Characterization of the white dots defect (“PIPS”) in “Doce de leite”

Resumo:

“Doce de leite” is a typical concentrated dairy product, widely consumed in Brazil and Argentina. It is common to fnd white dots ("PIPS"-coined from dots on dominoes tiles) in the mass of commercial samples, considered as a defect. The present work aimed to characterize PIPS in samples of pasty dulce de leche using diferent analytical techniques. Through microscopy techniques, it was possible to observe spherical aggregates whereby the surface of the PIPS is composed of a material needle-shaped. Using the energy dispersive X-ray spectroscopy (EDS) technique, calcium was found to be its major constituent. Applying infrared and Raman spectroscopy, it was possible to identify calcium associated with citrate. Moreover, with the aid of powder X-ray difraction (PXRD) the structure of calcium citrate hexahydrate (CCH) was identifed as the main component of PIPS. High performance liquid chromatography (HPLC) of PIPS hydrolysates used for aminoacids profling (aminogram) revealed that about 4% of PIPS was made by aminoacids, and highlighted higher concentrations for Glu, Pro, Leu and Asp. Solid state nuclear magnetic resonance (NMR) spectrum of 13C showed carbon chain signals of Glu, Asp and Leu. The 31P spectrum showed characteristic signals of inorganic orthophosphates. Thus, it was possible to conclude that PIPS have a crystalline structure composed mostly of CCH, but also by aminoacids and a small amount of phosphates that could be related to the presence of αs1-casein, suggesting the latter as the site surface for heterogeneous nucleation and crystal growth.

Conclusão:

The characterization of the white dots (PIPS) defect in “doce de leite” applying several analytical techniques on PIPS extracted from diferent commercial samples, led us to  conclude that they are formed by calcium citrate hexahydrate crystals resembling needles, presumably associated to the surface of a dense αs1-casein core. In “doce de leite” calcium citrate hexahydrate could precipitate also as the tetrahydrate moiety, in agreement with Vavrusova and Skibsted [25]. The severe heat treatment applied and water evaporation during the production (concentration) process seems to set the basis from where the defect could originate. The presence of low concentrations of inorganic phosphates identifed via NMR in solid state could be related with casein fractions, especially αs1-casein. In this regard, it is proposed to further investigate the possible role for the lactose and sucrose and of the αs1-casein in the PIPS formation using model systems.

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[147] Campos, N, S.; De Paula, I. L.; Perrone, I. T.; De Oliveira, L. F. C.; Doriguetto, A. C.; Wollfshoon-Pombo, A. F.; Stephani, R. Characterization of the white dots defect (“PIPS”) in “Doce de leite". European Food Research and Technology, 249:511–521, 2023.