Ethylene Concentrations in Controlled Atmosphere Storage
This project's purpose is to help determine the health of tree fruit in controlled atmosphere (CA) cold storage by monitoring the ethylene concentration. We thank Nate Reed of Stemilt for his continued support of this project.
- Concentrations of ethylene determined with Fourier Transform Infrared Spectrometer (FTIR)
- Ethylene is known as the ripening gas, it can be a good indicator of when a CA room should be flushed.
Although the double cold gas handling system uses carbon dioxide concentration to control when the room is flushed with new gases; in the future it might be possible to more precisely control the need for flushing rooms by monitoring both carbon dioxide and ethylene. It has been observed that fruit from rooms that are flushed too often have fruit that is less flavorful. Our work to date has shown that flushes occur well before the ethylene is at levels that are detrimental to fruit.
Classical Least Squares Model
Classical least squares is simply the method used to measure concentrations of ethylene. It is akin to using tracing paper to duplicate a drawing. In this case the computer varies the height of the trace and keeps track of how much ethylene is necessary to duplicate the trace.
Software provided by instrument maker (Midac) allows concentrations to be determined by fitting to known spectra.
We have developed the following procedures to ensure the quality of our data:
- Daily inspection of baseline in spectra.
- Two to three data downloads and thorough processing: examination of error.
- Once every three to four weeks calibration standards (ethylene, ammonia, carbon monoxide, and carbon dioxide) are run to ensure instrument and classical least squares performance.
Daily Change of Ethylene Concentrations
Ethylene concentrations change daily in small amounts unless a flush occurs. After a flush (February 8, 2000 on Figure 1) the ethylene concentration goes down and then builds back up slowly. The flush is determined by carbon dioxide concentration (ethylene currently is not used as an indicator of flush). It is possible that rooms in CA are being flushed too often and decreasing the flavor of fruit (flush carries away the acetates that give fruit its flavor). In the future it may be best to determine when a room needs to be flushed by looking at carbon dioxide and ethylene concentrations (only when both concentrations are high would a flush need to occur). None of the rooms we monitor show ethylene concentrations over 1000 parts per million (ppm), an ethylene level that is thought to be excessive.
Figure 1. Daily change of ethylene concentrations.
One Period of Ethylene Concentrations
Again, none of the rooms is indicating high (over 1000 ppm) ethylene (Figure 2).
Figure 2. One period of ethylene concentration by room.
Two Days Worth of Ethylene Concentration
Figure 3 data is from Feb. 15th and 16th, 2000. It shows a nice slow increase in ethylene concentration. In Figure 4 we show similar data, but from May 8th through the 15th when the fruit is more ripe.
Figure 3. Ethylene versus acquisistion number.
Ethlylene vs. One Week Period in May 2000
Figure 4 indicates that as fruit becomes more ripe, the rate of increase in ethylene is much higher compared with Figure 3 from three months prior (less ripe fruit).
Figure 4. Ethylene concentration, week of May 8 to 15, 2000.
Butyl Acetate in Red Delicous
This is an experiment performed here at our Lab with Red Delicious apples placed in a sealed, un-cooled bag for a period of one month (Figure 5). After one month, the gas in the bag has butyl acetate in it (one of the flavors that Red Delicious gives off). We never see the butyl acetate in the CA environment however; most likely because room flushes remove the very low vapor pressure butyl acetate. This ability to monitor the flavor compounds may make it possible to reduce the frequency of flushes so that flavor in the fruit could be increased.
Figure 5. Butyl acetate in Red Delicious.
Methyl Cyclo Propene with FTIR
Methylcyclopropene (MCP) is an experimental compound being promoted by Rohm and Haas as Ethylbloc. MCP is a means to slow down the ethylene production of certain variety of apples. We have demonstrated that our FTIR instrument can measure this compound in real time (Figure 6). We have secured some Department of Energy funding to further study the possibility of monitoring MCP concentrations in real time. It is vital to monitor the MCP concentration, because too much MCP will destroy the fruit.
Figure 6. Methylcyclopropene measured with FTIR.
- Daily monitoring indicates "health" of CA.
- FTIR technique has proven rugged and reliable.
- Continued daily monitoring of concentrations.
- Promising work with supplier of MCP.
Pete Eschbach and Jeff Griffin(1) and Nate Reed(2)
(1)Battelle Northwest Laboratories
P.O. Box 999, Richland, WA 99352
(2)Stemilt Growers Inc.
P.O. Box 2779, Wenatchee, WA 98807-2779
16th Annual Postharvest Conference, Yakima, WA
March 14-15, 2000