Modified Atmosphere Packaging (MAP), a vacuum-based preservation method that involves evacuating air from the package and replacing it with a tailored gas mixture, has proven highly effective in extending the shelf life of food products for several years. As a pollution-free preservation technique, MAP has gained significant international attention, particularly in the field of fruit and vegetable preservation. However, research on this technology is still limited in our country. Based on several years of study, we have found that MAP offers unique advantages in preserving fresh fruits and vegetables.
First, the technical aspects of MAP:
1. **Gas Supply Equipment**: MAP requires gas sources such as nitrogen generators, carbon dioxide machines, and oxygen generators. These are typically provided through gas cylinders or industrial production units.
2. **Packaging Materials**: Most MAP packaging is made from four common polymers: polyvinyl chloride (PVC), polyethylene terephthalate (PET), polypropylene (PP), and polyethylene (PE). These materials are chosen for their gas permeability and durability.
3. **Gas Composition**: The primary gases used in MAP are oxygen, carbon dioxide, and nitrogen—each of which is relatively inexpensive and easily available. Other gases like sulfur dioxide, dinitrogen dioxide, nitrogen oxide, ozone, and chlorine have also been studied, but their use is restricted due to safety concerns, legal regulations, and consumer acceptance.
Second, the application of MAP in preserving fruits and vegetables:
1. **Unique Characteristics of Fruits and Vegetables**: Unlike other foods, fresh fruits and vegetables continue to respire after harvest, consuming oxygen and releasing carbon dioxide. This makes it challenging to maintain a stable gas composition within the packaging. Traditional MAP methods involve filling the package with an optimal gas ratio and maintaining it, but for fruits and vegetables, a high-permeability film is necessary to balance the gas exchange between the product and the environment.
2. **Testing Methods for MAP in Fruits and Vegetables**:
- **Film Selection**: The selected film must have gas permeability matching the respiration rate of the specific fruit or vegetable.
- **Temperature Range Determination**: The storage temperature must be carefully controlled to ensure optimal preservation conditions.
- **Instruments and Materials**: Common tools include vacuum sealing machines, gas mixers, gas analyzers, colorimeters, and spectrophotometers.
Third, results and analysis:
- **Extended Shelf Life**: MAP significantly prolongs the storage life of fruits and vegetables, especially at low temperatures. In many cases, it can double the shelf life compared to conventional cold storage. It effectively prevents browning in mushrooms, maintains the white shell of certain produce, and preserves the red color of lychees.
- **Gas Concentration Changes**: Under low-temperature conditions, oxygen levels in the sealed bags gradually increase over time. For example, oxygen concentration rose from 3.4% to 8.5% after 4 days and stabilized at 12.1% after 12 days. In some cases, oxygen levels started at around 4%, increased to 5.4% after 4 days, and then balanced at about 4.8%.
Fourth, challenges in MAP preservation of fruits and vegetables:
1. **Limited Microbial Control**: While MAP can reduce oxygen and increase carbon dioxide to inhibit microbial growth, its effectiveness against fungi and bacteria in fruits and vegetables is limited. High COâ‚‚ levels (>60%) or very low Oâ‚‚ levels (<0.1%) are required for significant microbial control. However, moisture condensation inside the packaging often leads to spoilage, so constant temperature and moisture control are essential.
2. **Difficulty in Finding Optimal Conditions**: Each type of fruit or vegetable has different respiration rates and gas requirements, making it difficult to find a universal packaging solution. Currently, only a few versatile films can meet the needs of multiple products, limiting the widespread adoption of MAP in this area.
Overall, while MAP holds great promise for the preservation of fresh produce, further research and development are needed to overcome current limitations and improve its practical application.
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