Jennifer S. Timmons, M. S., Donald L. Palmquist, Ph.D. 
Department of Animal Sciences

Ration composition of lactating cows has been altered over recent years to supply the energy required to support enhanced milk production. One way to accomplish this is to supplement with a fat source such as roasted soybeans. Roasted soybeans contain approximately 20% fat that is high in polyunsaturated fatty acids: 50% to 55% linoleic acid (18:2) and 6% to 8% linolenic acid (18:3)Some of the unsaturated fatty acids in roasted soybeans bypass the rumen and are digested in the small intestine; these are directly incorporated into the milk fat. The polyunsaturated fatty acids are highly susceptible to oxidation; oxidation of milk fat leads to development of spontaneous oxidized flavor (SOF) in milk. The flavor in some cases occurs instantaneously from the cow and intensifies in susceptible milk during refrigerated storage Milk may taste normal at the farm, but develops a flavor characterized as tasting metallic, cardboardy or tallowy with time. The off-flavor may cause consumer rejection, a shorter shelf life and a decrease in the demand for milk. In 1996, Reiter Dairy / Dean Foods contacted OARDC about this off-flavor problem that had lead to rejection of tanker loads of milk. An initial survey showed that polyunsaturated fatty acids were higher in milk from herds fed whole soybeans. OARDC then obtained funding through Dairy Management Incorporated to investigate causative factors related to off-flavor development and to explore possible preventative measures. A commercial field study was performed to determine the role of roasted soybeans (RSB) and milk components in off-flavor development. Twenty commercial dairy farms in the vicinity of Wooster, Ohio were selected based on the feeding of roasted soybeans. Herd size ranged from 35 to 432 head with a RHA of 14,500 to 30,800 lbs/yr. Cows in herds were fed typical Ohio diets of haylage, corn silage, high moisture corn, and a range of 0 to 15% of the diet dry matter as RSB. Bulk tank milk was sampled after the AM milking prior to milk tanker pickup and analyzed for milk fat composition, vitamins, minerals and off-flavor development on 0, 3, and 8 days post-sampling. The development of SOF was related to an increase in the polyunsaturated milk fatty acids (linoleic and linolenic acids), caused by the feeding of RSB, a decrease in milk antioxidants (vitamin E and beta-carotene) with increasing time of storage, and increased concentrations of pro-oxidants (copper).

From the data an equation was developed to predict the development of off-flavor at 8 days post-sampling (approximate time for fluid milk to be bottled, shelved, and purchased by the consumer). The development of SOF was found to be predictable, with one half of the variation in flavor attributed to the amounts of xanthine oxidase (a pro-oxidative enzyme), copper and polyunsaturated fatty acids in the milk. According to these prediction equations, feeding RSB at 15% of the diet DM in the presence of low copper from feed and water would not develop off-flavor. However, in the presence of high copper the amount of RSB (% of diet DM) would need to be decreased below 15% of diet DM to prevent the development of off-flavor. Dairy processors using the prediction equation could pool milk according to its potential for development of SOF. Milk likely to produce oxidized flavor could be converted to alternative uses and milk with low susceptibility to oxidation could be used for fluid milk. These measures should increase shelf life of milk and diminish consumer rejection of milk due to off-flavor. It should be noted that spontaneous oxidized milk flavor is not a wide-spread problem currently because of the low concentration of polyunsaturated fatty acids in the milk supply (3 - 8%); further, homogenization greatly protects against oxidized flavor development. However, the potential for off-flavor development exists, especially with the increased feeding of RSB as a bypass protein and a fat source. Therefore, the cost of potential dumped milk must be weighed against the advantages of feeding high levels of RSB. Farmers feeding RSB need to be aware of the potential of milk with higher unsaturation to develop off-flavor and the repercussions this may have on the dairy industry in the future.