Amaranth proteins foaming properties: Film rheology and foam stability – Part 2

•Treatment at pH 2 significantly improves the foams stability of amaranth proteins.•Interfacial film showed higher viscoelasticity and flexibility at acid pH than at alkaline pH.•The relationship between interfacial and foaming properties of amaranth proteins has been better established at acidic pH than alkaline pH. In this work the influence of pH and ionic strength on the stability of foams prepared with amaranth protein isolate was analyzed. The behaviour observed was related to the physico-chemical and structural changes undergone by amaranth protein as a result of those treatments. The results obtained show that foams prepared at acidic pH were more stable than the corresponding to alkaline pH. At pH 2.0 the foams presented higher times and more volumes of drainage. This behaviour is consistent with the characteristics of the interfacial film, which showed a higher viscoelasticity and a greater flexibility at acidic pH than alkaline pH value, which in turn increased by increasing the concentration of proteins in the foaming solution. It is also important to note that the presence of insoluble protein is not necessarily detrimental to the properties of the foam. Detected changes in the characteristics of the interfacial film as in the foam stability have been attributed to the increased unfolding, greater flexibility and net charge of amaranth proteins at acidic conditions.