D. L. Palmquist
Department of Animal Sciences
Americans are becoming increasingly concerned about the content and composition of fat in the diets. Research in dairy cattle nutrition and metabolism is addressing this concern by exploring ways to change the composition of milk fat to a more desirable pattern from the consumer's viewpoint. Numerous examples of modified milk fat composition are given. Development of modified milk fat will depend upon market forces.
Americans and citizens of other developed countries are receiving so much information about "healthy" and "unhealthy" foods that they are becoming confused and paranoid about all that they eat. Nutritionists know that there are no unhealthy foods as such, but many persons consume unhealthy diets. In fact, some who are overly conscious about what they eat may have unhealthy diets because they are afraid to eat adequate amounts of such important foods as dairy products and red meat that provide important nutrients for any healthy diet.
Dairy products are often targeted as "unhealthy" because of perceived highly saturated fat. Careful nutrition research has shown that only two of the saturated fatty acids in milk, myristic and palmitic (C14:0 and C 16:0, respectively, in chemists' shorthand) may contribute to increasing blood cholesterol concentrations. However, these two fatty acids are important components of milk fat, usually constituting one-third to one-half of the fat in milk.
Dairy cattle nutritionists and milk chemists have found that the fatty acid composition (profile) of milk fat may be altered by feeding. In response to concerns of the public and human nutritionists, a multi-state cooperative research project was instituted in 1994 to quantify effects of feeding on milk fat composition. The project objectives go beyond this, to include biochemists who are study-ing how synthesis of milk fat is regulated in the mammary gland. In the future, milk fat composi-tion may be modified by molecular engineering of the mammary gland. Finally, milk chemists are cooperating to learn how modifying the composi-tion of milk fat may influence the quality, flavor, and stability of dairy products (such as the softness of butter). Currently, there are active cooperators from 11 states (from New York to California), two Canadian provinces, Australia, and Denmark. Ohio was a leader in initiating the project.
One of the simplest approaches to changing milk fat composition is to feed increased amounts of fat to the cow. This approach, however, is not as easy as it may seem, and certainly more difficult than changing the fat composition of pork or eggs. Fat in the diet of cows may have severe effects on proper digestion of feed. The maximum amount of fat that may be fed is only about 5% of the ration. Further, the composition of the fat must be relatively saturated or chemically treated in some way to limit its interaction with ruminal microbes. Unsaturated dietary fatty acids are biohydrogenated (saturated) by ruminal microorganisms. This is a major reason that fat from ruminants (cattle and sheep) is relatively saturated.
Several approaches to developing commercial ruminally inert fats have been successful, and researchers in Australia and at Clemson University have developed processes that not only prevent digestive interference, but also increase the amount of polyunsaturated fat absorbed by ruminants.
Seasonal variation in milk fat composition in the United States is in Table 1. Seasonal variation is significant, primarily caused by summer pasture feeding and decreased forage intake during hot summer weather, whereas regional variation is small. Several examples of effects of changing the composition of dietary fat supplements on milk fatty acid composition are shown in Table 2. All fat supplements reduced the content of fatty acids with 6 to 14 carbon atoms (C6:0 to C14:0), which are synthesized in the mammary gland, and increased C18:1, which is provided directly or indirectly by the supplemental fat. Contents of C16:0 and C18:0 varied with the type of fat supplement. The composition of milk fat when a ruminally-protected unsaturated fat was fed is in Table 3. Content of C12:0 to C16:0 fatty acids was reduced by 23%, with complementary increases in unsaturated fattyacids. This process is being used commercially in Australia to produce modified fat, milk, and meat products, but it is not used in the U.S. because of concerns of using formaldehyde in feed processing. An alternate approach to increase polyunsaturated fatty acids in milk fat is being investigated by Dr. Tom Jenkins at Clemson University. By reacting soy oil with amines, he produces fatty acid amides that are protected from ruminal microbial action. An example of including butylsoyamide in the diet of dairy cows is shown in Table 4. Contents of C4:0 to C8:0 are lower than in other tables, caused by differences in analytical technique. The unsaturated C18:2 protected by amide linkage was increased in the milk fat. Extensive ruminal metabolism of the soybean oil is indicated by the high amount of trans C18:1 found when the free oil was fed. This approach has not yet been fully developed and investigated.
Recently, we were informed of a problem of oxidized flavor in milk from an area in Western Ohio. Oxidized flavor is not an unusual problem at the end of winter. Investigation showed that the milk in question contained unusually high amounts of linoleic acid (C18:2). Linoleic acid is polyunsaturated, making it quite susceptible to oxidation, with development of oxidized flavor. With funds provided by the Ohio Dairy Research Fund, we are surveying 50 Ohio farms to determine whether the changed milk fat composition is related to feeding. This study is a first step to develop procedures for preventing this occurrence in the future. Preliminary results from samples taken during December 1995 to March 1996 are in Table 5. Whereas the "fat" supplements tended to cause changes similar to those shown in Table 2, whole soybeans clearly increased the contents of C18:2 and C 18:3 (linoleic and linoleic acids).
Other applications for information on milk fat composition are of interest. Currently, great interest has been shown for certain isomers of linoleic acid for their potent anti-cancer activity. These fatty acids, conjugated linoleic acid (CLA), are naturally-occurring in ruminant products (milk, beef, lamb) produced from ruminal microbial action on dietary linoleic acid (Jiang et al., 1996).
|Table 1. Seasonal variation in fatty acid composition of milk fat in the U.S.1|
|(% of total)|
|1 Barbano (1990).|
|Table 2. Effect of dietary fat source on composition of the principal milk fatty acids.1|
|Basal||Animal-vegetable blend||Calcium soap||Hydrogenated animal fat||Saturated fatty acids||Tallow|
|(% of total fatty acids)|
|1 Palmquist et al. (1993).
2 Source: Basal = no added fat; Animal-vegetable blend = commercial fat; Calcium soap = Megalac® (Church and Dwight Co.); Hydrogenated animal fat = Alifet® (Rouse Marketing); Saturated fatty acids = Energy Booster 100 ® (Milk Specialties Co.); Tallow = Booster Fat® (Balanced Energy Co.).
a,b,c Values with different superscripts differ (P < 0.05).
|Table 3. Effect of dietary protected canola supplement (PCS) on the fatty acid composition of milk.1|
|(% of total)|
|1 Ashes et al. (1992).
*P < 0.001.
|Table 4. Composition of fatty acids in milk samples taken from lactating Holstein cows fed a diet without added fat or diets containing 3.5% added soybean oil (SBO) or butylsoyamide (BS).1|
|(% of total fatty acids)|
|1 Jenkins et al. (1996).
a,b,c Means within a row lacking a common superscript differ (P < 0.05).
Feeding higher amounts of unsaturated fat to cows increases this fatty acid in milk from a low content of 0.4 to 0.5% of fatty acids up to 1.5 to 1.8%. There is interest to increase the amount of these naturally-occurring anti-cancer CLA in the diets of all Americans.
The very long-chain polyunsaturated fatty acids (n-3 or "omega-3") found naturally in fish oil may be lower than desirable in the diets of infants. In our research we have found that these fish oil fatty acids are less susceptible to biohydrogenation by ruminal microorganisms than are the usual unsaturated fatty acids found in forages and corn. We are interested to learn how these fish oil fatty acids may be fed to cows without disturbing ruminal function, and to determine the efficiency of their incorporation into milk fat. Some research from Denmark and Germany suggests that these fatty acids will be used by the cow for milk fat synthesis.
These are a few examples of how research is advancing to the next level of investigation -- improving food quality. Some of this research is ready for application. The next step is for industry to identify appropriate markets and to develop procedures to segregate milk with defined composition and guidelines to pay for increased costs of producing and processing specialty milk products. Some commercial dairies are beginning to move toward these developments.
|Table 5. Effect of dietary fat sources on milk fat composition in bulk tank milk from commercial Ohio dairy herds.|
|No fat||Whole soybeans||Fat1|
|1 Fat sources included: Purina High Fat, tallow, "liquid
fat", Rumafat, Megalac, cottonseed.
2 n = number of farms.
* Percent of total fatty acids.
Although most Americans need not be concerned about the composition of normal milk fat, modified fat dairy products will provide wholesome alternatives for those with real or perceived concerns.
Ashes, J. R., P. St. Vincent Welch, S. K. Gulati, T. W. Scott, and G. H. Brown. 1992. Manipulation of the fatty acid composition of milk by feeding protected canola seeds. J. Dairy Sci. 75:1090.
Jenkins, T. C., H. G. Bateman, and S. M. Block. 1996. Butylsoyamide increases unsaturation of fatty acids in plasma and milk of lactating dairy cows. J. Dairy Sci. 79:585.
Jiang, J., L. Björck, R. Fondén, and M. Emanuelson. 1996. Occurrence of conjugated cis-9, trans-11-octadecadienoic acid in bovine milk: effects of feed and dietary regimen. J. Dairy Sci. 79:438.
Palmquist, D. L., A. D. Beaulieu, and D. M. Barbano. 1993. Feed and animal factors influencing milk fat composition. J. Dairy Sci. 76:1753-1771.