The Bursting Problem of High-Temperature Retort Pouches, Completely Solved!

[Key Points] High-temperature retort pouches are a high-end product in flexible packaging. On one hand, profits are high: the profit per pouch is several times that of ordinary pouches of the same specification. On the other hand, risks are high: there are many problems caused by discoloration, reduced peel strength, pouch bursting and so on. In mild cases, downstream manufacturers return the goods and orders are lost; in severe cases, compensation must be paid for a large sum covering the contents of downstream products during storage. An analysis is now made of the frequently occurring bursting problem.

1. First, analyze the boiling pouch: water is filled into the pouch as specified, air is exhausted and the pouch is sealed.

After careful observation, it was found that the air had not been completely exhausted. Its heating process before 100℃ is shown as follows: the sealed glass ball represents the closed pouch; it can be seen that the expansion of the boiling pouch during heating is normal.

The reason is analyzed as follows: when water is filled and the pouch is sealed at room temperature, the pressure inside and outside the pouch is balanced at this time. During heating from room temperature to 98℃, the liquid water itself expands, the air inside the pouch expands, and part of the water vapor generated at this time also occupies part of the space, causing expansion.

At this time, the pressure inside the pouch will increase. To support the above conclusion, we used the ZY-1 positive pressure tester and the DK-98-1A water bath from Beijing Landmark to conduct a qualitative test. During heating, the pressure inside the pouch did indeed rise, and the degree of increase was related to the ratio of the filled sample and residual air to the maximum volume of the pouch, and to the amount of residual air. The higher the ratio of filled water to the pouch, the higher the pressure; the more residual air, the higher the pressure.

Pouch bursting is caused by the creep phenomenon of the polymer material at the seal under this hot state, which is insufficient to withstand the pressure; this strength is what we call hot burst strength. To compare cold and hot burst strength, we used the ZY-1 positive pressure tester at room temperature and 95℃ respectively to test the burst strength at different temperatures. A spot check of 20 groups showed that for well heat-sealed pouches, the burst strength at room temperature differs greatly from that at 95℃; the reason is analyzed to be the reduction in strength upon heating, which is a normal situation encountered in our production.

If the pouch is not well heat-sealed, its burst strength at room temperature is very small, and it is a defective product in production; although there is no leakage, it is more prone to bursting when heated.

1. Retort pouch problem: before the retort pouch is heated to 100℃, its changes are the same as those of the boiling pouch.

We examined the retort pouch when heated to above 100℃. Because there is currently no instrument in China that can observe the retorting process, and the sensor of the ZY-1 positive pressure tester cannot extend into the T-25C counter-pressure retort, we can only speculate that inside the retort, when heating exceeds 100℃, the pressure inside the pouch is always higher than the pressure outside the pouch (this speculation currently has no factual support; we are developing a retort with a transparent observation window in order to solve this problem).

1. Can pouch bursting be avoided?

Pouch bursting occurs when the pressure inside the pouch is higher than outside. Is there a way to avoid this situation? From the boiling process we learned that bursting is caused by the pressure difference formed when the internal pressure is high while the ambient pressure remains unchanged. Can it be avoided if the environmental changes are synchronized?

We closed the exhaust of the TS-25C counter-pressure retort, forming a closed environment from the moment the sample was placed inside, and set it to 115℃ (it could not be set higher due to the safety valve) for retorting. A total of 30 retort pouches were tested, and no bursting was observed. It can be imagined that this is equivalent to applying an external pressure to the pouch from the start of heating, balancing the internal and external pressures, so there was no bursting.

1. Conclusion:

(1) The retort bursting problem of high-temperature retort pouches is actually a problem of the hot burst strength of the pouch, generally caused by damage at the heat seal.

(2) If an external pressure is applied to the retort pouch in advance and this relative external pressure is maintained throughout the heating process, bursting should not occur; however, this requires high pressure resistance of the retort body. For example, at 135℃, the normal working pressure resistance is required to increase from 2.2kg to 3.2kg.

In this view, pouch bursting is related to the test method and also to the production method. Improving the high-temperature sterilization process is of great significance for reducing bursting. This article is intended to offer a modest contribution, and we hope it can help further improve food packaging.