Pressure during sterilization in canning containers
As noted, the sterilization of canned food in banks creates an overbalance of internal pressure over pressure in autoclaves. This is due to the fact that in the cans above the product mirror there is a gaseous medium consisting of a mixture of air and steam, and in an autoclave the atmosphere consists only of steam. Therefore, the excess pressure in the container over the pressure in the autoclave is determined by the air pressure in the cans.
Thus, the pressure during sterilization in a canning container of glass is greater than the pressure in the tin, and this is due to the fact that the air space in the glass jar is compressed when heated (the product volume increases, and the volume of the jar hardly changes). The volume of the vapor-air space in the can is practically unchanged (since both the volume of the product and the volume of the container increase due to the expansion of the lids and bottoms). The multiplier and indicates the degree of air compression during sterilization of canned food in tin containers. Practice (and calculations) showsthat this compression ratio is in the range of 2–5 and, therefore, the overpressure in glass containers reaches high values, at which the lids can be broken from the neck of the cans and even the destruction of the containers. Therefore, canned food in glass containers, especially in wide-mouth, always sterilized with back pressure.
The main reason that causes a significant (sometimes significant) pressure in the container over the pressure in the autoclave is the presence of air in the container, since the main measure that prevents the occurrence of excessive pressure is the removal of air from the can with the product before sinking. As noted in Chapter 4, such a process in the canning industry is called exhauster.
There are different ways of exhausing, but the most effective is thermal.
If the product temperature during packaging is increased (for example, by passing the liquid product through the heat exchanger steam pipes or heating the unbettled jar with the product in the heat exhauster): this means increasing and reducing the overall pressure.
If it was possible to raise the temperature of the product by rolling to 100 ° C, 0.1 would be zero and, consequently, neither in the tin nor in the glass container would there be absolutely any excess pressure during sterilization.Of course, practically this temperature during packing cannot be achieved.
It must be said that increasing the temperature of the product during the packaging not only reduces the overpressure in the can during sterilization, but also increases the vacuum in it during cooling after sterilization. The approximate relationship between the elasticity of water vapor during sealing (which is a function of the appropriate temperature) and the amount of vacuum during cooling can be expressed by the formula
For example, at a low temperature of the product at the time of rolling 45 ° C, the vacuum during cooling will be only. If, on the other hand, the temperature of the product is raised during roll-up to 89 ° C, then the vacuum during cooling will increase markedly and will be 0.07 MPa or 0.07 x 7,500 = 525 mm. Hg st.
The presence of vacuum in canned food has a positive value. First, it is an indicator of a reduced oxygen content in the bank and, consequently, a reduced level of undesirable oxidative processes in the product during storage. Secondly, in the presence of a vacuum, the bottoms of the cans turn out to be somewhat pressed inward by an overlap of atmospheric pressure over the inner, and the drawn in ends are the first visual sign of the good quality of canned food (as a rule, swollen ends indicate microbial spoilage,and those drawn in are of normal quality, although there are exceptions to this rule). Thirdly, during the transportation of canned food through hot climatic zones in banks, an increased (above atmospheric) pressure may occur. At the same time, their ends will swell and stresses will arise in the bank due to external pressure from the cans and containers located above the can. This is not safe for the strength of cans.
If, due to the vacuum, the ends of the cooled cans are somewhat drawn inwards, then as the storage temperature rises and, consequently, the pressure in the can, its ends do not swell, they will only straighten and unwanted stresses will not occur.
Thus, heat exhaustion is an extremely efficient technological process. The higher the temperature of the product during rolling, the lower the excess pressure during sterilization and the greater the vacuum during cooling.
At the same time, there are cases when the presence of a high vacuum in a container causes undesirable phenomena that complicate the production process and even lead to a product marriage. This mainly applies to the packing of very hot products into large tin containers, as is practiced, for example,when canning tomato paste by the so-called “hot pouring” method. As will be outlined below, with this method, a very deep emptying occurs, as a result of which, due to a significant overbalance of the external atmospheric pressure over the inner side, the lateral surface of the cans is pressed inward. Formed deep dents on the body, disfiguring the appearance of the container and unsafe for the strength of the longitudinal and sealing seams cans. This phenomenon has received the name of vacuum deformation and, unfortunately, till now not enough effective measures on the prevention of this kind of marriage have been found.
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