Milk is one of the most essential foods for humans, but also one of the most suitable environments for the growth of microorganisms Therefore, the production and processing of milk are heavily regulated. A dairy farmer must meet stringent federal and state regulations to produce Grade A, such as having healthy cows and adequate and sanitary facilities and equipment, and maintaining bacteria count of less than 100,000 per ML. Also, the milk should not have objectionable flavors and odors.
The cows in a dairy farm are mechanically milked, and the milk flows into an insulated stainless-steel bulk tank through sanitary tubes. The milk is stirred by an agitator and is cooled as milking continues. The raw milk is constantly agitated to maintain uniform milk fat distribution. The refrigeration capacity of the tank must be enough to cool 50 percent of its capacity from 32.2 to 10°C during the first hour, and from 10 to 4.4°C during the second hour. Milk is usually delivered to a dairy plant in a stainless-steel tank on a truck. The tank is adequately insulated so that refrigeration is not required during transportation.
The temperature rise of the tank filled with milk should not exceed 1.1°C in 18 h when the average temperature difference between the milk and the ambient air is 16.7°C. Horizontal storage tanks must also meet the same requirement. A dairy plant receives, processes, and packages the milk. The minimum amounts of milk fat and non-fat solids in the milk are regulated. For example, the minimum legal milk fat requirements are 3.25 percent for whole milk, 0.5 percent for low-fat or skim milk (the milk fat contents of low-fat and skim milk in the market are about 2.0 and 0.5 percent, respectively), 18 percent. For sour cream, and 36 percent for heavy cream. The minimum legal requirement for non-fat solids in milk is 8.25 percent. The stored milk is first standardized to bring the milk fat content to desired levels using a milk separator.
The separation of milk fat is easier and more efficient at higher temperatures. Therefore, milk is usually heated from the storage temperature of 4.4°C to about 20 to 33°C in warm milk separators. Only a portion of the milk needs to be separated. The remaining milk can be standardized by adding the required amount of skim milk or milk fat. To kill the bacteria, milk is pasteurized in a batch or continuous flow type system by heating the milk to a minimum temperature of 62.8°C normally by hot water or steam and holding it at that temperature for at least 30 min. Milk can also be pasteurized quickly by heating it to 71.7°C or above and holding it at that temperature at least 15 s. Fast pasteurization is very suitable for continuous-type systems.
The pasteurized milk is first cooled to the environment temperature by the plant’s cold water, and then it is cooled to 4.4°C or less by refrigerated water usually in plate-type heat exchangers. The entire cooling process should be completed in less than 1 h for sanitary reasons. The continuous-type pasteurizers are equipped, with a regenerator, which is a counter flow heat exchanger in which the incoming cold raw milk is heated by the hot pasteurized milk as it is cooled. The flow rates of both the pasteurized and the raw milk are the same, and thus the temperature drop of pasteurized milk will be practically equal to the temperature rise of the raw milk. The effectiveness of the regenerators is in the range of 80 to 90 percent.
The taste of milk depends on the feed of the cattle, and the milk is usually vacuum processed after pasteurization to reduce the undesirable flavors and odors in it. In this process, the temperature of pasteurized milk is raised to 82 to 93°C by injecting hot steam into it and then spraying the mixture into a vacuum chamber where it is cooled by evaporation to the pasteurizing temperatures. The vapor as well as the non-condensable gases responsible for the odors and off-flavor are removed by the vacuum pump.
Homogenization distributes milk fat throughout the body of milk and prevents the milk fat from collecting at the top because of its lower density. This is done by pumping the warm milk at 56 to 82°C to n high-pressure (usually 8 to 17 MPa) and shearing off the large globules by forcing the milk through homogenizing valves.
The homogenized milk is refrigerated again to about 4°C and is packaged in the familiar paperboard, plastic, or glass containers for distribution in refrigerated trucks. The paperboard carton is made of a 0.41-mm-thick paper layer with 0.025- and 0.019-mm polyethylene film laminated on the inside and outside, respectively. Other milk products such as yogurt, cheese, cream, and ice cream are produced by processing the milk further.
The most important process to preserve the quality of the milk is refrigeration. KoldKraft Refrigeration provides ice builder tanks which provides constant chilled water to remove the heat of milk in plate heat exchangers.
We also provide incubation rooms for making yoghurt and blast chillers to instantly cool down other milk products.