Introduction
Applying organic acids may help regulate resistance to biotic and abiotic stress during the postharvest stage. Oxalic acid naturally occurs in plants and affects numerous physiological functions during plant growth and development, including protection against pests and diseases. Oxalic acid can also improve the quality of produce and postharvest life.
Objective
The objective is to determine the effect of postharvest oxalic acid treatments on the postharvest physical and chemical attributes of cucumbers.
Material and Methods
Cucumber (Cucumis sativus L.) fruits from the spring and fall seasons were obtained immediately after harvest from a cooperating grower in South Georgia. Only healthy fruits free of visible mechanical damage were selected.
Fruits were treated by immersion in oxalic acid (Sigma-Aldrich, Inc., St. Louis, MO) solutions at 0, 2, or 4 mM (pH 6.0) for 10 min at 20 °C (68 °F), followed by rinsing with distilled water (pH 6.0) for 1 min. Treated fruits were held at 20 °C for 6 days or stored at 10 °C (50 °F) for 10 days, followed by 6 days at 20 °C.
Fruit-quality attributes were evaluated before and after storage, including water loss rate (at 20 °C and 70% RH), firmness, skin color, soluble solids content (SSC), flavonoids, Trolox antioxidant activity (TEAC), and total phenolics.
Results and Discussion
Oxalic acid treatments did not significantly affect fruit water loss, firmness, or total phenolic content. However, antioxidant capacity (TEAC) and flavonoid content differed by season.
In the fall (Table 1A), fruit held at 20 °C (not stored at 10 °C) had a higher water-loss rate than fruit stored at 10 °C, followed by 6 days at 20 °C. An opposite response in water-loss rate occurred in the spring.
In the spring (Table 1B), fruit firmness decreased, whereas flavonoid content increased after storage. In fall-harvested fruit, TEAC and total phenolic content decreased after storage at 10 °C, while in the spring, TEAC and total phenolic content increased after storage.
| Treatment stage | Fall season treatment | Water loss rate (%/day) | Firmness1 | TEAC2 (µM) | Flavonoids (quercetin equivalent [mg·ml–1]) | Total phenols (gallic acid equivalent [mg·L–1]) |
|---|---|---|---|---|---|---|
| Oxalic acid | 0 mM | 1.35 | 4.98 | 828 a | 3.7 | 45.8 |
| 1 mM | 1.26 | 5.00 | 793 a | 3.6 | 48.0 | |
| 2 mM | 1.37 | 4.95 | 811 ab | 5.3 | 48.5 | |
| 4 mM | 1.18 | 5.0 | 751 b | 3.7 | 46.5 | |
| Storage | 0 days, 20 °C | 899 a | 6.4 | 55.9 a | ||
| 6 days, 20 °C | 1.54 a | 4.99 | ||||
| 10 days, 10 °C | 795 b | 4.6 | 47.2 b | |||
| 10 days, 10 °C + 6 days, 20 °C | 1.04 b | 4.96 | ||||
| Significance | Oxalic acid (O) | 0.5123 | 0.275 | < 0.001 | 0.252 | 0.821 |
| Storage (S) | < 0.0001 | 0.365 | < 0.0001 | 0.0926 | < 0.0001 | |
| O x S | 0.269 | 0.883 | 0.088 | < 0.0001 | 0.0921 | |
| Means within the same column followed by the same letter are not statistically different according to Tukey’s HSD post hoc test (P ≤ 0.05). 1 Firmness: 1 = Very soft; 5 = very firm. 2 TEAC = Trolox equivalent antioxidant capacity. 3 Transformations were applied where necessary to improve the normality of residuals and to meet assumptions of homogeneity of variance. | ||||||
| Treatment stage | Spring Season Treatment | Water loss rate (%/day) | Firmness1 | TEAC2 (µM) | Flavonoids (quercetin equivalent [mg·ml–1]) | Total phenols (gallic acid equivalent [mg·L–1]) |
|---|---|---|---|---|---|---|
| Oxalic acid | 0 mM | 1.94 | 4.27 | 372 | 4.6 b | 36.9 |
| 1 mM | 1.96 | 4.27 | 412 | 6.6 b | 38.1 | |
| 2 mM | 2.26 | 4.36 | 420 | 7.0 ab | 34.4 | |
| 4 mM | 2.19 | 4.41 | 476 | 9.9 a | 38.7 | |
| Storage | 0 days, 20 °C | 278 a | 2.5 a | 18.4 a | ||
| 6 days, 20 °C | 1.87 a | 4.75 a | ||||
| 10 days, 10 °C | 420 b | 7.0 b | 37.0 b | |||
| 10 days, 10 °C + 6 days, 20 °C | 2.30 b | 3.90 b | ||||
| Significance | Oxalic acid (O) | 0.1393 | 0.882 | 0.259 | < 0.001 | 0.760 |
| Storage (S) | < 0.001 | < 0.0001 | < 0.0001 | < 0.0001 | < 0.0001 | |
| O x S | 0.008 | 0.248 | 0.497 | < 0.0001 | 0.576 | |
| Means within the same column followed by the same letter are not statistically different according to Tukey’s HSD post hoc test (P ≤ 0.05). 1 Firmness: 1 = Very soft; 5 = very firm. 2 TEAC = Trolox equivalent antioxidant capacity. 3 Transformations were applied where necessary to improve the normality of residuals and to meet assumptions of homogeneity of variance. | ||||||
Fruit color, measured as the hue angle, also varied by season. Only untreated fall fruit exhibited slightly higher hue values (greener coloration) than those treated with 2 mM oxalic acid (Table 2A). However, storage conditions did not significantly affect fruit skin color.
Oxalic acid treatments did not significantly affect fruit soluble solids content. Soluble solids remained relatively stable during storage at 10 °C but declined after 6 days at 20 °C only in fall-harvested fruit.
| Treatment stage | Fall Season Treatment | Skin color | SSC (%) | |||
|---|---|---|---|---|---|---|
| L* value | a* value | b* value | Hue angle (h°) | |||
| Oxalic acid | 0 mM | 27.9 ab1 | -7.6 ab | 9.0 ab | 130.5 a | 2.8 |
| 1 mM | 29.7 ab | 8.5 b | 10.6 a | 129.2 ab | 2.8 | |
| 2 mM | 30.3 a | -8.4 ab | 11.4 ab | 127.6 b | 2.9 | |
| 4 mM | 27.1 b | -6.8 a | 8.2 b | 129.8 ab | 2.9 | |
| Storage | 0 days, 20 °C | 29.4 | -7.5 | 8.8 | 131.1 a | 3.0 b |
| 6 days, 20 °C | 29.2 | -7.7 | 9.7 | 129.8 ab | 3.3 a | |
| 10 days, 10 °C | 28.1 | -7.9 | 10.0 | 128.8 b | 3.0 b | |
| 10 days, 10 °C + 6 days, 20 °C | 29.3 | -7.8 | 9.7 | 129.6 ab | 2.3 c | |
| Significance | Oxalic acid (O) | 0.0222 | 0.036 | 0.026 | 0.017 | 0.726 |
| Storage (S) | 0.270 | 0.809 | 0.365 | 0.002 | < 0.0001 | |
| O x S | 0.708 | 0.951 | 0.943 | 0.446 | 0.507 | |
| 1 Means within the same column followed by the same letter are not statistically different according to (ANOVA-Tukey’s HSD post hoc test (P ≤ 0.05). 2 Transformations were applied where necessary to improve the normality of residuals and to meet assumptions of homogeneity of variance. L*a*b* color space: L* = lightness value; a* = green/red values; b* = blue-yellow values. | ||||||
| Treatment stage | Spring Season Treatment | Skin color | SSC (%) | |||
|---|---|---|---|---|---|---|
| L* value | a* value | b* value | Hue angle (h°) | |||
| Oxalic acid | 0 mM | 29.4 | -9.3 a1 | 11.3 b | 129.7 | 3.2 |
| 1 mM | 32.1 | -11.6 b | 15.2 a | 128.0 | 3.0 | |
| 2 mM | 31.7 | -11.0 ab | 14.1 ab | 128.5 | 3.0 | |
| 4 mM | 31.8 | -10.6 ab | 14.7 ab | 128.1 | 2.9 | |
| Storage | 0 days, 20 °C | 31.3 b | -8.9 a | 10.2 b | 131.7 a | 3.2 a |
| 6 days, 20 °C | 30.5 bc | -8.5 a | 9.5 b | 131.9 a | 2.7 b | |
| 10 days, 10 °C | 28.9 c | -11.5 b | 15.0 a | 128.1 b | 3.2 a | |
| 10 days, 10 °C + 6 days, 20 °C | 35.9 a | -11.2 b | 16.1 a | 126.2 b | 3.3 a | |
| Significance | Oxalic acid (O) | 0.1592 | 0.036 | 0.054 | 0.350 | 0.484 |
| Storage (S) | < 0.0001 | < 0.0001 | < 0.0001 | < 0.0001 | < 0.0001 | |
| O x S | 0.348 | 0.770 | 0.366 | < 0.001 | 0.143 | |
| 1 Means within the same column followed by the same letter are not statistically different according to (ANOVA-Tukey’s HSD post hoc test (P ≤ 0.05). 2 Transformations were applied where necessary to improve the normality of residuals and to meet assumptions of homogeneity of variance. L*a*b* color space: L* = lightness value; a* = green/red values; b* = blue-yellow values. | ||||||
Conclusion
Overall, oxalic acid treatments did not result in consistent improvements in fruit greenness or firmness and, therefore, did not enhance fruit shelf life under the conditions evaluated.
