best antifungal activity compared to the other concentrations of all four essential oils tested as well as fungicide treatment for 30 days. Of all the treatment tested, 0.05% lemongrass oil, 0.25% and 0.5% lemon oil appeared to be the most promising treatments. However, these treatments need to be tested for antifungal effects, fruit quality, flavour and nutritional effects in large scale experiments before they can be applied as replacements to currently used fungicides. Also, essential oils are complex compounds; therefore it would be of interest to determine the active compound(s) of the lemongrass and lemon oils.