1. How Are Canned Foods and Retort Pouches Alike in Keeping Food Without Refrigeration?
Canned foods and retort curry pouches on supermarket shelves can both be stored without refrigeration. The basic process is the same for both: food is sealed in a container and sterilized by heating it in a high-pressure sterilization vessel [1]. The main difference lies in the container. Canned food uses metal cans, while retort foods use thin pouches or semi-rigid molded containers [1]. The term "retort" originally referred to a distillation flask used in chemistry, but in food processing, it means a high-pressure sterilization pot [1]. Does this mean the sterilization for pouches is weaker? To answer this, we must look at the specific temperatures and times used.
2. How Does Retort Sterilization Use High Heat to Kill Spore-Forming Bacteria?
A major challenge in food sterilization is bacteria that form spores, such as *Clostridium perfringens*, *Clostridium botulinum*, and *Bacillus cereus*. For spore-forming bacteria, the comparison is 400 minutes at 100°C and 4 minutes at 120°C [1]. Dividing 400 by 4 shows that the time is reduced to one-hundredth. Raising the temperature by just 20°C shortens the time dramatically. It also causes far less damage to the food itself [1]. This standard of "120°C for 4 minutes" is based on measures against *Clostridium botulinum* [2]. Retort foods are defined as foods sterilized by reaching a core temperature of 120°C for at least 4 minutes [1]. Being in a pouch does not mean the sterilization is weak. The "4 minutes" refers to the time after the center of the food reaches that temperature, not the total time in the pot [1]. Yet, the world’s first retort curry reportedly lasted only a few months. If the sterilization was this strong, what was missing?
3. Why Did the First Bon Curry Pouch Last Only 2 to 3 Months?
On February 12, 1968, Otsuka Food Industries (now Otsuka Foods) launched "Bon Curry" [3]. The JAF Mate article identifies this as the world’s first commercial retort curry [3]. It is reported that the company tried many times to perfect the pouch’s heat resistance, strength, and sterilization conditions before finalizing the product [4]. The origins of retort food are said to be in the 1950s, when the US Army developed them as portable food to replace cans. They later gained attention when used as space food in the Apollo program [3]. The pouch at launch was made of two layers: polyethylene and polyester. It was semi-transparent. Because it allowed oxygen and light to pass through easily, the shelf life at room temperature was only about 2 to 3 months [3]. Even if heat killed the bacteria effectively, if the pouch let air and light in, the contents spoiled faster. How did the manufacturer address this weakness?
4. How Did Adding Aluminum Foil Extend the Pouch's Shelf Life to 2 Years?
In the following year, 1969, the pouch changed to a three-layer structure: polyester, aluminum foil, and polypropylene. The shelf life extended to 2 years [3]. Since 2 to 3 months became 24 months, this is an increase of roughly 8 to 12 times. Metal foil does not let light pass through. Cans are also metal, so they share this trait. Adding aluminum foil made the pouch similar to cans in blocking light. However, no data was used here regarding the thickness of the aluminum foil or exactly how much each layer blocks light versus oxygen. It is safer to view the change in shelf life as the result of the entire switch to a three-layer structure. Pouch foods come in two types: transparent pouches where you can see the contents, and pouches using aluminum foil [1]. The difference in appearance is also a difference in how much light they let through. So, which is better: the can or the pouch?
5. What Are the Different Strengths of Cans and Retort Pouches?
Retort foods were developed as a container to replace cans. Their advantages are said to be that they are light, easy to handle, can be heated in a short time, and are easy to discard [1]. On the other hand, cans are metal and do not let light through. Pouches can block light if they use aluminum foil, but transparent pouches are also made [1]. In other words, pouches approached the can’s strength of blocking light by using aluminum foil, while keeping their own strengths of lightness and ease of heating. This does not mean all pouches are the same; their properties change depending on the type of pouch. What can we observe in our own kitchens?
6. What Two Things Can You Check on a Retort Pouch at Home?
If you have unopened retort food at home, you can check two things. First, look at the best-before date on the back of the pouch. You cannot tell how many months the shelf life is from the date alone. However, you can confirm that the length of the shelf life differs from pouch to pouch. Second, hold the pouch up to window light to see if it is transparent. Pouches come in types that use aluminum foil and transparent types where the contents are visible. If a pouch is not transparent, it may contain aluminum foil [1]. Try lining up several types and sorting them into those that let light through and those that do not. You cannot determine the material just by looking at the outside of the pouch. Please do not open, cut, or heat the pouches; just look at them. The thinking behind sterilization standards is found in the Ministry of Health, Labour and Welfare’s measures against botulism food poisoning [2].