Milk Quality Booklet
Introduction
This publication is intended to assist teachers of agriculture and inform students participating in the FFA Dairy Foods Career Development Event (CDE).
The author has extensive experience with state and national dairy foods contests.
The Career Development Events Handbook, published by the National FFA Organization, provides the rules of the event (available at http://www.ffa.org).
The information is relevant to careers such as:
Milk producer
Bulk milk hauler
Field representative
Milk sanitarian
Technician for milk processor or health authority
Technical sales representatives
Milk Production Overview
Milk production in the U.S. varies greatly by farm location, size, housing, feed, equipment, and management.
Most milk is produced under Grade A regulations, but some meets manufacturing milk standards.
The trend has been toward fewer but larger dairy farms since World War II.
USDA data (representing 74% of U.S. milk production) indicates:
1965: ~15 million cows on 1.1 million farms produced 44.8 billion pounds of milk.Average: 3,000 lbs/cow & 41,000 lbs/farm
2002: 9.1 million cows on 92,000 farms produced 125.5 billion pounds of milk.Average: 13,800 lbs/cow & 1,364,000 lbs/farm
Farms with >2,000 cows produce ~15% of total milk supply.
The International Dairy Foods Association (IDFA) publishes Dairy Facts annually
Milk Quality and Safety
Control of milk quality and safety is the responsibility of states and municipalities.
Most milk is regulated under the Grade “A” Pasteurized Milk Ordinance (PMO), published by the USPHS/FDA.
>90% of U.S. milk is produced under these rules.
The first FDA-written milk ordinance was published in 1924.
In 1946, the Conference of State and Territorial Health Officers requested a plan to certify interstate milk shippers.
In 1950, the first Interstate Milk Shippers Conference was held in St. Louis, Missouri.
This resulted in the National Conference on Interstate Milk Shipments (NCIMS) and adoption of the PMO.
States receiving milk accept inspections and testing from shipping states if PMO rules are followed.
No state can exceed PMO requirements.
NCIMS meets biennially to update the PMO with USPHS/FDA concurrence.
In Missouri, the State Milk Board is responsible for milk safety.
Sanitarians inspect producing farms at least twice annually and processing plants quarterly.
They use inspection forms and provide timeframes for correcting deficiencies.
FDA rating officers periodically check state/local health units to ensure correct enforcement of PMO requirements.
Inspectors collect samples for testing in approved laboratories following procedures in:
Standard Methods for the Examination of Dairy Products (SMEDP), American Public Health Association
Official Methods of Analysis, AOAC International
The FDA monitors laboratory proficiencies by sending "split samples" for testing (bacteria count, somatic cells, antibiotics, pesticides, freezing point).
Laboratories failing to produce acceptable results risk losing certified status.
The USDA has published General Specifications for Approved Plants in the Code of Federal Regulations.
These plants produce butter, cheeses, and dry milk products with a grade label (e.g., Grade AA butter).
Milk for these plants must be Grade A or meet requirements of USDA's Milk for Manufacturing Purposes and Its Production and Processing: Recommended Requirements (similar but less stringent than Grade A).
## Regulatory Standards for Grade A Milk
Temperature
On the farm, milk must be cooled to ≤ C (45°F) within 2 hours after milking.
Temperature of blended milk after subsequent milkings must not exceed C (50°F).
- The milk processor must maintain the temperature at ≤ C (45°F) except during processing.
Bacterial limits
Standard Plate Count (SPC) for individual producer's milk: ≤ 100,000/ml.
SPC for commingled milk: ≤ 300,000/ml (due to bacterial clumps breaking during transport).
SPC limit for pasteurized milk: 20,000/ml.
Coliform bacteria test is applied to pasteurized milk since they are killed by pasteurization, their presence indicates contamination after pasteurization.
- Coliform count limit: 10/ml.
Standard Plate Count (SPC) Test Procedure (Raw Milk)
Take a sample aseptically and place it into a sterile container. Keep the sample refrigerated and store it no more than 24 hours before testing.
Mix the sample and transfer 1 ml to 99 ml of sterile diluent, making a 1:100 dilution.
Transfer 1 ml of this diluted sample to 9 ml of diluent, making a 1:1,000 dilution.
Place 1 ml of each diluted sample onto a Petrifilm® plate that contains the nutrients needed to support growth of bacteria plus a water-soluble gelling agent.
Lower the transparent cover of the plate onto the sample and apply a spreading device so the spread sample is kept within an area of .
Incubate the plates aerobically at for 48 +/- 3 hours.
Select plates containing 25–250 colonies for counting because a number less than 25 is too low to give good precision, and a number over 250 introduces too much error in counting.
8. Multiply the number of colonies counted by the reciprocal of the dilution. For example, if the plate containing 1/100 ml of milk yields 260 colonies and the one containing 1/1,000 ml yields 30 colonies, use the count of 30 and multiply by 1,000 to provide an estimate of 30,000/ml. Note: Petri dishes and plate count agar may be used instead of the Petrifilm® plate for doing the test. The procedure is otherwise quite similar.
Somatic cell count (SCC) limit
Milk from uninfected mammary glands: typically < 100,000/ml.
Legal limit: 750,000/ml (higher in some countries).
High SCC indicates mastitis infection.
SCC is tested using automated, direct microscopic, or electronic methods.
Adulterants and Adulteration
Common adulterants in raw milk: antibiotics, extraneous matter, pesticides, and chemicals from cleaning/sanitizing.
These are usually present in traces but should be avoided.
Antibiotics
Used to treat bovine mastitis.
Administered via infusion into the quarter or by injection.
Drug suppliers determine milk withholding times (usually 72 hours after last treatment).
PMO requires testing of all tank loads for beta-lactam antibiotics.
Positive tests lead to testing of individual farms and penalties for offending producers.
This has decreased the incidence of positive antibiotic tests to <1 in 1,000 samples.
FDA's Center for Veterinary Medicine (CVM) monitors testing and evaluates rapid test kits. Test kits must include negative and positive controls.
Mastitis causes decreased milk production and protein content while increasing somatic cell content. Some Federal Milk Marketing Orders pay higher prices with low counts.
Antibiotics can be detected using screening tests (≤15 minutes) and confirmatory tests (high-performance liquid chromatography) to identify specific drugs.
Commonly used antibiotics: penicillin, cloxacillin, streptomycin, tetracyclines. Sulfa drugs are used sparingly. Most tests detect beta-lactam antibiotics.
Sensitivity of many tests: 5 ng/ml (5 parts per billion) or 0.008 International Units (IU) penicillin G or cloxacillin.
Penicillin infused into a single quarter: 100,000 IU.
PMO sets strict limits on the concentration of control included in each kit. The has an allowable error of 10%, i.e., .
Hypothetical Example Dilution
A treated cow releases 80,000 IU at next milking. In order to get is , equivalent to or about , we calculate . This approximately equals 2,666 gallons. So, cows giving more than lb would need to contribute to the supply to dilute below detectable limit. If only 50% of the antibiotic is lost during milking, from a typical cow, nearly gallons of milk is needed near gallons.
Data shows a low number of positive tests, especially in pasteurized milk (dilution accounts for this).
Producer samples were positive in about 1 of every 750 tests, versus tanker loads (mixed milk) at 1 of every 2,300 tests.
Positive tanker loads at plants are removed to account for the few incidence of pasteruized products with only one for each 14,500 tests.
The program has reduced the incidence of drug residue positive samples in raw milk.
Extraneous matter
Major sources: unclean udders and teats.
Milking machine falling off can draw in extraneous matter.
~80% of extraneous matter is dissolved in milk and is a major source of bacteria.
Filters are used in milk lines to remove sediment.
Sediment test detects insoluble matter (insoluble extraneous matter, clots of milk from mastitic mammary glands, somatic cells, and bacteria) in farm bulk milk tank samples after agitation.
470 ml (1 pint) of milk at 35–38°C is strained through a cotton lintine filter disc (1.02 cm diameter).
Amount of extraneous matter is estimated by comparison with USDA standard discs.
Processors now rely on tests for microorganisms instead of the sediment test.
Pesticides
Can get into milk from direct application to animals or contaminated feeds.
Chlorinated hydrocarbons (methoxychlor, DDT) were a concern in the past but are now restricted.
Organic phosphorus types (malathion, diazinon) are metabolized by cattle.
Pesticides are not permitted for direct use on animals and applications to crops are limited.
Pyrethrins, extracted or made synthetically, are permitted for use on dairy cattle except at the time of milking and are widely used as space sprays (low toxicity to humans and break down quickly).
Emphasis is placed on preventing fly breeding by manure removal and larvacide applications.
Public health officials monitor pesticide use on dairy farms and plants.
Pesticides are detected by chromatographic methods (FDA's Pesticide Analytical Manual).
Water
The most common adulterant.
Profitable when milk is priced by weight and only fat content determines the price.
Example: Dilution of milk with 4% milk fat with water to 3.5% milk fat:
Uniform Price: . Milk fat differential: .20
Producer A delivers milk with 4.0% milk fat
The Producer should be paid 15.00 + (5 * .20) = $16.00
The dilution factor is . Meaning the addition of approximately 14 pounds of water will produce 114 pounds of diluted milk.\frac{$15.00}{cwt} = $17.10.
Produces a net "profit" from adding water of 17.10 - 16.00 = $1.10
Component pricing (testing for protein and fat) prevents this profit.
Added water affects cheese and dry milk product yield.
Freezing point test detects water adulteration.
Milk has a constant freezing point of about .
Freezing point is a colligative property (affected by dissolved substances).
Osmotic pressure up means that the freezing point is down.
Thermistor-type cryoscope gives results in ≤5 minutes.
Enforcement involves farm inspection and authentic sample testing.
Original sample can be 0.004°C above the authentic sample.
Freezing point increases approximately 0.006°C per 1% added water.
Mycotoxins
Toxins produced by environmental microorganisms.
Aflatoxins, produced by Aspergillus flavus and Aspergillus parasiticus, are highly toxic.
is most toxigenic, followed by forms.
Cows convert toxin to .
The toxin disappears from the milk within 3 to 4 days following removal of a contaminated feed source.
Aflatoxins are checked with fluorescence under ultraviolet light (black light) to test for kojic acid produced by Aspergillus flavus.
All corn exported from the United States must be tested for aflatoxin.
Tests: quantitative affinity chromatography, enzyme-linked receptor binding assay, or competitive binding assay.
USDA Grain Inspection, Packers and Stockyards Administration provides testing service.
FDA will consider action if aflatoxin levels exceed 20 ppb for corn and other grains intended for immature animals or for dairy cattle.
Clean and Sanitary Cows and Equipment
Dirty cows and equipment are major sources of spoilage bacteria (psychrotrophs).
When controlled, bacteria in milk primarily come from mammary glands and grow poorly at refrigeration temperatures.
Cleanliness and preparation of cows before milking are highly important.
Pasture-raised cows are generally cleaner than confined cows.
Cleaning and sanitizing dirty udders before milking is essential.
Principles:
Remove all soil that might enter the milk.
Udders must be practically dry (water carries bacteria).
Disinfect teats with a sanitizer (hypochlorite or iodophor).
Dry the teats and area above to remove excess sanitizer.
Cleaning teats stimulates oxytocin secretion (milk let-down hormone).
Milking equipment must be designed and constructed for easy cleaning.
Clean equipment after each use.
Cleaned in place with minimal take-down.
Cleaning process
Rinse water is begun as soon as the lines have been drained.
Relocate pipe leading into the bulk tank to become to return line for the cleaning solutions
Circulate a solution of hot chlorinated alkaline cleaner through the milking machines, receiver jars, and milk lines.
Terminated with a water rinse.
Rate of flow is important (turbulence provides scrubbing).
Minimal rate of flow is 5 feet per second.
Acidified rinse is used periodically to remove residual minerals.
Allow equipment to drain dry.
Alkaline cleaners remove fat and proteins.
Acid-type cleaners dissolve and remove minerals.
Milk fat starts to solidify at ~93°F (34°C), so cleaning is done with hot water.
Mineral deposits are called milkstone (water spots).
Sanitize milking equipment just prior to milking with sodium hypochlorite at 150 mg/liter.
Minimal chlorine concentration at the end of use is 50 mg/liter.
Do not rinse after sanitizing.
Hot water at 180°F or higher is used in milk processing plants to sanitize milk lines and filling equipment.
Hypochlorite sanitizers are inactivated by soil residues, so equipment must be clean for sanitizers to be effective.
Composition of Milk
Milk is defined as the lacteal secretion from complete milking of one or more healthy cows and is practically free of colostrum (secretion after birth).
Official definition: ≥3.25% fat and ≥8.25% nonfat milk solids.
Holstein cows produce an average of ~3.65% milk fat and 8.7% nonfat milk solids (12.35% total solids).
Jersey cows produce nearly 5% fat and 9% nonfat milk solids.
Milk and dairy foods contribute minerals, bioactive lipids, and unique protein components.
Consumption helps reduce risks of osteoporosis, hypertension, excess body weight and fat, dental caries, and some cancers.
Milk Fat
Fat content can be determined using the Babcock test (volumetric) or ether extraction (gravimetric).
The Babcock test releases fat from its emulsified state and the protein and lactose are digested by concentrated sulfuric acid.
Addition of water and centrifugation assist in the separation process.
Tests can be used to construct a standard curve for calibration of electronic testing machines.
Electronic testers are used for rapid testing to determine concentration of protein and lactose.
Modern dairy laboratories use electronic testers (infrared analyzers).
Total solids can be estimated by summing fat, protein, and lactose, plus a value for other solids.
Market milk labels include “pasteurized,” “homogenized,” “vitamin D added,” and (for ≤2% fat) “vitamin A added.”
Proteins
75–80% casein, plus whey proteins.
Casein is composed mainly of alpha, beta, and kappa types and occurs suspended as a colloid containing a high percentage of milk’s calcium, some of the magnesium, and considerable phosphate.
Kappa casein is on the surface of micelles and is attacked by rennin (chymosin) in cheese making.
Casein coagulates when pH is lowered to neutralize negative charges on micelles.
Whey proteins are soluble and consist of alpha-lactalbumin, beta-lactoglobulin, bovine serum albumin, immunoglobulins, and proteose peptones.
Lactoferrin (iron-binding protein) is a valuable whey protein.
Whey proteins precipitate with high heat. This is the basis of producing ricotta and similar whey cheeses.
Lactose
Nearly all carbohydrate in milk is lactose (12-carbon sugar).
Glucose and galactose are combined by the cow to produce lactose (nearly 5% in milk).
Lactose is only 1/2 to 1/5 as sweet as sucrose.
Low solubility of lactose causes crystal formation in sweetened condensed milk and ice cream.
Lactose constitutes about 7% of human milk versus 5% of cow’s milk.
Infant formulas made from cow's milk must add supplemental lactose.
There is a strong positive correlation between the size of the brain in mammals and the concentration of lactose in mother’s milk.
Lactic acid is the main product of fermentation. Clean-tasting acid has no aroma.
Lactose-fermenting bacteria used to make cultured buttermilk, yogurt, cottage cheese, and many cheeses provide the delicious characterizing aromas (Lactococcus cremoris, Lactobacillus bulgaricus, Lactobacillus acidophilus, and Streptococcus thermophilus).
Psychrotrophic bacteria attack fat and proteins of milk, causing spoilage.
Some people digest lactose incompletely (lactase deficiency=ß-D-galactosidase).
Causes gas and flatulence in the colon, and influx of water causing diarrhea.
Processors may use microbially produced lactase to reduce the lactose content of certain milk products such as lactose-reduced milk.
Lactose can be removed by ultrafiltration.
Ripened cheeses contain virtually no lactose.
Minerals
Milk is the major source of dietary calcium (70% of total).
Humans need calcium and vitamin D to build and maintain healthy bones and teeth.
Milk is fortified with vitamin D at 400 International Units per quart.
Most teenage girls and adult women and adult men fail to meet calcium recommendations.
90% of human bone mass is achieved by age 17 which creates a public health problem that will result in osteoporosis among many persons as they pass midlife.
Low calcium consumption is correlated with a large increase in consumption of soft drinks and decrease in consumption of milk and milk products.
Vitamins
Vitamins are found in low concentrations in the tissues of plants and animals where they are necessary for normal metabolic reactions, each having specific functions.
Animals and humans must ingest most vitamins, although some are produced in part by bacteria in the alimentary canal.
Fat-soluble vitamins (A, D, E, K) are removed when fat is separated from milk along with a proportion of water-soluble vitamins supplied by the skim milk portion.
Vitamin A content varies with feeds and feeding practices (more in pasture-fed cows).
ß-carotene, provitamin A, is converted to vitamin A by the cow and the human that consumes the milk.
Milk contains enough to supply 25–30% of the needs of a person who consumes 1 quart of milk per day.
Fluid milk products that are reduced in fat content must have vitamin A added to make up for that lost in removal of fat to make those products.
Addition of 400 IU of vitamin D per quart of milk for drinking purposes is uniformly practiced.
Contains relatively small amounts of vitamins E and K.
Synthesis of Vitamins from B Complex occur from microorganisms of the rumen for thiamine and riboflavin equivalents.
Fresh Milk contains relatively high content of Vitamin C, but much of it is oxidized to an inactive form during commercial processing.
Enzymes
Enzymes are proteins that catalyze reactions without being used up.
Lipase breaks down milk fat, releasing fatty acids.
Hydrolytic reaction releases fatty acids from glycerol.
Short-chain fatty acids are volatile and easily detected by smell.
Long-chain fatty acids form soaps with calcium and magnesium (soapy sensation).
This condition is called hydrolytic rancidity.
Proteases break down proteins in cheeses (especially raw milk cheeses).
Phosphatase is tested for to check for adequate heat treatment to kill pathogenic microorganisms by pasteurization. If there is residual phosphatase can be quantified.
Bacteria produce exoenzymes that break down large molecules.
Psychrotrophic bacteria (Pseudomonas) produce proteases, lipases, and phospholipases.
These enzymes can remain active after pasteurization.
Proteases commonly cause bitterness, lipases/phospholipases cause rancidity, and combined activity causes unclean/fermented flavors.
Catalase in white blood cells indicates high somatic cell count.
Equipment Used in Milk Production
Materials and construction are approved by 3-A Sanitary Standards, Inc.
This organization started in the 1920s with manufacturers and users of equipment in dairies and public health officials standardize equipment design.
Recognized by the U.S. Public Health Service (FDA) in 1944.
Subcommittees represent dairy processors, equipment manufacturers, and sanitarians.
Third-party verification:
Equipment is inspected by certified conformance evaluators.
Stainless steel is preferred as a material.
Can be polished to a smooth finish, surfaces do not corrode easily, and there is little chance for copper to get into milk
Glass is excellent as weigh jars, receiver jars, and certain milk lines(inert and easily cleaned).
Stainless steel milk lines provide the advantage of not having gaskets.
Joints with gaskets can trap milk solids and bacteria. Loose joints permit air to be pulled into the line by the lowered pressure produced by the vacuum pump of the milking machine. This moving air carries bacteria with it and causes excess agitation of the milk in the line. Such agitation may activate lipase that can release fatty acids from damaged fat globule membranes causing milk to become rancid.
The Grade A Pasteurized Milk Ordinance limits to what Materials that may be used:
stainless steel or other equally corrosion-resistant metals
rubber
both flexible and inflexible plastics.
Plastic and rubber materials must be relatively inert, resistant to scratching, scoring, decomposition, crazing, chipping, and distortion. They are nontoxic, fat resistant, relatively nonabsorbent, and do not release component chemicals or impart flavor or odor to the product.
Copper is excluded because it is a catalyst of oxidation in milk.
Surfaces must be smooth and free of openings, easily accessible for cleaning, and self-draining.
Pipelines should be free of ''dead ends'' because they interfere with turbulent flow of cleaning solutions.
The 3-A specification requires cooling milk from to within the first hour and from to within the next hour.
When milk is picked up every other day, cooling capacity is rated at one-fourth the volume.
Whereas with every day pick up, the capacity is rated at one-half the tank’s volume.
Finally, the temperature of the milk during subsequent milkings shall not exceed . In the blend temper
Evaluation of Defects
Participants in the FFA Dairy Foods CDE evaluate milker unit parts for eight defects, four each with rubber and metal parts
Defects, including dirty, are marked if they appear on any surface, including “water spots” on equipment.
If a part is badly dented or damaged, checked or blistered, or has an open seam, it is likely to be dirty even though the smooth surfaces appear clean.
A split or hole in a hose will cause it to be marked leaky if it is positioned so that air may pass through it. Leaks cause the loss of vacuum and result in poor milker performance.
The defect is poorly fitted when a split occurs in the end of a hose or a rubber part does not fit the metal part onto or into which it belongs, for example, when it is obvious that an inflation does not fit into the shell of the teat cup.
To be criticized as badly dented or damaged, the defect must be great enough to affect cleaning or function.
Stainless steel seldom becomes corroded unless a very strong acid or hypochlorite solution is applied to it. Also, when attached to dissimilar metals or exposed to stray electrical current, stainless steel may corrode. Participants should observe carefully whether a dull or discolored appearance is caused by corrosion or merely reflects incomplete polishing of the metal part.
The presence of tiny cracks in any rubber part in any amount justifies marking the checked or blistered defect because this makes it impossible to remove soil and to disinfect the part. Blistering is almost never seen on today’s milking machine parts. Checks on rubber inflations occur only after extended use and are sure indications of low elasticity of the rubber, and therefore, poor milking function.
Openings between components of metal parts constitute open seams. To be criticized, the seam must be large enough to trap soil or milk solids. Openings on the outside of the claw that are no wider than a human hair are not to be criticized.
Flavor and Odor of Milk
The first line of protection of milk’s flavor is the milk producer who must manage the feeding, disease prevention, housing, cleaning, and milking for the dairy herd and must ensure that the milk is cooled quickly to prevent bacterial growth.
The first representative of the milk buyer and of the public health agency is the bulk milk hauler that must check each tank of milk for normal appearance, freedom from off odors, and acceptable cold temperature.
The next check on quality and safety comes at the receiving plant where a person checks the odor of the milk and samples it to be tested for antibiotics before off-loading from the tanker.
Although milk is among the most nutritious of all foods, it cannot be acceptable to consumers unless it has the rather bland and slightly sweet, clean flavor. Of course, pasteurization imparts a cooked flavor, but this flavor dissipates quickly leaving a pleasing fullness to the flavor.
The four taste sensations found in milk that cannot be detected by smell are bitter, flat, salty, and sour. Some odors, however, can be associated with these taste sensations. This is nearly always true of sourness.
Off flavors and odors of raw milk can be caused by feeds consumed by the cow, illnesses of the cow, or chemical changes that occur due to activities of enzymes that are either naturally in milk or produced therein by bacteria.
Participants in the Dairy Foods CDE are asked to score milk samples based on flavor and odor.
Evaluation of Defects
Caused by the cow's diet
Fresh raw milk should have very little aroma, and most of the aroma results from volatile substances derived from the feeds the cows have eaten.
Therefore, the feed flavor is the most common off flavor of milk. Generally, the more succulent the feed (e.g., fresh green pastures in the early spring), the more offensive the feed flavor of the milk. Onion and garlic are particularly offensive. Silage produces more off flavor than does hay. Since these off flavors have several possible causes, the flavors and odors that result vary with the type of feed. Feed flavors tend to disappear from the mouth and nose more quickly than do many of the more offensive flavors, including rancid and oxidized.
Caused by bacteria
The acid (sour) flavor of milk results from fermentation of milk’s sugar, lactose, by bacteria generally of the Lactococcus genus. Interestingly, this word means the milk (lacto) seed or sphere (coccus). Lactococci are so named because they were found in milk when first described. The main product of this fermentation is odorless lactic acid, which causes the sour flavor. The aroma that accompanies the sour taste is caused by other small molecules produced by these bacteria. Development of sourness requires the presence of fermenting bacteria along with poor cooling because these bacteria grow poorly at temperatures below ().
Sourness is quickly perceived by most persons, appears near the front of the tongue, and leaves the mouth feeling clean if not accompanied by other off flavors. Millions of bacteria are in milk that has a sour taste. A chemical test for the amount of acid in milk is described later in this booklet.
A flavor closely related to acid is malty. The bacteria responsible for this defect produce both sourness and a maltlike aroma. They are known as Lactococcus lactis subspecies maltigenes.
Some reports relate the malty defect in milk to consumption of certain pasture grasses. The offending compound has been reported to be 3-methyl-butanal (isovaleraldehyde). In general, similar small aldehyde molecules are responsible for a wide range of off flavors. As discussed in the section on enzymes, a group of off flavors is caused by bacteria, the psychrotrophs that grow at relatively low temperatures. These flavors can be bitter, unclean, rancid, fermented, and fruity. They result primarily from the degradation of proteins and fat. These bacteria grow slowly at the cold temperatures of milk stored on the farm