Plantings of pomegranates have been continuously increasing in California, the leading state for this crop production in USA. From 1000 ha in the 1980s to about 8000 ha in the last years. This increase is a consequence of worldwide increasing consumer demand for nutritional and functional foods. Pomegranates arils, the edible fruit part, comprise juice and seeds and are a rich source of sugars, pectins, ascorbic acid, amino acids, minerals, fibers, phytoestrogens, polyphenols, and flavonoids. The important antioxidant activity of pomegranate is well-known and many clinical studies have demonstrated that consuming the fruit helps to prevent coronary heart disease and some types of cancer.'Wonderful' is the most widely planted pomegranate cultivar in California because it offers the best ratio yield and quality of fruits. The arils are tender, deep crimson and with good flavour, the skin is of medium thickness making the fruit well suited for both fresh consumption and processing for whole arils. Pomegranate is a nonclimateric fruit, it does not ripen off the tree, thus it should be harvested when fully ripe to ensure the best organoleptic characteristics. In California, the harvesting period is from the beginning of September to the middle of November, but there is a commercial interest to prolong the postharvest life at least after Christmas when demand and prices are higher.
Chilling injuries, weight loss, and gray mold (Botrytis cinerea), responsible for quality decay, are the main concerns that jeopardize the pomegranate storability. Studies have demonstrated that chilling injuries, such as brown discoloration or scald of the skin and surface pitting, occur when pomegranate is stored at ≤5°C. The weight loss during storage is due to the water lost through the natural porosity of pomegranate skin, the shriveling symptoms on fruit are noticeable only when weight loss is ≥ 5% of initial weight.
Some studies have demonstrated that storing pomegranate at 7.5°C in 5kPa O2 and 15 kPa CO2 is an optimal combination to delay fruit senescence, inhibit the microbial development, and extent the postharvest life up to 20 weeks. Generally, B. cinerea infects the flowers or the crown of the young fruits in field, remains latent, and after harvest develops. Gray mold is favored when pomegranate is stored at 5-10 °C and <90% RH, the losses due to this disease are more than 30% of harvested fruits when postharvest fungicides are not applied. For this reason, in California fludioxonil is used, it is classified as 'reduced-risk' fungicide, in fact it is effective to control gray mold but increases the potential proliferation of resistant strains of the pathogens.
The research
The objectives of the study conducted by Palou et al. (2007) were: 1) to evaluate the effectiveness of common food additives and mixture to control gray mold on pomegranate artificially inoculated; 2) to test a combined treatment on naturally infected fruit and to determine its impact on decay control and fruit quality. For the first objective, 8 fungicide treatments were tested:
- control,
- dipping fruits for 3 min in a solution of 3% (w/v) potassium sorbate at 21°C,
- dipping fruits for 3 min in a solution of 3% (w/v) sodium bicarbonate at 21°C,
- dipping fruits for 3 min in a solution of 3% (w/v) sodium carbonate at 40.5°C,
- dipping fruits for 3 min in a mixture with 1.5% (w/v) potassium sorbate and 1.5% (w/v) sodium bicarbonate at 21°C,
- dipping fruits for 3 min in a mixture with 1.5% (w/v) potassium sorbate and 1.5% (w/v) sodium carbonate at 21°C,
- dipping fruits for 30 seconds in a solution of fludioxonil (0.6 g/L) at 21°C,
- dipping fruits for 30 seconds in a solution of fludioxonil (0.6 g/L) at 49°C.
For the second objective, the combined treatment consisted of following sequence of three treatments:
- 3% (w/v) potassium sorbate in pressure washer for 15 seconds at 21 °C
- dipping for 3 min in a solution with 3%(w/v) sodium bicarbonate and 200 mg/L sodium hypoclorite at 26.7°C
- dipping for 30 seconds in 0.6 g/L fludioxonil at 29.4 °C.
The experiment with artificially inoculated fruits showed that the development of B. cinerea was significantly influenced by postharvest treatments, storage conditions, and their interaction. The most effective antifungal treatments were potassium sorbate and fludioxonil regardless of storage conditions. Fludioxonil was more effective at 49°C than at 21°C, showing the benefit of heating the solution. Potassium sorbate lacked persistence, even if it was as effective as fludioxonil at 49°C after 4 and 8 weeks of storage, it was not after 15 weeks. The Gray mold was significantly inhibited by all treatments more in CA than in air.
In general, the B. cinerea incidence was lower in fruits stored in CA than in air; after 4, 8, and 15 weeks of storage the sporulation percentage on fruits treated with potassium sorbate and stored in air was 5, 50, and 90%, respectively, while on fruits stored in CA it was 0, 5, and 50%. After 4, 8, and 15 weeks, the sporulation percentage on fruits treated with fludioxonil at 49°C was 5, 10, and 20%, respectively, regardless of storage conditions.
The experiments with naturally infected fruits showed that the combined treatment reduced significantly the incidence of disease in both storage conditions, however the efficacy of treatment was greater on fruits stored in CA than on fruits stored in air. This treatment resulted effective only up to 6 weeks of storage, while after 14 weeks of storage the fruit quality was seriously compromised by the incidence of decay being too high.
In all experiments, the internal quality was not affected by treatments. CA storage greatly improved fruit storability due more to its fungistatic effects than to its effects on fruit condition.
Conclusions
The research has demonstrated that the integration of potassium sorbate with CA storage could offer an alternative to the synthetic fungicides for the management of pomegranate postharvest decay. Furthermore, this combination could be an interesting alternative both for organic productions and for productions addressed to markets with zero tolerance to agrochemicals. In addition, the application of food additives as fungicides should reduce the risks of resistance development.
Original work. Palou L., Crisosto C.H., Garner D., "Combination of postharvest antifungal chemical treatments and controlled atmosphere storage to control gray mold and improve storability of ‘Wonderful’ pomegranates", Postharvest Biology and Technology, 2007, Issue No. 43, pagg. 133-142. For more details: http://ucce.ucdavis.edu/files/datastore/234-1176.pdf