Milk fresh from the cow is virtually a sterile product. All post-milking handling must maintain the milk's nutritional value and prevent deterioration caused by numerous physical and biological factors. In addition, equipment on the farm must be maintained to government and industry standards. Most cows are milked twice a day, although some farms milk three or four times per day. The milk is immediately cooled from body temperature to below 40°F (5°C), then stored at the farm under refrigeration until picked up by insulated tanker trucks at least every other day. The milk tanker driver records the amount of milk and notes the temperature and the presence of any off-odors. If the milk is too warm or has an off-odor, it will not be picked up, and the farmer will have to feed it to his animals or dump it. When the milk is pumped into the tanker, a sample is collected for later lab analysis.
When the milk arrives at the milk plant, it is checked to make sure it meets the standards for temperature, total acidity, flavor, odor, tanker cleanliness, and the absence of antibiotics. The butterfat and solids-not-fat content of this raw milk is also analyzed. The amounts of butterfat (BF) and solids-not-fat (SNF) in the milk will vary according to time of year, breed of cow, and feed supply. Butterfat content, solids-not-fat content, and volume are used to determine the amount of money paid the farmer.
Once the load passes these receiving tests, it is then pumped into large refrigerated storage silos (nearly half-million pounds capacity) at the processing plant.
All raw milk must be processed within 72 hours of receipt at the plant. Milk is such a nutritious food that numerous naturally occurring bacteria are always present. The milk is pasteurized, which is a process of heating the raw milk to kill all "pathogenic" bacteria that may be present. A pathogen is a bacteria that could, if allowed to grow and multiply, make humans sick. It should be noted that pasteurization is not sterilization (sterilization eliminates all viable life forms, while pasteurization does not). After pasteurization, some harmless bacteria may survive the heating process. It is these bacteria that will cause milk to "go sour." Keeping milk refrigerated is the best way to slow the growth of these bacteria. Some bacteria do not cause spoilage, but are actually added to milk or cream after pasteurization to make "cultured" products such as cheese, cottage cheese, yogurt, buttermilk, acidophilus milk and sour cream.
There are different ways to pasteurize milk. The "batch" method heats the milk to at least 145° and holds it at that temperature for at least 30 minutes.
Since this method may cause a "cooked" flavor, it is not used by some milk plants for fluid milk products.
High Temperature/Short Time (HTST) pasteurization heats the milk to at least 161° for at least 15 seconds. The milk is immediately cooled to below 40° and packaged into plastic jugs or plastic-coated cartons. Most milk plants have at least one HTST processor. This piece of equipment is considered the "heart" of the plant.
Butterfat content accounts for several different types of products. Whole milk, 2%, 1%, Nonfat, and Half & Half are some examples. A machine called a separator separates the cream and skim portions of the milk. A separator is really a large centrifuge that spins about 2,000 rotations per minute. The different types of milk products are then "standardized" by blending the components (skim milk, raw milk, cream) in the correct proportions to yield the desired end-products. Water is never added to lower the butterfat content of fluid milk. Excess cream is used to make ice cream and butter.
Milk is homogenized to prevent the cream portion from rising to the top of the package. The expression "cream rises to the top," is accurate because cream is lighter in weight than milk. The cream portion of un-homogenized milk would form a cream layer at the top of the carton. A "homogenizer" forces the milk under high pressure through a valve that breaks up the butterfat globules to such small sizes they will not "coalesce" (stick together). Homogenization does not affect the nutrition or quality of the product; it is done entirely for aesthetic purposes.
Vitamin quantities may be reduced by the heating process and removal of the butterfat. Therefore, to replace the natural nutrition of nature's perfect food, liquid vitamins are added to fortify most fluid milk products. Many states have milk standards that require the addition of milk solids. These solids represent the natural mineral (i.e. calcium, iron), protein (casein), and sugar (lactose) portion of nonfat dry milk. You will see this shown as an ingredient on those products needing fortification.
Quality Control personnel conduct numerous tests on the raw and pasteurized products to insure optimum quality and nutrition. A sample is analyzed for the presence of microbiological organisms with a standard plate count (SPC) and ropey milk test. The equipment used to analyze butterfat and solids-not-fat is calibrated on a regular basis to insure a consistent, quality product that meets or exceeds government requirements.
All milk products have a sell-by date printed on the package. This is the last day the item should be offered for sale. However, most companies guaranty the quality and freshness of the product for at least 7 days past the date printed on the package. Samples of each product packaged each day are saved to confirm that they maintain their freshness 7 days after the sell-by date.
Once the milk has been separated, standardized, homogenized and pasteurized, it is held below 40°F in insulated storage tanks, then packaged into gallon, half-gallon, quart, pint, and half-pint containers. The packaging machines are maintained under strict sanitation specifications to prevent bacteria from being introduced into the pasteurized product. All equipment that comes into contact with product (raw or pasteurized) is washed daily. Sophisticated automatic Clean-in-Place (CIP) systems guarantee consistent sanitation with a minimum of manual handling, reducing the risk of contamination.
Once packaged, the products are quickly conveyed to a cold storage warehouse. They are stored there for a short time and shipped to the supermarket on refrigerated trailers. Once at the store, the milk is immediately placed into a cold storage room or refrigerated display case.