Chemistry of the flavor deterioration of sterilized concentrated milk Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0p096979p

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  • Deterioration of the flavor of sterilized concentrated milk (SCM) is recognized as the principal limiting factor to commercial acceptance of this product. Although a number of volatile compounds have been identified in SCM, quantitative information on these compounds is lacking. It is therefore difficult to ascertain the significance of these compounds. The purpose of this investigation was to determine the identity of additional flavor compounds of stored SCM and to determine the concentrations of the major flavor compounds. Vacuum steam distillation was utilized to recover volatile flavor compounds from samples of SCM. The distillates were extracted with ethyl ether, and components of the ethereal flavor concentrates were separated by gas-liquid chromatography (GLC). Major components whose identity was unknown were collected from the GLC effluent, and were analyzed by capillary column GLC and mass spectrometry. A system for transferring trapped components directly onto a capillary GLC column was developed. 2-Furfural, which had not previously been identified in SCM, and 2-furfurol, which had not been identified as a component of any stored milk product, were identified in stored SCM. Commercial samples of SCM were placed in controlled storage, and subjected to flavor panel evaluation and a number of quantitative determinations at selected intervals of storage. The concentration of the odd-numbered n-methyl ketones, C₃-C₁₁, and of o-aminoacetophenone were determined by measuring the absorbance of their respective 2, 4-dinitrophenylhydrazone derivatives. The concentration of 2-furfurol was determined by a gas entrainment, on-column trapping GLC technique. Acid degree values were obtained by titration of SCM milk fat. Hydroxymethylfurfural (HMF) values were determined by measuring the absorbance of the HMF thiobarbituric acid reaction product. The methyl ketones and o-aminoacetophenone were found to exceed their flavor threshold concentrations after 13 weeks storage of SCM at 27°C. The concentration of 2-furfurol exceeded its threshold concentration after 26 weeks at 27°C, but not after 13 weeks. Acid degree values increased slowly, but did not reach significant levels through 26 weeks at 27°C. Hydroxymethylfurfural values increased slowly through the first 13 weeks at 27°C storage, followed by a marked increase during the second 13 weeks. Heat degradation of thiamine was studied as a possible source of volatile flavor compounds. Heating of thiamine solutions in phosphate buffer at pH 6.7 resulted in the production of volatile components of potential flavor significance. The identity of these components was studied by gas entrainment, on-column trapping GLC, collection of components, and capillary column GLC in conjunction with mass spectrometry. The system developed for the transfer of trapped components directly onto the capillary GLC column was utilized. Hydrogen sulfide, 2-methyl furan, 2-methyl thiophene and a compound which appeared to be a dihydro-2-methyl thiophene were identified as volatile heat degradation products of thiamine.
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