Dynamics of Colourimetric Parameters of Dry Skim Milk During Storage

Introduction. During storage, skim milk powder undergoes complex physicochemical and structural changes, leading to a deterioration in its technological stability. Such changes are not always detectable using standardised product quality assessment methods. Therefore, the search for integrated indicators that allow for a rapid assessment of the depth of degradation processes and justification for component processing is relevant.

Purpose. To study the dynamics of the colorimetric characteristics of skim milk powder and reconstituted skim milk during storage at different temperatures.

Materials and Methods. The study involved skim milk powder samples stored at temperatures of 4 ± 2, 25 ± 2, and 45 ± 2°C. Colour characteristics were assessed using the CIELab colour space parameters, the calculated whiteness index, and colour difference using digital image processing. Results: It was found that for dry samples, an increase in storage temperature was accompanied by an increase in the yellowness index (from 6.36 ± 0.01 to 9.80 ± 0.75), a decrease in lightness (from 89.06 ± 0.41 to 83.41 ± 0.93) and a decrease in the whiteness index (from 87.33 ± 0.36 to 80.62 ± 1.21). The maximum value of colour difference corresponded to 6.88 ± 0.80 and was recorded for samples stored at 45 ± 2°C. Dry milk stored at 25 ± 2°C was characterised by less pronounced transformations: the colour difference changed in the range of 1.10 ± 0.32–1.79 ± 1.05. The dynamics of changes in the colour space parameters was nonlinear. A different mechanism for the formation of colour characteristics was revealed for the reconstituted samples, implying a decrease in yellowness (from 5.02 ± 1.11 to 2.32 ± 0.32), lightness (from 85.41 ± 0.78 to 76.69 ± 1.47), and whiteness (from 84.47 ± 0.64 to 76.52 ± 1.71), while the colour difference reached a value of 9.14 ± 2.29.

Conclusions. It has been shown that colorimetric parameters can be considered as integral indicators of the degradation of skim milk powder quality. Product colour characteristics reflect a complex of physicochemical and structural transformations during storage: the formation of Maillard reaction products, lactose crystallisation, moisture redistribution, and the degree of product hydration during reconstitution. It has been revealed that the contribution of colour space coordinates to the formation of colourimetric parameters depends on the aggregate state of the sample being studied.

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