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Milk Composition
Walter Hurley
09/01/1997
General Milk Composition
Sow productivity often is measured in terms of the number of pigs produced and weaned per year. Colostrum and milk are key to the survival and growth of the suckling pig. Milk is the major, and often the only, source of nutrients for the young pig suckling the sow. While the solids in milk generally get most of the attention, water is of major significance to the neonate and milk is the only source of this nutrient. Total solids of sow milk is approximately 19% (or ~80% water), but may range from 17 to 25 %. An interspecies comparison can sometimes help put these numbers into perspective. Sow milk is fairly thick compared to cow milk (at ~12.5% solids) and human milk (also ~12.5% solids).
The major components of milk solids are fat, lactose, protein, and minerals. Sow milk contains 7 or 8 % fat, however, fat in milk can be extremely variable. Milk fat is affected by breed, diet, stage of lactation, and other factors. Fat in sow milk is much higher than in cow milk (~3.5-5.0%) or human milk (~4.5%). The newborn pig utilizes milk fat for deposition of body fat, while milk fat can be used for energy in older suckling pigs. Lactose is the major sugar in milk and is the least variable component of milk. Lactose is used by the suckling pig as a major source of energy. Breed and stage of lactation can affect milk lactose content to a limited degree, but generally diet does not affect lactose. Content of lactose in sow milk (~4%) is similar to that in cow milk (~4.9%), but much lower than in human milk (~6.8%). Milk proteins supply the amino acids necessary for growth of the suckling pig. Total protein in sow milk is approximately 6%. This compares to ~3.8% for cow milk, while human milk is quite low in total protein at ~1.1%. Milk protein is affected by breed and stage of lactation, but generally not much affected by diet. The other major component of milk is the ash or mineral content. Sow milk contains 0.6 to 1.0% ash, with the majority of that as calcium and phosphorus. Cow milk is similar at 0.7%, while human milk is lower at 0.2%. Milk Proteins Milk protein is actually a complex mixture of many different proteins.
The major milk proteins are the caseins. These are primarily responsible for the formation of the curd in response to the acid and enzymes of the stomach. Other proteins in milk are collectively called the whey proteins. These include two major whey proteins which are produced in the mammary gland, called beta-lactoglobulin and alpha-lactalbumin. These proteins are digested in the intestine to supply amino acids, but are generally not as digestable as the caseins. Cow, pig and human milks each have alpha-lactalbumin, but the presence of beta-lactoglobulin is species specific; cow and pig milks have it, while human milk does not. Two minor whey proteins found in milk are absorbed directly from the blood serum and include serum albumin and immunoglobulins. Both of these are in relatively low concentrations in milk, but immunoglobulins are in high concentration in colostrum. These colostral immunoglobulins are particularly critical for the health of the newborn pig (to be discussed in a later Topic-of-the Week). Another minor whey protein in milk is lactoferrin, an iron-binding protein known for its antimicrobial activity. Lactoferrin is synthesized in the mammary gland. There are many other proteins found in milk, although these are usually found in very low concentrations compared with the caseins and the major whey proteins.
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