Litchi film packaging preservation technology

Litchi can be said to be one of the subtropical fruits of the Chinese specialties that are famous in the world. It has bright colors, unique flavor, and rich nutrition. It is the top grade in Lingnan fruit. However, the lychee's “one-day color change, two days fragrant, three-day flavor change” nature will quickly reduce its freshness, and it will also reduce consumer appeal and food value. According to statistics, the annual loss of litchi caused by spoilage is about 20% of the total output. The difficulty of storage and transportation of fresh lychee has limited its sales, and many consumers have still not tasted the taste of real fresh lychee. To this end, many research institutes and colleges and universities at home and abroad have studied the laws of postharvest physiological quality and storage and preservation techniques of litchi, and have made some progress. At present, the main methods of preservation include foam boxes plus ice, atmospheric temperature-controlled storage and preservation, chemical agent preservation, low-temperature storage and preservation, and quick-freeze preservation. Litchi planting grows rapidly in China, but the planting area is scattered, and the storage and preservation facilities are relatively backward. According to our country's situation, fresh air conditioning is the future direction of litchi preservation. The choice of modified atmosphere brewing materials is very important, the use of film packaging of fruits, can effectively regulate the gas composition of the packaging environment, but also can effectively prevent the loss of fruit moisture, so choose the right modified atmosphere packaging materials can effectively extend Litchi preservation period.

1 Physiological characteristics of litchi after harvest

Most studies have shown that litchi is a non-respiratory peak fruit. Within 1 to 4 days after harvesting, the respiratory intensity decreased significantly and there was a valley. Normal breathing maintains life activities, thereby enhancing the resistance to pests and diseases, and is conducive to storage. However, excessive respiration will consume large amounts of nutrients, causing over-ripening and causing diseases, and deteriorating quality. Within a certain temperature range, the lower the storage temperature, the lower the respiration of the fruit. Reducing the O2 concentration in the environment will inhibit the respiration, but it is usually necessary to reduce the concentration of O2 to about 5% and the respiration of the fruit will be significantly reduced. O2 concentration is too low will promote anaerobic respiration, increased respiratory substrate consumption, while the accumulation of alcohol, acetaldehyde, physical disease. Increasing the CO2 concentration also inhibits respiration, and high CO2 poisons the cells. Appropriately increasing the concentration of CO2 and reducing the concentration of O2 can reduce the respiratory intensity of litchi, reduce nutrient consumption, and maintain flavor. Practice has proved that Litchi can achieve better preservation effect under the environment of 5% O2 concentration and 5% CO2 concentration.

1. 2Ethylene release and ripening aging

Seen from the physiological process of plant development, fruit ripening is the last stage of sexual reproduction cycle, and it is the beginning of post-harvest fruit ripening and aging. Ethylene production and release initiates and induces plant senescence. During ripening and ripening of litchi fruit, ethylene was gradually produced and released. Ethylene is a recognized ripening hormone that activates polyphenol oxidase (PPO) and peroxidase (POD) activities in plant tissues, increases plasma membrane permeability, destroys phenolic compounds, and initiates enzymatic browning reactions, leading to peels. Browning. Therefore, ethylene absorbers are usually added to litchi packaging to delay the aging of litchi.

2 Causes and Influencing Factors of Browning and Rot of Litchi

2.1 Causes of litchi browning

a. Enzyme. Litchi browning is mainly enzymatic browning. Polyphenol oxidase PPO is generally considered to be an important enzyme involved in litchi browning process. In the fast freezing storage of litchi, low temperature inhibits the activity of PPO, and the browning of the pericarp is prolonged. The peroxidase POD activity was observed in the cytoplasm of some thick-walled cells in the harvested red litchi skin, and the POD activity of the pericarp was higher than that of the pulp. POD is involved in catalyzing the oxidation of phenolic substances and causes browning of the peel.
b. Anthocyanins. It is generally believed that the change of anthocyanin is an important factor that causes changes in the appearance color of flowers and fruits. Changes in the pH value will cause changes in the structure of the anthocyanins, resulting in changes in color. At pH 7.01, the color changes from brown to dark brown. The most stable pH of anthocyanins is around 3.5. Litchi pH after picking is generally between 3 and 5 and gradually increases by 5-7 during storage, resulting in browning. In addition, as the pH increases, the anthocyanins themselves also degrade faster.
c. Lost water. The comparison experiment was conducted between the film packaging and the control group without packaging, and stored under the same conditions. As a result, litchi of the control group had basically browned and lost commercial value in 3 days, while litchi of the packaging group still had commercial value after 5 days. The above shows that there is a close relationship between dehydration and browning of the peel. Because with the dehydration of the skin, the pH value of the pericarp is gradually increased, the peel will be browned, the package will reduce the loss of water, the pH of the pericarp will increase slowly, and the browning of the peel will also be delayed.

2.2 Litchi rot causes

There are many reasons for the deterioration of lychee rot, which is mainly caused by rot fungi, including Aspergillus niger, Aspergillus flavus, Aspergillus fumigatus, Aspergillus tetraspiri, Aspergillus nidulans, Cylindrospora, Coococcus spp., Penicillium sp. Cyclospora sphaeroides and Pythium and other fungi. In addition, Helminthosporium hawaii is also a fungus that causes litchi rot. Some litchi species in China are particularly sensitive to this bacterium. They lurk on the surface of the skin during fruit development or before and after harvest, or invade and injure the fruit from the wound. In addition, some molds, bacteria will also reproduce on the surface of the skin and deep inside the fruit, so that the fruit becomes acid rot. In addition, alcohol and acetaldehyde produced by hypoxic respiration of lychee will also make lychee rot.

2.3 The external factors that affect litchi browning and rot

a. temperature. Litchi rot is mainly caused by the activity of microorganisms, and physical factors that affect the life activities of microorganisms include: temperature, moisture, osmotic pressure, ultrasonic waves, microwaves, radiation, and pressure, among which the temperature effect is the largest. The appropriate temperature can promote the growth of microorganisms, and unsuitable ones can weaken their life activities. Low temperature can effectively inhibit browning, because low temperature can reduce the activity of PPO and POP, reduce transpiration, and reduce water loss. Low temperature can effectively inhibit the metabolic activity of microorganisms and fruits, indirectly play an anti-corrosion effect, and also prolong the life of the fruit. In addition, low temperatures can also inhibit the production of ethylene. The choice of temperature should minimize the physiological activity of the fruit without causing frostbite. Litchi cryogenic storage temperature is 2 ~ 40 °C. b. humidity. Litchi can inhibit water loss effectively under high humidity. Therefore, increasing the humidity of the litchi environment can effectively prevent the browning of litchi skin. Select packaging materials to choose those materials that have a high barrier to moisture, so that the moisture emitted by the litchi fruits will increase the humidity of the packaging environment.

3 Litchi plastic film packaging preservation technology

3.1 Selection of Thin Film Materials

When selecting a film, the desired film must be considered in terms of its permeability (breathability and moisture permeability), cold resistance, strength, etc. according to the requirements for preservation of litchi.
a. Permeability. Litchi's modified atmosphere preservation technology usually uses plastic bags to add low-temperature methods. Fresh-keeping bags play an automatic atmosphere adjustment role. If the oxygen permeability of the film is too low, the O2 concentration in the packaging environment will become lower and lower, resulting in anaerobic respiration of the litchi. The low CO2 penetration rate of the film will cause the CO2 concentration in the packaging environment to be too high, which will cause the litchi in the package. The selected film must have a proper barrier to O2, CO2.
Since loss of water is a major cause of litchi browning, the film used for packaging litchi must have high moisture resistance. Selecting a film with a low moisture permeability can increase the humidity in the package environment and prolong browning time. Among the packaging films, there are films with high moisture barrier properties such as LDPE HDPE, PP, PVC, PVDC, and PC.
b. Cold resistance and puncture resistance. Litchi requires cold resistance when it is stored at low temperature. Good cold resistance is LDPE, BOPP, soft PVC, PET. Litchi surface uneven, there are many scale-like protrusions, so the packaging material should have good puncture resistance. Among the commonly used plastic films, PP, BOPP, PA, etc. have better puncture resistance

3.2 Analysis of Permeability of Plastic Film

Litchi was packed in plastic film. The air permeability and moisture permeability of the film had a great influence on the preservation of litchi. From the Fick's first law of diffusion and Henry's law, the steady-state gas diffusion rate through a monolayer film is:
J= -Pg(P2-P1)/L ( 1)
Where: J is the diffusion rate of the gas; Pg is the permeability coefficient; P2, P1 is the partial pressure of the diffusion gas on both sides of the membrane.
For a multi-layered structure composed of n-layer films, under steady-state transmission conditions, the diffusion rate through each layer is equal and equal to the total diffusion rate, and the total driving force for gas diffusion is each layer. The sum of the driving forces of diffusion can be obtained from equation (1):
L/Pg=L1/Pgl+L2/Pg2+......+Ln/Pgn ( 2)
Where: t is the permeability coefficient of the composite membrane; L1...Ln is the thickness of each membrane; Pg1...Pgn is the permeability coefficient of each membrane.
The plastic film used for packaging litchi has a certain degree of permeability to O2 and CO2. The transmission rate of the two should not only reduce the respiration of litchi and achieve the preservation effect, but also not cause the physiological damage of litchi. Practice has proved that litchi maintains good preservation effect under the environment of 5% CO2 concentration of 5% O2. When litchi is packaged in a suitable plastic film, after a certain period of time, the gas composition in the package is balanced. With O2 breathing: C6 H12O6 + 6 O2 = 6 CO2 + H2 O, it can be seen that the molar amount of O2 absorbed by the O2 respiration and the released CO2 is equal, so when the gas composition within the packaging reaches equilibrium, the film is required to O2 Diffusion rate with CO2 is equal. O2 accounts for 21% in the air, and CO2 accounts for 0.03%. To make litchi in the environment of CO25% and O25%, the ratio of the oxygen permeability coefficient of the film to the coefficient of permeation of carbon dioxide according to formula (1) should be 1: 3 or so. Under normal circumstances, fruit packaging, single-layer film can not meet the ratio of oxygen permeability and permeability of CO2 ratio, and requires two or more layers of packaging. The ratio of the two is determined by the ratio of the thickness of each layer of film.

3.3 Designing Thin Films and Theoretical Calculations

According to the analysis of the above materials, the LDPE/BOPP fresh-keeping bag was used as the composite film packaging of litchi, with a thickness of 12 to 18 μm and a size of 25 cm×25 cm; each bag contained 1 kg of litchi. Take storage temperature T = 4 °C.

3.3.1 The air permeability coefficient of the composite film looks up the CO2 coefficient and the oxygen permeability coefficient of LDPE and BOPP. It is also known that the thickness of LLDPE is L1=12(μm), and the thickness of BOPP is L2=18(μm). Thickness L = 30 (μm). The above data was substituted into equation (2) to obtain the CO2 permeation coefficient of the composite film Pg1=419460 (m1.μm∥m2.24h.101000Pa), and the oxygen permeability coefficient of the composite film Pg2=138800 (m1.μm/m2.24h.101000Pa). According to the formula (1), the oxygen permeability of the film is 138800×0.125×16%/30 (ml/m2.24h). At 4°C, O2 is 5% and CO2 is 5%. The respiration rate of litchi is 5 .26 O2mg/kg. h, which is 5.26×24×22.4. /32 (ml/m2.24h), the calculation result is the former