Getting to know milk and how it can be consumed
An increasing number of consumers wish to know more about what they eat and drink. The present document provides technical information about milk, including aspects that are currently under debate such as the potential risks involved in consuming homogenised milk (Vista Alegre farm milk is not homogenised). A second document in this corner of our website offers more practical information about the milk from our farm itself.
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This document looks at the composition and quality of raw bovine milk. Forthcoming documents will look at the composition and quality of yoghurts and cheeses made with cows’ milk.
1- Definition:
Milk can be defined as the liquid secreted by the mammary glands of female mammals, from the four teats on each udder in the case of dairy cattle. In legal terms the definition also states that “secreted milk should not be adulterated nor altered and should not contain colostrum* and should be collected by regular, complete, uninterrupted and hygienic milking procedures from healthy, well fed cows”. (* Colostrum, also known as beestings, is the milk secreted by a cow a few days before calving and, particularly, during the first few days following calving. It is more yellow in colour and contains more protein and antibodies than the milk normally given by dairy cows).
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2- The composition of milk:
2.1- Chemical composition:
Milk varies in composition from cow to cow and herd to herd, but on average contains the following:
Water 87.0 %
Proteins 3.3 – 3.4 %
Fats 3.5 – 3.9 %
Lactose 4.7 %
Salts (minerals) 0.8 %
(Dry matter 12.5 %)
Water: water is present in milk in two forms:
- free water: most water in milk is free water, which is important for physical, chemical and microbiological processes. The differing consistency of different cheeses is the result of regulating the amount of free water present
- bound water: this is water bound to proteins
Proteins: there are two main groups of proteins in milk:
- casein (from the Latin caseus, “cheese”): this protein is composed of amino acids and phosphorus. It coagulates under the action of rennet or acidification at pH 4.6. Casein, also defined as “a nitrogenous material that can clot”, is a very thick and complete molecule present in milk as micelles or small particles of calcium phosphocaseinate in suspension, that clot or coagulate due to an increase in acidity (acid curdling) or due to the action of a rennet (rennet curdling). This is of interest for cheese makers as each type of coagulation produces different effects in milk and cheeses and, in fact, many cheeses combine both processes (acidity of milk and enzymatic action of rennet). This is why there are so many ways to make cheese.
- lacto-whey proteins: these are the proteins that remain in the whey when cheese is made because they do not clot or coagulate. They include lactalbumin, proteases, peptones and globulin which all precipitate when heated. They are also referred to as nitrogenous materials that do not clot. (Curds are actually a precipitate of the proteins that do not clot when heated).
Fats: these are found in milk as small globules that can be seen under a microscope. The diameter of the globules varies according to the type of milk: goats’ milk is the easiest to digest because its fat globules are the smallest. The fat globules in cows’ milk are bigger which is one of the main reasons given to justify homogenisation of milk, given that the process reduces the size of the fat globules, theoretically making milk easier to digest (but with potentially negative side-effects, see section 4.3). The fat content in milk gives cheeses their aromas and the higher the fat content the more cheese can be made per unit of milk and the better the consistency of cheese, plus the fact that the fat content also prevents excessive concentration of casein. When yoghurt is made with pasteurised milk that has not been homogenised, the top layer of the yoghurt has a distinctive, creamy flavour.
Fats are made up of different components:
- fatty acids: around 90% of the fats in dairy cow milk are fatty acids. There are two sorts:
* saturated fatty acids: 50-60%. According to current scientific thinking, these fatty acids, when ingested in large quantities, are bad for our health.
* non-saturated fatty acids: 30-40%. These are considered to be healthy, although consumers are advised to be wary of milks that are enriched or fortified with these fatty acids and to check the independence of the research undertaken about them to promote them and whether or not they originate in milk itself
The way in which dairy cows feed alters the amount of each sort of fatty acid in milk. Forage based diets increase the amount of non-saturated fatty acids, including Omega 3, omega 6 and omega 9. So what is all the fuss about these omegas? During recent years some of the big dairies are giving a lot of publicity to the presence of omega 3 in their milk implying that this is beneficial for those that consume it. As such it is worth pointing out that:
• omega 3 fatty acids are considered to be essential fatty acids, in other words, the human body cannot make them from other substances. We have to ingest them.
• the results of some research suggest that omega 3 is good for reducing incidence of cardiovascular illnesses (as blood takes longer to clot).
However, the potential benefits of omega 3 depend on the amount of omega 6 ingested at the same time:
• research suggests that historically humans ingested omegas 6 and 3 in a one to one ratio (one unit of omega 6 to every unit of omega 3).
• nowadays, however, in a typical Western diet the ratio is 10:1 and even up to 30:1 (10-30 units of omega 6 for each unit of omega 3), with negative consequences for health.
• for health reasons, the ratio should not surpass 4:1, in other words, we should not ingest more than 4 unit of omega 6 for each unit of omega 3.
Thus, it is of utmost importance not only to tell consumers how much omega 3 is in a litre bottle or pack of milk, but also how much omega 6 is present.
Omega 9 in not an essential fatty acid as the human body can synthesise it, but its presence in food is not negative. Omega 9 is also beneficial to health when present or ingested in the right amount.
Some research indicates that the dairy cattle feeding regimes based mainly on protein concentrate to increase milk yields that are associated with intensive livestock farming result in a higher saturated fatty acid content in milk, whilst cows that graze give milk with a higher concentration of non-saturated fatty acids, in particular omega 3.
- other components: substances such as carotenoids, tocopherols and aldehydes are also present in milk. They are usually found in small quantities but are of great importance as they heavily influence the final quality of, for example, cheese. Natural pastures and organic production favour the presence of these substances.
Lactose: the sugar contained in milk (and which is found in no other organism in nature) is formed by two simple sugars, galactose and glucose. Lactose ferments easily and when in contact with certain microorganisms (such as lactic bacteria and bacterial enzymes) it partially or completely turns into lactic acid. This is the process by which milk is made into yoghurt and is of interest to those consumers that cannot tolerate lactose. There four types of fermentation:
- lactic (very important)
- propionic (very important)
- alchoholic
- butyric
Fermentation is a very important phenomenon in cheese making as it affects the qualities of each particular cheese (flavour and aroma).
Mineral salts: Calcium, phosphorus, sodium, magnesium and potassium are present, amongst others. The most important in cheese making are calcium and phosphorus. As far as calcium and phosphorous are concerned, it is important that these are present both in adequate quantities and in a balanced relation between the two. Thus, a lack of or an excess of either of these macrominerals can affect the absorption of the other. An adquate calcium-phosphorous ratio would be at most 1.5:1, however it is lower in cows' milk than human milk (2:1) which does not favour the absorption of calcium. In predominantly acid curds, nearly all the calcium and phosphorus is lost in whey as they do not bind to the casein proteins due to the high acidity of milk. Cheese made when milk is not very acidic retains much more calcium and phosphorus (Manchego cheese and fresh Burgos cheese). Livestock fodder has a big influence on the presence of different mineral salts.
Biocatalysts: even though these are present in very small amounts they are of interest for health reasons or to favour certain processes and reactions when making cheese, for example. They include:
• Liposoluble vitamins (soluble in fat) A and D are found in cream and cheese
• Hydrosoluble vitamins (soluble in water) B1, B2 and PP that are partially eliminated in whey
• Enzymes that produce or cause certain important biochemical reactions that effect aromas and flavours.
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2.2- Biological elements:
Microbial cells:
- milk that is supplied by milking cows in good hygienic conditions contains a microbial flora that is enriched with a large variety of micro organisms during processing. They play an essential role during cheese making and ripening. However, raw milk contains both useful and harmful germs. The art of turning milk into an edible or drinkable product such as pasteurised milk, yoghurt or cheese consists in using the properties of useful germs and limiting the development of harmful ones.
- Microbes (bacteria) are microscopic organisms. To complete their life cycle they need food (in this case, milk) and an adequate temperature: 15-40ºC is ideal and at 2-4ºC they become inactive but do not die (not even when frozen). However, they do die if milk is heated to over 70-80º for a certain time.
- These microbes are found everywhere, in particularly large numbers in infected areas (mastitis), at the end of the digestive tube (excrements and manure) and in dirty humid places.
Somatic cells: these are formed in cows themselves: 2% are epithelial cells of desquamation and 98% are white globules or leukocytes that come from filtration of blood (these are one of principal defences of mammary glands when infected). Milk from cows that do not have mastitis usually have a Somatic Cell Count (SCC) under 300.000 cells/ml. The SCC has an impact on milk production: milk production falls 3.5% when the SCC reaches 400.000 cells/ml, and 10.5% when the SCC reaches 1.500.000 cells/ml. A zero SCC is not desirable as it would suggest malfunctioning of a cow’s defence system.
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3- Factors that have an influence on the quality of raw milk
26-36% of the factors that have an influence on the quality of raw milk are related to the animals themselves and are hereditary and the rest are related to environmental conditions. The fat content of milk is the component that varies most.
Cattle breeds: in general, animals from lower altitudes give more milk than hill cattle, but the latter give higher quality milk or higher yields in terms of litres of milk used to make a kilo of cheese. Local rustic breeds therefore usually give a lower quantity of milk but this is of a higher quality, whilst pure-breeds give a lot of milk but of lower yield in terms of, for example, fat content.
Lactation cycle: the milk given at the end of a lactation cycle is richer in fats and proteins but cow gives less milk. The quantity of milk given rises until the third or fourth month after a cow stops producing colostrum (a few days after calving).
Fodder: A varied diet, good grazing practice and continuous exercise guarantee healthy cows and good quality milk and processed dairy produce. In terms of organoleptic quality (that related to the senses: sight, smell, taste...), fodder based on concentrates leads to a loss of singularity and aromas. For more information see the document “Feeding the cows on the Vista Alegre farm”.
Physical conditions and management: a cow that suffers stress, is tired, is cold, etc uses its reserves to recover, will produce less milk and will be more likely to fall ill or have infections.
Milking: milking should be done as calmly as possible, without brusqueness and at a set time each morning and night. As each cow is milked, the first spurts of milk from each teat should be rejected (in case coliform bacteria are present). It is also important to thoroughly milk each cow in order to extract all fat content.
Possible pollutants include metals, detergents, disinfectants, internal anti-parasites and other pesticides, antibiotics and chemotherapeutic products.
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4- Processes used to conserve raw milk for human consumption
Raw milk freshly given by cows contains some elements that are ideal for human consumption, for human and health and nourishment. Others, however, need to be removed. Thus, ideally:
• all harmful micro organisms should be destroyed
• all enzymes should be de-activated
• all the original properties and quality of milk should be conserved intact.
No existing method of processing milk achieves all these objectives, but all try to optimise the possibility of supplying healthy, nutritious milk.
4.1- Pasteurisation:
Objectives: the main objectives of pasteurisation are to:
- guarantee the innocuous nature of milk
- prevent alteration of milk during a given period of time
- guarantee it’s nutritional value
- lengthen the period in which milk can be kept and marketed
Milk is heated to a specific temperature for a set length of time and then cooled immediately, a process that slows microbial growth.
Systems:
Low temperature pasteurisation: milk is heated to 63-65ºC during 30 minutes.
Advantages:
* it is cheap
* no foam appears on the surface of the milk
* the milk and cream do not separate
* the nutrient composition of milk is preserved with very few changes (see below)
* the structure of milk is preserved
Disadvantages:
* it is a slow process
* as milk is heated slowly more vitamins oxidise and thermophilic germs may
proliferate
High temperature short time (HTST) pasteurisation: milk is heated to 72ºC during 15 seconds
Very-high temperature pasteurisation: milk is heated to 80ºC during 3-5 seconds.
The general inconveniences of pasteurisation include:
* consequences for clotting with rennet: heating milk to the temperatures required for pasteurisation have a negative impact on clotting with rennet as precipitation of the calcium content occurs and CO2 molecules become detached as acidity falls. This is why calcium salts (particularly calcium chloride, CaCl2) are added to pasteurised milk to be used in cheese making.
* reduction in the size of casein micelles: this has a negative impact on the capacity of milk to clot, curds are not firm and whey is very slow to separate, which is why calcium chloride, calcium phosphate (Ca3(PO4)2) or lactic acid bacteria are added. Even so whey, separates slowly.
* Changes in nutritional values: data concerning changes in the nutrient value of pasteurised milk is contradictory. For example, both “high” and “low” losses in vitamins are mentioned in research. A loss of up to 20% in iodine or the transformation of lactose in beta-lactose which is more soluble and is absorbed more rapidly in the body are also quoted.
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4.2- Sterilisation:
Objectives: the main objective of sterilising milk is to totally destroy all microbes present and thus achieve indefinite conservation.
Systems:
• sterilisation in air and water tight containers: at 110-118ºC for 20-24 seconds
• sterilisation through ultra-heat treatment (UHT): at 135-150ºC for 2-10 seconds
Although this milk is long life, one of the big disadvantages of sterilisation is that it has a greater impact on the nutritional quality of milk, although, once again, available scientific data is contradictory. Thus, some research suggests huge negative impacts in nutrient values, whilst others suggest that changes are only minor in the nutrient value of proteins (lysine, for example) and vitamins.
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4.3- Homogenisation:
Three reasons are usually given to justify homogenisation of milk:
• to prevent cream separating from water in milk
• to lengthen the life of milk
• to facilitate digestion of milk
Raw milk from dairy cows is a form of emulsion, a mixture of fat globules, different solids and water. The fat globules gradually separate out from the water and rise to the surface of the milk as a layer of cream, leaving what is essentially the equivalent of skimmed milk below. This is due to the fact that fat globules are not as dense as water. In milk in particular, fat globules tend to group in clusters of around a million globules which separate out even more quickly than individual globules. In the past, such separation was perceived to be inconvenient by some people, as milk had to be stirred or a bottle of milk shaken in order to mix the cream with the milk. Other consumers considered the existence of a layer of cream to be the way by which to gauge the adequacy of the fat content in milk.
In 1899, the French inventor Auguste Gaulin patented a machine that homogenised milk by breaking down fat globules into smaller, more uniform particles that would resist the process of separating out and rising to the surface of milk. Many different machines have since been invented to reduce the size of fat particles in milk using a decreasing amount of energy.
Initially, the consumer population was not convinced by homogenisation and it was not until 1919 that significant quantities of homogenised milk were bought. One of the arguments then used by companies homogenising milk was its easier digestion.
Nowadays, homogenisation has two phases:
• milk is forced through narrow pores or tubes of decreasing diameter. As the diameter falls but the velocity of the milk flow is maintained, the pressure exerted rises and fat globules are broken down in the resulting turbulence and cavitation (formation of gas in bubbles of a liquid that flows through a space in which the pressure of the liquid falls below the pressure of its vapour). The higher the pressure, the smaller the particles.
• The fat globules that begin to cluster together again include fragments of casein and whey in their walls and some become completely surrounded by protein. Because these new globules, altered in terms of their chemistry, also tend to cluster, a second phase of homogenisation breaks them up and guarantees that all fats and solids are in suspension.
Potential positive and negative impacts of homogenisation:
• The viscosity of milk increases
• Milk is whiter and takes longer to develop those smells associated with sour milk
• It is less stable when heated
• It is more likely to oxidise in light
• It tastes less of milk but is considered to have a creamier taste in the mouth
• Homogenised milk may be more digestible
• Homogenised milk can be sold with any fat content distributed equally throughout the milk.
• Some scientific research suggests fat globules with a high protein content increase the incidence of allergic reactions, although the data is contradictory.
• A link has been suggested between homogenised milk and arteriosclerosis: cows’ milk contains an enzyme the size of a large molecule known as bovine xanthine oxidase (BXO), which normally adheres to the fat globules in milk. In its natural state, before homogenisation, it is not easily absorbed by the intestinal walls. However, following homogenisation, when the particles are smaller they are considered to be more easily absorbed and thus have greater access to the blood circulation system. Some teams of scientists suggest that XOB in blood causes hardening of the arteries by causing plasmogen depletion. Once again, however, there is contradictory data and other teams of scientists do not support this hypothesis.
• The smallest particles of homogenised milk could be vessels for substances that bypass digestion. Thus, proteins that are normally digested in the stomach or intestines are not degraded and are absorbed in blood with possible implications in terms of cancer. Again, the scientific evidence for this is also disputed.
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4.4- Raw milk:
The obvious advantage of raw milk is that it keeps all its nutritional value intact. Its big disadvantage is the presence of biological elements that are harmful to our health, although the huge improvements made both on the farm (management and hygiene in cow barns and milking parlours) and in household kitchens have meant consumer demand for raw milk is increasing.
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5- Conclusions:
Not all milk is the same, even though some of the dairies claim so. Milk from permanently stabled cows that consume a lot of compound feed is not the same as milk from cows that graze part or most of the time and that feed mainly on forage. The milk from a farm that pays little attention to cleanliness and hygiene in the cow barn and milking parlour is not the same as milk from a farm that gives full priority to hygiene. Lastly, the milk that is pasteurised is not the same as homogenised or sterilised milk.
Apart from the differences consumers can perceive from the smell or taste of milk, measuring the content of given elements in milk also reveals it’s quality: the content of saturated and non saturated fatty acids, vitamins, calcium, etc. It is to their benefit that consumers be given information as to whether different elements present in milk (omega 3, calcium...) are naturally present in that quantity or have been added to milk at one stage or another of processing and transformation (“enriched milk”) and, if so, its origin.
It maybe the case that, as with other foodstuffs, less importance should be given to the length of time during which a bottle of milk may be stored before consumption and instead of bulk buying, priority should be given to the contribution of liquid milk in terms of health and nutrition and smaller amounts be bought on a more frequent basis.
The accumulation of contradictory data concerning the health implications due to the different ways in which cows are fed and their milk processed does not only lead to confusion amongst consumers, but also amongst the farming community. For this reason, the Vista Alegre farm has opted to farm organically, as naturally as possible and pasteurise but not homogenise milk whilst simultaneously proposing research into the exact nutritional quality of our milk (see the document “Milk from the Vista Alegre farm”).
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