General situation of development
Canned food was invented by a Frenchman, N.Ap-pert 18 10 published the monograph "Method of Preserving Food in Sealed Containers", which proposed the method of food preservation, namely boiling water cooking, and pointed out that all kinds of foods sealed in bottles can be preserved for a long time, and the main foods used for canning are meat, vegetables and fruits. This was once called Apel technology. After discovering microorganisms in 1864, Louis Pasteur, a Frenchman, confirmed that the main cause of all food spoilage was the growth and reproduction of microorganisms. 1874, an autoclave using external steam and control facilities was invented, which shortened the sterilization time and improved the safety of operation, and the storage technology of storage tanks was widely popularized. From 1920 to 1923, Bigelow and C.O.Ball put forward a mathematical method to determine the relationship between the reasonable sterilization temperature and time of canned food according to the data of microbial thermal resistance and heat transfer characteristics of canned containers and canned food. Stambo further put forward the sterilization theory of canned food in 1948.
China's canning industry started at 1906, and Shanghai Taifeng Food Company invested by overseas Chinese is the first cannery in China. Since then, some canneries and empty canneries have been built in Shanghai, Ningbo and other provinces, with a maximum annual output of only over 7,000 tons. 1949, the annual output of canned food in China dropped to only 484 tons. Later, with the economic recovery, the canned food output increased steadily, rising to 1952 tons. Later, a number of key factories in the canning industry were newly built and expanded, and the total output rose from 1959 to 159600. With the change of canned food export trend, the construction of raw material base, the breeding of canned food varieties, the introduction of foreign advanced technology and equipment, and the formulation of main canned food quality standards, process operation points, testing methods and factory hygiene management system, China's canned food industry has reached a new level. By 1986, the total output of canned food in China had reached 164 1000 tons, of which canned fruit accounted for about 40-45%, ranking first.
In the 1980s, the world's total canned food output was about 40 million tons, of which the United States produced about12, and other major producers were the Soviet Union, Japan, Britain and France. The main canned fruits abroad are pineapple, peach, pear and apple. China is dominated by pineapples, oranges, peaches and apples. Canned fruits with special features include bayberry, loquat, litchi, longan, kiwi, kumquat and hawthorn.
Principle of fruit canning
The canned fruit can removes part of the air in the can, seals the can, and then kills the harmful microorganisms in the fruit and its soup in the can container to achieve "commercial sterility". Using the sealing performance of canned containers, external microorganisms can no longer enter the cans for infection and can be stored for a long time. Thermal sterilization is usually used. (See Fruit Processing). Fruit raw material belongs to acidic food because it is rich in organic acids, and its pH value is often lower than 4.5. Under this low pH condition, the heat resistance of microorganisms is greatly weakened. Therefore, most canned fruits can be killed by boiling water or sterilization temperature below boiling point.
Processing technology of canned fruit
Due to the different types, varieties, product types and product specifications of fruits, the processing technology is also quite different, but this difference is mainly manifested in the pretreatment technology of raw materials before canning, while the processing technology after canning is similar. The main process flow is as follows:
Selection and classification of raw materials
Different canned fruits have specific requirements for raw materials, so some fruits form special varieties for canning. In order to ensure the quality of canned fruit, facilitate processing operation, improve production efficiency and reduce raw material consumption, raw materials must be classified. There are two grading methods: size grading and quality grading. Generally, the size classification is divided into one grade according to the maximum transverse diameter of fruit every 5 ~ 10 mm, and small fruits such as cherry, bayberry, litchi, longan, strawberry, loquat and plum. Generally, it is divided into three levels or two levels by visual inspection. Quality grading is divided into different quality grades according to the color, maturity, shape and flavor of fruits. The grading processing of fruits has an important influence on the quality of canned food.
Shampoo or perm?
Cleaning before fruit processing needs to remove dust, sediment, some microorganisms and residual pesticides on the surface of fruit. Washing method, generally soaking first, and then washing with running water or clear water. The fruits sprayed with pesticides should be soaked in 0. 1 ~ 1.0% dilute hydrochloric acid solution first, and then washed with clear water. Small soft fruits, such as bayberry and strawberry, should be washed in small batches or with compressed air in the sink. Some fruits need to be blanched for peeling. For example, oranges are often blanched in hot water at 95 ~ 100℃ for 45 ~ 60 seconds. The loquat was blanched at 85 ~ 90℃ for 5 ~ 15 seconds.
Peeling and coring
Peeling fruits can improve the color, fragrance and quality of products. The method of peeling should adopt different methods according to the variety and maturity of fruit. There are three common peeling methods: manual, mechanical and chemical peeling. Mechanical peeling method generally adopts peeling machine, such as pineapple, apple and pear. Chemical peeling method usually uses hot sodium hydroxide solution to peel, such as peach, apricot, plum, kiwi fruit and so on. Some peaches with higher maturity can be peeled by steam jet heating. No matter which peeling method is adopted, the principle is to remove the skin, keep the fruit smooth, and prevent excessive peeling from increasing the loss of raw materials. Enzymatic browning will occur quickly after most fruits are peeled, so the peeled fruits should be immediately soaked in dilute acid, dilute salt water or mixed solution of acid and salt for color protection. Varieties that need to be pitted or cored can be pitted or manually removed. The concentration and time of alkali peeling are different according to the types, varieties, maturity and alkali temperature of fruits. Varieties that need to be precooked should be thoroughly cooked in boiling water, peeled and pitted, and then quickly cooled to inactivate enzyme activity and remove air from pulp tissue, so as to minimize the time from peeling to canning, sealing and sterilization.
Cutting and trimming
According to the types and sizes of raw materials and product specifications, large fruits should be sliced or diced. Pineapples are generally cut into disks, fans or fragments; Peaches, apples and pears can be cut into two halves or four petals. After the fruit is cut, the spots, pits, rot, discoloration and burrs on the fruit block should be cleaned up. There is a lot of air in pineapple, apple and other pulp tissues, which will lead to discoloration of products, corrosion of metal can walls and reduction of vacuum degree of cans, so vacuum degassing treatment must be taken to prevent it.
can
Comprises two parts of filling fruit and adding sugar liquid. Trimmed fruits or fruit pieces that meet the requirements of canning should be canned separately according to their sizes, shapes and colors, and the colors, sizes and shapes of fruits in the same can should be uniform. The weight of each canned fruit should be accurately weighed according to the product specification to ensure a stable quantity. Then add sugar solution, and the amount of sugar solution is generally controlled to be slightly higher than the specified net weight, so as to avoid the shortage of the net weight of the can, but a top gap of 6 ~ 8 mm (that is, the distance between the surface of the can contents and the bottom of the lid) must be left. The concentration of sugar solution can be calculated according to the soluble solid content of the fruit itself before canning, the weight of pulp in each can, the net weight of the product, the concentration of sugar solution when opening the can and the actual amount of sugar solution added in each can.
Exhaust and seal
After canning, the can should be ventilated and then sealed quickly. There are two exhaust modes: heating exhaust mode and vacuum exhaust mode. The former is heated by the exhaust box after being canned, and when the central temperature of the tank reaches the specified requirements, it is immediately taken out and sealed; The latter can be vacuumed and sealed at the same time with a vacuum sealing machine. The vacuum degree of vacuum can sealing machine is generally controlled between 300-500mm Hg, which depends on the can temperature, the size of the can and the product. There are semi-automatic, full-automatic and vacuum automatic can sealing machines for sealing. The edge of the sealed tank must meet the specified size and specifications, and the surface must be smooth and level, and there must be no defects such as cracks, ridges, sharp edges and folds, otherwise the sealing performance will not meet the requirements.
Sterilization and cooling
Canned fruit is an acidic food, and its sterilization conditions (i.e. sterilization temperature and sterilization time) should be different according to product type, pH value of canned contents, size of canned food, thermal conductivity of packaging container, close arrangement of fruit pieces in canned food, presence or absence of soup in canned food and sanitary conditions of technological process. There are two sterilization methods: water bath heating and steam heating. Water bath heating is more uniform and faster than steam heating, so it is widely used. It is best to use rotary atmospheric continuous sterilization cooler as sterilization equipment. The sterilization intensity can kill the microorganisms in the jar, make the pulp cooked properly, and improve the tissue state and flavor. Excessive heating sterilization is harmful to the quality of fruit. Therefore, under the premise of ensuring the safe storage of canned food, the sterilization temperature should be reduced as much as possible and the sterilization time should be shortened. After sterilization, canned food must be cooled quickly in time to reduce the influence of continuous heating on the quality of canned food. The cooling method can be directly immersed in flowing cold water or drenched with cold water for cooling. Cooling water should be kept clean and hygienic. Generally, the temperature in the tank should be 37 ~ 40℃ after the tank is evenly cooled. The glass tank should be cooled in stages at 80℃, 60℃ and 40℃ to prevent sudden cooling and bursting. After cooling, the water drops outside the tank should be wiped off in time, so that the water on the surface of the tank can be evaporated and dried at a slight temperature. Then put in storage for inspection, and the temperature of the inspection warehouse is 35 ~ 37℃. A week later, after inspection and sampling inspection, it was labeled and packaged into finished products.