Post-processing of fermented milk to stirred products: Reviewing the effects on gel structure

The structure of stirred fermented milk products such as yoghurt, Greek-style yoghurt and fresh cheese is an essential criterion for consumer acceptance. It is determined by various factors ranging from the milk base composition to starter culture to mechanical forces occurring during filling. The effect of upstream processing parameters and fermentation conditions on the structure of stirred fermented milk gels have been extensively studied and reviewed by different authors. This review examines the effect of mechanical stresses in different unit operations downstream of fermentation, or post-processing, on the milk coagulum, often referred to as a microgel suspension. The process steps can be indispensable, e.g. pumping, cooling and filling, and concentration for concentrated milk products, or optional, e.g. mechanical or thermal treatment. They are evaluated with regards to their impact on the structure of microgel suspensions. In-situ laboratory scale experiments in rheometers provide an insight into structural changes of milk gels due to shearing. The structural properties, including rheological parameters, of the microgel suspension depend on the cumulative effect of the type, magnitude and duration of the mechanical stresses occurring in the different post-processing unit operations. The latter can be optimised to achieve the desired gel properties and save raw material, mainly protein and stabilisers. It can also be adjusted for low viscosity at high protein contents. Thus, the efficiency of the process can be improved by considering the design of the process downstream of fermentation.