Recent studies have shown that when milk fat is heated, a homologous series containing the n-alkyl members of methyl ketones with odd numbers of carbon in their chains are produced (48; 44; 38; and 3). The same series of compounds also is found in evaporated and dried whole milk and in these products the concentration increases during storage (70 and 46). It is believed by some investigators that the methyl ketones play an important role in flavor deterioration of milk fat and in the aforementioned concentrated products. At the present time, however, there is disagreement on the factors influencing methyl ketone production; some workers relate their formation to autoxidation (25), while there are others who report that heat and water are essential in the reaction (48; 38 and 3). Finally, a recent report indicates that anhydrous milk fat will give rise to methyl ketones when heated in the absence of oxygen (44). The purpose of this investigation was to study the effect of various factors on the qualitative and quantitative composition of methyl ketones in heat treated milk fat. It is anticipated that the resulting information will contribute to a more thorough understanding of the reactions leading to ketone production in the fat; hence, to development of suitable processing measures for prevention of this type of deterioration in dairy products. Milk fat was prepared from raw cream two days after milking. It was washed free from phospholipids, centrifuged at 30,000 x G for 20 minutes and degassed at two to five microns pressure for one hour. The fat was then heat treated in sealed vials at various temperatures and time periods under controlled conditions. The samples were quantitatively analyzed for methyl ketones by direct conversion of the ketones to 2, 4-dinitrophenylhydrazine (DNP) derivatives in the intact fat sample. The derivatives were isolated from the fat, separated and identified by a combination of column and paper chromatographic methods and by their absorption spectra. Methyl ketone formation in heated milk fat was shown to be non-oxidative. A plateau in ketone production was approached in the 120°C to 140°C range when the time of heat treatment was 30 minutes. Added water enhanced total methyl ketone production at 140°C but not at 200°C. Air did not hinder ketone production. Maximum ketone production (1.733 mM/kg fat) was noted after three hours of heat treatment at two to five microns pressure, and 140°C. Milk fat centrifuged at 30,000 x G for 20 minutes and degassed at two to five microns pressure for one hour was found to contain 0.27% water. This quantity of water is sufficient for hydrolysis of β-ketoesters assuming them as the precursors of the methyl ketones. Conventional methods of preparing "anhydrous" milk fat were not adequate for removal of trace amounts of water. When milk fat was dried over calcium hydride (35) prior to heat treatment, total ketone formation was significantly reduced indicating that water is a limiting factor in ketone formation. A homologus series of n-alkyl methyl ketones (C₃, C₅, C₇, C₉, C₁₁, C₁₃, C₁₅) was isolated from heat treated samples. The ketones produced in large amounts were the C₃, C₇, and C₁₅. When the heat treatment was for three hours or longer the C₄ ketone was detected and composed approximately 11% of the total. The possible origin of the C₄, ketone is discussed. The reaction of intact fat with DNP-hydrazine and the subsequent isolation and identification of methyl ketones were quantitatively evaluated. / Graduation date: 1964

No description available.

Graduation date: 1961

Call number: LD2668 .T4 1956 M43 / Master of Science

Stale flavor development has been recognized as a defect of stored dry milk powders for several years. Recently, stale flavor development has been found to occur upon storage of sterilized concentrated milk, and is recognized as the principal limiting factor to commercial utilization of this process (Seibert, 1963). Some attempts have been made to identify the volatile flavor components of sterilized concentrated milk (Patel et al, 1963; Bingham, 1964). The flavor components responsible for the stale flavor defect as it occurs in sterilized concentrated milk have not been identified, however. The purpose of this work was to identify the compounds responsible for the stale flavor defect of sterilized concentrated milk. It was felt that this information was essential to an understanding of the staling phenomenon, which in turn might eventually lead to correction of the defect. Commercial samples of sterilized concentrated milk were obtained. Stale flavor development was hastened in some of the samples by storing them at 21°C. Subjective flavor panel evaluation of stored and fresh samples revealed significant differences between the two. Gas chromatographic analysis of the volatile head space components by the procedure described by Morgan and Day (1965) revealed only minor differences between the fresh and stale samples. It was reasoned, therefore, that the compounds responsible for the stale flavor defect were primarily of a less volatile nature, A technique for isolating the higher-boiling flavor components was developed. This technique consisted of lyophilization of the sterile concentrated milk, uniform wetting of the lyophilized milk with water to 10% moisture, solvent extraction of the fat and flavor components from the moistened milk powder, and reducedtemperature, reduced-pressure steam distillation of the flavor components from the extracted fat. The resulting flavor extract was studied by gas chromatography in conjunction with mass spectrometry A base-treated pre-column was used in front of the regular gas chromatography column to remove fatty acid peaks from the chromatograms. A technique, which consisted of repeatedly trapping (from several successive chromatograms) particular regions of the effluent from a non-polar column onto a short section of packed column and re-chromatographing the trapped components on a polar column, was developed to build up the concentration of flavor components and to improve the separation of components for mass spectral analysis. The following compounds were positively identified in the flavor extract from stale sterile concentrated milk: 2-heptanone, 2-nonan.one, 2-undecanone, 2-tridecanone, benzaldehyde, napthalene, a dichlorobenzene, L-decalactone, benzothiazole, and o-aminoacetophenone. Acetophenone was tentatively identified. Of these compounds, 2- heptanone and the dichlorobenzene were positively identified in the extract from fresh sterile concentrated milk, and L-decalactone was thought to be present. The ketones and L-decalactone undoubtedly make some contribution to the stale flavor defect (USDA, 1964). The identification of o-aminoacetophenone in stale sterilized concentrated milk supplements its identification in stale nonfat dry milk powder (Parks, Schwartz and Keeney, 1965), and further implicates it as an important compound in the stale flavor defect. This compound possesses a characteristic "grape-like" odor. Benzothiazole has not previously been identified in milk products. It possesses a characteristic "rubber-like" odor. Its possible significance in the stale flavor defect will require further study. / Graduation date: 1966

Digitized by Kansas Correctional Industries

No description available.

No description available.

No description available.

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. / Graduation date: 1968