Graduate Thesis Or Dissertation
 

Identification of some compounds contributing to the stale flavor defect of sterilized concentrated milk

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  • 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.
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