Heat-Induced Volatile Compounds in Milk

Abstract

Milk, preheated at 82C for 30min, was heated to 146C for four seconds and cooled to 5C in a tubular heat exchanger. Immediately after heat treatment, the milk was vacuum distilled at 30C in a semicontinuous, reduced pressure, glass apparatus. Raw milk was distilled in a similar manner for a control. Continuous liquid-liquid ethyl ether extractions were employed to recover the compounds from the aqueous distillates. Gas chromatography, mass spectrometry, infrared spectrophotometry, and odor confirmation were used to characterize the compounds in the ethereal concentrates.The following compounds were found to be heat induced: The C3,4,5,7,8,9,10,11,13n-methyl ketones, the C8,10,12 delta-lactones, benzaldehyde, furfural, phenylacetaldehyde, vanillin, oct-1-en-3-ol, n-heptanol, 2-butoxyethanol, maltol, acetophenone, benzonitrile, benzothiazole, and diacetyl. Other food products could yield these heat-induced compounds provided the basic materials from which they originate were present during the heat treatment. The following novel compounds were found in both heated and raw milk: A dichlorobenzene, a trichlorobenzene, methyl iodide, and diacetyl.Benzonitrile, dichlorobenzene, trichlorobenzene, and methyl iodide were present in the ethereal concentrates in trace amounts.The concentration of diacetyl in heated and raw milk was determined by a modified gas entrainment, on-column trapping GLC technique. The amount of diacetyl in raw milk was 5 ppb, while the amount in the heated milk was 38 ppb, which is above the average flavor threshold for diacetyl in milk. It is suggested that diacetyl contributes to the rich or heated note in the flavor of heated milk.