1. What Three Conditions Do Microbes Need to Grow, and How Does Cold Help?
One major cause of food spoiling is that microorganisms multiply and change the food. It is explained that bacteria need three things to grow: temperature, moisture, and nutrients [1]. Ordinary foods already have moisture and nutrients. If the temperature is comfortable for humans, all the conditions are met.
The speed of growth varies greatly by the type of bacteria. Under conditions close to optimal, Vibrio parahaemolyticus bacteria are said to divide once every 8 minutes, while Staphylococcus aureus divides once every 27 minutes [3]. If we assume this speed continues for 1 hour in a simple calculation, dividing every 8 minutes results in roughly 180 times the original amount. Of course, they do not always grow at this speed. The temperature range where they grow easily also differs by bacteria, and some sources use 20–50°C (68–122°F) as a guideline [2].
The Ministry of Health, Labour and Welfare sets a guideline that refrigerators should be at 10°C (50°F) or below, and freezers at −15°C (5°F) or below. Making things cold slows down how they multiply, but it does not kill the bacteria [1].
2. What Decides Whether Microbes Cause Fermentation or Spoilage?
When microorganisms change food, the change is called fermentation if it is favorable for humans, and spoilage if it is unfavorable. An explanation from Tokyo University of Pharmacy and Life Sciences states that this distinction is based on human evaluation [4]. The "Encyclopedia of Food Safety" also explains that among the actions of microorganisms, those useful to human life are called fermentation, and those harmful are called spoilage [5]. In other words, both involve food changing due to microbes, but the name depends on whether we view the change as useful or harmful.
Moreover, this line shifts depending on people and culture. The same encyclopedia cites natto as an example of a food that some people like and others dislike [5]. We call yogurt fermentation and changed rice spoilage because most people evaluate those changes that way.
While the "Encyclopedia of Food Safety" defines fermentation as useful and spoilage as harmful [5], how this distinction connects to daily safety decisions is a separate issue, which we will handle in the next section.
3. Can You Tell Spoiled Food by Its Smell?
Bacteria that cause spoilage create smelly substances through metabolism, so it is said that we can easily notice them through strange odors or changes in appearance [3]. On the other hand, bacteria that cause food poisoning may look and smell almost the same as normal food, even while present in the food [3]. Other materials explain that even when food poisoning bacteria multiply, there is often no change in smell or taste [6].
From this, two opposite misunderstandings become clear. Just because something does not smell does not mean it is safe. Conversely, just because something is rotten does not mean you will definitely get food poisoning. It is safer to think of the smell of spoilage and the danger of food poisoning as separate things.
Even if there are no abnormalities in smell or appearance, you cannot judge it as safe.
4. What Are the Visible and Hidden Parts of Mold?
The body of mold is made of thin, thread-like hyphae and spores. The colors you see on the surface of food are mostly the color of the spores. Below the surface and inside the food, hyphae may be spreading [7]. If you compare it to a flower, what you see is the flower, and there is a part spreading like roots underneath. However, hyphae are not real roots.
Materials from Shinagawa Ward point out that hyphae can penetrate the invisible parts and that some types produce mycotoxins. They recommend throwing away the entire item rather than removing just a part [8]. Not all mold produces poison; it is only "some types." The Ministry of Agriculture, Forestry and Fisheries also publishes information on mycotoxins.
On the other hand, mold is not always a villain. There is blue mold used in making cheese, and koji mold essential for fermented foods [8]. When mold changes food, if the change is favorable for humans, it is called fermentation; if unfavorable, it is called spoilage. This connects with the distinction made in Section 2.
5. What Do Expiration Dates and Best-Before Dates Mean?
There are two types of date labels. The "expiration date" is the "date until which it can be eaten safely," and it is attached to perishable foods like bento boxes, prepared bread, and fresh sweets. The "best-before date" is the "date until which quality such as taste is maintained," and it is attached to less perishable foods like snack cakes and canned goods [9].
The best-before date is the limit for unopened food stored according to the label. Just because it has passed does not necessarily mean there is an immediate food hygiene problem. It is said that food past its expiration date should not be eaten [10]. Once opened, it is a different matter.
These two are also determined differently. According to guidelines from the Consumer Affairs Agency, expiration dates are set prioritizing safety tests such as microbial tests, while best-before dates are set prioritizing quality tests such as physical and chemical tests and sensory inspections [11].
The expiration date is the date for safe eating, shown on perishable foods like bento and prepared bread. The best-before date is the date for maintained quality, shown on less perishable foods like snacks and cans [9]. According to the Consumer Affairs Agency guidelines (March 2025), expiration dates prioritize safety tests, and best-before dates prioritize quality tests [11].
6. Which Dates Are on the Food Labels in Your Home?
With an adult, look at food packages at home and see what date labels they use. In Japan, common labels include “expiration date” and “best-before date.” Which one do bento boxes, bread, snack cakes, and canned goods have? If you first guess "which one is more perishable" and then check, you can feel the meaning of Section 5 of this article. You do not need to open them.
If you want to know more, explanations of date labels from the Consumer Affairs Agency, pages on food poisoning prevention from the Ministry of Health, Labour and Welfare, and information on mycotoxins from the Ministry of Agriculture, Forestry and Fisheries are available in Japanese. Some can be traced from the source list of this article.