The fantasy journey of microbes: Why does food become good or bad? ── “PanSci TALK: How to preserve fresh food”-PanSci

This article was commissioned by the Food and Drug Administration of the Ministry of Health and Welfare, and executed by a pan-scientific plan

Transcript / Chen Tingwei

How does food start to spoil? By “freezing” fresh food, can the microbes on the food be dealt with so that the food will not “spoil”? And is the way we use the refrigerator really correct?

The second part of this year’s food safety lecture series “PanSci TALK: How to store fresh food?The first speaker, Assistant Professor Chen Bangyuan from the Department of Food Science of Fu Jen Catholic University, this time led everyone to start from the beginning.Food MicrobiologyDiscuss the basic principles of food preservation from the perspective of

Assistant Professor Chen Bangyuan, Institute of Food Science, Fu Jen Catholic University. Photo/By PanSci

Is fresh food alive or dead?

“First of all, fresh foods in daily life can be divided into two categories: “animal” and “plant-based.” Animal foods, such as meat and aquatic products, have the biggest feature.The organism itself has died, The enzymes in the body will automatically start to decompose its own tissues after the animal dies. Therefore, even if the influence of external microorganisms can be eliminated through processing, vacuum, etc., the animal food will still be degraded. And fresh plant foods such as fruits and vegetables,Tissue still aliveThe respiration in the body generally continues, but the nutrients and water that are continuously supplied in the planting environment have been lost. With the prolonged storage time, the condition of plant foods will gradually lose water and nutrients and deteriorate.

Fresh food. Photo/Anelka @ Pixabay CC0 Public Domain

Since the food will change, how do you judge that the food has reached the point where it is not edible? “Generally speaking, when the appearance of food can be found to have relatively drastic changes such as spoilage, acidification, mold, discoloration, mucus production, etc., we will think that it is completely unsuitable for eating, and these changes are mostly caused by “microbes”. Yes, it includes several types of bacteria, molds and yeasts that are discussed in food science.” Mr. Chen Bangyuan said that if we want to perfect the preservation of food, we need to limit the growth conditions of microorganisms in the environment and minimize the growth of microorganisms. In order to extend the storage time of food.

The invisible army of microorganisms, the foundation of food microbiology

“Bacteria exist everywhere in life. If you provide the right conditions, they can grow very fast, for exampleStaphylococcus aureusIt can split every 12 to 15 minutes, and it can grow to 1 million in 8 hours; for ordinary meat or fish, there are about 10 to 10,000 bacteria per gram. Regardless of the food, as long as the bacteria count reaches 5 million, it can probably be judged to be spoiled. Teacher Chen Bangyuan explained, “If the food is contaminated at the beginning, that is, it is contaminated with more bacteria, the rate of spoilage will naturally be much faster than that of uncontaminated food.”

Under normal circumstances, bacteria cannot be seen with the naked eye, and the true face of the bacteria can only be seen through an electron microscope.Photo/By Chen Bangyuan’s briefing

“The main content of food microbiology is to focus onFactors required for the growth of microorganisms to control environmental conditions and develop suitable food preservation methods. ” Factors that affect the growth of microorganisms in foodSubstances can be divided into intrinsic factors, extrinsic factors, microbial factors, and so on.

Intrinsic factors: the influence of one’s own conditions

The factors inherent in food that naturally affect the growth of microorganisms, we call them intrinsic factors, mainly include:

For microorganisms to survive and multiply on food, they need high enough water activity in the environment, which is the ratio of “water available to microorganisms”. Take honey as an example,Honey is liquid, but its water is almost covered by sugarMicroorganisms cannot obtain water, so it is naturally very difficult to survive; but there are still a few microorganisms that can dormant in it, such as the spores of botulinum. There are few probiotics, and botulinum spores can be fatal if they invade.

“Generally, fresh foods have very high water activity, so methods such as pickling and drying are used to reduce the water activity of foods and prevent the proliferation of microorganisms,” said Mr. Chen Bangyuan.

The pH value in the environment will affect the operation of biological enzymes and also affect the growth of microorganisms. Most microorganisms growThe most appropriate pH range is between pH 6.5 and 8.5. Except for a few acid-producing bacteria (lactic acid bacteria, acetic acid bacteria, etc.), the general pH range of bacterial growth is in the pH range of 4.0~9.0, in a slightly alkaline environment; relatively speaking, yeast (suitable range of pH 1.5~8.5), Mold (suitable range pH value 1.5~11.0) prefers acidic environment. Teacher Chen Bangyuan mentioned that general vegetables are relatively alkaline and are more likely to cause spoilage due to bacteria; fruits are mainly acidic and are often spoiled by molds or yeasts.

  • Nutrient contents in food
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Quite simply, microorganisms need to obtain energy to survive, and the more nutrient sources they love, the more food they love.

  • The presence or absence of antimicrobial substances Antimicrobial agents

Some natural foods contain “antibacterial ingredients”, which can inhibit the growth of certain microorganisms. The most common one is garlic, whose essential oil has an antibacterial effect.

Garlic itself has antibacterial ingredients, which can help food not to be affected by microorganisms. Picture/MALIZ ONG @ PublicDomainPictures, CC License
  • Biological barrier of food

If the food can beTry to maintain a complete structure, It is also helpful to resist bacteria. A whole fruit will not reveal nutrients and water, and the external microorganisms cannot access it and cannot proliferate. For another example, compared with the whole meat, the structure of ground meat has been destroyed and the inside is more susceptible to bacterial contamination. Therefore, the rate of spoilage will be much faster than that of whole meat.

  • Oxidation/Reduction potential

Each ingredient in food has its own oxidation-reduction potential, and various microorganisms also have their own preference for oxygen.The greater the positive value of the oxidation-reduction potential, the more prone to the “oxidation state”, which is beneficial to Pseudomonas, halophilic bacteria, etc.Aerobic bacteriaGrowth; the greater the negative value of the oxidation-reduction potential, the more the “reducing state”, which is favorable for Listeria, yeast, etc.Anaerobic bacteriaGrow.

External factors: fueling the flames in all directions

The environment where food is located also affects the growth of microorganisms, called extrinsic factors, which are also the main control content of food microbiology. The main external factors include:

  • Temperature of the environment where the food is located
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The relevant laws and regulations for food preservation have restrictions on the temperature of the environment where each food is located. It is worth noting that the preservation of food is not limited to low temperature, normal temperature or even high temperature, and there are also certain conditions that can preserve food. The most suitable growth temperature for microorganisms is 4-40 degrees. In winter, some beverages of supermarkets will be placed in an incubator at about 60 degrees, which is to store food at high temperature, providing you with more choices that meet your needs. However, because some types of bacteria can form spores in cans, these spores will be awakened by high temperature, so such high-temperature drinks are usually not kept overnight.

  • Relative humidity

As mentioned earlier, the water activity of food will affect the growth of microorganisms, and of course the humidity outside the food will also have an effect. Most microorganisms like humid environments. Bacteria and yeasts like food to be soaked in water, while molds do not prefer too much water.

  • Gas composition

Generally, air is composed of 20% oxygen and 80% nitrogen. Nitrogen is a colorless and odorless inert gas, which is not easy to interact with general compounds or microorganisms. Therefore, vacuum or vacuum nitrogen filling is often used in food preservation (after removing the air, fill in Nitrogen) way.

Don’t really talk about buying potato chips to send air. That’s food preservation technology. Picture/[email protected], CC License

Microbial factors: mutual assistance and competition

Next, Assistant Professor Chen Bangyuan mentioned the interaction between microorganisms.

“Food contains many different microorganisms, which will help or compete with each other. For example, compared with fresh milk, yogurt is less prone to severe deterioration, because yogurtIt contains a lot of lactic acid bacteria, Other bacteria are not easy to multiply in it; but yogurt that is left for too long will multiply more lactic acid bacteria, which may make the yogurt too sour and not good to drink. “

Fence technology: control various factors to preserve food

Hurdle Technology will combine the aboveThe various factors obtained can extend the shelf life by controlling water activity, pH, and microbial interaction. For example, fresh fish sold on the market are often preserved by the three-removing method, vacuum and freezing-removing the three most vulnerable parts of the head, scales and internal organs, and then wiping them dry, vacuuming the outer packaging and freezing, combining several The concept of “external factors” extends the shelf life of food as much as possible.

Secrets of Food Preservation in the Refrigerator

For the average person, “temperature” is a condition that is easier to control than water activity, pH, and microorganisms. So, won’t it be spoiled if the food is thrown into the refrigerator? Mr. Chen Bangyuan explained the application of refrigerators with a piece of bacteria classification in each temperature range (as shown in the figure below).

The growth curve of microorganisms in different temperature ranges.Photo/By Chen Bangyuan’s briefing

As you can see, 15~50℃ is a dangerous zone where microorganisms multiply rapidly. The regulations stipulate that the product temperature of frozen food should be kept below -18℃; for refrigerated food, it should be below 7℃ and above the freezing point, and severe temperatures should be avoided. change.

General household refrigeratorRefrigerated layerMost of them are set at 4~5°C. At this temperature, most bacteria cannot grow or grow very slowly, leaving only low-temperature bacteria, yeasts and molds, so these microorganisms will be the main reason for the spoilage of refrigerated food. “For this reason, if the cooked food is not to be eaten right away, it should be as fast as possible to cool down to below 15°C to avoid the food being exposed to the dangerous 15~50°C for too long.” Teacher Chen Bangyuan said.

Take rice as an example. If you don’t eat it immediately after heating, you should leave the temperature range where microorganisms (Cactus bacilli) are easy to grow as soon as possible (the dark orange bottom in the picture).Photo/By Chen Bangyuan’s briefing

If you want to prevent all microorganisms from growing, you have to set the external temperature to about -10°C – of course, it is still difficult to guarantee 100%.

Came to the refrigeratorfreezer, The low temperature below the melting point will freeze the water inside the food (that is, the decrease in water activity mentioned above), prevent the growth of microorganisms, reduce the activity of enzymes, and achieve the preservation of food in freezing. “However, even if the freezing temperature is controlled at -10°C where microorganisms cannot grow at all, and the food does not spoil after thawing, it does not mean that the food will not deteriorate at all. The enzymes (internal factors) of the food itself may still continue at -10°C. Operation for a long time will still affect the flavor of the food.” This is why frozen food still has a limit on the shelf life.

And Mr. Chen Bangyuan also mentioned one of the shortcomings of frozen stored food: the cooling rate of general household refrigerators is slow, and it is easy to form in the crystals, Causing plant tissues and animal tissues to rupture, so many frozen foods will lose water and taste changes. “In addition, the moisture of frozen food is easy to sublime, and the meat may even change color. Therefore, if you want to freeze food, especially meat, it is recommended to use vacuum packaging as much as possible.”

The freezing curve of food, the slower the cooling rate, the easier it is to form larger ice crystals.Photo/By Chen Bangyuan’s briefing

Precautions for food refrigeration

Finally, Mr. Chen Bangyuan talked about the precautions for food refrigeration. “Using the natural plant characteristics of fresh fruits, cold storage should beTry to maintain the original biological structure and reduce tissue damageFor example, grapes can be maintained relatively well without cleaning the stalks; other fruits, such as tropical fruits, have no hard skin, such as bananas, loquats, etc., which are easy to frostbite and cannot be refrigerated. “

The shelf life of fresh aquatic products and meat is usually 3 days. He reminded everyone to keep the food surface as dry as possible if they choose to “refrigerate”. “Meat and fish are especially prone to exudation of tissue fluid, so the bottom layer of meat in supermarkets will have absorbent pads.Reducing the moisture on the surface of the meat can prevent bacteria from accumulating and growing; But if it is purchased from the traditional market, there is usually no absorbent pad, so please keep it as dry as possible during storage. “

For more about the preservation of fresh food at home, welcome to read the second half of the lecture documentary “Fresh food is bought home, how to store it? 》

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