Milk Production and Biosynthesis

Milk is the source of nutrients and immunological protection for the young cow. The gestation period for the female cow is 9 months. Shortly before calving, milk is secreted into the udder in preparation for the new born. At parturition, fluid from the mammary gland known as colostrum is secreted. This yellowish coloured, salty liquid has a very high serum protein content and provides antibodies to help protect the newborn until its own immune system is established. Within 72 hours, the composition of colostrum returns to that of fresh milk, allowing to be used in the food supply.

The period of lactation, or milk production, then continues for an average of 305 days, producing as much as 9000-10,000 or more kg of milk. This is quite a large amount considering the calf only needs about 1000 kg for growth.

Within the lactation, the highest yield is 2-3 months post- parturition, yielding 40-50 L/day. Within the milking lifetime, a cow reaches a peak in production about her third lactation, but can be kept in production for 5-6 lactations if her health and milk yield are still good.

Graph representing Milk Yield (kg/day) and Weeks of Lactation. Milk Yield declines at -9% per month after peak lactation

About 1-2 months after calving, the cow begins to come into heat again. She is usually inseminated about 3 months after calving so as to come into a yearly calving cycle. Heifers are normally first inseminated at 15 months so she's 2 when the first calf is born. About 60 days before the next calving, the cow is dried off. There is no milking during this stage for two reasons:

- milk has tapered off because of maternal needs of the fetus
- udder needs time to prepare for the next milking cycle

The life of a female cow can be summerized as follows: 

 Age 

0                          Calf born
15 mos                Heifer inseminated for first calf
24 mos                First calf born - starts milking
27 mos                Inseminated for second calf
34 mos                Dried off
36 mos                Second calf born - starts milking
Cycle repeats for 5-6 lactations. 

Automatic Milking

Milking of cows on-farm is usually done twice per day in milking parlors. Modern milking units cn be applied automatically in robotic milkers, or manually in herring-bone style or rotary milking parlor configurations. The milking unit simulates the suckling action by oulsating a rubber inflation to massage the teat and remove the milk. Vacuum is applied to create the pulsating opening and closing of the rubber inflation, but the vacuum is not applied directly to the teat. Milk is withdrawn at 37°C and must be immediately cooled to < 4°C. 

 

Diagram of an Automatic Milking Machine showing both the Massage Phase and the Expansion Phase

Effects of Milk Handling on Quality and Hygiene

Cleanliness

The environment of production has a great effect on the quality of milk produced. For many reaosns including cow and udder health, food safety and dairy food quality, the production of the highest quality milk (lowest microbial content) should be the goal. Hygienic quality assessment tests include sensory tests, dye reduction tests for microbial activity, total bacterial count (standard plate count), sediment, titratable acidity, somatic cell count, antibiotic residues, and added water.

The two common dye reduction tests are methylene blue and resazurin. These are both synthetic compounds which accept electrons and change colour as a result of this reduction. As part of natural metabolism, active microorganisms transfer electrons, and thus rate at which dyes added to milk are reduced is an indication of the level of microbial activity. Methylene blue turns from blue to colorless, while resazurin turns from blue to violet to pink to colourless. The reduction time is inversely correlated to bacterial numbers. However, different species react differently. Mesophilics are favoured over psychrotrophs, but psychrotrophic organisms tend to be more numerous and active in cooled milk. 

Temperature

Milk production and distribution in the tropical regions of the world is more challenging due to the requirements for low-temperature for milk stability. Consider the following chart illustraing the numbers of bacteria per millilitre of milk after 24 hours: 

5°C 2,600
10°C 11,600
12.7°C 18,800
15.5°C 180,000
20°C 450,000

Traditionally, this has been overcome in tropical countries by stabilizing milk through means other than refrigeration, including immediate consumption of warm milk after milking, by boiling milk, or by conversion into more stable products such as fermented milks. 

Mastitis and Antibiotics

Mastitis is a bacterial and yeast infection of the udder. Milk from mastitic cows is termed abnormal. Its SNF, especially lactose, content is decreased, while Na and Cl levels are increased, often giving mastitic milk a salty flavour. The presence of mastitis is also accompanied by increases in bacterial numbers, including the possibility of human pathogens, and by a dramatic increase in somatic cells. These are comprised of leukocytes (white blood cells) and epithelial cells from the udder lining. Increased somatic cell counts are therefore indicative of the presence of mastitis. Once the infection reaches the level known as "clinical' mastitis, pus can be observed in the teat canal just prior to milking, but at sub-clinical levels, the presence of mastitis is not obvious. 

Somatic Cell Count (000's/ml) Daily Milk Yield (kg): 1st Lactation
0-17 23.1 29.3
18-34 23.0 28.7
35-70 22.6 28.0
71-140 22.4 27.4
141-282 22.1 27.0
282-565 21.9 26.3
566-1130 21.4 25.4
1131-2262 20.7 24.6
2263-4525 20.0 23.6
>4526 19.0 22.5

Antibiotics are frequently used to control mastitis in dairy cattle. However, the presence of antibiotic residues in milk is very problematic, for at least three reasons. In the production of fermented milks, antibiotic residues can slow or destroy the growth of the fermentation bacteria. From a human health point of view, some people are allergic to specific antibiotics, and their presence in food consumed can have severe consequences. Also, frequent exposure to low level antibiotics can cause microorganisms to become resistant to them, through mutation, so that they are ineffective when needed to fight a human infection. For these reasons, it is extremely important that milk from cows being treated with antibiotics is withheld from the milk supply.

The withdrawal time after final treatment for various antibiotics is shown below (Note: for illustration only, practitioners shoudl consult individual drug therapy recommendations):

Amoxcillin 60 hrs.
Cloxacillin 48 hrs.
Erythromicin 36 hrs.
Novobiocin 72 hrs.
Penicillin 84 hrs.
Sulfadimethozine 60 hrs.
Sulfabromomethozine 96 hrs.
Sulfaethoxypyridozine 72 hrs.

Anti-Microbial Systems in Raw Milk

There exists in milk a number of natural anti-microbial defense mechanisms. These include:

  • lysozyme - an enzyme that hydrolyses glycosidic bonds in gram positive cell walls. However, its effect as a bacteriostatic mechanism in milk is probably negligible.
  • lactoferrin - an iron binding protein that sequesters iron from microorganisms, thus taking away one of their growth factors. Its effect as a bacteriostatic mechanism in milk is also probably negligible.
  • However, lactoperoxidase is significant - an enzyme naturally present in raw milk that catalyzes the conversion of hydrogen peroxide to water. When hydrogen peroxide and thiocyanate are added to raw milk, the thiocyanate is oxidized by the enzyme/ hydrogen peroxide complex producing bacteriostatic compounds that inhibit Gram negative bacteria, E. coli , Salmonella spp., and streptococci. This technique is being used in many parts of the world, especially where refrigeration for raw milk is not readily available, as a means of increasing the shelf life of raw milk. Good sanitation and hygiene practices remain critical to produce good quality milk! See, for example, the commercial product Stabilak, http://www.censa.edu.cu/index.php/2015/12/10/stabilak/?lang=en. This method should only be used in situations when technical, economical and/or practical reasons do not allow the use of cooling facilities for maintaining the quality of raw milk. Use of the LP-system in areas which currently lack an adequate infrastructure for collection of liquid milk, would ensure the production of milk as a safe and wholesome food, which otherwise would be virtually impossible. The method of activating the LP-s in milk is to add about 10 ppm (parts per million) of thiocyanate (preferably in powder form) to the raw milk to increase the overall level to 15 ppm (around 5 ppm is naturally present). The solution is thoroughly mixed for 30 seconds and then an equimolar amount (8.5 ppm) of hydrogen peroxide is added (generally in the form of a granulated sodium carbonate peroxyhydrate). The activation of the lactoperoxidase has a bacteriostatic effect on the raw milk and effectively extends the shelf life of raw milk for 7–8 hours under ambient temperatures of around 30oC or longer at lower temperatures. This allows adequate time for the milk to be transported from the collection point to a processing centre without refrigeration." (Benefits and potential risks of the lactoperoxidase system of raw milk preservation: report of an FAO/WHO technical meeting, FAO Headquarters, Rome, Italy, 28 November - 2 December 2005.)

Milk Biosynthesis

Milk is synthesized in the mammary gland. Within the mammary gland is the milk producing unit, the alveolus. It contains a single layer of epithelial secretory cells surrounding a central storage area called thelumen, which is connected to a duct system. The secretory cells are, in turn, surrounded by a layer of myoepithelial cells and blood capillaries.

Diagrams of the Udder Quarter, Alveolus, and Secretory Cell

The raw materials for milk production are transported via the bloodstream to the secretory cells. It takes 400-800 L of blood to deliver components for 1 L of milk.

  • Proteins: building blocks are amino acids in the blood. Casein micelles, or small aggregates thereof, may begin aggregation in Golgi vesicles within the secretory cell.
  • Lipids:
    • C4-C14 fatty acids are synthesized in the cells
    • C16 and greater fatty acids are preformed as a result of rumen hydrogenation and are transported directly in the blood
  • Lactose: milk is in osmotic equilibrium with the blood and is controlled by lactose, K, Na, Cl; lactose synthesis regulates the volume of milk secreted

The milk components are synthesized within the cells, mainly by the endoplasmic reticulum (ER) and its attached ribosomes. The energy for the ER is supplied by the mitochondria. The components are then passed along to the Golgi apparatus, which is responsible for their eventual movement out of the cell in the form of vesicles. Both vesicles containing aqueous non-fat components, as well as liquid droplets (synthesized by the ER) must pass through the cytoplasm and the apical plasma membrane to be deposited in the lumen. It is thought that the milk fat globule membrane is comprised of the apical plasma membrane of the secretory cell.

Milking stimuli, such as a sucking calf, a warm wash cloth, the regime of parlour etc., causes the release of a hormone called oxytocin. Oxytocin is relased from the pituitary gland, below the brain, to begin the process of milk let-down. As a result of this hormone stimulation, the muscles begin to compress the alveoli, causing a pressure in the udder known as letdown reflex, and the milk components stored in the lumen are released into the duct system. The milk is forced down into the teat cistern from which it is milked. The let-down reflex fades as the oxytocin is degraded, within 4-7 minutes. It is very difficult to milk after this time.