Effect of Various Direct Ultra-High Temperature Heat Treatments on Flavor of Commercially Prepared Milks

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Abstract
Grade A milk (3.25% fat) was direct UHT processed at 132.2, 143.3, and 137.2.degree. C with 2 min preheating and at 137.2.degree. C without preheating for 12 s and aseptically packaged in 246-ml Brik Pak cartons in a commercial plant. Samples from each treatment were stored at 25 and 2.degree. C for 24 wk. Changes in acid degree value praralleled the development of stale flavor most closely with correlation coefficient higher than any of the other tests. Dissolved oxygen might have affected rate of flavor change for UHT milks stored at room temperature. Increases in acidity were slightly greater in UHT samples stored at room temperature than under refrigeration. A combination of acid degree value, dissolved oxygen, and titratable acidity correlated most closely to stale flavor. There was little difference in flavor and concentration of volatile compounds among various heat treatments. Concentrations of acetaldehyde, propanal, n-hexanal, 2-pentanone, 2-hexanone, and 2-heptanone increased more at room temperature than in refrigerated storage; in general, these increases paralleled the stale flavor. Concentrations of the carbonyl compounds measured were far below flavor threshold values when UHT milks were criticized as stale. However, a combination of acetaldehyde, acetone, 2-pentanone, and 2-heptanone were related most closely to stale flavor and could have an effect on the flavor when all were present.