Dairy Bacteriology: An Introduction to Bacteria
Scientists use two names to describe each kind of bacteria. The first is the genus name and second is the species name. When the names of the species and genus are written, the are italicized, or underlined. The genus name usually refers to the group to which the bacterium belongs, somewhat like our human family names, except it is listed first. Many times the genus and species names (in the Latin or Greek language) are selected to describe some general feature of the bacterium. For example, the word used to describe the genus name, Streptococcus, tells us that it is a sphere-shaped cell (coccus) and that it occurs in chains (strepto). The species name is more specific and usually refers to the activity or habit of the organism. The species name lactis tells us that is associated with milk. To illustrate, then, we have the most common bacterium in dairy work: Streptococcus lactis.
Once one becomes familiar with the various types of bacteria important to your work, "nicknames" are often used to describe the species of bacteria. For example, Streptococcus lactis becomes "Strept. lactis". If you want to refer to more than one species of bacteria that have some common characteristics, you can use another nickname, like Streps, referring to those several species of bacteria with characteristics like those found in the genus Streptococcus. One nickname commonly used in the dairy industry is E. coli which is short for Escherichia coli. With sometimes difficult names to pronounce, it is no wonder that people prefer bacterial nicknames!
Bacteria Are Very Small
One of the most important things to remember about bacteria is their extreme smallness. The fact that they cannot be seen with the unaided eye is one of the chief reasons they are not given the prime consideration they should by people in the dairy and food industries. The average bacterial cell is 1/25,000 of an inch in length and even smaller in diameter. In other words, one could place 25,000 bacteria cells, side by side, on an inch-long line. By contrast, if 25,000 people were lined up shoulder to shoulder, they would make a line over 18 miles long. For us to see these incredibly small living things, a microscope with a magnification of over 800 power or more is needed. In contrast, most binoculars used to observe sporting evens magnify objects about 7 to 10 power.
So if these bacteria are too small to see with the eye, how does one know they are present in a food? The process we use is to plate the food being examined to determine if bacteria are present. One takes a sample of food being examined and places a small portion of it on an agar that contains food on which bacteria will grow. The agar, a gelatin-like substance containing the bacterial food, is actually placed in a Petri plate, a shallow round dish with a cover. A small portion of the food being examined is spread over the surface of the agar. The amount of food being "plated" depending on the suspected number of bacteria in the food. For foods containing only a few bacteria, up to one gram (g) or milliliter (mL) will be "plated". For foods heavily contaminated with bacteria, one-millionth or a gram or mL of the food would be plated. The food is diluted with sterile water to achieve this small amount on the agar in the Petri plate. If bacteria are present they grow rapidly producing offspring that within 12 to 48 hours will produce a "mound" of bacteria in one spot. We can see this mound and call it a colony. The assumption is that each colony originated from one bacterial cell 12 to 36 hours ago. If this assumption is true--sometimes it is not-likely one can calculate the number of bacteria in the original food placed on the agar in the Petri dish by knowing how much food was placed on the plate originally.
If one were to look at bacteria through a microscope, one would notice that the bacteria come in a variety of shapes. The most common are cocci (cock'eye), bacilli (bah-sill'eye) and sprialla (spi-rill'-lah). The cocci-shaped bacteria are spheres, the bacilli are rod-shaped, while spirilla are shaped like corkscrews. Some bacteria have other shapes, but these bacteria are generally of little importance to the food and dairy industries.
Good & Bad Bacteria
Bacteria can be classified by their habits as they relate to human activities. The overwhelming majority of bacteria are harmless to humans. These bacteria are important to humans because they play a role in the ecology of life, by decomposing wastes, both natural and man-made, for example and created nitrogen fertilizer at the root zones of certain crops.
Bacteria can also be used purposely by people to make foods. For example, the group of various bacteria collectively called the lactic acid bacteria are used for the manufacture of cultured dairy foods like sour cream. To manufacture sour cream, the species of bacteria Streptococcus lactic is added directly to the 18% cream. These bacteria grow in the cream incubated at 70 degrees producing lactic acid. The lactic acid causes the cream to thicken and cause the flavors which are ascribe to sour cream. Careful selection of the right bacterial type to be used in food manufacture has lead to a variety of cultured food, foods to which carefully selected species of bacteria have been added for their manufacture. For example, most all of our cheeses owe their unique flavors and textures to bacterial growth. San Francisco sourdough bread would not be sour (acid taste) if it were not for the lactic acid bacteria called Streptococcus sanfranciscus. growing in the dough during the time the yeast is growing and causing the dough to rise.
Then there are bacterial types that are capable of spoiling foods -spoilage bacteria- or causing sickness or death in people -pathogens. These bacteria are in the minority, but they are well known. Occasionally, one species of bacteria can be categorized as either beneficial or harmful. Here's a case in point: We use bacteria called Streptococcus lactis to make buttermilk. We encourage its growth by adding it directly to the milk and allowing it to "sour" the milk--that's buttermilk. On the other hand, if our fresh milk is soured -spoiled- by these bacteria, then these bacteria are considered to be "harmful" spoilage agents.
What Bacteria Eat
Bacteria like about the same things to eat that people like. They like meat, cake, bread, water, and milk. Some have food requirements that are very much like those of humans because they need performed proteins, vitamins, and so on. These types of bacteria are called "fastidious".
Other bacteria can get along quite well on the simple chemicals, like nitrogen, minerals and an energy source such as sugar. These types of bacteria can then manufacture all the proteins, vitamins, fats, and carbohydrates they need from these very simple foods.
Just as we have championed milk as "Nature's most nearly perfect food," bacteria also find milk to be one of the best places in which to thrive, for milk furnished almost everything needed for growth, vitamins, proteins, sugar, and water.
To consume the food in its environment, the bacteria draw the molecules that make up the food and that are dissolved in the water through their outer membrane and into the inside of their cell. Here the food is digested by bacterial enzymes. The "waste material" of digestion is then passed out through the cell into the environment surrounding the cell.
It is this "waste material" that passed out of the cell that causes the changes in our food. In the cases of some pathogenic bacteria, the some of the waste materials are toxins that produce disease and illness. With the bacteria used in making buttermilk or other cultured dairy foods, the waste is lactic acid, which, when its concentration is just right (0.9%), causes the milk to curdle and taste sour--deliciously tasty to some!
Harmful Toxins Produced by Some Bacteria
Most everyone is familiar with the instances of food poisoning. One of the most common is caused by a bacterium called Staphylococcus aureus, an organism that produces a heat-stable toxin during its growth in some foods. When a food containing the toxin produced by these bacteria is consumed, the person becomes very sick for 24 to 48 hours. However, death rarely results.
There have been cases of food poisoning of this type from eating dairy products. The point here is that although these staph bacteria are killed by pasteurization, the toxin is not destroyed. Thus, control of the growth of the bacterium is essential, for if it allowed to grow in food, it could produce the toxin. Of course, the best control is to keep the organism out of the food in the first place. That's why we wash hands after leaving the restrooms, wash and sanitize equipment that comes in contact with food, etc. Obviously, the food industry is doing a great job at keeping this and other food-poisoning bacteria from processed foods since but 5% of food poisoning problem originate at the food processing plant.
Among the pathogenic bacteria that cause disease in man are Brucella abortus, which causes undulant fever, and various species of Salmonella which cause a disease called salmonellosis. It should be remembered that various food poisoning and pathogenic bacteria can inhabit the udder of a cow, and some can cause illness in people if the milk is consumed unpasteurized. There is no assurance that cow or goat milk that is tested and shown to be pathogen free on one day cannot acquire harmful bacteria the next day. Frequently, there is no outward sign in the cow that indicates that this has occurred.
How Bacteria Multiply
Bacteria multiply by splitting into halves, a process called binary fission. Under the most favorable conditions one bacterial cell will divide into two cells in about 20 to 30 minutes. Twenty minutes later, these two cells will elongate and split into four cells. Then after 20 more minutes, each of the four cells will divide into eight cells and so on. It's called a logarithmic progression ("log growth", as the bacteriologist call it). For example, 1 cell ‘ 2 cells ‘ 4 cells ‘ 8 cells ‘ 16 cells ‘ 32 cells ‘ 64 cells ‘ 128 cells ‘ 256 cells ‘ 512 cells ‘ 1024 cells, etc. In the previous examples one bacterial cell, multiplying about every 20 minutes, increases it's number in less than 3 hours to around 1020 cells. In 36 hours of continuous, unrestricted growth, there would be enough bacteria to fill 200 five-ton trucks! Obviously, bacteria do not multiply indefinitely, so what does control bacterial growth? One factor is temperature.
Different Bacteria have Different Temperature Requirements
Bacteria like different temperatures for growth. The largest and most common group is called mesophilic (mess-o-fill'-ik). These bacteria are somewhat like people in that they prefer moderate temperatures for growth. With this group the "best"--that is, the most rapid growth is around 70 to 98 degrees. The precise "best" growth does vary with the species of bacteria. The mesophiles can also grow down to 45 degrees and up to 110 degrees, but do so more slowly.
In the bacterial world, some like it hot! These bacteria live and multiply best at approximately 130 degrees F. but can grow anywhere between 110 and 190 degrees F. They are referred to as the thermophilic (ther-mo-fill'-ik) group.
Contrary to the belief of some people, cold or freezing does not always kill bacteria. In most cases it just stops or slows down their growth. Extended freezing, however, will slowly kill them. Psychrophilic (sigh-crow-fill'-ik) bacteria will grow from 32 to 90 degrees F. with most having their "best" growth around 50 to 70 degrees. Because they grow better NOT best than the mesophilic bacteria at refrigerated temperatures--32 to 45 degrees--, this group is most often responsible for spoilage in refrigerated foods.
So how does one control bacterial growth with temperature. If you have a food that is given a "light" heat treatment, like pasteurization, the food must be kept cold so that the growth of any spoilage bacteria surviving the pasteurization process is slowed. (The pasteurization process is designed to kill all pathogens, but not all spoilage bacteria). Obviously cold storage does not stop all bacterial growth since spoilage does eventually occur. But the colder you store the product the longer it will take for the spoilage bacteria to grow and spoil the food. In the dairy and perishable food industries we say, "Life begins at 40"--(degrees, that is). Keep the food at 40 or less and you will get the shelf life you need with a properly processed food.
Examples of Control of Bacteria
There are other ways that one can control the growth of bacteria. Bacteria need water to grow and even though some of them have the ability to resist long drying out periods, keeping things dry will stop growth and in some instances will kill them. Therefore, it is a good policy to keep utensils and some equipment dry when not in use. Remember, too, that the bacteria responsible for spoilage of foods (mesophilic and psychrophiles) can be killed by hot water. Ten minutes at 150 degrees F. will be sufficient. And, germicides such as chlorine and quaternary ammonium compounds are also effective.
Bacteria must eat. And milkstone, which is a dried milk-mineral deposit, and other mineral deposits are good food sources. Therefore, it is necessary to keep all equipment that comes in contact with the food scrupulously cleaned, and sanitized. Likewise the environment in which the food is processed also must be kept cleaned and sanitized. Bacteria from the environment can be "transported" to the food processing areas on hands, feet, clothing and by other unclean equipment. Bacteria can even "float" in on air currents and splashing water can dislodge bacteria from surfaces and make them airborne. These airborne bacteria can eventually contaminate cleaned surfaces.