The so-called sterilization of food, as the name suggests, is to kill all the microorganisms in the food. However, what can be called "sterilization" in canned foods is different from the sterilization of the bacteria. The latter refers to absolute sterility and thus the term "sterilization" is used. If the sterilization of canned foods is to such an extent, the temperature and time of sterilization will be greatly increased. This will inevitably affect the quality of food. That is to reduce the flavor and nutrition of the food, and thus lose the edible value. In order to ensure the color, aroma, taste and nutritional value of the food, the sterilization of canned food can only require that the food does not contain human health or harmful pathogenic bacteria after heating to a certain extent. At the same time, it can inhibit the activity of non-pathogenic microorganisms that make food corrupt under normal storage conditions. Thereby achieving the shelf life specified by the can storage. This sterilization of canned food is also referred to as "commodity sterilization." (1) Preheating stage: It is necessary to rapidly heat the material by steam circulation, and it is required to raise the temperature in the autoclave to 100 ° C ~ 106 ° C in 5-6 min.
According to research, there are many factors affecting the sterilization effect of canned food, such as the degree of contamination of food before sterilization, food composition, heat transfer, and initial temperature of can. Briefly introduce the following:
(1) The degree of contamination of food before sterilization
From raw material processing to filling sterilization, foods are subject to different levels of microbial contamination, and the higher the pollution rate, the longer the sterilization takes at the same temperature. Different types of microorganisms have different degrees of heat resistance, yeast 40 ~ 70 ° C, thermophilic bacteria 75 ~ 80 ° C, botulinum A, B spores to 100 ° C after 6 hours or at 120 ° C for 4 minutes Can kill. The more microbial spores, the higher the temperature required for sterilization and the longer the sterilization time.
(2) Food ingredients
Canned foods contain sugar, salt, protein, fat and other foods that can affect the heat resistance of microorganisms and contain phytoncides, such as peppers, onions, etc., which have the effect of inhibiting or killing microorganisms.
The acidity in the food has a great influence on the heat resistance of the microorganism, and the undissociated organic acid molecule easily permeates into the living cells of the bacteria and dissociates into ions, thereby transforming the internal reaction of the cells and causing cell death. Therefore, foods with high acidity generally have a lower sterilization temperature and a shorter time.
(3) Heat transfer
When the can is heat-sterilized, the heat transfer mode mainly includes conduction and convection.
1 Types and types of canned containers: Tin-plated steel cans have a faster heat transfer rate than glass cans, and small cans transfer heat faster than large cans. With the same volume of cans, flat cans transfer heat faster than short cans.
2 Types of food and canned state: Liquid foods transfer heat faster, but the heat transfer rate of sugar liquid, salt water or seasoning liquid decreases with the increase of its concentration. Solid foods such as: luncheon meat, crab meat, etc., have a slow heat transfer rate. The blocky food plus soup is faster than the soup without heat transfer. Larger blocks are smaller than blocklets and heat transfer is slow. Tightly packed heat transfer is slower.
3 The position of the sterilization pot and the position of the can in the sterilization pot: The rotary sterilization has a larger sterilization effect than the static sterilization, and the time is short. The can is kept away from the intake line in the sterilizing pot, and the heat transfer is slow when the temperature in the pot has not reached equilibrium. The air removal amount in the pot, the accumulation of condensed water, and the structure of the sterilization basket all affect the sterilization effect.
4 cans of initial temperature
The temperature of the center of the can before sterilization is closely related to the sterilization effect. Before the sterilization, the initial temperature of the food in the tank should be increased (such as increasing the temperature of the food and the soup when filling the tank, and sterilizing after the exhaust seal), which is more important for the cans which are not easy to form convection and slow heat transfer.
There are strict requirements for the sterilization process of canning. There are different process curves for different varieties. It can be divided into four stages: heating, heating, heat preservation and cooling in chronological order. There are different requirements for temperature, time and pressure in different stages. At present, most of the sterilization of canned food adopts the method of steam heating and cold water cooling, and the main equipment for sterilization is a sterilization kettle. The sterilization process requirements are as follows.
(2) Heating stage: The temperature in the kettle rises at a certain rate, and rises to 127 ° C in about 10 minutes, and the corresponding vapor pressure will reach 1X105Pa.
(3) Insulation stage: keep the pot temperature at 121 °C, the steam pressure in the kettle is kept constant at 1X105Pa, and the holding time varies with the type of material, generally ranging from 15 to 30 minutes.
(4) Cooling stage: the pot temperature is reduced from 12l °C to below 40 °C within a certain period of time. A very important requirement at this stage is to keep the pressure in the kettle basically unchanged. However, while the cold water is added to cool the kettle, the pressure in the kettle is often greatly affected. When a large amount of cold water is injected into the kettle, the pressure will be rapid. Decline, need to be controlled in time. In addition, there are corresponding requirements for cooling time.
Most microorganisms have an optimum pH in the range of 6-7. In acidic and highly acidic foods, the heat resistance of microorganisms and their spores is greatly reduced. Therefore, the acidity of the food is different, and the corresponding sterilization temperature is also different. At present, the heat sterilization temperature of canned food can be roughly divided into two categories:
1. Low-temperature sterilization Some foods such as fruits and some vegetable foods cannot withstand high-temperature heating, and the tissue morphology of fruits and vegetables becomes soft at high temperatures. The color scent and flavor are reduced. Therefore, for such high-acid foods, the temperature can be lower than 100 ° C, depending on the variety and specifications. Usually in 10-30 minutes. However, the use of such temperature and time for low-acid canned fruits and vegetables cannot achieve the purpose of "commodity sterilization". In order to achieve both the quality of the food and the purpose of sterilization, a low temperature gap sterilization method can be employed. That is, the can is first sterilized at a temperature of 60-70 ° C. After being taken out, it is left at room temperature for a certain period of time to allow the microorganisms remaining in the tank to multiply and develop. The cans are then sterilized at a low temperature to achieve the purpose of sterilization of the product.
2. High temperature sterilization of meat, aquatic products and certain canned vegetables. Such low-acid foods use higher temperatures because of their high heat resistance. Usually higher than 100 ° C, up to 121 ° C. The time should be extended accordingly. About 60-90 minutes. The long-term sterilization of certain foods will cause great loss of product quality and nutrients. For this reason, the "high temperature short time" sterilization method is also used in the canning industry. The temperature is greater than 121 °C. Commonly used are 127 ° C, 135 ° C, up to 150 ° C time in minutes to seconds. This sterilization is best for the sterilization effect of fluid foods and cans using a rotary sterilization device.
In addition, according to the sterilization device, the sterilization mode of the can is divided into: intermittent sterilization and continuous sterilization.