Physical Structure of Milk Fat
The positional specificity of fatty acid location on the glycerol molecule (designated as sn-1, sn-2 or sn-3) is responsible for the plastic, spreadable nature of butter, for high digestibility of saturated fatty acids, and possibly for some aspects of the hypercholesteremic potential of milk fat (German et al., 1997). Most natural fats (notable exceptions: milk fat and lard from pigs) have a preponderance of unsaturated fatty acids in the central (sn-2) position of glycerol. Two-thirds of the fatty acids in milk fat are medium- and long-chain saturated fatty acids. The sn-1 position is almost entirely occupied by long-chain fatty acids, whereas sn-3 is dominated by 4:0, 6:0, and 18:1. In fact, nearly all of butyric and caproic acids are found exclusively in the sn-3 position. These are rapidly hydrolyzed by gastric lipases, releasing these rather water-soluble fatty acids to aid in emulsification of the bulk lipid in the stomach for ready digestion in the intestine. Also, these short-chain fatty acids are rapidly absorbed directly into the blood, followed by rapid clearance and oxidation by the liver. Thus, they are a quick energy source, especially important for the newborn.
Saturated fatty acids tend to be less digestible than unsaturates. An exception occurs if the fatty acid resides in the sn-2 position of glycerol, because a large portion of these will be absorbed as the 2-monoglycerides, which are more digestible than fatty acids. Thus, because a high percentage of saturated fatty acids in milk fat are in the sn-2 position, these nutritionally unpopular fatty acids have a high digestibility in humans.
As noted above, the combinations of different fatty acids in triglycerides leads to a very wide array of different structures. Triglyceride (TG) structure is often described partially by the triglyceride number, the sum of the carbon atoms in the fatty acids of the glyceride. Thus, tributyrin (all positions esterified with butyric acid) has a TG number of 12; tristearin (3 x 18) has a TG number of 54. These are the theoretical limits for TG number in normal milk fat. In milk fat, the TG number commonly ranges from 34 to 54, and there are characteristic patterns within this range (Palmquist et al., 1993). Molecules with lower TG number tend to be more liquid, whereas molecules with high TG number and mostly saturated fatty acids make up a high melting fraction. The melting range of milk fat is approximately 40°C to + 40°C; as temperature increases within this range, there is an increasing amount of liquid fat supported in a three-dimensional crystalline (solid) matrix. This contributes to the unique plasticity of milk fat and is the main reason that it is not possible to replicate the spreadability characteristics of butter with hydrogenated vegetable oils. This property also contributes to the desirable mouth feel of butter (German et al., 1997).