Gas chromatographic analyses of head-space vapors of steam distillates from milk enabled me to monitor part per billion changes in concentrations of acetaldehyde, propanal, methyl sulfide, acetone, butanone, n-pentanal, and n-hexanal before and after milks were exposed to sunlight in a clear glass flask and fluorescent light in a plastic jug, n-Pentanal and n-hexanal increased about the same under either light (19.6 to 68.5 ppb and 19.1 to 83.6 ppb, respectively). Acetaldehyde increased much more under sunlight than under fluorescent light. Methyl sulfide increased slightly in concentration under 44 h of fluorescent light (6.8 to 11.6 ppb) but did not change appreciably during exposure to 1 h of sunlight. Changes in concentrations of the same compounds, except acetone and butanone, were determined in raw; laboratory pasteurized; and plant pasteurized, homogenized, Vacu-Therm-treated milk exposed to sunlight for 20 min. Except for n-pentanal, all carbonyl compounds in pasteurized, homogenized, Vacu-Therm-treated milk changed less after exposure to 20 min sunlight than did the same compounds from either raw or laboratory pasteurized milk. n-Pentanal was relatively higher in exposed-pasteurized milk than in either raw or laboratory pasteurized milk. Differences between concentrations of volatile materials in exposed raw and in laboratory pasteurized milk were slight. Exposing skimmilk to sunlight for 20 min produced more acetaldehyde than similarly treating homogenized-pasteurized milk, indicating that nonfat fractions are precursors of acetaldhyde. Increases in all other carbonyl compounds could be attributed to the fat.
1Contribution 906, Department of Dairy and Poultry Sciences, Kansas Agricultural Experiment Station, Manhattan.