Fruit coating is mostly used for pre-marketing treatment and short-term storage of fruits. However, coating treatment only plays an auxiliary role, and the decisive role of fruit maturity, mechanical damage and storage environmental conditions can not be ignored.
General situation of development
The international application research of fruit coatings began with 1924, starting with waxing citrus fruits. The research and application of fruit coating developed rapidly in 1930s and 1950s, and it was widely used in business. In 1980s, the United States, Japan, Italy, Australia, South Africa and other countries widely used paints to treat citrus fruits before they went on the market. It also applies to bananas, pineapples, busy fruits and avocados. For deciduous fruit trees, such as apples, pears, peaches, apricots, plums, cherries, grapes, raspberries, blackberries, cranberries and strawberries, there have been a lot of researches and applications on fruit coatings, but they are only widely used in commerce in apples, peaches and nectarines.
The application of fruit coating in China started late. From 65438 to 0962, Shantou, Guangdong introduced a set of waxing machine and fruit wax liquid from Japan, and copied the waxing machine. After the 1970s, the Institute of Forest Products Chemical Industry of China Academy of Agriculture and Forestry Sciences, Nanjing Institute of Forest Products Chemical Industry, Guangxi Institute of Chemical Industry, Chongqing Normal University and other units all carried out research on fruit coating, made various types of coating films, and mixed with hormones and preservatives. The anti-corrosion and fresh-keeping effect of the coating film was enhanced, and experiments were carried out on a variety of fruits, and certain results were obtained. Zhejiang province has also developed Chinese herbal medicine coatings.
Coating type
There are many kinds and formulas. The requirements for coatings should be tasteless, non-toxic, pollution-free, without side effects, low cost, convenient use and good effect. Wax coatings are widely used in commerce, mainly as sunscreens for various products such as paraffin, carnauba wax and polyethylene-based waxes, resins and shellac. Most of these formulas are patented. (1) shellac coating. All kinds of shellac (shellac) coatings were developed by Nanjing Institute of Forest Products and Chemical Industry, Jiangsu Province, and used for apples. (2) Chinese herbal medicine coating, which is mixed according to 1: 3 when used; Alpinia officinarum, a Chinese herbal medicine developed by Zhejiang Agricultural University, was mixed with a certain proportion of water, boiled to get juice, and then mixed with bleached shellac to form a coating film, which was used for the preservation of wide-peel citrus. ③ Film-coated fruit preservative. Developed by Jinan Fruit Research Institute, Shandong Province, it is mainly made of sucrose esters of high-carbon fatty acids. The concentration of sucrose ester is 0.5 ~ 1%. Red star and golden apple can be stored for 1 ~ 4 months at room temperature. ④W2 coating. The concentration of 1∶5 developed by Nanjing Institute of Forest Products and Chemical Industry in Jiangsu Province has obvious fresh-keeping effect. ⑤ Emulsified wax coating. For citrus in Jiangsu, the fruit was coated by foam method. The film thickness is 10 ~ 40 microns, and about 500 kilograms of fruit can be coated per kilogram of film. The preservation effect is obvious. ⑥ In Japan, vegetable oil is added to polymer solutions such as starch and protein to make mixed coating, and citrus and apple are sprayed to prolong the shelf life of fruits by 3-5 times and keep them fresh. This kind of coating is edible and harmless to human body.
It is also widely used to use paint as a carrier and add preservatives and colorants to improve the freshness and appearance of fruits, but such substances must meet the requirements of food hygiene and can only be used after inspection. 1 preservative. 2,4-DDT, carbendazim, thiophanate and baking soda are widely used. For example, in Nanning, Guangxi, 2.5% baking soda was added to 200ppm2, and the citrus was soaked in 4 drops for 0.5 ~ 1 min. Hunan Qianyang soaked citrus with 500 or 700 times 250ppm of 50% thiophanate, or 50% carbendazim 1000 times 250ppm of 2,4-drops. There are fungicides such as o-phenol and benzimidazole abroad. ② Colorant. In the United States, dialkylaminoalkoxy benzene is used as a colorant to induce the accumulation of lycopene in the peel, which makes the color dark and orange or red, and is used to treat citrus and other fruits.
Fruit coating method
Fruits treated with paints can be divided into small-scale treatment and large-scale treatment. In small-scale treatment, the fruit is soaked in the prepared coating liquid, or cotton cloth is soaked in the coating liquid and evenly coated on the surface of the fruit. It is required that the concave part and fruit handle of the fruit should be evenly coated, and the excess coating liquid on the surface of the fruit should be wiped off. Large-scale processing, all with fruit coating machinery. The main methods of fruit coating are dip coating, brush coating, spray coating, foaming and atomization. ① Dip coating method. A method of soaking fruit in a tank filled with coating liquid for about half a minute to form a thin coating on its surface. This method is the simplest, but it consumes more paint. ② Brush coating method. The brushing process is completed by the brushing machine erected above the conveying system. The fruit paint is distributed on the brush through the moving rod system, and then the surface of the fruit conveyed under it is brushed. In order to prevent damage to the fruit, the brush should keep the lowest effective speed (< 100 rpm), and the bristles should be soft, preferably a horse brush. ③ Spray method. Spray the fruit coating liquid on the running fruit from the hydraulic or steam pressure nozzle installed above the conveying system. The spraying rate can be adjusted by changing the nozzle model and system pressure. ④ Foam method. With the help of the foam generator erected above the fruit conveying system, the coating is coated on the fruit surface in the form of foam, and after the water is evaporated or dried by the fruit dryer, a coating with uniform texture and reasonable thickness is formed on the fruit surface. The spraying rate can be adjusted by the lifting height of the coating in the agitator and the lifting of the overflow plug. This is an ideal method. ⑤ atomization method. Paint is atomized by a paint atomizer and applied to the fruits on the conveying system.
The thickness of the coating film formed after fruit coating should be appropriate. If it is too thick, it will lead to the lack of oxygen in the fruit, weaken the respiration, worsen the flavor and quality of the fruit, accelerate the aging and even rot. At the same time, the appropriate coating should be selected according to the types and varieties of fruits.
Fruit coating machine
Most of the new wax spraying machines used in the world are composed of fruit cleaning, wiping and drying, wax spraying, low-temperature drying and grading packaging, which are continuously operated and completed at one time. The waxing classifier manufactured by American machinery manufacturing company consists of five parts: soaking tank and wooden tank elevator, water washer, dryer and waxing machine, roller conveyor belt and dryer, roller conveyor belt and classifier, citrus classifier and sorting box. Fruit coating is 4 ~ 5 tons per hour.
The trial-produced orange fruit coating and grouping machine in Shaoyang, Hunan, China, consists of four parts: fruit pouring trough, fruit coating machine, dryer and grouping machine. The whole machine can be folded into three sections, with fruit coating per hour 1. 1 ~ 1.5 tons.