Milk quality is essential to the dairy industry and the health of consumers. Quality begins at the farm and continues throughout milk processing, including the alfalfa hay they eat.
Off-flavors can indicate problems with feeding or cleaning procedures. The cowy flavor is often associated with poor barn aeration, while feed flavors may reflect low-quality feed. High sediment levels are related to dirty equipment and cows.
- Poor Nutrition
A healthy dairy cow produces milk that contains adequate levels of protein, fat, and carbohydrates. Variations in fat percentage and composition are controlled by genetics, hormones, and diet (Jenness, 1985). Carbohydrates are deposited in the mammary gland from the digestive tract as glycogen that is stored in cells for use as fuel when necessary. Proteins are synthesized and secreted from mammary cells. Proteins are made up of chains of amino acids, the building blocks of living tissue. The quality of proteins is reflected in their ability to form stable crystals, have the best mechanical properties, and have good nutritional value.
Most segments of the dairy industry require that milk have an SCC and SPC of less than 100,000 cfu per milliliter to be acceptable for processing. Consistent application of proper milking system cleaning and udder hygiene, as well as good mastitis prevention and control practices, should allow dairy producers to achieve these standards.
SPC counts are a measure of the number of aerobic bacteria present in raw milk. The test is done by plating the sample on a solid agar, incubating it at 32 deg C, and counting the number of bacterial colonies that grow on the plate. High SPCs are usually associated with dirty milking equipment, poor sanitation, contaminated water, or cows that have subclinical or clinical mastitis.
A Preliminary Incubation Count (PI) measures the number of psychotropic or cold-loving bacteria in milk. This test is not a regulatory one, but it is useful to monitor bulk tank milk to detect on-farm problems/deficiencies and to help identify potential sources of poor milk quality in milk used for processed dairy products. A high PI can also indicate inadequate cooling of the bulk tank or poor storage conditions.
- Poor Environment
A clean milk supply relies on the sanitary condition of dairy cows and their milking facilities. A high bacteria count indicates that these conditions are not as clean as they should be, and the cows may have ingested bacteria from contaminated water or manure. This can lead to off-flavors or odors that affect milk quality and safety.
A standard plate count (SPC) measures the total number of bacterial colonies in a sample of milk. It is an indicator of milk quality and can be used to determine if the milk is fit for human consumption. A high SPC may indicate a dirty milking system, poor milking practices, or udder disease in the dairy cow. A low SPC is typically a result of consistent cleaning of the milking system, good milking practices, and proper udder hygiene in the herd.
Bacterial contamination can also be introduced in the milking environment through improper stall design, urea addition to the diet, or the presence of antibiotics. These contaminants can then be deposited in the udder during milking. Often the presence of these bacteria is not recognized until off-flavors or rancid odors are observed in the milk after it leaves the farm.
Farmers are able to recognize off-flavors and odors in their milk and are concerned about the quality of the product that they produce. This is because their livelihood depends on milk sales, and if they produce a substandard product, it could negatively impact their financial situation. Moreover, they are aware that milk adulteration can occur at different stages of the production process, and the quality of the finished product is ultimately dependent on how well the milk is handled and stored in transit to the markets.
- Poor Health
Keeping a healthy cow in good shape is the key to milk quality. A cow’s diet and health directly affect the components of milk — protein, fat, and carbohydrate content. Cows with poor health produce less milk and lower-quality milk than healthy cows. Sick cows also produce higher somatic cell counts, which can degrade the fat and protein components of milk. This taints the milk and leads to deductions in pay.
The quality of milk can be monitored with a Standard Plate Count (SPC), which measures the number of typical coliforms that grow in a plate of milk on Violet Red or MacConkey’s agar after being incubated aerobically for 48 hours at 32degC. Milk with SPCs above 10,000/ml is generally rejected. A high SPC may result from a dirty milking system, contaminated water, soiled cows or mastitis, improper cleaning of milking equipment, and poor sanitation.
In addition, a high SPC could indicate a problem with feeding or a change in the diet of the cow. A “barny” odor may be related to inadequate barn aeration, and an off-flavor similar to wild onion or garlic could be caused by a change in the cow’s feed.
A change in the odor or taste of milk should always be investigated to determine its cause. Many of these issues can be corrected by simple changes in management, feeding, milking, and sanitation. If a quality problem is detected, it is important to correct it as soon as possible to prevent other problems and maintain a consistent, nutritious milk supply.
- Poor Equipment
In addition to the health of the cow, milk quality is greatly impacted by the cleanliness and maintenance of milking equipment. Many countries have regulatory agencies that monitor bulk milk and require farmers to pay a premium based on their milk quality. Milk is routinely tested for bacterial counts (total bacterial count, TBC) and somatic cell count, milk protein, fat, lactose, and solids nonfat (SNF).
A TBC, also known as a Standard Plate Count, measures the total number of aerobic bacteria present in a sample of milk after being incubated on a standard medium under aerobic conditions for 48 hours. A high TBC indicates that bacteria are present in the milk and can be associated with poor herd health, contaminated stalls, inadequate plant cleaning, mastitis, intramammary infections, or improper teat washing.
An SCC, or coliform count, is determined by plating a sample of milk on a Violet Red Bile or MacConkey’s agar to see if typical coliform colonies will grow. A high coliform count suggests contaminated milk and can be associated with a dirty farm, poor milking practices, sanitary procedures, and improper feeding of dairy cows.
Another indicator of milk quality is the freezing point of the milk, which is a measure of how much water is present in the milk. If the milk has a low freezing point, it may indicate the presence of added water that can negatively impact milk quality.
TBC, SCC, coliform count, and freezing point are all indicators of milk quality. To help improve milk quality, it is important to ensure that all milking equipment is clean and sanitized after each use. Regular checks of hoses, claw gaskets, pulsators, and milk storage tanks are necessary to prevent the build-up of contaminating residues in the system.
- Poor Storage
Milk is perishable and should be stored in a cool, dry place to ensure that bacteria don’t spoil it. A glass of fresh milk should not be left out for more than 15 minutes, or it will start to deteriorate and can quickly become sour. In fact, the number of bacteria can double every minute in warm temperatures, so it’s best to pour the amount you need and then immediately put it back in the refrigerator.
Bacterial quality is measured in several ways, including Standard Plate Count (SPC), PIC, LPC, and coliform counts. The higher the count, the more bacterial defects that are present in the milk and the greater the risk of these defects carrying over into products made from the milk. A high SPC can indicate mastitis and/or poor pre-milking udder hygiene. It can also be due to dirty equipment or marginal cooling.
A coliform count indicates the presence of typical bacterial contaminants that enter the milk from a contaminated environment. A coliform count that is higher than 10,000 cfu/mL may indicate a herd with subclinical or clinical mastitis, as well as poor dairy management and/or milking practices.
A number of factors affect the quality of milk, but it is important to monitor the farm’s quality. The more attention and care given to hygiene, sanitation, nutrition, and milking procedures at the farm level, the better the results will be when the milk is transported, processed, or consumed. Farmers can also use their senses to detect off-flavors and odors, which may reveal problems with milking equipment, sanitation, milking routines, or other factors. These can provide clues that a problem with milk quality is developing, such as off-flavors or rancid odors.
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