Freshness cards belong to externally controlled food preservatives. Generally, use paper as a carrier and have some slow-release agents inside, releasing gases that can act as insurance and sterilization for food. According to the main chemical substances inside the fresh-keeping card, there are generally several categories:

1. Sodium metabisulfite

Sodium metabisulfite (SMBS), has a molecular formula of Na2S2O5 and a relative molecular weight of 190.12. It is a colorless crystal or white to slightly yellow crystalline powder with a sulfur dioxide odor. When heated to 150 ℃, it will decompose and release sulfur dioxide. If left in the air for a long time, it can decompose due to the release of SO2, and finally generate sodium sulfate. The reaction formula is as follows:

Sodium metabisulfite absorbs water to generate sodium bisulfite:


Sodium bisulfite is oxidized:


Sodium bisulfate generated by oxidation can also reacts with unabsorbed sodium metabisulfite or sodium bisulfite:



Sodium metabisulfite itself can also slowly decompose:


From the above reaction formula, it can be seen that the preservation card containing sodium metabisulfite placed in a closed food bag can absorb the moisture and oxygen inside the bag, while releasing sulfur dioxide. Sulfur dioxide not only has a bactericidal effect, but also can bleach, and is generally used for the preservation of light colored fruits.

Sodium metabisulfite is a commonly used food additive widely used in the food industry. At present, sodium metabisulfite is often used in the processing and preservation of light colored vegetables in China, such as dried potatoes, mushrooms, laver, kelp, bamboo shoots, pickled Chinese cabbage and canned mushrooms. In the biscuit industry, sodium metabisulfite is used as a dough improver, and Americans also add sodium metabisulfite to the dough to weaken its gluten effect by releasing sulfur dioxide, thereby improving the characteristics of the dough and producing high-quality hard semi sweet, sweet biscuits, and some delicious salty snack biscuits.

Due to the toxicity of sodium metabisulfite, the oral LD50 (calculated as SO2) of mice is 600-700mg/kg. China has very strict requirements for the use of sodium metabisulfite, which can be used for candies, biscuits, sugar, rock sugar, maltose, candy, glucose, liquid glucose, bamboo shoots, mushrooms, and canned mushrooms, with a maximum usage of 0.45g/kg. The residual amount (calculated as SO2) of preserved fruits, bamboo shoots, mushrooms, canned mushrooms, grapes, and concentrated black currant juice is less than 0.05g/kg; The residual amount of biscuits, sugar, vermicelli, and other varieties is less than 0.1g/kg; The residual amount of liquid glucose shall not exceed 0.2g/kg.

Food Freshness Cards

2. Chlorine dioxide

Chlorine dioxide is an inorganic compound with the chemical formula ClO2. It is a yellow green to orange yellow gas at room temperature and pressure. When chlorine dioxide encounters hot water, it decomposes into hypochlorous acid, chlorine gas, and oxygen. It is also easily decomposed under light, and its solution is relatively stable in cold and dark places. It is a strong oxidizing agent, and its effective chlorine is 2.6 times that of chlorine. It can react violently with many substances. The reaction formula is as follows:

1) Reacting with hydrogen sulfide:

In addition, the deodorization of chlorine dioxide is due to its ability to undergo dehydration reactions with odorous substances (such as – SOH, – NH2, etc.) and rapidly oxidize the odorous substances into other substances. Moreover, it can prevent the decomposition of methionine into ethylene and also destroy the formed ethylene, thereby delaying decay, while killing microorganisms without reacting with fatty acids and not damaging food structure.

2) Reacting with water:


The generated hypochlorous acid can serve as the main component of common disinfectants and can kill microorganisms such as viruses, bacteria, fungi, etc.

From the above reaction formula, it can be seen that the preservation card containing chlorine dioxide placed in a closed food bag can absorb the moisture inside the bag, while releasing chlorine gas, oxygen, and hypochlorous acid. Chlorine dioxide, chlorine gas, and hypochlorous acid all have bactericidal effects, and hypochlorous acid can also bleach.

Chlorine dioxide is generally used at low concentrations, and at extremely low concentrations (0.1ppm), it can kill many pathogenic bacteria such as Escherichia coli and Staphylococcus aureus. Even under the interference of organic matter, it can completely kill all microorganisms such as bacterial propagules, hepatitis viruses, bacteriophages, and bacterial spores when used at concentrations of tens of ppm. It can be directly used for sterilization and preservation of fruits, vegetables, and meat. Soaking fruits and vegetables in chlorine dioxide solution for a while can kill microorganisms without reacting with fatty acids, without damaging the fiber tissue of vegetables, and without any damage to the taste and nutrition of fruits and vegetables, without the need for further cleaning with clean water. In the field of circulation, some fruits and vegetables that are not suitable for washing can be packed together with solid chlorine dioxide (adsorption or reactive) in packaging boxes. This can slowly release chlorine dioxide for a long time, which not only sterilizes but also achieves freshness preservation. Fish, chicken, and poultry soaked in chlorine dioxide solution can not only eliminate fishy odor, but also effectively control microbial growth, extend storage period, and maintain a delicious taste. Treating poultry eggs with chlorine dioxide also has a good preservation effect and does not affect the hatching of the eggs.

Chlorine dioxide is used as a preservative for the preservation of fruits and vegetables at a concentration of 10mg/L according to GB2760 regulations; A concentration of 50mg/L is used for fish processing.

3. Ethanol (concentration 75%)

Ethanol is an organic compound with the simple formula CH3CH2OH or C2H5OH and the molecular formula C2H6O, commonly known as alcohol. Ethanol is a volatile, colorless, transparent liquid with low toxicity at room temperature and pressure. The aqueous solution of ethanol has a wine like aroma and is slightly irritating, with a sweet taste. The sterilization principle of alcohol is mainly to absorb the water from bacterial proteins, dehydrate and solidify, thereby achieving the goal of killing bacteria. The effect of alcohol varies depending on its concentration. 75% concentration of alcohol is the concentration of medical disinfectant alcohol in clinical practice. This is because high concentrations of alcohol can form a protective film on the surface of bacteria, preventing them from entering the body and making it difficult to completely kill the bacteria. If the alcohol concentration is too low, although it can enter bacteria, it cannot coagulate the protein in their body, nor can it completely kill the bacteria.

There are many patented technologies in China that place alcohol flake food preservation cards into food bags, which can solve the problems of poor preservation effect of existing food in packaging and process, and food being easily contaminated by preservation materials, leading to inedibility. Freshness preservation cards generally include: film layer, substrate layer, second film layer, etc., which are basically carried by fabric material and mainly kept fresh with alcohol as the main raw material. According to the volume of different food bags, alcohol preservation cards of different sizes can be designed. After slow-release, a layer of sterilized alcohol vapor is formed around the food inside the bag.

In summary, the ingredients that can be used for food preservation cards should have the following characteristics:

1. Sustained release: After food is packaged and sealed, it can slowly release effective ingredients;

2. Disinfectability: The released effective ingredients should have a certain degree of sterilization and disinfection ability;

3. Not reacting with the main components in food or acceptable reactants: basically not reacting with various components in food, even if reacting, its products should meet food safety standards or be more conducive to food preservation.

4. Atmosphere regulation: It should be able to absorb oxygen inside food packaging bags, creating preservation conditions that are not conducive to the survival of bacteria.