Combination of Hot Water and Ethanol to Control Postharvest Decay of Peaches and Nectarines

Evaluation of a novel water-soluble decanoic acid formulation as a fruit sanitizer

Fruits irrigated with contaminated water can transmit various pathogens. High sugar content in fruits such as black cherry (BC) fruit encourages microbial proliferation. A novel water-soluble decanoic acid (WSDA) was evaluated as a fruit sanitizer and compared with other traditional fruit sanitizers such as ethanol, bleach, or dishwasher surfactants. WSDA sanitizer killed yeasts, molds and bacteria including E. coli microbes effectively as other sanitizers with (4 log cycle reduction) of microbial load. Furthermore, the bacterial sanitization mechanism i.e. bactericidal or bacteriostatic was evaluated for alcohol, bleaching and WASDA solutions. E. coli was selected as the model pathogen used for such comparison. Results indicated that the mechanism of action for the three sanitizer solutions against E. coli was bactericidal. The problem with most used fruit sanitizers is their negative influence on fruit quality in terms of physical, mechanical and taste properties. In addition, some led to toxicological and ecological concerns. Thus, studies were conducted to explore the changes in the exocarp cell structure of BC fruit upon exposure to WSDA and other sanitizers using microscopic investigation. WSDA could have a very mild or gentle effect on the BC fruit cells compared to other sanitizers. Alcohol, bleaching and dishwasher surfactant changed the cellular structures and the intercellular spaces. Sanitizers may also affect fruit swelling. WSDA showed an increase in percent weight gain but it was significantly (p < 0.05) much lower than dishwasher surfactant and bleaching solution. BC Fruit flesh firmness and hardness were investigated upon exposure to different sanitizer solutions. BC fruit treated with WSDA showed the highest firmness values. Some liquid sanitizers could affect fruit quality in terms of fruit taste. Sensory evaluation in terms of the sanitizer's smell, texture and hedonic of BC fruit after soaking in different sanitizers was carried out. All sensory parameters of BC fruit soaked with WSDA were similar with insignificant differences (p > 0.05) compared to BC fruit soaked in tap water. However, the sensory parameters were significantly different (p < 0.05) when compared with alcohol, bleach and dishwasher surfactant. This ensures that WSDA was superior to other evaluated sanitizers in terms of physical, mechanical and fruit quality.

Rapid Detection of Monilinia fructicola and Monilinia laxa on Peach and Nectarine using Loop-Mediated Isothermal Amplification

Monilinia laxa and M. fructicola are two causal agents of brown rot, one of the most important diseases in stone fruit. Two species cause blight on blossoms and twigs and brown rot on fruit in pre- and postharvest. Both species are distributed worldwide in North and South America, Australia, and Japan. In Europe, M. laxa is endemic, while M. fructicola was introduced in 2001 and it is now widespread in several countries. Currently, both species coexist in European stone fruit orchards. Monilinia spp. overwinter in cankers and mummified fruit. Mummy monitoring during winter permits growers to understand which species of Monilinia will be prevalent in an orchard during the following season, permitting planning of an appropriate crop protection. Traditionally, the identification has been carried out using morphological features and even with polymerase chain reaction (PCR)-based assays that requires time and well-equipped laboratories. In this study, two isothermal-based methods were designed to identify these pathogens in a faster way than using traditional methods. The loop-mediated amplification (LAMP) assays were validated on some isolates of Monilinia spp. coming from the mummy monitoring according to the international European and Mediterranean Plant Protection Organization standard (PM7/98), taking into account specificity, sensitivity, repeatability, and reproducibility. The sensitivity of both assays was checked by monitoring (at different time points) two nectarine varieties artificially inoculated and stored at two different temperatures. The reliability of both LAMP assays against the quantification of the inoculum was compared with previously published quantitative PCR assays. Both LAMP methods were able to detect a low number of cells. These LAMP methods could be a useful tool for monitoring brown rot causal agents in the field and during postharvest.

The impact of the postharvest environment on the viability and virulence of decay fungi

Postharvest decay of fruits, vegetables, and grains by fungal pathogens causes significant economic losses. Infected produce presents a potential health risk since some decay fungi produce mycotoxins that are hazardous to human health. Infections are the result of the interplay between host resistance and pathogen virulence. Both of these processes, however, are significantly impacted by environmental factors, such as temperature, UV, oxidative stress, and water activity. In the present review, the impact of various physical postharvest treatments (e.g., heat and UV) on the viability and virulence of postharvest pathogens is reviewed and discussed. Oxidative injury, protein impairment, and cell wall degradation have all been proposed as the mechanisms by which these abiotic stresses reduce fungal viability and pathogenicity. The response of decay fungi to pH and the ability of pathogens to modulate the pH of the host environment also affect pathogenicity. The effects of the manipulation of the postharvest environment by ethylene, natural edible coatings, and controlled atmosphere storage on fungal viability are also discussed. Lastly, avenues of future research are proposed.

Effect of ethanol treatment on physiological and quality attributes of fresh-cut eggplant

BACKGROUND

Fresh-cut eggplants, as other vegetables, have relatively short shelf life because of the large amount of tissue disruption and increased metabolism. There is a very rapid onset of enzymatic browning and tissue softening with consequent decrease in sensorial and nutritional quality. To reduce respiration and maintain the quality, various treatments have been applied to find the optimum conditions that provide more fresh and natural fresh-cut produce after minimal processing. The objective of this study was to investigate the effects of ethanol vapour treatment on physiological and quality attributes of fresh-cut eggplant during the extension of shelf life.

RESULTS

The fresh-cut eggplant treated with ethanol vapour showed that respiration rate and occurrence of enzymatic browning were reduced, and higher total phenol content was maintained during 8 days of storage at 10 degrees C. The polyphenol oxidase and peroxidase in fresh-cut eggplant were also inhibited significantly by ethanol treatment. The ethanol treatment reduced the weight loss and maintained the integrity of cell membranes, as confirmed by the low value of electrolyte leakage.

CONCLUSION

The ethanol treatment applied for fresh-cut eggplant was a practical approach to reduce the activity of physiological metabolism and maintain the fresh quality of fresh-cut eggplant. The experimental results revealed that ethanol treatment was effective for extending the shelf life of fresh-cut eggplant as a cheap, environmentally acceptable method.

Use of the Weibull model to describe inactivation of dry-harvested conidia of different Penicillium species by ethanol vapours

AIMS

This study aimed at modelling the effect of ethanol vapours, in the range 0.7-7.5 kPa, on the inactivation of dry-harvested conidia of Penicillium chrysogenum, Penicillium digitatum and Penicillium italicum.

METHODS AND RESULTS

Survival curves were modelled by a Weibull model: log (N/N(0)) = -1/2.303 (t/alpha)(beta). The shape parameter beta was different from one in all cases, indicating that the classical first-order kinetics approach is the exception rather than the rule. Survival curves exhibited upward concavity (beta < 1) with the notable exception of P. chrysogenum at ethanol vapour pressures 0.7 and 1.5 kPa. The scale parameter alpha (h) varied greatly depending on the ethanol vapour pressure and on the species.

CONCLUSIONS

For safety reasons, it is recommended not to exceed an ethanol vapour pressure of 3.3 kPa. At 2.8 kPa, more than 4 log(10) reductions in viable conidia were achieved for all the species after 24-h exposure.

SIGNIFICANCE AND IMPACT OF THE STUDY

Ethanol has GRAS status in the USA and represents an interesting alternative to fungicides. The effectiveness of ethanol vapours to inactivate dry-harvested conidia of some Penicillium was demonstrated in this study.

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.

Effect of hot water treatments on quality of highbush blueberries

Highbush blueberries, cv 'Burlington', were treated with 22, 45, 50, or 60 degrees C water for 15 or 30 s along with an untreated control. Fruit were then stored for 0, 1, 2, or 4 wk at 0 degrees C and 2 or 9 d at 20 degrees C prior to evaluation of microbial population and fruit quality. After 4 wk of storage, the hot water treatment at 60 degrees C resulted in 92% marketable berries, followed by 90% at 50 degrees C, 88% at 45 degrees C, and 83% at 22 degrees C compared with 76% in untreated controls. Decay incidence was reduced to 0.6%, 1.2%, 1.4%, or 2.8% with 60, 50, 45, or 22 degrees C water treatments, respectively, compared with 5.1% in controls following 4 wk at 0 degrees C and 2 d at 20 degrees C. After an additional 7 d at 20 degrees C, decay in fruit treated at 60 degrees C for 15 or 30 s remained at 1.8% and 0.4%, respectively, compared to 37.4% in controls. Weight loss of berries treated with hot water was 0.4% against 3.8% in controls, and shriveled and split berries were also reduced compared to controls (P<0.001). Aerobic plate count and yeast and mold count were reduced by 0.45 to 0.7 log at 60 degrees C for 30 s. Botrytis cinerea and Colletotrichum sp. were the dominant fungal pathogens causing decay of Burlington blueberries during storage. Hot water treatments also immediately induced an increase in ethanol and reduced fruit titratable acidity and soluble solids content, but had no significant effect on fruit firmness, pH, or most flavor volatile concentrations.

Effects of potassium sorbate on postharvest brown rot of stone fruit

The effect of potassium sorbate (K-sorb), a low-toxicity chemical, to control Monilinia spp. was investigated. Preliminary in vitro studies found the MIC of K-sorb for conidial germination and mycelial growth was, respectively, 260 and 1,250 mg/ liter. Immersion of naturally infected peach and nectarine fruit in a solution (15 g/liter) of K-sorb for 120 s reduced brown rot by over 80% in four of five trials. Although treated fruits showed a significant reduction in firmness with respect to the control, they did not reach the overripe stage and retained acceptable quality parameters. In an attempt to elucidate the mechanism of action for K-sorb, the inhibition of enzymatic activity by K-sorb was also tested. In a radial diffusion assay, the addition of K-sorb to agarose reduced polygalacturonase (PG) activity across the concentrations considered. The greatest reduction (54.3%, with respect to the control) was obtained at a sorbate concentration of 15 g/liter. PG kinetic activity of Monilinia laxa observed by a spectrophotometric assay peaked after 40 min in all samples tested. PG activity was significantly higher in the control than in the samples with increased K-sorb concentrations. In conclusion, based on these findings, K-sorb can be recommended as a low-toxicity antifungal compound against Monilinia spp. in peaches and nectarines with its mode of action probably depending in part on the inhibition of PG activity in M. laxa.

Residue levels and storage responses of nectarines, apricots, and peaches after dip treatments with fludioxonil fungicide mixtures

Mature apricots (Prunus armeniaca), nectarines [Prunus persica var. nectarine (Ait.)], and peaches [P. persica (L.) Batsch.] were subjected to a 2 min dip treatment with warm water at 48 degrees C or with fludioxonil (FLU) at 100 mg L-1 and 20 degrees C or at 25 mg L-1 FLU and 48 degrees C and then stored at 5 degrees C and 90-95% relative humidity (RH) for 1 week plus 1 additional week at 18 degrees C and approximately 80% RH. Fruit residue uptake was determined as a function of fungicide concentration, dip temperature, treatment time (only on nectarines), and fruit storage conditions. FLU residue level was closely related to fungicide concentration and treatment temperatures and was dependent on fruit species. FLU residues showed great persistence over both storage and shelf life. Fruit dipping in water at 48 degrees C effectively reduced decay development in cvs. 'May Grand' nectarines and 'Pelese' apricots but was ineffective in cvs. 'Red Top' and 'Sun Crest' nectarines during 7 days of storage compared with nontreated fruit. Decay rates in cvs. 'Glo Haven' peaches and 'Fracasso' apricots were very low in fruit dipped in water at both 20 and 48 degrees C. Fungicide treatments at 20 and 48 degrees C resulted in the total or almost total suppression of decay in all cultivars. During shelf life, fruit became very prone to decay, averaging 25.7-100% depending on the cultivar. Fruit dipping in hot water effectively reduced decay in 'Pelese' and 'Fracasso' apricots, 'Sun Crest' peaches, and 'May Grand' nectarines as compared to control, but was ineffective in 'Glo Haven' and 'Red Top' peaches. Fungicide treatments at 20 degrees C were more effective than hot water in most cultivars. The combination of FLU with water at 48 degrees C further improved the fungicide performance. Indeed, reduced levels (a fourth) of active ingredient were required to achieve a control of decay comparable to that for treatment at 20 degrees C. Residue levels in fruit after treatment with 100 mg L-1 FLU at 20 degrees C or with 25 mg L-1 FLU at 48 degrees C averaged approximately 0.6-2 mg kg-1, which were notably lower than the maximum residue limit (5 mg kg-1) allowed in the United States for stone fruit.

Fungicidal Activity of Plant Volatile Compounds for Controlling Monilinia laxa in Stone Fruit

Nine plant-volatile compounds were tested for their activity against Monilinia laxa, the cause of brown rot in stone fruit. In vitro trials on conidial germination and mycelial growth showed a consistent fungicidal activity of trans-2-hexenal, carvacrol, and citral, whereas trans-cinnamaldehyde, hexanal, (-)-carvone, eugenol, 2-nonanone, and p-anisaldehyde exhibited a progressively lower inhibition. The best inhibitor of conidial germination was trans-2-hexenal (effective dose for 50 and 90% inhibition [ED₅₀ and ED₉₅] = 7.53 and 9.4 μl/liter, respectively; minimal inhibitory concentration [MIC] = 12.3 μl/liter], whereas carvacrol was the best inhibitor of mycelial growth (ED₅₀ and ED₉₅ = 2 and 3.4 μl/liter, respectively; MIC = 6.1 μl/liter). The three most active compounds in in vitro studies also were tested in vivo as postharvest biofumigants. The best control of brown rot was with trans-2-hexenal (efficacy ranging from 46.2 to 80.3%, depending on cultivar), whereas citral and carvacrol resulted in a lower efficacy of 40 and 32.9%, respectively. Fumigation with trans-2-hexenal at concentrations that stopped decay did not cause any visible disorders to plum, whereas it was phytotoxic to apricot, peach, and nectarine and produced off-odors or off-flavors in all species of stone fruit tested.

Impact of Postharvest Hot Water or Ethanol Treatment of Table Grapes on Gray Mold Incidence, Quality, and Ethanol Content

The influence of brief immersion of grape berries in water or ethanol at ambient or higher temperatures on the postharvest incidence of gray mold (caused by Botrytis cinerea) was evaluated. The incidence of gray mold among grape berries that were untreated, or immersed for 1 min in ethanol (35% vol/vol) at 25 or 50°C, was 78.7, 26.2, and 3.4 berries/kg, respectively, after 1 month of storage at 0.5°C and 2 days at 25°C. Heated ethanol was effective up to 24 h after inoculation, but less effective when berry pedicels were removed before inoculation. Rachis appearance, epicuticular wax content and appearance, and berry shatter were unchanged by heated ethanol treatments, whereas berry color changed slightly and treated grape berries were more susceptible to subsequent infection. Ethanol and acetaldehyde contents of grape berries were determined 1, 7, and 14 days after storage at 0.5°C following treatment for 30 or 90 s at 30, 40, or 50°C with water, or 35% ethanol. Highest residues (377 μg/g of ethanol and 13.3 μg/g of acetaldehyde) were in berries immersed for 90 s at 50°C in ethanol. Among ethanol-treated grape berries, the ethanol content declined during storage, whereas acetaldehyde content was unchanged or increased. Untreated grape berries initially contained ethanol at 62 μg/g, which then declined. Acetaldehyde content was 0.6 μg/g initially and changed little during storage.

Survival of spores of Rhizopus stolonifer, Aspergillus niger, Botrytis cinerea and Alternaria alternata after exposure to ethanol solutions at various temperatures

AIMS

To quantify and model the toxicity of brief exposures of spores of Rhizopus stolonifer, Aspergillus niger, Botrytis cinerea and Alternaria alternata to heated, aqueous ethanol solutions. These fungi are common postharvest decay pathogens of fresh grapes and other produce. Sanitation of produce reduces postharvest losses caused by these and other pathogens.

METHODS AND RESULTS

Spores of the fungi were exposed to solutions containing up to 30% (v/v) ethanol at 25-50 degrees C for 30 s, then their survival was determined by germination on semisolid media. Logistical, second-order surface-response models were prepared for each fungus. Subinhibitory ethanol concentrations at ambient temperatures became inhibitory when heated at temperatures much lower than those that cause thermal destruction of the spores by water alone. At 40 degrees C, the estimated ethanol concentrations that inhibited the germination of 50% (LD(50)) of the spores of B. cinerea, A. alternata, A. niger and R. stolonifer were 9.7, 13.5, 19.6 and 20.6%, respectively.

CONCLUSIONS

Ethanol and heat combinations were synergistic. Control of spores of these fungi could be accomplished with much lower temperatures and ethanol concentrations when combined compared with either used alone. Botrytis cinerea and A. alternata were less resistant to the combination than A. niger or R. stolonifer.

Effect of hot water and hydrogen peroxide treatments on survival of salmonella and microbial quality of whole and fresh-cut cantaloupe

Cantaloupe melon has been associated with outbreaks of salmonellosis. Contamination might be introduced into the flesh from the rind by cutting or by contact of cut pieces with contaminated rinds. Our objectives were to investigate the efficacy of hot water or hot 5% hydrogen peroxide treatments in reducing the population of native microflora and inoculated Salmonella on cantaloupe rind and transfer to fresh-cut tissue during cutting. Whole cantaloupes, inoculated with a cocktail of Salmonella serovars to give 4.6 log CFU/cm2 and stored at 5 or 20 degrees C for up to 5 days, were treated with hot water (70 or 97 degrees C) or 5% hydrogen peroxide (70 degrees C) for 1 min at 0, 1, 3, or 5 days postinoculation. Aerobic mesophilic bacteria and yeast and mold on treated whole melon and fresh-cut pieces were significantly (P < 0.05) reduced by all three treatments. Treatments with hot water (70 and 97 degrees C) caused a 2.0- and 3.4-log CFU/cm2 reduction of Salmonella on whole cantaloupe surfaces irrespective of days of postinoculation storage prior to treatment up to 5 days at 5 or 20 degrees C, respectively. Treatment with 5% hydrogen peroxide (70 degrees C) caused a 3.8-log CFU/cm2 reduction of Salmonella. Fresh-cut pieces prepared from untreated inoculated melons and those treated with 70 degrees C hot water were positive for Salmonella. However, fresh-cut pieces prepared from inoculated whole melon dipped in water (97 degrees C) or hydrogen peroxide (70 degrees C) for 60 s were negative for Salmonella, as determined by dilution plating onto agar medium, but were positive after enrichment at days 3 and 5 of storage at 5 degrees C. The ability to detect Salmonella in fresh-cut pieces was dependent on the initial level of inoculation. The results of this study indicate that the use of hot water (97 degrees C) or heated hydrogen peroxide to reduce the population of Salmonella on contaminated whole cantaloupes will enhance the microbial safety of the fresh-cut product.

Near-Harvest Applications of Metschnikowia fructicola, Ethanol, and Sodium Bicarbonate to Control Postharvest Diseases of Grape in Central California

The yeast Metschnikowia fructicola, ethanol, and sodium bicarbonate (SBC), alone or in combinations, were applied to table grapes on vines 24 h before harvest to control the incidence of postharvest diseases. In four experiments, all significantly reduced the total number of decayed berries caused by Botrytis cinerea, Alternaria spp., or Aspergillus niger after storage for 30 days at 1°C followed by 2 days at 20°C. In three experiments, a mean gray mold incidence (caused by B. cinerea) of 34.2 infected berries per kilogram among untreated grape was reduced by Metschnikowia fructicola at 2 × 10⁷ CFU/ml, ethanol at 50% (vol/vol), or SBC at 2% (wt/vol) to 12.9, 8.1, or 10.6 infected berries per kilogram, respectively. Ethanol, SBC, and SO₂ generator pads were similarly effective. M. fructicola effectiveness was not improved when combined with ethanol or SBC treatments. Ethanol and yeast treatments did not harm the appearance of the grapes. M. fructicola and SBC left noticeable residues, and SBC caused some visible phytotoxicity to the rachis and berries. Ethanol applied at 50% (vol/vol) reduced epiphytic fungal and bacterial populations by about 50% compared with controls. M. fructicola populations persisted on berries during storage when applied alone or after ethanol treatments, whereas SBC reduced its population significantly.

Effects of Wounding, Inoculum Density, and Biological Control Agents on Postharvest Brown Rot of Stone Fruits

The effects of wounding, inoculum density, and three isolates (New, Ta291, and 23-E-6) of Trichoderma spp. and one isolate (BI-54) of Rhodotorula sp. on postharvest brown rot of stone fruits were determined at 20°C and 95% relative humidity (RH). Brown rot was observed frequently on wounded nectarine, peach, and plum fruits inoculated with two spores of Monilinia fructicola per wound, and occasionally on unwounded nectarine and peach fruits inoculated with the same spore load. Brown rot was observed on wounded plums only. A substantial increase in lesion diameter of brown rot was also recorded on wounded nectarines and peaches inoculated with suspensions of ≤20 spores and ≤200 spores per wound, respectively, compared with unwounded fruit. At concentrations of 10⁷ and 10⁸ spores per ml, all Trichoderma isolates substantially reduced brown rot on peaches (63 to 98%) and plums (67 to 100%) when fruits were inoculated with M. fructicola following the application of a biological control agent. Similarly, at 10⁸ spores per ml, the yeast BI-54 also suppressed brown rot on peaches completely and on plums by 54%. Significant brown rot reduction was also achieved with the isolate New at a concentration of 10⁸ spores per ml, even when the biocontrol agent was applied 12 h after inoculation with M. fructicola and under continuous conditions of 95% RH. The isolates Ta291 and 23-E-6 also reduced brown rot significantly under drier (50% RH) incubation conditions. These isolates provided the best control of brown rot on plums when they were applied 12 h earlier than inoculation with M. fructicola. Satisfactory control of brown rot on plums inoculated with M. fructicola at 8 × 10⁴ spores per ml was achieved with New at 10⁶ spores per ml and with Ta291 at 10⁷ spores per ml. Measures taken to avoid injuring fruit will greatly reduce brown rot of stone fruit at any spore load for plum, but only at ≤50 spores per mm² for peach, and at ≤5 spores per mm² for nectarine. This study identifies two isolates (Ta291 and New) of Trichoderma atroviride, one isolate (23-E-6) of T. viride, and one of Rhodotorula sp. that show potential for further development as biocontrol agents of postharvest brown rot of stone fruits.