Graduate Thesis Or Dissertation
 

Heat induced compounds in milk

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  • Milk, preheated at 82°C for 30 minutes, was heated to 146°C for four seconds (UHT-treated) and cooled to 5°C in a tubular heat exchanger. Immediately after heat treatment, 20 gallons of heated milk were vacuum distilled at 30°C in a semi-continuous, reduced pressure glass apparatus. Twenty gallons of non-heated milk were distilled in a similar manner to serve as 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 flavor concentrates. A technique for collecting and transferring packed column gas chromatographic fractions to capillary columns for mass spectral analysis was developed. The following compounds were identified in UHT-treated milk (the underlined compounds appeared to result from the heat treatment): the C₃, ₄, ₅, ₇, ₈, ₉, ₁₀, ₁₁, ₁₃ n-methyl ketones, the C₈, ₁₀, ₁₂ delta-lactones, acetaldehyde, hexanal, benzaldehyde, furfural, phenylactaldehyde, vanillin, the C₆, ₈, ₁₀ n-alkanoic acids, ethanol, oct-1-en-3-ol, n-heptanol, 2-butoxyethanol, diacetyl, maltol, acetophenone, ethyl acetate, benzothiazole, toluene, naphthalene, a dichlorobenzene, a trichlorobenzene, methyl iodide, benzonitrile and chloroform. The following compounds were identified in non-heated milk: C₃, ₄, ₅, ₇, ₉ n-methyl ketones, C₁₀, ₁₂ delta-lactones, hexanal, benzaldehyde, C₆, ₈, ₁₀ n-alkanoic acids, ethanol, diacetyl, ethyl acetate, methyl palmitate, diethyl phthalate, a dichlorobenzene, a trichlorobenze and methyl iodide. The concentration of diacetyl in UHT-treated and non-heated milk was determined by a modified gas entrainment, on-column trapping GLC technique. The amount of diacetyl in non-heated milk was 3 ppb while the amount in the UHT-treated was 38 ppb. The diacetyl concentration of UHT-treated milk decreased approximately 40% over 16 days storage at 4°C. The average flavor threshold for diacetyl in milk was found to be 12 ppb. It seems therefore that the UHT-treatment increased the diacetyl concentration from a subthreshold level to above the average flavor threshold. It is suggested that diacetyl contributes to the "rich", "heated" note in the flavor of heated milk.
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