Postharvest Physiology of Strawberry Fruits Treated With Sodium Dehydroacetate

Sodium dehydroacetate (NaDHA) was most effective in extending the shelf life of fruits of strawberry cultivars that have the poorest holding capacity. Decreased respiration of treated fruits is attributed to the fungicidal property of NaDHA or inhibitory effect of the chemical on a respiratory enzyme of the fruit. A 0.5% NaDHA solution retarded ripening. Various types of fungicides have been evaluated to reduce decay and rot of harvested strawberry fruit (3, 5, 7, 8). Many of these chemicals were effective as a fungicide; however, the wettable powder fungicides left an undesirable visual residue on the fruit. The Na salt of dehydroacetic acid was reported to be an effective fungicide (5, 6, 7, 9) and was appealing due to its solubility in water, lack of visual residue and relatively high (65 ppm) residue tolerance on strawberries. A 0.5% solution of NaDHA (sodium dehydroacetate) was found to be effective in controlling storage decay caused by species of Rhizopus, Alternaria and Botrytis, Sodium dehydroacetate has been reported to be effective in extending the shelf life of cubed squash (2) and reducing discoloration of snap beans (4). This compound is also used in pharmaceutical materials to prevent decomposition. Since NaDHA exhibited potential as an effective fungicide, a study was made to determine the effect of this compound on the shelf life, respiration rate and rate of ripening of the harvested strawberry fruits. Materials and Methods Fruits of 'Midway'. 'Surecrop', 'Sunrise' and 'CatskilT cultivars of strawberry (Fragaria ananassa Duch.) were used. The fruits were harvested in the early pink stage of ripeness and were either dipped in or sprayed with various concentrations of NaDHA for 60 seconds. Two types of controls were followed: one set was kept dry and the other set was dipped in water for 60 seconds. The fruits were enclosed in a jar at 20°C and humidified air was metered over the sample. After 4 days of the holding period, the fruits were evaluated for their marketability and for their ripeness. Marketability was based on visual quality using guidelines at the level of retail market. Respiration rate of whole fruit was measured daily by either the Claypool and Keefer method (1) or with a MSA infrared C02 analyzer. Each sample contained 25 fruits and the study was repeated 6 times over a 2-year period. Oxygen uptake of 3 x 8 mm circular tissue slices from the surface and internal pulp section of a light pink fruit of 'Surecrop' was measured separately with the GME differential respirometer. The skin of the surface tissue was removed. One gm. of the tissue slices was washed in a 0.2 M phosphate buffer (pH 7.4) and placed in Warburg flasks containing phosphate b u f f e r . P r e l i m i n a r y studies indicated that maximum effectiveness with NaDHA was obtained at pH 7.4. Sodium dehydroacetate at various concentrations was placed in the side arm of the Warburg flask. After initial 02 uptake readings were made for 20 minutes, the NaDHA was dipped into the main well containing the strawberry tissues. The readings were continued at 10-minute intervals for an additional 60 minutes. 'Received for p u b l i c a t i o n J u n e 10, 1970. Published with the approval of the Director of the \Vtst Virginia Agricul tural F.xperiment Station us Scientific Paper No. : ! 22. This study is piirt of the Northeastern Regional Research Project NEM-33 and was p a r t l y supported by the Regional Research F u n . Results and Discussion The cultivars responded differently in their marketability to the NaDHA treatment. The percentage of marketable fruit of all cultivars except 'CatskilF was considerably lower when the fruit was dipped in water as compared to those that were held dry (Table 1). Treatment of the 'Catskill' sample with 0.25 and 0.5% solution resulted in a 2and 3-fold increase, respectively, in marketable fruits. 'Surecrop' responded only slightly to a 0.25% solution, whereas a 2-fold increase in marketable fruit was noted with the treatment of 0.5% solution. Marketability of 'Sunrise' was drastically reduced when dipped in water, but treatment with 0.25% NaDHA prevented this deleterious condition. Treatment of 'Sunrise' sample with 0.5% NaDHA resulted in about a 1.5-fold increase in marketable fruits when compared with the dry sample. 'Midway' was also affected very seriously when dipped in water and the NaDHA treatment of either 0.25 or 0.5% concentration was of no value when compared with the dry sample. In the preliminary study, a slight benefit of 'Midway' was noted with the NaDHA treatment. Sodium dehydroacetate treatment appears to be more effective with cultivars that have the poorest holding capacity. Among the cultivars studied 'Catskill' had a very low percentage of marketable fruits when held dry. Next in sequence was 'Surecrop' followed by 'Sunrise'. The effectiveness of NaDHA possibly may be related to physical and chemical propert ies of the various cult ivars. The respiration rate of all the fruit tissues increased with holding and was affected by NaDHA. The degree of response to the treatment differed among cultivars, however, sine* the pattern was the same, the average relative respiration rate of all cultivars for each treatment is plotted in Fig. 1 using the first day rate as the base. The slope of increase of water-dipped fruit was slightly greater than those held dry. Sodium dehydroacetate had an inhibitory effect on the degree of increase. An exception to this was noted with the 'Midway' cultivar, in which the respiration rate of the samples treated with 0.25% concentration did not differ greatly from that of samples held dry. The inhibitory effect of NaDHA was greater at the higher concentration. The final rate of these samples had increassed to about 80%> of that of the control. Table 1. Percentage of marketable f r u i t s of strawberry cu l t iva rs .after 3 days at 20°C following treatment.