Skimmed milk powder is a complex mixture of compounds, but the main component is casein, a protein. Proteins are made up of amino acids, which are linked together by covalent bonds.

Here's why:

* Covalent bonds involve the sharing of electrons between atoms. This is the type of bond that holds together the atoms within a molecule.

* Ionic bonds involve the transfer of electrons between atoms, resulting in the formation of positively and negatively charged ions that attract each other.

Since the building blocks of proteins are held together by shared electrons, the bonds within a casein molecule are covalent.

While skimmed milk powder also contains minerals like calcium and phosphorus, these are present in the form of ionic compounds. These compounds are formed by ionic bonds, but they are a smaller component of the overall powder compared to the protein content.

Therefore, while skimmed milk powder contains both covalent and ionic bonds, the predominant type of bond is covalent due to the presence of protein.