Stuart C. Wooley and Dr. Darrell J. Weber, Botany and Range Science
The bane of all fruit growers and sellers is that fruit rots either before and during transport or on the store shelf. This creates income loss for the grower and grocer and reduces quality of produce. For example, some fruits like strawberries have a shelf life of 5-7 days, while raspberries only last 2-3 days. The consumer then does not have time to buy the fruit before it rots. If transport from field to store shelf was accelerated or storage during transport was improved, shelf-life would be extended, resulting in increased revenue to all involved in production and distribution.
There are two main reasons for decreased shelf life: post-harvest handling and transport, and ripening (senescence). Post-harvest handling, transport and shelving, contaminates the fruit with fungal pathogens and damages it, resulting in high losses for the producer. It also stresses the fruit. Stress and damage exacerbate normal physiological processes at the cellular level accelerating ethylene production. This causes the fruit to ripen faster than normal resulting in shortened shelf life. In oxygenated atmospheres, ethylene causes respiration to increase leading to tissue degradation or technically, rotting. This combination of harvesting, packaging and physiology make fruit storage and transport very delicate.
Oxygen drives cellular respiration. Thus, if O2 levels increase, respiration increases producing more CO2 as well as increasing ethylene levels. This causes fruit degradation. However, in a controlled atmosphere it is possible to replace the O2 with CO2 and decrease respiration and ethylene levels to below normal by changing the respiratory processes. Pressurizing the system can decrease respiration by decreasing the partial pressure of oxygen, water loss and inhibiting fungal spore germination and hyphal growth.2,4
However, at very low O2 levels, fermentation occurs producing ethanol, acetaldehyde, and resulting bad taste. High levels of CO2 cause physiological damage to the fruit. Therefore, an optimal range for each cultivar is needed. According to previous controlled atmosphere studies, appropriate levels for O2 are between 5-1.25% and CO2 levels between 1-10%. 3,5 The optimal pressure is usually case specific for each cultivar as each fruit is different in its metabolism and ripening rate.3 In preliminary tests it was determined that about 5 lbs was appropriate for purposes of this experiment.
The strawberries and tomatoes were stored at room temperature, 8′ and 1′ C in 48 airtight containers. Six were at 3 lbs, six at 1 lb, 18 at 5 lbs of air, including 18 controls (3 per treatment). The air controls were not pressurized. Previous trials under CO2 atmospheres were not very successful at preserving fruit, mostly due to accumulation Of CO2 in the tissue and resulting physiological and cosmetic damage, especially in the strawberries. Several tests including taste testing and appearance were run at 5, 12 and 22 days for each treatment. They were checked every day to equilibrate pressure and provide observations.
The strawberries were taste tested at 5, 12, and 22 days. At five days, all of the strawberries at room temperature had a poor taste. Those at 8′ had a fair taste and at 1′ they tasted good. At 12 days the strawberries at room temperature had completely turned to mush. The strawberries at 8′ and 1′ had a fair to good taste. At 22 days the 8′ strawberries were poor in taste and the ones at 1′ were fair to good. The strawberry leaves began to change to a brownish green shortly after introduction to the treatment.
The tomatoes were also tasted at 5, 12, and 22 days. At five days they were all reasonably tasty. The best tasting was the no-pressure-room-temperature group. At 12 days the room temperature tomatoes at 1 and 3 lbs were not good. The 5 lbs and no pressure sets were fair in their flavor. The tomatoes in the 8′ and l’ were good. At 22 days most in the room temperature were rotten and mushy due to bacteria. Mold was also beginning to grow. They had split and were oozing water. The 8′ set were edible, but not incredibly appealing. The no pressure at 8′ was the best. The 1′ were only fair in taste.
Nothing presented itself that gave the impression that pressure did anything to improve the storage life of tomatoes or strawberries other than to inhibit mold, which it did up to a point. It seems like the refrigerated no pressure did better. Some treatments (CO2) caused the skins to literally slide off the fruit. In all cases, pressure seemed to cause the fruits disintegrate to pulp and not retain firmness.
There are a few things to try if there were another study. Try other fruits with thicker skins, e.g. mangoes, watermelon, muskmelon; try more pressure variations, use dry air, fresher fruit (the fruit was from a California wholesaler in Salt Lake City ), or pressurize slowly to allow the fruit to equilibrate to avoid crushing. These are possible solutions to some of the problems experienced in this trial.
- USDA Publication 66. United States Department of Agriculture Publications, GPO Washington, DC. 1
- H.G. Kirk et.al.,1986. Low-pressure storage of Hibiscus cuttings. Effects on stomatal opening and rooting. Annals of Botany 58:389-96.
- Devon Zagory and A.A. Kader. 1988. Modified atmosphere packaging of fresh produce. Food Technology. (9):70-77.
- K.F. Chau and A.M. Alvarez. 1983 Effects of low pressure storage on Colletotrichum gleosporioides and postharvest infection of papaya.
- Steve J. Smith et. al. 1987. Production of modified atmospheres in deciduous fruits by the use of films and coatings. HortScience 22(5):772-6.