Modified atmosphere preservation method

There are many methods of modified atmosphere, which are different due to different equipment conditions and gas concentration index requirements. Generally speaking, it can be divided into the following categories: natural oxygen reduction, nitrogen filling, optimal concentration index gas replacement, reduced pressure atmosphere, modified atmosphere packaging.

2. 1 natural oxygen reduction method

(1) Mechanism: Natural oxygen reduction depends on the respiration of fruits and vegetables, so that the environmental O2 decreases and CO2 increases, which is also called modified atmosphere storage (MA storage for short).

(2) Features: The biggest advantages of this method are simple process and low cost of oxygen reduction equipment, which is suitable for popularization in economically underdeveloped areas. However, MA storage can not accurately control the gas composition, and the oxygen reduction speed is slow, and the preservation effect is limited. At the same time, this method requires high management, and it is easy to cause respiratory disorders due to too low O2 or too high CO2, which endangers the storage safety of agricultural products.

(3) Application: Natural oxygen reduction method is widely used in large-scale commercial modified atmosphere storage of agricultural products. Specifically, there are bagging method, big account method and silicone rubber window method.

2.2 Nitrogen-filled oxygen reduction method

(1) Mechanism: Nitrogen charging is to reduce oxygen by replacing the gas in the silo.

(2) Features: This method can rapidly reduce oxygen, and generally can reach the specified gas concentration within 24 hours or a little longer.

(3) Application: Some people sealed litchi with 0.0329mm low-density vinyl plastic film, then vacuumized, filled with N2, and stored at 5℃. The results showed that after 40 days of storage, the good fruit rate was 70%, and the sugar content only decreased by 4.2%. In contrast, the samples placed in the refrigerator at 5℃ without any treatment lost their commercial value within 13 days. After testing, the O2 concentration in the account was between13% and15% at the initial stage of storage. During storage, the concentration of O2 was between 8% and 10%, and the concentration of CO2 remained at about 3%.

2.3 Optimal concentration index gas replacement method

(1) mechanism: gas replacement with optimal concentration index refers to a modified atmosphere method, also known as controlled atmosphere storage (CA storage for short) or controlled atmosphere refrigeration (C.A storage method for short), which artificially prepares O2, CO2 and other gases into mixed gases according to the optimal concentration index, and at the same time, pumps out the original gases in the storage environment to maintain the optimal concentration index.

(2) Features: This method, which is developed on the basis of cold storage and accurately controls the composition of environmental gases, has obviously better fresh-keeping effect on fruits and vegetables than other methods, but this method requires high equipment and is expensive, and requires the establishment of a complex controlled atmosphere refrigerator.

(3) Application:

2.4 Reduced pressure modified atmosphere method

Reduced-pressure modified atmosphere is to use a vacuum pump to pump out part of the gas in a closed storage room to reduce the pressure, and at the same time, reduce the pressure and humidify the fresh air from outside and input it into the storage room. This way is to create a low oxygen environment by reducing the gas density. By continuously pumping air and inputting fresh air, it can discharge the metabolites of fruits and vegetables outdoors and delay aging. Usually, after decompression, the pressure in the warehouse drops to 10-70mmHg.

(1) mechanism: by vacuumizing, water evaporates and takes away a lot of heat, which can be quickly cooled and stored to achieve the purpose of precooling.

(2) Features: Pre-cooling and gas control under reduced pressure can be realized in parallel in the same warehouse, with rapid reduction of oxygen and rapid discharge of harmful gases such as CO2 and C2H4. Can realize ultra-low oxygen storage, and is particularly beneficial to preventing oxidation of fish, shrimp and meat and inhibiting germination of agricultural products seeds. The preservation effect is obvious. It is reported that they have made a breakthrough in the study of litchi preservation under reduced pressure, reaching a preservation period of 60 days and a good fruit rate of over 90%.

(3) Application: For every 65,438+0% of water evaporated in fruit and vegetable products, the temperature can be lowered by 6℃, and the cooling from 30℃ to 5℃ takes about 4% of water, which takes 30 minutes. As long as it is properly humidified, water will not be lost and flavor quality will not be affected. It takes about 30 minutes to reduce the temperature of meat from 24℃ to 10℃, and the weight is reduced by 4.8%. The product temperature of cereal agricultural products is reduced from room temperature to 4-8℃ within 30 minutes, and can be further dried (dehydrated by about 6%). It can be seen that in the early stage of decompression storage, the evaporation loss caused by decompression of stored products, especially meat products and cereal agricultural products, can play a good role in precooling.

2.5 Modified Atmosphere Packaging Method

Modified atmosphere packaging is a relatively new technology in food packaging at present, including deoxidation packaging and inflatable packaging.

(1) deoxygenated packaging

① Mechanism: Deoxygenation packaging is to seal the packaging by continuously filling nitrogen and vacuumizing to remove oxygen in the packaging container.

② Features: Generally, there will still be 2%-3% oxygen residue after this treatment, so it is often necessary to add a small deoxidizer package or deoxidizer tablet into the packaging bag. Commonly used deoxidizers include: reduced iron powder, ferrous salt, platinum film, glucose oxidase, ascorbic acid, oleic acid, etc.

③ Application: This method is not suitable for the packaging of fresh fruits and vegetables, because there is basically no oxygen in the packaging container.

(2) Inflatable packaging

① Mechanism: Inflatable packaging is to seal the packaging container after filling one or more gases. Commonly used filling gases are oxygen, nitrogen and carbon dioxide.

② Features: It is very important to choose the appropriate gas and gas composition for the packaging object. For fresh fruits and vegetables, we should not only reduce oxygen and suppress respiratory intensity, but also consider their tolerance to hypoxia and high carbon dioxide. It is generally believed that O22%-5% and CO23%-8% in gas are suitable for most fruits and vegetables. At the same time, fresh fruit and vegetable products packed in air must be kept at a proper low temperature to prevent the respiratory intensity from being too high due to rising temperature, so that the products are deprived of oxygen and lead to respiratory disorders.

③ Application: For fresh meat and meat products, it is best to use 60%CO2 and 40%N2 for inflatable packaging. Hypoxia can prevent the formation of oxygenated hemoglobin, which is beneficial to keep the bright red of raw meat. At the same time, it can also prevent the growth of aerobic microorganisms and the oxidative deterioration of unsaturated fatty acids in meat products. It is verified that the ham packed with 60%CO2 and 40%N2 gas is superior to the ham packed in vacuum in terms of oil leakage, oxidative deterioration and storage time. However, some studies believe that a small amount of oxygen should be reserved to inhibit the reproduction of anaerobic microorganisms.