ABSTRACT: Yogurt base was prepared from reconstituted skim milk powder (SMP) with 2.5% protein and fortified with additional 1% protein (wt/wt) from 4 different milk protein sources: SMP, milk protein isolate (MPI), micellar casein (MC), and sodium caseinate (NaCN). Heat-treated yogurt mixes were fermented at 40 °C with a commercial yogurt culture until pH 4.6. During fermentation pH was monitored, and storage modulus (G′) and loss tangent (LT) were measured using dynamic oscillatory rheology. Yield stress (σyield) and permeability of gels were analyzed at pH 4.6. Addition of NaCN significantly reduced buffering capacity of yogurt mix by apparently solubilizing part of the indigenous colloidal calcium phosphate (CCP) in reconstituted SMP. Use of different types of milk protein did not affect pH development except for MC, which had the slowest fermentation due to its very high buffering. NaCN-fortified yogurt had the highest G′ and σyield values at pH 4.6, as well as maximum LT values. Partial removal of CCP by NaCN before fermentation may have increased rearrangements in yogurt gel. Soluble casein molecules in NaCN-fortified milks may have helped to increase G′ and LT values of yogurt gels by increasing the number of cross-links between strands. Use of MC increased the CCP content but resulted in low G′ and σyield at pH 4.6, high LT and high permeability. The G′ value at pH 4.6 of yogurts increased in the order: SMP = MC < MPI < NaCN. Type of milk protein used to standardize the protein content had a significant impact on physical properties of yogurt.
Practical Application: In yogurt processing, it is common to add additional milk solids to improve viscosity and textural attributes. There are many different types of milk protein powders that could potentially be used for fortification purposes. This study suggests that the type of milk protein used for fortification impacts yogurt properties and sodium caseinate gave the best textural results.