This page describes the properties of milk fat. There is a brief introduction to General Fat Definitions and Chemistry, followed by sections on Milk Fat Chemistry, Milk Fat Physical Properties, Deterioration of Milk Fat, and the Influences of Heat Treatments on Milk Fat Properties. For more details on milk fat composition and properties see references by Fox and McSweeney (1998), Parodi (2004), van Boekel and Walstra (1995), Walstra et al. (1999), Weihrauch (1988).
General Fat Definition and Chemistry
Fats are made from individual fatty acid molecules attached to glycerol, a 3-carbon backbone. The most common type of fat is called a triglyceride, or triacylglycerol, which contains 3 fatty acids attached to the backbone and resembles a fork without the handle.
Because there are many different fatty acids that can be attached to the backbone, there are many different types of triglycerides or fats. Fat compounds can also be diglycerides that have 2 fatty acids or monoglycerides that have 1 fatty acid on the glycerol backbone. Mono- and diglycerides are used as emulsifiers, compounds that keep the fat and water from separating in foods such as ice cream.
Individual fatty acids can range in length from 4 to 22 carbons, and may be straight or branching chains. Carbon atoms have 4 bonding sites. Fatty acids may be saturated, which means that each carbon has a single bond to another carbon and 2 hydrogen atoms, or fatty acids may be unsaturated, which means that a carbon has two bonds to the adjacent carbon, called a double bond, and a single bond to another carbon and a hydrogen atom. A monounsaturated fat has 1 double bond and a polyunsaturated fat has 2 or more double bonds in the carbon chain. The bonds in unsaturated fatty acids can be either cis or trans, depending on the direction of the continuing carbon chain on each side of the double bond. A cis bond means that the fatty acid chain continues on the same side of the bond, forming a U shape, and a trans bond means that the fatty acid chain continues on opposite side of the bond, forming a Z shape.
The shorthand description for fatty acids is to list the number of carbons followed by a colon and the number of double bonds. For example, 4:0 is a 4 carbon chain with no double bonds, and 18:1 is an 18 carbon chain with 1 double bond. For unsaturated fatty acids the designation c is used for cis bonds and t is used for trans bonds. For example, C18:2 c,c is an 18 carbon chain with 2 double bonds that are both cis. The terms omega-3 or omega-6 refer to the carbon, either the 3rd or 6th carbon starting at the end of chain not attached to the glycerol backbone, at which the first double bond appears. The Greek letter omega, Ω , is used to identify the omega position.
Other fatty compounds include phospholipids and sterols. The phospholipids have a basic triglyceride type structure but there is a phosphate group at the 3rd position on the carbon backbone. The phosphate group, a combination of phosphorus and oxygen, provides phospholipids with surface properties that are active at the interface between compounds soluble in water and those that are not, like fats. Phospholipids are important components of cell membranes. Phospholipids make up approximately 1% of the fat in milk. The two most abundant phospholipids are phosphotidyl choline and sphingomyelin. Sphingomyelin has been shown to have a protective effect in some cancers. Sterols, such as cholesterol, are complex chemical compounds that are important components of hormones.
Milk Fat Chemistry
Milk contains approximately 3.4% total fat. Milk fat has the most complex fatty acid composition of the edible fats. Over 400 individual fatty acids have been identified in milk fat. However, approximately 15 to 20 fatty acids make up 90% of the milk fat. The major fatty acids in milk fat are straight chain fatty acids that are saturated and have 4 to 18 carbons (4:0, 6:0, 8:0, 10:0, 12:0, 14:0, 16:0, 18:0), monounsaturated fatty acids (16:1, 18:1), and polyunsaturated fatty acids (18:2, 18:3). Some of the fatty acids are found in very small amounts but contribute to the unique and desirable flavor of milk fat and butter. For example, the C14:0 and C16:0 ß-hydroxy fatty acids spontaneously form lactones upon heating which enhance the flavor of butter.
The fatty acid composition of milk fat is not constant throughout the cow's lactation cycle. The fatty acids that are 4 to 14 carbons in length are made in the mammary gland of the animal. Some of the 16 carbon fatty acids are made by the animal and some come from the animal's diet. All of the 18 carbon fatty acids come from the animal's diet. There are systematic changes in milk fat composition that are due to the stage of lactation and the energy needs of the animal. In early lactation, the animal's energy comes largely from body stores and there are limited fatty acids available for fat synthesis, so the fatty acids used for milk fat production are obtained from the diet and tend to be the longer chain 16:0, 18:0, 16:1 and 18:2 fatty acids. Later in lactation more of the fatty acids in milk are formed in the mammary gland so that the concentration of the short chain fatty acids such as 4:0 and 6:0 are higher than they are in early lactation. These changes in fatty acid composition do not have a great impact on milk's nutritional properties, but may have some effect on processing characteristics for products such as butter.
Milk fat contains approximately 65% saturated, 30% monounsaturated, and 5% polyunsaturated fatty acids. From a nutritional perspective, not all fatty acids are created equal. Saturated fatty acids are associated with high blood cholesterol and heart disease. However, short chain fatty acids (4 to 8 carbons) are metabolized differently than long chain fatty acids (16 to 18 carbons) and are not considered to be a factor in heart disease. Conjugated linoleic acid is a trans fatty acid in milkfat that is beneficial to humans in many ways. These issues are discussed in the Milk and Human Health section.
The fatty acids are arranged on the triglyceride molecule (Figure 1) in a specific manner. Most of the short chain fatty acids are at the bottom carbon position of the triglyceride molecule, and the longer fatty acids tend to be in the middle and top positions. The distribution of the fatty acids on the triglyceride backbone affects the flavor, physical, and nutritional properties of milk fat.
Milk Fat Physical Properties
Milk fat melts over a wide temperature range, from approximately -40°F (-40°C) to 104°F (40°C). This is best illustrated by the firmness of butter at refrigerator temperature versus room temperature. At refrigerator temperature butter is approximately 50% solid, but is only about 20% solid at room temperature, which is why it spreads more easily as the temperature increases. The melting properties of milk are a result of the melting points of the individual fatty acids that make up milk fat and their arrangement on the triglyceride molecule.
The triglycerides of milk fat are in the form of globules. The globules are surrounded by a protein and phospholipid membrane that stabilizes the globules in the serum (water) phase of milk. The native globules range in size from less than 1 µm to over 10 µm. The uneven size distribution allows the larger globules to float in a process called creaming, thus resulting in a “cream line” at the top of the container. Milk is homogenized to reduce the size of the large globules to less than 1 µm, create a uniform distribution of globules throughout the serum phase, and minimize creaming.
Deterioration of Milk Fat
Milk fat can be degraded by enzyme action, exposure to light, and oxidation. Each of these processes proceeds through different mechanisms. For further information see the references cited at the top of this page.
Enzymes that degrade fat are called lipases, and the process is called lipolysis. Milk lipases come from several sources: the native milk, airborne bacterial contamination, bacteria that are added intentionally for fermentation, or somatic cells present in milk. Lipases remove fatty acids from the glycerol backbone of the triglyceride. Usually the action of lipase causes undesirable rancid flavors in milk. Pasteurization inactivates lipases and increases the shelf life of milk. However, in some cheeses, such as blue cheese and provolone, a small amount of lipolysis is needed to achieve the characteristic flavor.
Light induced degradation can happen fairly rapidly in milk and produces a characteristic off-flavor. The majority of this off-flavor is caused by protein degradation. Storing milk in opaque containers minimizes this process. Milk fat can also be degraded by a classical chemical oxidation mechanism, the attack on double bonds in the fatty acids by oxygen. Oxidation of the unsaturated phospholipids in milk produces off-flavors that are described as painty, fishy, or metallic.
Influence of Heat Treatments on Milk Fat
Milk fat has a wide melting range, and is fully melted at 104°F (40°C). Typical high temperature short time (HTST) pasteurization conditions do not affect the functional and nutritional properties of milk fat. Higher heat treatments may stimulate oxidation reactions and cause fat deterioration and off-flavors. High heat treatments such as ultra high temperature (UHT) pasteurization can disrupt the milk fat globule membrane proteins and destabilize the globules, resulting in their coagulation.