Milk and Milk Products Technology

MILK FAT

• Milk fat contains approximately 65% saturated, 30% monounsaturated, and 5% polyunsaturated fatty acids.
• Milk Fats are made up of fatty acid molecules attached to glycerol. Triglyceride, or triacylglycerol is the most common type of fat. 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. A method to denote fatty acids is to write the number of carbons followed by a colon and the number of double bonds.
• Other fatty compounds include phospholipids and sterols. 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.
• 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.

Physical properties of milk fat

• The melting point of Milk fat is over a wide temperature range, of approximately -40°F (-40°C) to 104°F (40°C). This is evident from 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.
  

Table 2. Principal fatty acids found in milk triglycerides

	Molecular formula	Chain length	Melting point
Butyric	CH3(CH2)2COOH	C4	–8°C
Caproic	CH3(CH2)4COOH	C6	–2°C
Caprylic	CH2(CH2)6COOH	C8	16°C
Capric	CH3(CH2)8COOH	C10	31.5°C
Lauric	CH3(CH2)10COOH	C12	44°C
Myristic	CH3(CH2)12COOH	C14	58°C
Palmitic	CH3(CH2)14COOH	C16	64°C
Stearic	CH3(CH2)16COOH	C18	70°C
Arachidonic	CH3(CH2)18COOH	C20
Oleic	CH3(CH2)7CH=CH(CH2)7COOH	C18: 1	13°C
Linoleic	CH3(CH2)4(CH=CH.CH2)2(CH2)6COOH	C18: 2	–5°C
Linolenic	CH3.CH2(CH=CH.CH2)3(CH2)6COOH	C18: 3