Mammary epithelial cells, located in the mammary gland, are where all the action of milk synthesis occurs. Milk synthesis can be considered in three main stages – extraction, synthesis and secretion. That is, extraction of precursors and components from the blood, synthesis of the protein, lactose and lipid components of milk, secretion of the components into the alveoli of the mammary gland.
“It takes about a pint of blood to make a millilitre of milk. So if you look at the highest-producing cow in the world, she produced 85kg of milk per day at peak, so she needed 40,000 litres of blood going through the mammary gland to make this.” Dr Sam Peterson, senior lecturer at Massey University, says.
Before the magic in the mammary gland can happen, we must first back-track up the digestive system to the rumen. Once feed is consumed and enters into the rumen, it is broken down into the various precursors of milk, namely carbohydrates, protein, lipids, minerals and vitamins, by rumen bacteria. These components diffuse into the blood to be transported to various animal tissues. Enter stage one of milk synthesis – extraction.
Glucose, amino acids and fats are actively taken up by the gland.
“In terms of the extraction of vitamins and minerals we don’t know much about that. But we do know that shortages of them occur in the animal because too much goes into milk, causing neural and muscular disorders like white muscle disease and grass staggers.”
Synthesis of milk components
Protein synthesis and secretion
Precursors: Amino acids from dietary protein.
Extraction: Rumen bacteria breakdown the proteins in the cow’s feed source into amino acids. These amino acids are used as building blocks for the bacteria to rebuild their own protein. This bacterial protein moves into the intestines to be broken down into amino acids again. These diffuse into the blood and are transported into the mammary gland and made available for milk protein synthesis or to the liver to be excreted.
“From the point of view of feeding cows in New Zealand, it doesn’t really matter what quality protein you feed your cows, there’s no point in buying expensive protein. Feed them the lowest quality protein because it gets broken down and rebuilt anyway. In most cases we probably have too much protein in our pastures. Excess protein in the diet ends up as urea, which is expensive to get rid of.”
Peterson says high-protein diets are detrimental to the physiological cost of running a cow. The more protein in the system, the more urine is made, meaning the kidneys have to work harder. There is also an environmental consequence to this process.
Enzymes, structural and controlling proteins as well as the main milk proteins (casein and whey proteins) are made in the mammary epithelial cell.
Main milk proteins:
– Casein – has three variants α, β and ĸ. α itself has two variants αS1 and αS2 (which are the topic of the A1 and A2 milk debate)
-Whey proteins – α-lactalbumin and β-lactoglobulin
Synthesis and secretion: Much like building plans provide the blueprint for the construction of a house, DNA provides the blueprint for the synthesis of milk proteins from amino acids. Once synthesised, the milk proteins are secreted into the alveolar lumen in the mammary gland where milk constituents are collected.
The milk protein α-lactalbumin is also made, and makes up part of the enzyme responsible for lactose synthesis.
“If you study mammary cells and discover they are making lots of lactoferrin, then those cells aren’t active at the time in terms of making milk. They may be undergoing maintenance or drying off. Those cells producing α-lactalbumin are active tissues.”
Lactose synthesis and secretion
Precursors: Glucose, volatile fatty acids (VFAs – acetate, propionate and butyrate) and galactose
Extraction: VFAs diffuse down a concentration gradient into the mammary gland. They are able to cross membranes without transporters.
Glucose is the major precursor of lactose, but in a pasture-fed dairy cow any glucose that arrives in the rumen will end up as VFAs.
“For a NZ dairy cow, in order to make lactose she needs glucose, and all of the glucose in its blood has to be made in the liver, from propionic acid in particular. It’s also worth noting that glucose uptake is not controlled by insulin. This means that the udder can take so much glucose that the cow becomes hypoglycaemic.”
Synthesis and secretion: In order to change the amount of lactose that is made, the amount of glucose going into the system needs to be altered. Glucose determines the amount of lactose made, which determines milk volume.
“That’s the crunch for farmers, getting glucose precursors into the cow, having the liver make the glucose, having other tissues not use it.”
Lactose synthase, the enzyme responsible for making lactose, is made from α-lactalbumin and galactosyl transferase. Lactose is made in the Golgi-body of the cell which draws in large amounts of water. The ions that follow water around cause more water to be drawn into the cell in an attempt to balance the osmotic effect. However, a sodium pump on the basal membrane of the cell pumps out sodium, setting up an electrical charge across the apical membrane. It’s the potential difference created by the electrical charge across the membrane that inhibits the entry of positive ions and the accompanying extra water which enables milk to be concentrated. This results in high potassium and low sodium and chlorine in milk, giving milk a positive charge. The larger the difference in charge on either side of the membrane, the more lactose will be secreted into the lumen.
“The positive charge in milk is relevant for farmers when it comes to electronic mastitis detectors. When the charge in milk changes due to the membrane being damaged, there will be more salt in the milk due to leakages and, hence, lean towards a more negative charge.”
Milk fat
Precursors: VFAs, non-esterified fatty acids and triacylglycerols.
Extraction: Non-esterified fatty acids are extracted in the same way as VFAs. Triacylglycerols, however, must be broken into smaller components to enable them to cross the membrane into blood. Once lipids are broken down into their constituents, they diffuse down a concentration gradient in the blood directly to the mammary gland, or after modification in the liver.
Synthesis and secretion: Humans produce milk that has longer chain fatty acids and is either saturated or unsaturated depending on what the mother eats. If she consumes unsaturated fats, that is what she will produce in her milk. Ruminants however, have milk fat containing large amounts of saturated fats with a few mono-unsaturated fats.
“The thing about lipids is that any type of fat a cow eats is going to be saturated by the bacteria in the rumen.”
Bacteria break all the double and triple bonds and create their own saturated fats, much in the same way as they create their own protein. In order to modify this to alter the milk fat composition, ‘protected’ lipids can be fed. These protected lipids are coated in a protective layer to bypass rumen digestion and go straight to the intestine.
Milk fat is secreted by the cell in droplets surrounded by a part of the cell membrane. This wrapping is part of the reason cream easily separates from milk. The process of homogenisation at the factory breaks the wrapping membrane so the molecules of milk fat can disperse within the milk, instead of floating on top as cream.
Altering milk composition
Milk composition is determined by genetics and feed. If feed is changed, the proportions of milk components can alter quite rapidly, affecting the end products, and therefore the manufacturing processes.
“Occasionally you see farmers that have got less than half a percent of milk fat and wonder what’s gone wrong. Something has happened at the nutrition level to throw things out of whack.”
The process of synthesis and secretion of milk components is not the same day-to-day.
When feeding particular feeds there is an accompanying population of bacteria in the rumen to digest it, and when feed is changed the bacterial population must respond to the changes and will change the products they produce, altering the composition of milk. For example, feed that alters the ratio of acetate and propionate has an effect on the amount of glucose produced; less propionate produced means less glucose, so lower milk volume.
Alternatively, lower acetate, can result in less fat in the milk.
What a cow is fed, and how much, can alter the amount and composition of the milk produced. Traditionally in NZ, livestock has grazed similar ryegrass and clover pastures, so all the milk produced is consistent, except for seasonal variation, Peterson says.
“With increasing use of mixed rations and supplements like palm kernel extract, factories will experience bigger differences in composition of milk delivered to them; this may alter the processing requirements or change what can be made from the milk.”
Peterson says as diets become more complex, farmers are more likely to need the advice of nutritionists to get the best value from their investment in feed.
