Chlorine Dioxide Sachets for the Control of Postharvest Decay of 'Crimson Seedless' Table Grapes

Table grapes are of increasing importance in the world. Gray mold (Botrytis cinerea) is the main postharvest decay problem of table grapes and limits their shelf-life. Sulfur dioxide (SO₂) is traditionally used as an antimicrobial postharvest, in packages or storage rooms. Concerns about harm to human health from SO₂ encourage evaluation of alternatives. Chlorine dioxide (ClO₂) has proven effective to control microorganisms on produce. Our objectives were to assess the ability of ClO₂ to control rot and influence quality. ClO₂ gas was generated inside packages from sachets (2.5 by 4 cm, containing 0.1, 0.2, or 0.3 g Curoxin^TM; Worrell Water Technologies, VA). SO₂ was generated from sachets (4.5 by 6 cm, containing 0.3 g sodium metabisulfite; OSKU S.A., Chile).  Our first experiment evaluated decay control, while the second evaluated berry quality. Experiment 1—plastic clamshells with 500 g of ‘Crimson Seedless’ berries (6 g/berry) were used. Grapes were inoculated by injection of conidia 0.5 cm deep into berries. One inoculated berry was placed in the center of each clamshell with nothing (control) or one sachet (SO₂, 0.1, 0.2, or 0.3 g ClO₂).  Clamshells were placed inside an expanded polystyrene box with a plastic liner and stored for: 1) one month at 0 °C; or 2) one month at 0 °C + 1 week at 10 °C. After 1 month, decay incidence in clamshells (control or with SO₂, 0.1, 0.2, and 0.3 g ClO₂ sachets) was 22.8%, 4.8%, 3.4%, 1.7%, or 0.8%, respectively. After 1 month + 1 week, incidence in control, or SO₂, 0.1, 0.2, and 0.3 g ClO₂ sachets was 38.3%, 5.4%, 8.2%, 5.5%, or 4.1%, respectively. Natural incidence was similarly reduced. Some rachis injury was observed, particularly with 0.3 g ClO₂. SO₂ retarded aerial mycelia growth on inoculated berries. ClO₂ retarded it after 1 month, but not after 1 month + 1 week. Experiment 2—grapes were prepared as before, without inoculation, with nothing (control) or a sachet (SO₂, 0.1 or 0.3 g ClO₂), and stored for: 1) 1 week at 0 °C, 2) 1 month at 0 °C, or 3) 1 month at 0 °C + 1 week at 10 °C. Within each period, color (LCh), rachis appearance, shatter, soluble solids, pH, acid content, firmness, and berry size did not differ significantly.  Berry appearance, particularly controls, declined during storage. Unlike experiment one, ClO₂ (0.3 g) caused no rachis injury. ClO₂ effectively controlled decay with minimal harm to grapes.