From a chemical perspective, fats are primarily triglycerides, i.e. ester-like compounds composed of three fatty acids and glycerol.
Milkfat is characterised by a very broad distribution of around 400 different fatty acids, ranging from butyric acid (4C atoms) to behenic acid (22C atoms). In addition to saturated (medium and long-chain) fatty acids, it also contains the physiologically important unsaturated and short-chain fatty acids in amounts of up to 40%. Also of particular interest are the conjugated linoleic acids (CLAs) produced in ruminants through biohydrogenation in the rumen of unsaturated fatty acids contained in the feed. These are said inter alia to have a cancer-inhibiting effect. Milkfat also contains mono- and diglycerides, phospholipids, free fatty acids, cholesterol, fat-soluble vitamins A, D, E and K, and enzymes, as well as aromatic and decomposition compounds.
The composition of milkfat is influenced by various factors, the most important of which are:
Feed: influences the proportion of saturated and unsaturated fatty acids, CLA content, and vitamin A and E content.
Heredity, state of health, age of animals: influence the susceptibility of unsaturated fatty acids to lipolysis and oxidation.
In What Form is Milkfat Present in Milk?
Milkfat is present in milk in the form of globules with a diameter of approx. 1-10 µm. These globules are encased in and protected by a membrane consisting primarily of emulsifying substances, especially phospholipids, lipoproteins, proteins and cholesterol, which contribute to a uniform distribution of fat globules in the aqueous phase (cf. figure).
The membrane is formed in the final stage of lactation; the milkfat is completely protected when it leaves the teat. If sufficient care is not taken, the fat-globule membrane can even be damaged during milking, which results in free fat.
The fat globules are reduced in size through deliberate or unintentional exposure to strong mechanical or thermal effects. An example of unintentional exposure would be through inappropriate pumps. Intentional reduction in size occurs when milk is homogenised. Since the total surface area of the fat globules increases, this results in a shortage of primary membrane material. Milk proteins from the serum arrive at the newly formed fat-globule surface to form the so-called secondary fat-globule membrane, and have an emulsifying effect. In addition to casein and whey proteins, this can also cause enzymes such as lipases to bind more frequently to the fat-globule membrane. To prevent the lipases from damaging the milkfat, pasteurisation – which also inactivates these enzymes – must be carried out immediately after homogenisation.
How Does Feed Affect Milkfat Composition?
Grassland fodder – or maize silage plus concentrate-based feeding
Feeding cattle plenty of fresh grassland fodder leads to milkfat with a high proportion of unsaturated fatty acids. A diet with large amounts of maize silage and concentrates results in a reduction in unsaturated fatty acids. This raises the melting point of the milkfat and its hardness, since the long-chain fatty acids of the milkfat are absorbed from the feed. Depending on its quality, hay also leads to a certain reduction in unsaturated fatty acids compared to fresh grassland fodder, but to a lesser extent than maize silage and concentrates.
The rumen bacteria in the cow’s digestive tract partially hydrogenate the polyunsaturated fatty acids (linoleic and linolenic acids). Because a double bond is normally preserved, the oleic acid in the milkfat constitutes the most important unsaturated fatty acid. What’s more, cows possess a so-called desaturase enzyme which can re-convert stearic acid into oleic acid. This mechanism ensures that the melting point of the milkfat always remains below the body temperature of the cow (38–39 °C).
Unsaturated fatty acids are found in abundance in high-quality young green fodder, but are less well represented in dried fodder, beets and maize silage. With dried fodder, quality plays a major role. Extremely high percentages of oleic acids (up to 30%) are obtained during alpine summering.
The details listed in the table below are based on a comprehensive Agroscope study dating from 1997. Extreme values such as those that may occur in alpine summering milk are not taken into account here, since the production percentages are low when measured against Swiss milk production as a whole.
With the help of high-quality grass silage, high-quality hay, and oilseeds (oilseed rape, sunflower, flaxseed) fed in quantities of 0.4 to 1.0 kg per day, the percentage of unsaturated fatty acids in milk can be raised even in winter, thereby reducing seasonal differences in milkfat composition. Adding special fats to the cows’ feed can increase the linoleic and linolenic acid content of their milk from a normal 2.5% to as much as 8%, and the oleic acid content to as much as 35%.
What is meant by fat deterioration?
Fat deterioration encompasses the following processes:
The formation of free fat (generally irreversible)
Lipolysis of the milkfat (formation of free fatty acids and mono- and diglycerides)
Oxidation of the unsaturated fatty acids (formation of volatile decomposition products such as aldehydes, ketones, esters, lactones and alcohols, as well as short-chain mono- and dicarboxylic acids)
In addition to the adverse sensory changes produced by fat deterioration, such as flavour defects (rancid, metallic, oxidised, tallowy, fishy) and cap or plug formation or butter separation, fat deterioration also causes technological drawbacks.
As emulsifiers, the fatty acids and mono- or diglycerides formed reduce the separating effect during centrifugation. In serious cases, they can even increase the fat content of the buttermilk or whey.
Mechanical Fat Deterioration
All mechanical processes contribute to the formation of free fat, and hence to fat deterioration. In raw milk products, free fat is subject to attack by microbial lipases or lipases in the milk itself, enabling the formation of free fatty acids. All mechanical treatments should therefore be performed as gently as possible.
Microbiological Fat Deterioration
Microorganisms, particularly psychrotrophic bacteria, are able to form lipases and proteases that can be very heat-resistant. If enzymes are formed in fairly large quantities over several days during cold storage of raw milk or raw cream, thermal treatments with temperatures above 90°C must be carried out. Even in these conditions, however, complete inactivation of the microbial enzymes cannot be entirely guaranteed. Long (cold-) storage periods for raw milk or raw cream must therefore be avoided.