Team:UNITN-Trento/Project/Fruit ripening

We have tested several type of fruit to verify that our engineered bacteria are able to produce ethylene (E. coli NEB10β cells transformed with BBa_K1065001) and methyl salicylate (cell transformed with BBa_K1065106) for enhancing or slowing down the ripening of fruit, respectively.

The flasks containing the induced culture were kept at 37 degree in agitation and connected to a sealed jar (or a desiccator) where the fruit was exposed to either ethylene or methyl salicylate (MeSA). Negative controls were either a fruit kept in a airtight jar or a fruit placed in a jar connected to a flask with non-induced E. coli NEB10β cells.

Within the same experiment we used fruit assumed to be at the same ripening stage (i.e.: the same bunch of bananas).

The results obtained varied according to the fruit tested. We tested: bananas, plums, cherries and heirloom tomatoes, blackberries, kiwifruit and others. Results were evaluated qualitatively by observing skin color changes and pulp texture or firmness.

In some cases we were able to detect different levels of fragrance, however this results were too subjective and we discarded them.

We were also able to perform a iodine coloration test on starch-containing fruit.

We also performed quantitative evaluations of ethylene effects by fruit compression strength test and by a fruit color measurement test.

Click on the images to view them enlarged.

Cherry tomatoes are successfully ripened

Type of fruit Cherry tomatoes Duration of experiment 8 days Exposition to Ethylene 1 days Container Fruit in 0.5 L jar

Ripened plums become red

Type of fruit Plums Duration of experiment 4 days Exposition to Ethylene 4 days Container Fruit in 1 L jar

Heirloom tomatoes ready to be eaten

Type of fruit Heirloom tomatoes Duration of experiment 9 days Exposition to Ethylene 9 days Container Fruit in 1 L jar

Cherry tomatoes are successfully ripened

Type of fruit Cherry tomatoes Duration of experiment 7 days Exposition to Ethylene 1 day Container Fruit in 0.5 L jar

Cavendish bananas are successfully ripened

Type of fruit Cavendish banana Duration of experiment 6 days Exposition to Ethylene 6 days Container Essicator (10.3L)

"Fried Green Tomatoes"

Type of fruit Date tomatoes Duration of experiment 7 days Exposition to Ethylene 2 day w/, 3 days w/o, 2 days w/ Container Fruit in 0.5 L jar Note: Lycopene synthesis probably inhibited by the high temperature (>30 degrees) in the lab as suggested in the literature

In summary: E. coli NEB10β cells transformed with BBa_K1065001 (our ethylene producing device) successfully ripen all the fruit tested when compared to the negative control, except for a few cases (Test with Date Tomatoes) where no significant effects were observed. We believe that this experiment did not work because the temperature in the laboratory was over 30 degrees, causing a decreased activity of lycopene synthesis enzymes (Mary Lu Arpaia et Al., Fruit Ripening & Ethylene Management 2010, 9-10).

Starch accumulates in apples during growing season and is hydrolyzed to sugar in the last stages of maturation and development (Krotkov and Helson 1946). Hydrolysis occurs first in the core area and progresses outwards (Phillips and Poapst 1952). Starch in cut sections of the fruit reacts with a solution of iodine-potassium iodide to produce a blue-black color. The intensity and pattern of the color developed is an indication of the stage of maturation.

A compression test was used to determine the maturation stage of kiwifruit and bananas. All these measurements were done in the laboratory of eng. Alessandro Pegoretti in the Industrial Engineering Department. In this test the fruit is subjected to a crushing load to measure the resistence of a sample to compression strength. The test allows to measure the strength necessary to break the material analyzed, as an indication of maturation stage.

The test helped us to quantify the stage of maturation of kiwifruit and bananas, as another confirmation that our ethylene producing device works as predicted. However the test worked better on kiwis rather than bananas probably because the bananas used started already from a more advanced maturation stage.

We used this test to quantify the difference between the color of ripe and unripe fruit's pulp. All measurments were done in the laboratory of eng. Sara Spilimbergo in the Industrial Engineering Department.

We estimated the color of kiwifruit using a high resolution miniature spectrometer, connected to a optical fiber.

The instrument measures three parameters L* (lightness), a* (redness) and b* (yellowness). The L* parameter indicate a range between black (0) and white (100), the a* parameter indicate a range between red (+a*) and green (-a*, complemental color of red), the b* parameter indicate a range between yellow (+b*) and blue (-b*, a complemental color of yellow). A shift in the color toward a darker green and a decrease in lightness are an indication of kiwi ripening.

Click on the images to view them enlarged.

Date tomatoes stay green

Type of fruit Date tomatoes Duration of experiment 7 days Exposition to MeSA BBa_K1065106) 2 day w/, 3 days w/o, 2 days w/ Container Fruit in 0.5 L jar

Cherry tomatoes unripen

Type of fruit Cherry tomatoes Duration of experiment 7 days Exposition to MeSA (BBa_K1065106) 7 days Container Fruit in 0.5 L jar Note: One of the control was probably at a different maturation state ad day1.

Pure MeSA has a toxic effect on cerry tomatoes

Type of fruit Cherry tomatoes Duration of experiment 8 days Immersion in MeSA pure 8 days Container Fruit in 0.5 L jar Note: The immersion in MeSA pure killed the fruit.

E. coli NEB10β cells transformed with BBa_K1065106 (our methyl salicylate producing devices) did not show any difference when compared to the negative control. We were not able to determine if MeSA actually inhibited fruit ripening because the experiments were not long enough to allow ripening of the control. However, the final concentration of the inhibitor in the gas phase in the jar could be lower than the needed concentration. We are now in the process of better estimating these values.

Exposure to pure MeSA coused a toxic effect on cherry tomatoes. We also tested the effect of pure MeSA dissolved in LB but we obtain inconsistent results. For example in a few cases we observed an unexpected change color in the skin of plums and banana. MeSA was reported infact to slow down ripening at high concentration (0.5mM) and to accelerate the process at lower concentration (0.01mM) (Ding, C. and Wang, Y. 164, Plant Science 2003, pp. 589-596). We think that under the used conditions its concentration was above the threshold required to inhibit ripening.