Modifying the atmospheric pressure under which button mushrooms are packaged helps retain their freshness for a long time.
MUSHROOMS ARE FRAGILE AND HARD TO KEEP. Consumers will put up with wild mushrooms that are a bit bruised, but they want ordinary button mushrooms to be nice and white, with a short stem, small cap, and gills that are covered by a continuous veil, for they know that mushrooms can rapidly change character. A few days is all it takes for mushrooms to darken, for their stems to lengthen, for their ink-black gills to be exposed, and, worse still, for their taste and texture to be denatured.
How can mushroom producers keep their products fresh for longer periods of time? The success of various kinds of ready-to-eat salad greens, washed and packaged under controlled atmospheric pressure, has encouraged food processing firms to take an interest in the problems associated with bringing mushrooms to market. Having shown that the shelf life of baskets of mushrooms could be extended by altering the preservation atmosphere, a team of researchers from the Institut National de la Recherche Agronomique (INRA) station at Montfavet working with food technologists from the Association de Développement et de la Recherche dans les Industries Agro-Alimentaires et d’Emballage-Conditionnement studied various plastic films to determine which one does the best job of sealing in freshness.
Living, Breathing Mushrooms
The researchers began by analyzing button mushrooms kept at room temperature and were able to confirm that cold temperatures reduce microbial and physiological degradation. The usual form of commercial display calls for mushrooms to be stored for a day or two at 2°C (36°F) and then put out for sale at room temperature for a day. This means they will be fresh for only a brief period after purchase: At 11°C (52°F), in 90% relative humidity, mushrooms remain presentable for three to five days, but at 13°C (55°F) this period is reduced to three days.
The development of a system of refrigerated storage and transportation links (“cold chains”) for delivering fresh prewashed salad greens to the consumer has made it possible to offer button mushrooms in plastic wrapping that significantly extends their sell-by date. But in order to know what kind of atmosphere is optimal for such packaging, it is necessary to understand how mushrooms change under different conditions.
In the complete absence of oxygen, mushrooms are colonized by potentially dangerous microorganisms such as Clostridium botulinum. In the presence of oxygen, on the other hand, mushrooms continue to breathe and change. Since 1975 it has been known that the color and texture of mushrooms depend on the atmosphere is which they develop. In particular, increasing carbon dioxide concentration while reducing oxygen concentration lowers the respiration of fungal cells and, for this reason, retards degradation.
To determine under which atmosphere mushrooms are best preserved, G. Lopez Briones and his colleagues at the Montfavet INRA station measured the effect of various concentrations of oxygen and carbon dioxide on mushrooms stored at a temperature of 10°C (50°F). They observed a correspondence between carbon dioxide concentration and the phytotoxic effect of this gas (detected by the increased respiratory intensity of the mushrooms on being put back into contact with air), which damages the cell membranes and thus favors their exposure to darkening enzymes and their substrates. From the point of view of color, the best atmospheres are those in which the carbon dioxide and oxygen concentrations are lower than 10%.
By contrast, texture is best preserved when the carbon dioxide concentration is higher than 10%, for mushrooms have a superstructure that resists modification. In specimens preserved for a week at 10°C (50°F), in an atmosphere containing 15% carbon dioxide, the veils are not ruptured, preserving the button form preferred by French consumers; the higher the carbon dioxide content, the longer the cap remains closed. Finally, excessive humidity causes rapid degradation, so if you put button mushrooms in the refrigerator, don’t close the bag.
Slowing Maturation
A compromise must be found: The carbon dioxide concentration should be not too high, or the mushrooms will not remain white, nor should it be too low, or they will develop too rapidly. Carbon dioxide concentrations between 2.5% and 5% and oxygen concentrations between 5% and 10% appear to be optimal.
How can such atmospheres be created? The Montfavet team compared new microperforated polypropylene films and stretchable polyvinyl chloride films. Mushrooms were placed in baskets, some wrapped and some not, and stored at temperatures between 4°C (39°F) (the legally mandated temperature for cold storage of ready-to-eat salad greens) and 10°C (50°F) for eight days. After eight days of storage at the high end of this range the veils on 85% of the unwrapped mushrooms had opened, whereas maturation was slowed at all temperatures in those that were covered with both types of film. But the old polyvinyl chloride type, being less permeable to humidity, was more successful at retarding development. The challenge now facing researchers is to maximize this impermeability.