2.1　Know Your Enemy: Bacteria, Viruses, and Parasites

The things that cause food poisoning fall into three broad groups. They differ in how they multiply and in what kills them. If you mistake one for another, you may keep using measures that take effort but do not work.

Bacteria: They Grow in Food

Bacteria are tiny living things made of a single cell. When nutrients, water, and the right temperature are all present, they multiply in food by doubling: 1 becomes 2, and 2 become 4. Some bacteria produce toxins in food. A toxin is a poison that bacteria make. Even when heat kills the bacteria, the toxin may remain.

Viruses: They Do Not Grow in Food

A virus is a tiny particle, tens of times smaller than a bacterium. Viruses can multiply only inside the living cells of people or animals. So no matter how much time passes, they do not multiply on food. Instead, a very small amount is enough to infect a person. The virus that matters in sushi restaurants is norovirus. It withstands refrigeration, freezing, and acid, and alcohol does not work well against it. What reliably works is heat and sodium hypochlorite (Chapters 3 and 4).

Parasites: They Do Not Grow, but Freezing and Heat Kill Them

A parasite is a small animal that lives inside the body of another living thing. Parasites do not multiply in food either. The typical example is Anisakis, a larva that looks like a white thread 2 to 3 cm long. Freezing and heat kill it, but vinegar, salt, wasabi, and soy sauce do not.

Table 2-1 sums up the differences between the three. Remember it this way. Bacteria multiply, and freezing does not kill them. Viruses do not multiply, but freezing does not kill them either. Parasites do not multiply, and freezing kills them.

Infection Type and Toxin Type, Kudoa, and Molds and Yeasts

Bacterial food poisoning falls into two types. The infection type occurs when you eat live bacteria and they multiply in your intestines. Examples are Vibrio parahaemolyticus, Salmonella, and enterohemorrhagic E. coli. The toxin type (intoxication) occurs when you eat a toxin that was produced in the food. Examples are Staphylococcus aureus, Clostridium botulinum, and Bacillus cereus. The way bacteria multiply is called binary fission: 1 cell splits into 2.

Besides Anisakis, another parasite is Kudoa (Kudoa septempunctata), which is found in olive flounder (hirame). You can see Anisakis with your eyes, but you cannot see Kudoa. Kudoa dies when frozen at −20°C for 4 hours or more, or heated to a core temperature of 75°C for 5 minutes or more.

The fourth group is molds and yeasts. Both belong to the fungi. Molds grow by spreading thread-like filaments, and yeasts multiply by budding. They can grow in drier places than bacteria can, and they are fairly tolerant of acid. In a sushi restaurant, they grow on dried foods, sweet vinegar, and wooden tools. Freezing stops their growth, and heat kills them. However, some molds produce mycotoxins that heat does not destroy. That is why you do not use moldy ingredients.

2.2　Six Conditions for Bacterial Growth and the Temperature Danger Zone

Six conditions decide whether bacteria grow: nutrients, water, temperature, time, acidity, and oxygen. Acidity is expressed as pH. pH is a number that shows how acidic or alkaline something is. 7 is neutral, and the lower the number, the more acidic. The lower the pH, the harder it is for bacteria to grow.

Every hygiene procedure in a sushi restaurant exists to cut off one of these six. Fish flesh is full of nutrients and water, a feast for bacteria. You cannot take away the nutrients and water. So the reins the chef holds are temperature, time, and the acid in vinegar.

The Temperature Danger Zone: A Lunch Box in a Summer Kitchen

Most bacteria that cause food poisoning grow best at around 35 to 37°C, close to human body temperature. The range of temperatures in which bacteria grow actively is called the temperature danger zone. In Japanese hygiene guidance, it is 10 to 60°C. Many standards outside Japan use 5 to 60°C, and the U.S. FDA uses 5 to 57°C. A lunch box left in a kitchen on a summer morning may be spoiled by noon. That is because it sat in this temperature range for half a day.

10°C in Japan, 5°C Elsewhere: Mesophiles and Psychrotrophs

Most food poisoning bacteria are mesophiles. These are bacteria that grow at 20 to 45°C and grow best at around 35 to 37°C. Japan uses 10°C as its dividing line because most refrigerated storage standards under Japan's Food Sanitation Act are 10°C or below. The standard for seafood eaten raw is also 10°C or below. Vibrio parahaemolyticus also grows much more slowly at 10°C or below, and almost stops at 4°C or below (Chapter 3).

However, some bacteria, such as Listeria, grow slowly even at 0 to 5°C. Bacteria that can grow at 10°C or below are called psychrotrophs. This is why many countries outside Japan draw the line at the lower temperature of 5°C (Chapter 5).

Oxygen: The Pitfall of Vacuum Packing

Bacteria fall into three types by how they respond to oxygen. Aerobes cannot grow without oxygen. They include Pseudomonas, the main cause of spoilage in refrigerated fish. Obligate anaerobes cannot grow when oxygen is present. They include Clostridium botulinum and Clostridium perfringens. Facultative anaerobes can grow either way. Many food poisoning bacteria, such as Vibrio parahaemolyticus, belong to this group. Vacuum packing holds back the spoilage bacteria that like oxygen, so food keeps longer. But at the same time, it creates a favorable environment for bacteria that dislike oxygen.

2.3　Doubling and Doubling Again: Why Time Is Frightening

The time it takes for bacteria to split once and double in number is called the doubling time. It is also called the generation time. Under good conditions, Vibrio parahaemolyticus doubles in about 10 minutes, E. coli and Salmonella in about 20 minutes, and Staphylococcus aureus in about 30 minutes.

Bacteria that double every 20 minutes become 8 times as many in 1 hour, 512 times in 3 hours, and about 260,000 times in 6 hours. Vibrio parahaemolyticus, which doubles every 10 minutes, becomes about 260,000 times as many in 3 hours, and about 16.8 million times in 4 hours. It is like folding a sheet of paper in half: each fold doubles the thickness. Time works not by "adding" but by "multiplying."

Time Adds Up

One more important point: time adds up. When you put food back in the refrigerator, bacteria grow more slowly. But the bacteria that multiplied while the food was out do not go away. Suppose fish was out for 30 minutes during morning prep, 1 hour during the lunch rush, and another 30 minutes in the evening. In total, it spent 2 hours in the temperature danger zone. Guidelines in many countries say, "Throw it out once the total time in the temperature danger zone exceeds 4 hours." This also comes from the idea of adding up the time.

The Lag Phase and the Number of Bacteria That Make People Sick

The numbers in Table 2-2 are calculations for good conditions. In reality, bacteria that move into a new environment do not start splitting right away. First there is a lag phase, when they get ready. Next they enter the log phase, when they double again and again. Finally they reach the stationary phase, when growth stops. The curve in Figure 2-2 shows these three stages.

The lag phase lasts longer at lower temperatures, and also when the bacteria have been weakened by refrigeration. This is the point of taking food out in small amounts and "putting it back in the refrigerator before the food temperature rises." If the food temperature does not rise, it is hard for bacteria to get out of the lag phase.

The number of Vibrio parahaemolyticus needed to make a person sick is generally said to be 100,000 to 1,000,000 or more. Suppose a fish with 100 bacteria on its surface sits on a summer prep counter for 3 hours. If they double every 10 minutes, the calculation gives tens of millions, far beyond the level that makes people sick. The sense you should have is this: "Time at room temperature has turned into doubling numbers."

2.4　Water Activity and pH: Why Salt, Sugar, and Vinegar Hold Back Bacteria

Water Activity: The Water Bacteria Can Use

Water that is bound to salt or sugar cannot be used by bacteria. The proportion of free water that bacteria can use is called water activity. Its symbol is Aw, and pure water is 1.00. Raw fish has a water activity of about 0.99, almost the same as pure water.

Most food poisoning bacteria cannot grow when water activity falls below 0.90 to 0.91. But Staphylococcus aureus can grow down to about 0.86, so you cannot let your guard down even with salty foods. Molds can grow down to about 0.80. Salt and sugar protect food not by killing bacteria. They protect it by pulling water around themselves and reducing the water bacteria can use. When you cure fish with salt, water is drawn out, and at the same time the remaining water binds to the salt, so the water activity falls (Chapter 13).

pH: Each Kind of Bacteria Has Its Own Limit

Along with temperature, pH strongly affects how bacteria grow. Most food poisoning bacteria grow best at pH 6.5 to 7.5, and the lower the pH, the harder it is for them to grow. However, where growth stops differs for each kind of bacteria. Clostridium botulinum does not grow at 4.6 or below, and Vibrio parahaemolyticus does not grow below 4.8. On the other hand, if other conditions are right, Bacillus cereus can grow down to about 4.3, Listeria to about 4.4, Staphylococcus aureus and pathogenic E. coli to about 4.0, and Salmonella to about 3.7. "Most bacteria do not grow at pH 4.6 or below" is wrong. 4.6 is the value for Clostridium botulinum.

Tolerance of acid also differs by bacteria. Vibrio parahaemolyticus is weak against acid, while enterohemorrhagic E. coli O157 survives a long time even in acidic conditions. Some everyday pH values: rice vinegar is about 2.5 to 3.0, freshly cooked rice is 6 to 7, and vinegared rice is 4.0 to 4.5.

Stacking the Effects: Why Vinegared Rice Spoils Less Easily

Vinegared rice spoils less easily than plain rice because two mechanisms overlap. The main player is acetic acid. It lowers the pH and gets inside bacteria as undissociated molecules, holding back the growth of many bacteria. Salt and sugar lower the water activity, but the water activity of vinegared rice does not fall far enough to stop bacteria. They play a supporting role. By stacking defenses that are not enough on their own, we build practical safety. This is called the hurdle concept (Chapters 1 and 6).

However, vinegared rice does not "never spoil." It only "spoils less easily." Rice carries spores of Bacillus cereus, which is fairly tolerant of acid. If you leave the rice in a hot place for hours, it goes past its limit.

2.5　Chilling, Freezing, and Cooking: What They Can and Cannot Do

Refrigeration does not kill bacteria. A refrigerator at 0 to 5°C almost stops the growth of most bacteria. Even so, the number of bacteria does not go down. Some bacteria also grow slowly even at refrigerator temperatures.

Freezing does not kill bacteria. Bacteria only go dormant, and they start multiplying again when thawed. What freezing reliably kills is parasites. Think of it this way: the state before freezing is the state after thawing.

Cooking is not all-powerful. Most bacteria die when heated to a core temperature of 75°C for 1 minute or more. But some toxins made by bacteria are not destroyed by boiling or grilling. Also, some bacteria found on rice form spores, which lie dormant inside a heat-resistant shell, and they survive even boiling (Chapters 3 and 9). That is why "keep bacteria off" and "keep them from growing" come first (Chapters 4 and 5).

The Numbers for Refrigeration, Freezing, and Cooking, and Spores and Toxins

Even at refrigerator temperatures, Listeria and Pseudomonas keep growing at around 0°C. As a rule of thumb in fisheries science, fish loses freshness about 2 times faster at 5°C than at 0°C, and about 4 times faster at 10°C. In freezing, almost all microorganisms stop growing at −18°C or below. Some die during freezing, but most survive. Neither norovirus nor toxins are destroyed by freezing. Anisakis dies at −20°C or below for 24 hours or more. To be sure, start counting after the center of the fish reaches −20°C. Heating to a core temperature of 75°C for 1 minute or more kills bacteria that do not form spores, and Anisakis. Heating to 85 to 90°C for 90 seconds or more also destroys norovirus.

Two things withstand heat. The first is spores. Bacillus cereus, Clostridium perfringens, Clostridium botulinum, and others form spores when conditions turn bad. A spore is a dormant state wrapped in a thick shell. It is extremely resistant to heat, drying, and disinfectants, and it does not die even at 100°C. When food cools into the temperature danger zone, spores wake up and start to multiply. Spores of Bacillus cereus come from soil and are widely found on rice. A common cause of trouble is leaving cooked rice at room temperature for a long time.

The second is toxins. Staphylococcus aureus produces enterotoxin as it multiplies in food, and this toxin causes violent vomiting. Even heating at 100°C for 30 minutes does not destroy it. The bacteria die at 75°C, but the toxin remains. So the answer is not "heat it later." First, do not move bacteria from your hands to the food, by washing your hands and taking care of cuts. Second, keep food cold so the bacteria do not multiply enough to make toxin (Chapters 3 to 5). The vomiting toxin of Bacillus cereus and the histamine in red-fleshed fish are also not destroyed by heat (Chapter 3).

2.6　Spoilage and Aging: What "As Long as No Bacteria Get In" Really Means

The phrase "the fish has gone bad" mixes together changes with different causes. The most important thing is to think about spoilage and aging separately.

Spoilage: Bacteria Break Down the Flesh

Spoilage is a change in which bacteria break down the proteins in fish flesh and produce foul-smelling substances. A sharp smell like ammonia, a strong fishy smell, and a sulfur-like smell appear, and the surface turns slimy.

Aging: The Fish's Own Enzymes Create Umami

Fish flesh contains enzymes from the time the fish is alive. Enzymes are substances that drive changes inside the body. Enzymes keep working even after the fish dies. Their work slowly softens the flesh and increases the umami compounds. Aging means skillfully advancing only this change, under conditions that keep bacteria from growing.

Table 2-3 sums up the differences between the two. Spoilage is caused by bacteria, and aging is driven by the fish's own enzymes. If an ammonia smell appears, that is spoilage, not aging.

"As Long as No Bacteria Get In"

Among chefs there is a saying: "As long as no bacteria get in, fish will never rot." This saying is half right. There are no bacteria inside the flesh of a healthy, living fish. Bacteria live on the skin, the gills, and in the intestines, and when the fish dies, they move from there into the flesh. So remove the gills and guts early, fillet the fish with a clean knife and cutting board, wipe the surface dry, and keep it cold. Then you can greatly hold back bacterial growth in the flesh. Aging is possible because of this idea.

But even if no bacteria get in, the fish's own enzymes keep working. In time, the umami compounds turn into bitter compounds. The flesh breaks down, and the fat oxidizes on contact with air, giving off a smell like old oil. "As long as no bacteria get in, it will not rot" may be true. But "as long as no bacteria get in, it will stay delicious" is not. Aging comes to an end once the peak of flavor has passed, before spoilage begins (Chapter 10).

Telling Apart Spoilage, Autolysis, and Lipid Oxidation

In Grade 1, we think of "going bad" as three separate things.

The first is spoilage. Bacteria break down proteins and amino acids, producing ammonia, amines, and hydrogen sulfide. A compound unique to sea fish is trimethylamine, the main source of fishy smell. Bacteria make it from trimethylamine oxide, an odorless compound that is naturally present in the flesh. As spoilage progresses, the pH moves back toward the alkaline side. The main players are psychrotrophic spoilage bacteria, which grow even at refrigerator temperatures.

The second is autolysis. This is breakdown by the food's own enzymes, and it proceeds even if there is not a single bacterium. Enzymes that break down the proteins in the flesh go to work. At the same time, ATP, the source of energy, is broken down step by step. ATP passes through inosinic acid, an umami compound, and becomes hypoxanthine, which is bitter. This is mainly the work of the fish's own enzymes. However, bacterial enzymes also join in at the last step, when hypoxanthine is formed. Refrigeration slows this down, but does not stop it (Chapter 10).

The third is lipid oxidation. Fish fat is rich in unsaturated fatty acids. They react with oxygen and produce a smell of old oil and a yellowish color. The causes are oxygen, light, heat, and iron. It has nothing to do with bacteria, and it proceeds even inside a freezer (Chapter 12).

Aging is a technique that advances only autolysis, under conditions that hold back bacterial growth and lipid oxidation. It increases inosinic acid and softens the flesh. In other words, it is the management of autolysis without spoilage. Spoilage does not happen if no bacteria get in, but autolysis and lipid oxidation happen even without bacteria. This is why "as long as no bacteria get in, it stays delicious" does not hold true.