1. What Freezing Does to Microorganisms and Food Components
The Japan Frozen Food Association explains that freezing food suppresses the growth of microorganisms and prevents changes in food components, allowing for long-term storage [1]. An article from the Ministry of Agriculture, Forestry and Fisheries also states that storing food at −18°C (about 0°F) or below suppresses the growth of bacteria and slows quality changes caused by oxidation and enzyme reactions [2].
What is suppressed here is the "growth" of bacteria. Freezing slows growth; it does not sterilize food. The Association’s manual includes a diagram showing the relationship between storage temperature and bacteria. It shows that bacteria causing food poisoning grow near 0 to 10°C (32 to 50°F) but do not grow near −18°C (about 0°F), which is the temperature used in the manual’s comparison for a home freezer. However, it is reported that some low-temperature bacteria or exceptional cases have grown down to around −10°C (14°F) [1].
2. Where the Storage Temperatures −18°C and −15°C Come From
You often hear "−18°C (about 0°F) or below" as the storage temperature for frozen food. However, according to the Japan Frozen Food Association, the standard in the Food Sanitation Act is "−15°C (about 5°F) or colder." This is a food-safety standard intended to prevent bacteria from growing. The −18°C (about 0°F) standard is for keeping food tasty for a long time, and the purposes are different [3].
The guideline of −18°C (about 0°F) or below is said to have been set in the "Voluntary Handling Standards for Frozen Foods," established in 1971. It is not a law, but a voluntary agreement within the industry [3]. However, the calculation or specific reason for choosing −18°C (about 0°F) was not written in the materials we read this time. If you want to know the reason, it would be good to read further in the Association’s materials.
Safety Note: According to the Japan Frozen Food Association (2014), the Food Sanitation Act has a standard of −15°C (about 5°F) or colder, which is a food-safety standard intended to prevent bacterial growth. Note that while −18°C (about 0°F) is often called a "safety standard," it is actually a quality standard.
3. How Freezing Speed Affects Ice Crystal Size
When freezing food, the range between −5 and −1°C (23 to 30°F) is called the "maximum ice-crystal formation zone." This is a temperature range where large ice crystals form easily. Large ice crystals are explained to damage food tissues and cause deterioration [4].
Therefore, "rapid freezing" is used to pass through this temperature zone quickly. The Ministry of Agriculture, Forestry and Fisheries article describes a method to pass through this zone in about 30 minutes to reduce tissue damage [2]. If ice crystals are smaller, food damage is also less [4].
Whether home freezers are "rapid" in this sense is not clear from the materials. It is better not to assume they are the same as factory equipment.
4. How Frost Forms When Ice Turns Into Gas Without Melting
The Ministry of Agriculture, Forestry and Fisheries article explains that when the freezer door is opened and closed, the temperature inside fluctuates. This causes fine ice on the food to sublime. Sublimation is the phenomenon where ice becomes gas directly without becoming water [2].
Sometimes, laundry hung outside on a cold morning dries while still frozen. This is also an example of ice becoming gas. However, the inside of a freezer is narrow and closed, so the moisture has a different path in a closed freezer.
The Japan Frozen Food Association manual explains that temperature fluctuations in the freezer cause moisture in the food to evaporate as water vapor. This vapor then freezes again and attaches as frost to the food surface or the inside of the packaging [1]. Panasonic’s explanation also states that frost is moisture that has escaped from the food [5]. In other words, frost is moisture that has escaped from the food.
5. What Freezer Burn Is and How Long to Store Frozen Food
Panasonic’s explanation states that when moisture is lost from food, it dries out and causes "freezer burn." This leads to dryness, loss of flavor, and changes in smell due to oxidation [5]. It is not burned by fire, but dried out by cold.
The Japan Frozen Food Association states that frozen foods are managed to be kept at −18°C (about 0°F) or below, and their quality is maintained for about 1 year after manufacturing. It also writes that preservatives are not needed because bacterial activity is suppressed in freezing [1].
However, this is a guideline for when the industry manages the process from manufacturing to distribution. It is not the time limit for a home freezer where the door is opened and closed many times. Its quality depends on freezer temperature, door use, and wrapping.
6. How to Investigate the Frost in Your Own Freezer
With an adult, look inside the freezer. Do not take out the food. Look for places with a lot of frost and places with almost none, and write down what you see in words. Also, look at how frost forms on the inside of the bag.
Do not leave the door open; close it in a short time. Avoid touching cold surfaces with bare hands for a long time. Consider whether the differences by location are related to temperature fluctuations by comparing your observations with the explanations in the materials.
Safety Note: Do this observation with an adult.
If you want to know more, the official websites of the Japan Frozen Food Association and the Ministry of Agriculture, Forestry and Fisheries have explanations about the basics of frozen food and storage. Try comparing the −15°C and −18°C standards in the original materials.