Although the vitamin riboflavin is synthesized by micro-organisms in the paunch of the cow, its concentration in milk can be altered somewhat by altering the quantity of it in the feed. The riboflavin content of milk also may vary with the breed of the cow, and has been reported to be influenced by other factors. Reports, therefore, of its concentration in milk show a wide variation, but all of the results agree in indicating that milk is an excellent source -of the vitamin. Thus C. A. Elvehjem, in a review on the B vitamins, gives the average riboflavin content of milk as 2 mg. per liter; Sharp and Hand also give 2 mg. per liter, and state that winter milk may be 10 to 20 percent lower than that produced by cows on pasture. Arthur D. Holmes and associates at the Massachusetts station found that there were 1.43 mg. of riboflavin per liter of milk from cows on winter rations, and that there were no immediate increases in this amount when these cows were put to pasture.
Riboflavin is remarkably stable toward heat except in the presence of light. It is not destroyed in the homogenization, pasteurization, or boiling of milk. But, according to Sharp and Hand, "about one-fourth of the jiboflavin in a bottle of milk is converted to lumiflavin," which is devoid of vitamin activity; "by exposure to bright sunlight for 1 hour." R. R. Williams and V. H. Cheldelin at the University of Texas demonstrated that as much as 26 percent of the riboflavin in milk may be destroyed by exposure to light for 5 minutes at 100 C.
Walter J. Peterson, F. M. Haig, and A. O. Shaw, at North Carolina State College, exposed fresh samples of milk in pint bottles at temperatures between 60 and 72 F. on sunny days at times between midmorning and midafternoon in direct sunlight on an open porch. The original riboflavin content of the samples varied from 0.83 to 2.60 ( average 1.90) mg. per liter. The losses of riboflavin during periods of 30, 90, 120, and 210 minutes were, respectively, 28, 50, 66, and 72 percent. There was no loss of riboflavin in control samples of milk stored in the dark at room temperature for 24 hours, nor when stored for 7 days in a refrigerator; and, when samples that had been exposed to light were placed in the dark in a refrigerator, further loss was extremely small.
Olof E. Stamberg and D. R. Theophilus, at the Idaho Agricultural Experiment Station, made an extensive study of the losses of riboflavin in milk as a result of exposure to light and on methods of preventing this loss. They reported :
"The results of this study show that direct sunlight exposure of milk is very detrimental to the riboflavin content. It is a common practice to deliver milk bottles on doorsteps where they stand fully exposed for long periods. A shady place or lightproof box or cabinet should be provided unless protective types of containers are adopted.
"The results of this work show that as much as 40 percent of the riboflavin in milk was destroyed after 2 hours' exposure to direct sunlight in clear quart bottles even though the milk was quite cool. Good shade, brown glass bottles, or paper containers gave very good protection. One type of paper container was less efficient in this respect than three other types.
"After milk was exposed to sunlight and subsequently stored in the dark in a refrigerator, there was no further loss of riboflavin for the 20 hours investigated.
"Temperature is an important factor, and the photolysis of riboflavin at near freezing temperature was low as compared to near boiling temperature. The photolysis of riboflavin in raw milk was generally greater than in pasteurized milk and least in homogenized milk.
"Milk kept in a store showcase close to a window lost some riboflavin, but there was practically no loss in milk stored in a showcase where most of the light came from low intensity electric lights.
"As much as 11-12 percent of the riboflavin was lost due to daylight when milk was allowed to simmer for 30 minutes in covered glass pan or uncovered aluminum pan in a bright room, but there was practically no loss when a covered aluminum pan was used."