Effects of moderate electric fields on fungal load and shelf-life of chestnuts

Abstract

The chestnut (Castanea sativa Mill.) is a seasonal product that needs a post-harvest treatment to control fungal and insect worms development in order to extend its shelf-life. Studies have demonstrated the feasibility of ohmic heating (OH) and the effects of its moderate electric fields during microbiological inactivation treatments for food applications. This emerging thermal technology can be an alternative to chestnut conventional hydrothermal treatment (HT) or to its chemical fumigation. The aim of this study was to evaluate OH capacity to extend shelf-life of chestnuts fruits by eliminating its fungal load. Five treatments were carried out: I) unprocessed chestnuts (control), II) HT (50 ℃, 45 min), III) OH at 35 ºC, IV) OH at 45 ºC and V) OH at 55 °C. The ohmic processing was conducted in a 0.5 M NaCl solution at an electric field of 9 (V/cm) for 2.00, 2.83 and 3.33 min, respectively for each temperature. Then, the chestnuts were stored for 60 days under different atmospheric conditions: i) 25°C with 40% RH, ii) 25 °C with 90% RH, and iii) 5 °C with 70% RH. Once a week the chestnuts were visually inspected for fungal and insect larvae development. Additionally, fungal load of chestnuts (log CFU/g) was determined by plating serial dilutions in RBC agar. The results showed significant differences (p>0.05) in counts of fungi between unprocessed and treated samples, in particular when the storage of chestnuts was done at 5 ºC. The OH treatment conducted at 55 ºC showed the better results; at 5 ºC and 70% RH, no fungi could be detected in samples for 60 days; while at room temperature (25 ºC), no fungi were detected for 24 days. After this period, 3.8 to 5.1 log CFU/g were detected in fruits without any visible formation of molds. The HT performed better then OH at 35 ºC, but fungi could be detected after 24 days of storage at 5 ºC (3.8 to 4.9 log CFU/g). In conclusion, OH showed to be an effective alternative HT, allowing to reduce postharvest decay of chestnuts, increase its shelf-life, and thus meet foodsafety international trade regulations.