-In this paper, the formation of insolubility in milk powder is described, and the factors affecting the reaction are discussed. The zone in which casein proteins are most sensitive to heat is in the moisture range that occurs during spray drying. The rate of reaction has been determined and shows differences of orders of magnitude with the moisture content. The effect of chemical reagents on the extent of reaction gives insights into the chemical bonds that inhibit hydration. Information on the drying of casein products and individual proteins is used to infer that the withdrawal of moisture from casein proteins and the concentration of divalent cations during drying lead to alterations in the conformation of proteins that have an adverse effect on their rehydration during reconstitution. Further work to fully characterise these effects is required.spray drying / powder / casein / insolubility / kineticsRésumé -Insolubilité des poudres laitières -une mini-revue. L'apparition de l'insolubilité des poudres laitières est décrite et les facteurs affectant cette réaction sont discutés dans cet article. La zone dans laquelle les caséines sont les plus sensibles à la chaleur se situe dans la gamme d'humidité intervenant au cours du procédé de séchage par atomisation. La vitesse de réaction a été déterminée et montre des différences d'ordres de grandeur des taux d'humidité. L'effet des réactifs chimiques sur l'étendue de la réaction donne un aperçu des liaisons chimiques qui inhibent l'hydratation. On peut déduire de l'information obtenue sur le séchage des caséines et des protéines individuelles que l'élimination de l'humidité des caséines et la concentration en cations divalents au cours du séchage conduisent à des modifications de la conformation des protéines qui peuvent avoir des effets négatifs sur leur réhydratation au cours de la reconstitution des poudres. Un travail ultérieur est nécessaire pour caractériser complètement ces effets.séchage par atomisation / poudre / caséine / insolubilité / cinétique

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SummaryThe use of X-rays to evaluate food microstructure has developed considerably in recent years. In this work we used X-ray micro tomography (XMT) to visualize the microstructure of loose-packed and compacted samples of spray-dried skim milk powder (SMP) and whole milk powder (WMP) and to quantify the proportion of both interstitial and occluded air voids in each sample. Visual scrutiny of the acquired XMT images showed details of the microstructure of the milk powders such as the spherical morphology of the particles, the size of the particles and internal air voids of various sizes. Within loose-packed powders, the proportion of air voids was higher (13% average) in WMP than in SMP. This was reversed in the compacted powders, in which the proportion of air voids was higher in SMP. The disparity in the proportion of air voids in both loose-packed and compacted samples of SMP and WMP was attributed to the size, shape and surface properties of the particles.

When milk was preheat-treated after either evaporation or homogenization with fat present the degree of heat stability induced in the resulting evaporated milk (EM) was considerably less than with the normal process in which the preheat treatment was carried out first. When the preheat treatment was carried out after both evaporation and homogenization, no heat stability was induced by the preheat treatment. Removal of the whey protein fraction from the milk increased the heat stability of both the EM homogenized before preheat treatment and the EM made by the normal process.

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