Randolph Beaudry, Department of Horticulture, Michigan State University, East Lansing, MI 48824, USA
Nihad Alsmairat, Department of Horticulture and Crop Science, University of Jordan, Amman 11942, Jordan
The use of high CO2 levels to reduce the decay of blueberry fruit can be traced back to the 1920s, when it was realized that levels of CO2 of 10% or more were effective at suppressing the growth of fungal decay organisms. Early applications of high CO2 date to the 1930s and included the use of dry ice for the shipment of berry fruit in trains traversing the North American continent. One of the problems associated with CO2 use on blueberries is that it is known to cause softening and even result in browning of the fruit tissues. We believed that, like most plant responses, there would be significant variation in the degree of browning and softening between different cultivars. In that several new cultivars have recently been released from blueberry breeding programs, we set out to test our theory and get an idea how much improvement there might be in storability if cultivar selection could be optimized for storage and shipping.
Current applications of CO2 for blueberry fruit include controlled atmosphere (CA) and modified atmosphere packaging (MAP). The distinction between the two is that CA is monitored and maintained by machinery, whereas MAP is maintained by matching the rate of CO2 production by the fruit via respiration and the permeability of the package to CO2. MAP typically involves an initial atmosphere modification at the time of packaging. CA is applied in stationary rooms and in containers used in sea shipments. MAP is applied in pallet-sized packages in which a portion of the atmosphere is removed and then replaced with CO2. The latter technique has been found to be useful for holding fruit at or near the site of production and for cross-country and trans-oceanic shipments.
One of the peculiar features of CA (and MAP when the CO2 exchange is via perforations in the package) is that the sum of O2 and CO2 will equal approximately 21% if O2 is not directly controlled. That is, if air, rather than nitrogen is the source of O2. In CA, after an initial addition of CO2 to achieve a target level (e.g., 12% to 15%), the fruit in the environment continues to produce CO2 and deplete O2 until flushing is required to remove excess CO2. Flushing (with air) displaces CO2 and O2 - so O2 is removed by both respiration and by flushing. This process is repeated each time CO2 gets too high, maintaining a relatively constant CO2 level, but eroding the O2 partial pressure until the two gasses sum to 21%. In MAP, the diffusion of O2 and CO2 molecules through package perforations is roughly equal, so as the CO2 escapes, the O2 enters at roughly the same rate. The result is that for both systems, the concentration of CO2 targeted directly affects the O2 level. If the CO2 level is 12%, the O2 level will tend toward 9%.
Using the fixed relationship between O2 and CO2, we constrained them to sum to 21% using the following combinations: 19/2; 18/3; 16.5/4.5; 15/6; 13.5/7.5; 12/9; 6/15; and 0/21 for CO2/O2, respectively, in per cent. We evaluated nine highbush blueberry cultivars (hand-harvested at the full-blue stage from the Southwest Michigan Research and Extension Center near Benton Harbor, Mich.). Cultivars evaluated were Duke, Toro, Brigitta, Ozarkblue, Nelson, Liberty, Elliott, Legacy and Jersey. Fruit were held at 0 °C for eight weeks to simulate an unusually long ocean shipment and storage regimen.
We found that cultivar affected the response to CO2. Fruit from most of the cultivars softened in response to an increasing proportion of CO2 while others were relatively unaffected or experienced a slight increase in firmness (Fig. 1). There was extensive browning after the long storage period, but some cultivars were significantly more affected than others (Fig. 2). As with firmness, CO2 was more damaging to some cultivars than others, in some cases doubling the extent of browning relative to air-stored fruit. It appeared that a concentration of CO2 near 12% was generally acceptable for all cultivars, but higher concentrations would be beneficial for some. CO2 suppressed decay for all varieties as expected, but the effect of cultivar was much greater. Liberty and Legacy had the lowest decay rates (approximately 2% to 3%) under CA, and Elliot and Jersey had the greatest decay rates (approximately 15%). Under conditions of 12% CO2 and 9% O2, we found that ‘Liberty’ and ‘Toro’ stored extremely well, ‘Duke’, ‘Brigitta’, and ‘Legacy’ stored well, ‘Elliott’ stored moderately well, and ‘Ozarkblue’, ‘Nelson’ and ‘Jersey’ stored poorly.