Why does vacuum-packed food need to be refrigerated for storage?

Vacuum packed food

According to a recent article in “China Food News”, it has been highlighted that failing to refrigerate or freeze some vacuum-packed food poses a greater risk compared to leaving them unsealed.

The power of vacuum packaging

Vacuum packaging is a form of “modified atmosphere packaging” (MAP), which also includes techniques like nitrogen flushing. The deterioration of food is closely associated with the presence of oxygen. Firstly, in an oxygen-rich environment, fats undergo oxidation and develop an unpleasant taste known as “rancidity.” Secondly, the proliferation of aerobic bacteria in food can lead to its spoilage.

Vacuum sealing effectively eliminates oxygen from the food. By utilizing a vacuum packaging machine to extract the air, it inhibits the oxidation of fats and hinders the growth of aerobic bacteria. As a result, the food can maintain its freshness and quality for a significantly longer period, preserving its original state.

Vacuum packaging machine

VacuumPackaging vacuum-packaged food

The structure of a vacuum packaging machine typically comprises vacuum chamber, frame, electrical system and vacuum system.

During the operation, the vacuum pump displaces air from the chamber until the predetermined level of vacuum is attained. Once the desired vacuum level is reached, the vacuum pump ceases operation, and the pressurization valve opens. This causes the airbag to inflate and apply pressure to the sealing area. Concurrently, the heating element is activated, sealing the packaging bag.

After the sealing process, the electrically controlled air release valve opens automatically, facilitating the release of air and completing one cycle of the packaging operation.

Understanding the dynamics of bacteria and the menace of botulinum toxin

In the natural environment, a diverse array of bacteria exists. When air is present, aerobic bacteria thrive while anaerobic bacteria are suppressed. However, once the vacuum sealing process is carried out, aerobic bacteria are inhibited, creating an environment where anaerobic bacteria can flourish effortlessly.

Although the growth of anaerobic bacteria may not result in sourness, foul odor, or stickiness in food, they possess the ability to produce toxins. Of particular importance is Clostridium botulinum, commonly referred to as “botulinum bacteria” or “botulism,” which is known for its toxin production.

Botulinum bacteria is widely present in nature and can be found in various food sources such as vegetables, fish, meats, and poultry. While the bacterium itself is not highly hazardous or resilient, routine cooking methods are generally effective in eliminating it. However, its true threat lies in its ability to form spores. These spores are remarkably resistant and can survive for several hours even in boiling water under neutral conditions. 

Once the conditions become favorable (such as room temperature, lack of adequate preservatives, and a non-acidic environment), the spores can germinate and produce botulinum toxin, which is regarded as one of the most potent biological toxins known, with a lethal dose in the microgram range.

How can the food industry prevent botulism poisoning?

Although Clostridium botulinum is widely present and its toxin is highly potent, the actual occurrence of poisoning cases is relatively low. This can be attributed to several factors. Firstly, in home cooking or dining at restaurants and cafeterias, most meals are freshly prepared and consumed immediately, leaving little room for the proliferation of botulinum bacteria. Moreover, in industrial food production, adherence to proper guidelines helps ensure prevention. In essence, the basic techniques employed by the food industry to avoid the formation of botulinum toxin in cooked foods include the following:

  • Low-moisture foods, such as nuts and biscuits, have minimal water content, making it unfavorable for bacterial growth. The utilization of vacuum packaging not only helps inhibit oxidation but also prevents the formation of botulinum toxin.
  • High-acid foods (pH < 4.6) can be safely cooked as regular heat kills botulinum spores, preventing toxin formation.
  • Neutral, high-moisture processed foods can effectively prevent botulinum toxin production by adding nitrite additives, such as in ham sausages.
  • Neutral, high-moisture processed foods, whether vacuum-sealed or not, can be safely stored refrigerated or frozen. The low temperature inhibits Clostridium botulinum growth, preventing botulinum toxin formation.
  • Neutral, high-moisture foods without preservatives can be safely packaged under sterile conditions after undergoing ultra-high temperature (above 135°C) heating, effectively eliminating Clostridium botulinum spores and preventing botulinum toxin formation.
  • Neutral, high-moisture, preservative-free foods can be canned using high temperatures (≥121°C) for over 20 minutes to kill Clostridium botulinum, preventing botulinum toxin growth during room temperature storage.

Ready-to-eat foods obtained from reputable sources generally meet these requirements. Neutral foods, such as ham, beef, and soy products, undergo thorough sterilization at ultra-high temperatures, while high-acid foods like sauerkraut and pickles are boiled to eliminate spores completely. Once vacuum-sealed, these foods can be safely stored at room temperature without any concerns.

In reality, certain small shops and households lack the capability for ultra-high temperature sterilization. Vacuum packaging and transportation without proper temperature control and preservatives pose a potential risk for botulism toxin formation and poisoning.

vacuum-packaged food

Vacuum packaging cooked food enhances flavor and texture, but refrigeration is essential post-vacuum sealing to minimize safety risks.

When choosing vacuum-packed food, carefully observe for any signs of bulging, indicating microbial growth and gas production. Additionally, pay attention to the storage instructions. If the label indicates room temperature storage, it suggests that the food has undergone high-temperature treatment or contains preservatives. For items labeled as requiring refrigeration, or those without labels or self-vacuumed foods, it is crucial to refrigerate or freeze them accordingly.

Coolink, a TOP 3 Global HVAC Vacuum Pump Manufacturer & Quality Industrial Vacuum Pump Supplier from China

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