Lait 66 (1986) 79-98
DOI: 10.1051/lait:198615

Etude de la thermostabilité des laits concentrés reconstitués

A. LAGAUDE^a and B. TARODO DE LA FUENTE<sup>b

a</sup>  Ecole Nationale Supérieure Universitaire de Technologie BP 5085, Dakar Fann, Sénégal
^b  Laboratoire de Technologie alimentaire, Centre de Génie et Technologie alimentaires, Université des Sciences et Techniques du Languedoc, place E.-Bataillon - 34060 Montpellier, France

Abstract - Studies on the heat stability of reconstituted concentrated milks
The heat stability study of reconstituted concentrated milks was performed on skim milk powders of " High Heat " (H.H.) and " Medium Heat " (M.H.) type. The heat stability was estimated by coagulation time measurement at the laboratory and by a visual inspection alter industrial sterilization. The coagulation time measurements of reconstituted milks were in accordance with results observed in industrial sterilization.
The comparison of nitrogenous and inorganic compound contents of four samples, together with the Whey Protein Nitrogen Index determination (W.P.N.I.) did not allow to determine the milk ability to undergo a sterilization. Only the natural concentration of urea was correlated to coagulation time of milk.
Each sample studied was characterized by its coagulation time vs. pH curve, when natural pH and maximum stability pH of different milks were virtually similar. At pH lower than natural, the coagulation time vs. pH curves had a different shape for each milk, whereas at higher pH, the shape was the same for the four samples. This phenomenon could indicate a different destabilization process on both sides of the natural pH.
The addition of CaCl₂ during reconstitution only induced a moderate destabilization at pH below 6.58 and did not bring about noticeable changes of the coagulation time at pH above 6.58. On the other hand, it seems that the total calcium, soluble calcium and ionic calcium contents of reconstituted condensed milks were not related to coagulation time.
The addition of increasing quantifies of Na₂HPO₄ to the H.H. samples modified slightly maximal coagulation times whereas M.H. milks had their maximal coagulation time considerably increased; this leads to think that preheating and phosphate addition bring into the same phenomenon.
If the increase of the ionic strength was obtained by addition of KCl and not by addition of CaCl₂ or Na₂HPO₄, the maximum stability occured at higher pH and coagulation time observed at acid pH decreased when ionic strengh increased.
Urea addition improved heat stability until an optimal content, depending on the samples, was reached beyond which a considerable loss of stability appeared. Urea addition did not induce any pH modification, change of the buffer capacity, change of soluble phosphate, soluble calcium or ionic calcium concentrations. On the other hand, the effect of urea addition was similar for all samples although their W.P.N.I. were very different. It results from these considerations that the urea could act on the protein colloïdal phase rather than on the soluble phase.

Résumé - L'étude de la stabilité thermique des laits concentrés reconstitués a été effectuée à partir de poudres du type " High Heat " (H.H.) et " Medium Heat " (M.H.) à 0 % de matières grasses. La thermostabilité a été appréciée par la mesure du temps de coagulation (T.C.) au laboratoire et par examen visuel après stérilisation industrielle. Les modifications dues aux variations de pH, de la force ionique, ainsi qu'à l'addition de calcium, de phosphate ou d'urée ont été notées. L'évolution du calcium ionique, du calcium soluble et du phosphore soluble a été suivie au cours des différents essais.

Key words: Concentrated milk / Reconstituted milk / Heat stability

Mots clés : Lait concentré / Lait reconstitué / Thermostabilité