Controlled—modified atmosphere storage

Methanethiol has a very low flavor threshold value (0.02 ppb in water 15), and possesses a strong, offensive, fecal-like odor (18). It also is readily converted to very unpleasant oxidized sulfurous off-flavor compounds with low detection thresholds, such as dimethyl disulfide and dimethyl trisulfide (18,25). Thus, suppression or modulation of methanethiol formation in cruciferous vegetables would le to a reduction of off-flavors associated with methanethiol-related compounds, and enhance the utility of controlled and modified atmosphere storage technologies for these vegetables. 

Pariasca, J.A.T. et al.. Effect of modified atmosphere packaging (MAP) and controlled atmosphere (CA) storage on the quality of snow pea pods (Pisum sativum L. var. saccharatum). Postharvest Biol. TechnoL, 21, 213, 2000. 

In addition to the traditional canning, freezing, pickling and dehydration processes, several new processing techniques have surfaced in recent years. Irradiation with gamma rays, storage with controlled and modified atmospheric environment, reverse osmosis and ultrafiltration will be discussed in subsequent chapters. The emphasis will be on chemistry of quality improvement of the processed fruit and vegetable products. 

Controlled atmosphere (CA) or modified atmosphere (MA) storage refers to reduction of O2 and/or elevation of C02 to levels different from those in air. The addition of carbon monoxide (CO) or removal of C2H4 may also be involved. CA implies a greater degree of precision in maintaining specified levels of O2 and C02 than MA. 

Literature is scarce on the effects of controlled atmosphere (CA) and modified atmosphere (MA) storage on carotenoid content and colonr retention in yellow and red vegetables. Sozzi et al. (1999), reported a lower content in total carotenoids and lycopene in tomatoes, which had been stored in 3% O2 or 20% CO2 than after storage in air (control). No differences were fonnd after storage in low O2 and high CO2. When the samples were transferred to air, the total carotenoid and lycopene contents were lower in the tomatoes, which were subjected to different storage treatments from those stored in air all the time. 

Storage Tests. Tests with about 35L samples were made in gas- and vapor-proof plywood cabinets (holding two 35L sample boxes each) in which temperature, relative humidity and atmospheric composition could be controlled accurately. The sterilized sample boxes were equilibrated at the storage relative humidity prior to the tests to avoid moisture absorption by the wood during tests (22 ). Modified atmospheres were obtained by flushing cabinets continuously with the desired gas mixture. 

The influence of dimethyl disulfide on flavor quality of modified atmosphere stored broccoli florets was further demonstrated by data shown in Table VI which revealed that aroma assessments of undesirable sulfurous aromas in broccoli florets more closely corresponded with concentrations of dimethyl disulfide than with methanethiol. In this experiment broccoli florets were treated by dipping in various solutions containing either ascorbic acid (500 ppm), sodium hy oxide (0.01 M), or phosphoric acid (0.1 M). Broccoli florets dipp in distilled water were used as the control samples. Samples held in sealed Curlon 850 pouches were analyzed after storage for 4 days at 10°C. Ascorbic acid was included in this study because it acts as a reducing agent for methanethiol under anaerobic conditions in model systems (25). 

Food sensitive to oxidation and/or microbial spoilage can be stored in an atmosphere of protective gas or a gas mixture (N2, CO2, CO, etc. modified or controlled atmosphere storage). This is often a suitable method for lengthening the shelf life of food. 

Yahia EM, editor. 2009. Modified and Controlled Atmospheres for Transportation, Storage and Packaging of Horticultural Commodities. Boca Raton, FL CRC Press/Taylor Francis. In press. 

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