Lluís Palou

Control of Postharvest Blue and Green Molds of Oranges by Hot Water, Sodium Carbonate, and Sodium Bicarbonate

Control of citrus blue mold, caused by Penicillium italicum, was evaluated on artificially inoculated oranges immersed in water at up to 75°C for 150 s; in 2 to 4% sodium carbonate (wt/vol) at 20 or 45°C for 60 or 150 s; or in 1 to 4% sodium bicarbonate at room temperature for 150 s, followed by storage at 20°C for 7 days. Hot water controlled blue mold at 50 to 55°C, temperatures near those that injured fruit, and its effectiveness declined after 14 days of storage. Sodium carbonate and sodium bicarbonate were superior to hot water. Temperature of sodium carbonate solutions influenced effectiveness more than concentration or immersion period. Sodium carbonate applied for 150 s at 45°C at 3 or 4% reduced decay more than 90%. Sodium bicarbonate applied at room temperature at 2 to 4% reduced blue mold by more than 50%, while 1% was ineffective. In another set of experiments, treatments of sodium bicarbonate at room temperature, sodium carbonate at 45°C, and hot water at 45°C reduced blue mold incidence on artificially inoculated oranges to 6, 14, and 27%, respectively, after 3 weeks of storage at 3°C. These treatments reduced green mold incidence to 6, 1, and 12%, respectively, while incidence among controls of both molds was about 100%. When reexamined 5 weeks later, the effectiveness of all, particularly hot water, declined. In conclusion, efficacy of hot water, sodium carbonate, and sodium bicarbonate treatments against blue mold compared to that against green mold was similar after storage at 20°C but proved inferior during long-term cold storage.

Antimicrobial Edible Films and Coatings for Fresh and Minimally Processed Fruits and Vegetables: A Review

The use of edible films and coatings is an environmentally friendly technology that offers substantial advantages for shelf-life increase of many food products including fruits and vegetables. The development of new natural edible films and coatings with the addition of antimicrobial compounds to preserve fresh and minimally processed fruits and vegetables is a technological challenge for the industry and a very active research field worldwide. Antimicrobial agents have been successfully added to edible composite films and coatings based on polysaccharides or proteins such as starch, cellulose derivatives, chitosan, alginate, fruit puree, whey protein isolated, soy protein, egg albumen, wheat gluten, or sodium caseinate. This paper reviews the development of edible films and coatings with antimicrobial activity, typically through the incorporation of antimicrobial food additives as ingredients, the effect of these edible films on the control of target microorganisms, the influence of antimicrobial agents on mechanical and barrier properties of stand-alone edible films, and the effect of the application of antimicrobial edible coatings on the quality of fresh and fresh-cut fruits and vegetables.

Improving control of green and blue molds of oranges by combining Pantoea agglomerans (CPA-2) and sodium bicarbonate

The potential of using Pantoea agglomerans (strain CPA-2) alone, or in combination with sodium bicarbonate or sodium carbonate solutions, for control of Penicillium digitatum (green mold) and Penicillium italicum (blue mold) on oranges was investigated under ambient (20 °C) and cold storage (3 °C) conditions. P. agglomerans controlled both pathogens on oranges at 2 × 108 cfu ml-1. The biocontrol agent was found to be completely tolerant to 2% sodium bicarbonate at room temperature, although its culturability was reduced by > 1000-fold after 30 min in 2% sodium carbonate. The efficacy of P. agglomerans for control of green mold was improved when combined with sodium bicarbonate, resulting in complete and 97.6% reduction of decay incidence at 3 °C and 20 °C, when compared to untreated controls. Satisfactory results were also obtained with the combined treatment for control of blue mold. P. agglomerans grew well inside wounds on oranges at both 20 °C and 3 °C. In contrast, it showed a reduced growth on the surface of intact fruit. Sodium bicarbonate at 2% concentration did not noticeably affect antagonist population development. Thus, use of bicarbonate treatment at 2% followed by the antagonist P. agglomerans CPA-2 could be an alternative to chemicals for control of postharvest diseases on oranges.

Effect of Gaseous Ozone Exposure on the Development of Green and Blue Molds on Cold Stored Citrus Fruit

The effects of gaseous ozone exposure on in vitro growth of Penicillium digitatum and Penicillium italicum and development of postharvest green and blue molds on artificially inoculated citrus fruit were evaluated. Valencia oranges were continuously exposed to 0.3 ± 0.05 ppm(vol/vol) ozone at 5°C for 4 weeks. Eureka lemons were exposed to an intermittent day-night ozone cycle (0.3 ± 0.01 ppm ozone only at night) in a commercial cold storage room at 4.5°C for 9 weeks. Both oranges and lemons were continuously exposed to 1.0 ± 0.05 ppm ozone at 10°C in an export container for 2 weeks. Exposure to ozone did not reduce final incidence of green or blue mold, although incidence of both diseases was delayed about 1 week and infections developed more slowly under ozone. Sporulation was prevented or reduced by gaseous ozone without noticeable ozone phytotoxicity to the fruit. A synergistic effect between ozone exposure and low temperature was observed for prevention of sporulation. The proliferation of spores of fungicide-resistant strains of these pathogens, which often develop during storage, may be delayed, presumably prolonging the useful life of postharvest fungicides. In vitro radial growth of P. italicum, but not of P. digitatum, during a 5-day incubation period at 20°C was significantly reduced by a previous 0.3 ± 0.05 ppm ozone exposure at 5°C for 4 days. Inoculum density did not influence the effect of gaseous ozone on decay incidence or severity on oranges exposed to 0.3 ± 0.05 ppm ozone at 20°C for 1 week. Susceptibility of oranges to decay was not affected by a previous continuous exposure to 0.3 ± 0.05 ppm ozone at 20°C for 1 week. A corona discharge ozone generator was effective in abating ethylene in an empty export container.

Terpene Down-Regulation in Orange Reveals the Role of Fruit Aromas in Mediating Interactions with Insect Herbivores and Pathogens

Plants use volatile terpene compounds as odor cues for communicating with the environment. Fleshy fruits are particularly rich in volatiles that deter herbivores and attract seed dispersal agents. We have investigated how terpenes in citrus fruit peels affect the interaction between the plant, insects, and microorganisms. Because limonene represents up to 97% of the total volatiles in orange (Citrus sinensis) fruit peel, we chose to down-regulate the expression of a limonene synthase gene in orange plants by introducing an antisense construct of this gene. Transgenic fruits showed reduced accumulation of limonene in the peel. When these fruits were challenged with either the fungus Penicillium digitatum or with the bacterium Xanthomonas citri subsp. citri, they showed marked resistance against these pathogens that were unable to infect the peel tissues. Moreover, males of the citrus pest medfly (Ceratitis capitata) were less attracted to low limonene-expressing fruits than to control fruits. These results indicate that limonene accumulation in the peel of citrus fruit appears to be involved in the successful trophic interaction between fruits, insects, and microorganisms. Terpene down-regulation might be a strategy to generate broad-spectrum resistance against pests and pathogens in fleshy fruits from economically important crops. In addition, terpene engineering may be important for studying the basic ecological interactions between fruits, herbivores, and pathogens.

Inhibition of Penicillium digitatum and Penicillium italicum by hydroxypropyl methylcellulose-lipid edible composite films containing food additives with antifungal properties.

New hydroxypropyl methylcellulose (HPMC)-lipid edible composite films containing low-toxicity chemicals with antifungal properties were developed. Tested chemicals were mainly salts of organic acids, salts of parabens, and mineral salts, classified as food additives or generally recognized as safe (GRAS) compounds. Selected films containing food preservatives were used for in vitro evaluation (disk diameter test) of their antifungal activity against Penicillium digitatum (PD) and Penicillium italicum (PI), the most important postharvest pathogens of fresh citrus fruit. Mechanical properties and oxygen (OP) and water vapor permeabilities (WVP) of selected films were also determined. Film disks containing parabens and their mixtures inhibited PD and PI to a higher extent than the other chemicals tested. Among all organic acid salts tested, potassium sorbate (PS) and sodium benzoate (SB) were the most effective salts in controlling both PD and PI. The use of mixtures of parabens or organic acid salts did not provide an additive or synergistic effect for mold inhibition when compared to the use of single chemicals. Barrier and mechanical properties of films were affected by the addition of food preservatives. Results showed that HPMC-lipid films containing an appropriate food additive should promise as potential commercial antifungal edible coatings for fresh citrus fruit.

Effect of continuous exposure to exogenous ethylene during cold storage on postharvest decay development and quality attributes of stone fruits and table grapes

The influence of continuous exposure to exogenous ethylene on fruit quality and on the development of postharvest brown rot of stone fruits and gray mold of table grapes during long-term cold storage was investigated using selected cultivars of table grapes (nonclimacteric) and climacteric (peach, plum, nectarine, and apricot) and nonclimacteric (sweet cherry) stone fruits. Depending on the experiment, climacteric stone fruits were exposed to concentrations of ethylene of 0, 0.1, 1, 3, 10, or 100 m ll 1 during storage at 0, 5, or 10 8C for up to 28 days; sweet cherries were exposed to 0, 0.01, 0.1, or 1 m ll 1 ethylene during storage at 0 or 5 8C for 21 days; and table grapes were exposed to 0, 0.125, 0.25, 0.5, or 1 m ll 1 ethylene during storage at 0 or 5 8C for up to 60 days. Neither incidence nor severity of brown rot were affected by constant ethylene exposure on stone fruits wound-inoculated with Monilinia fructicola . Similarly, ethylene did not affect gray mold nesting ability on table grapes artificially inoculated with Botrytis cinerea . Furthermore, ethylene exposure neither influenced external quality attributes (skin color on peaches and cherries, skin pitting and stem browning on cherries, and rachis browning on table grapes) nor internal quality attributes (flesh firmness, soluble solids concentration, and titratable acidity on all fruit, and flesh color and internal breakdown on climacteric stone fruits). The only exceptions were flesh softening of apricots, which in every test was significantly enhanced by exogenous ethylene, and flesh mealiness in experiments with ‘Elegant Lady’ peaches, the appearance of which was delayed by ethylene exposure in one case. In conclusion, no general commercial benefit could be expected from actively removing ethylene from cold storage rooms or transport containers containing peaches, plums, nectarines, sweet cherries, or table grapes. # 2002 Elsevier Science B.V. All rights reserved.

Curative and preventive activity of hydroxypropyl methylcellulose-lipid edible composite coatings containing antifungal food additives to control citrus postharvest green and blue molds.

Edible composite coatings based on hydroxypropyl methylcellulose (HPMC), lipid components (beeswax and shellac), and food preservatives with antifungal properties were evaluated in vivo on clementine mandarins cv. Clemenules, hybrid mandarins cv. Ortanique, and oranges cv. Valencia. Their curative and preventive activity against citrus postharvest green (GM) and blue molds (BM), caused by Penicillium digitatum (PD) or Penicillium italicum (PI), respectively, were determined. Fruits were artificially inoculated before or after the application of the coatings and incubated up to 7 days at 20 degrees C. Selected food preservatives included mineral salts, organic acid salts, parabens, and 2-deoxy-d-glucose. Inoculated but uncoated fruits were used as controls. For curative activity, HPMC-lipid edible composite coatings containing sodium benzoate (SB) were most effective in reducing the incidence and severity of GM on clementine mandarins cv. Clemenules (86 and 90%, respectively). On this cultivar, the reduction in GM incidence by the SB-based coating was twice that of potassium sorbate (PS)-based coating. On mandarins cv. Ortanique, PS- and SB-based coatings reduced the incidence of GM and BM by more than 40 and 21%, respectively. However, the HPMC-lipid coating containing a mixture of PS and sodium propionate (PS + SP) exhibited a synergistic effect in the reduction of the incidence of GM (78%) and BM (67%). Coatings with parabens modestly reduced disease incidence and severity. On oranges cv. Valencia, coatings with food preservatives better controlled BM than GM. Coatings containing SB + PS and SB + SP reduced the incidence and severity of BM by 85% and 95%, respectively. PS- and SB- based coatings controlled GM more effectively than coatings formulated with other food preservatives. In every cultivar, fruit coated before inoculation did not show any incidence or severity reduction of both GM and BM (preventive activity). In every test, the antifungal action of the coatings was fungistatic rather than fungicidal.

Evaluation of food additives as alternative or complementary chemicals to conventional fungicides for the control of major postharvest diseases of stone fruit.

To evaluate potential alternatives to conventional fungicides to control decay, more than 20 food additives and generally regarded as safe compounds were tested at three concentrations in in vivo primary screenings with several cultivars of California peaches, nectarines, and plums that had been artificially inoculated with seven major postharvest pathogens: Monilinia fructicola, Botrytis cinerea, Geotrichum candidum, Alternaria alternata, Penicillium expansum, Mucor piriformis, and Rhizopus stolonifer. Overall, the best compounds were 200 mM potassium sorbate (PS), 200 mM sodium benzoate (SB), 200 mM sodium sorbate, 100 mM 2-deoxy-D-glucose, 400 mM sodium carbonate, and 250 mM potassium carbonate. Sodium and ammonium molybdates, acid lactic, and hydrogen peroxide were somewhat effective but were phytotoxic to fruit skin tissues. However, the best compounds lacked effectiveness and persistence when tested against brown rot in small-scale trials of 60-s dips in aqueous solutions at ambient temperatures; PS and SB reduced brown rot incidence by less than 40%. Rinsing treated fruit with tap water reduced the efficacy of the compounds by up to 30%. In contrast, heating the solutions to 55 or 60 degrees C significantly increased treatment efficacy. Brown rot incidence and severity were reduced by 35 and 25%, respectively, on PS-treated peaches after 7 days of incubation at 20 degrees C. However, treatment efficacy was not superior to that with water alone at these temperatures. In semicommercial trials, mixtures of fludioxonil with PS, SB, or 2-deoxy-D-glucose applied as fruit coatings on a packing line were not synergistic in their effect on brown rot, gray mold, and sour rot.

The monoterpene limonene in orange peels attracts pests and microorganisms.

Plant volatiles include terpenoids, which are generally involved in plant defense, repelling pests and pathogens and attracting insects for herbivore control, pollination and seed dispersal. Orange fruits accumulate the monoterpene limonene at high levels in the oil glands of their fruit peels. When limonene production was downregulated in orange fruits by the transgenic expression of a limonene synthase (CitMTSE1) in the antisense configuration, these fruits were resistant to the fungus Penicillium digitatum (Pers.) Sacc. and the bacterium Xanthomonas citri subsp. citri and were less attractive to the medfly pest Ceratitis capitata. These responses were reversed when the antisense transgenic orange fruits were treated with limonene. To gain more insight into the role of the limonene concentration in fruit responses to pests and pathogens, we attempted to overexpress CitMTSE1 in the sense configuration in transgenic orange fruits. Only slight increases in the amount of limonene were found in sense transgenic fruits, maybe due to the detrimental effect that excessive limonene accumulation would have on plant development. Collectively, these results suggest that when limonene reaches peak levels as the fruit develops, it becomes a signal for pest and pathogen attraction, which facilitate access to the fruit for pulp consumers and seed dispersers.