Milk pasteurized at 73, 80 and 90°C for 16.2 s and homogenized than exposed to 50-foot-candle intensity of fluorescent light in clear glass bottles was compared for flavor and concentrations of acetaldehyde, propanal, n-pentanal and n-hexanal with similarly treated milk in foil-covered glass bottles. Flavor (hedonic scaling by five judges) was influenced by pasteurization temperatures, storage time and exposure to light. Milk pasteurized at 73°C and held in foil-covered bottles through 10 d at 2°C had the most acceptable flavor. However, when milk was pasteurized at this temperature but exposed to light, it had the least desirable flavor during 10 d. At 14 d, flavor score of the 73°C, unexposed milk declined, and that of the irradiated milk increased so that both were almost identical. At pasteurization temperatures of 80 and 90°C, the adverse effect of irradiation was either reduced or eliminated and the incidence of oxidized flavor lessened. Poorer flavor at these pasteurization temperatures from unexposed milks reflected greater intensities of cooked flavor. Concentrations of acetaldehyde, propanal, n-pentanal and n-hexanal increased much more in the light-treated samples than those kept in the dark. However, high-heat treatment (90°C) lessened those increases in propanal and n-hexanal but enhanced increases in acetaldehyde and n-pentanal.
Six brands of milk sold at Manhattan, Kansas, retail outlets were evaluated for quality on the day the milk was delivered, and again after being held in display cases for a week. Five of the six were in one gallon plastic jugs; the other one was in a 1/2 gallon carton. Only freshly delivered milks were analyzed for chemical composition. Both freshly delivered and stored samples were examined for bacteria, temperature, flavor and some volatile materials. The study continued five weeks, with fresh samples collected weekly. Only 3 (all from one processor) of the 30 samples were below the 3.25% legal fat limit for Kansas (considering a 0.1% allowance for Babcock testing); they were 2.4, 3.1, and 3.15%. Temperatures of milks held in the display cabinets were generally lower than milk as delivered; only 2 of the 30 samples exceeded 4.5 C (40 F). Fourteen of the milks as delivered exceeded 4.5 C (40 F), but only one 8.5 C (47 F) exceeded the 7 C (45 F) legal limit. Most bacterial counts (SPC) of freshly delivered milks were within legal limits, but three of the brands of week-old milks had consistently high counts, and most often were >300,000/ml. Psychrotroph counts of milk from the same 3 brands were consistently >300,000/ml after a week in the display case. Only two SPC and four psychrotroph counts from the other three brands exceeded 20,000/ml after one week in the display case. All milks showed less than one coliform per ml. Flavors tended to deteriorate after one week storage except in two brands that remained good. One of the two was cartoned milk. Gas liquid chromatographic (GLC) analysis of milks showed increases in acetaldehyde, n-pentanal and n-hexanal, which paralleled increases in off-flavors, in milk held in the display cases. There was no apparent relationship between methyl sulfide concentration and tendency of milk to deteriorate in flavor during one week display-storage.