Analysis milk, heat treatment

Sberveglieri, G., Comini, E., Faglia, G., Niederjaufner, G., Benussi, G.R,Contarini, G., Povolo, M. (1998) A novel electronic nose based on semiconductor thin films gas sensor to distinguish different heat treatments of milk. In Hurst, W.J. (ed) Seminars in Food Analysis. Chapman Hall, New York, pp 3 67-76. 

Downey et al. (14) used a statistical approach to classify commercial skim milk powders according to heat treatment. They used 66 samples of commercially produced skim milk powder including high-heat, medium-heat, and low-heat powders. Principal component analysis (PCA) was applied to the normalized spectral data, with the use of wavelengths as principal variables and class values as supplementary variables. Factorial discriminant analysis (FDA) was performed on the PC scores. Ten components were needed to correctly classify all samples in the calibration development set 91% of those in the evaluation set were correctly identified. Three samples of the medium-heat class were incorrectly classified, but the authors pointed out difficulties in the exact definition of the heat treatment classes, particularly the medium-heat class. 

The side chains of proteins can undergo post-translational modification in the course of thermal processes. The reaction can also result in the formation of protein cross-links. A known reaction which mainly proceeds in the absence of carbohydrates, for example, is the formation of dehydroalanine from serine, cysteine or serine phosphate by the elimination of H2O, H2S or phosphate. The dehydroalanine can then lead to protein cross-links with the nucleophilic side chains of lysine or cysteine (cf. 1.4.4.11). In the presence of carbohydrates or their degradation products, especially the side chains of lysine and arginine are subject to modification, which is accompanied by a reduction in the nutritional value of the proteins. The structures of important lysine modifications are summarized in Formula 4.95. The best known compounds are the Amadori product -fructoselysine and furosine, which can be formed from the former compound via the intermediate 4-deoxyosone (Formula 4.96). To detect of the extent of heat treatment, e. g., in the case of heat treated milk products, furosine is released by acid hydrolysis of the proteins and quantitatively determined by amino acid analysis. In this process, all the intermediates which lead to furosine are degraded and an unknown portion of already existing furosine is destroyed. Therefore, the hydrolysis must occur under standardized conditions or preferably by using enzymes. Examples showing the concentrations of furosine in food are presented in Table 4.13. 

Several investigations on the stability of chloramphenicol during thermal treatment have been also carried out. Chloramphenicol has been found quite stable to heating when added to water or milk. Even after 2 hours of boiling, no more than 8% loss of its amount could be observed by physicochemical methods of analysis (16). Application of microbiological methods has shown, however, a 33.3-35% reduction in the activity of chloramphenicol after heating the milk or water samples at 100 C for 30 min (3). 

These studies demonstrate that treatments that have been reported to modify the heat stability of milk also may change the distribution of tracer milk proteins between the physical phases of milk. Analysis of these interactions may provide useful information about the mechanisms of these effects. In unpublished experiments, we have extended this approach for studying homogenized milk-based systems, also containing a lipid phase, to investigate lipid-protein interactions. Dual-label experiments, for example in milk containing M-/3-L and 3H-methyl-/c-casein, could be applied to the isolation and characterization of protein complexes in milk. Labeled caseins could prove valuable as probes for elucidating micelle structure. 

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