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The colour of powder and reconstituted milk (Hunter's L-, a- and b-values), fluorescent oxidation products, and two peak areas (corresponding to β-lactoglobulin and α-lactalbumin) from size exclusion chromatography were measured 9 times during accelerated storage at 50 °C and a water activity of 0.31 on 15 medium-heat whole milk powders manufactured at the same production plant. The powder and reconstituted milk gradually turned darker and more yellow with increasing storage time, but the Hunter a-value (redness value) was ambiguous, suggesting that during lactose crystallization, β-carotene migrates with free fat to the surface of the powder particles. Fluorescent oxidation products were formed at a constant rate from carbonyl compounds and protein amino groups. β-Lactoglobulin and α-lactalbumin were subject to progressive denaturation. Using canonical variable analysis, milk powders were best distinguished by their colour and by their content of β-lactoglobulin and α-lactalbumin. Age classification of powders of unknown origin should follow two different strategies: fresher powders (corresponding to accelerated storage times up to 25 days) should be distinguished by the amount of fluorescent oxidation products, the content of α-lactalbumin and powder brightness, whereas for older powders (accelerated storage times from 25 to 43 days), the colour of the reconstituted milk should be measured also. An interaction was found between storage time and powder which indicated a more erratic behaviour of the powders as storage time was prolonged. |
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