The effect of concentrated buttermilk on cheese milk rennet-induced coagulation and rheological properties at various buttermilk to skim milk ratios

Buttermilk (BM) is still undervalued since its incorporation in dairy products negatively impairs physicochemical properties. Concentrating BM could mitigate these adverse outcomes. This study aimed to characterize the impact of 2-fold BM concentration by reverse osmosis (RO) and ultrafiltration (UF) on the rennet-induced gelling characteristics of milk containing varying BM to skim milk (SM) ratios. Oscillatory rheology was used to determine the gel coagulation properties (rennet coagulation time [RCT], storage modulus [G’] at 2 × RCT, maximal firming rate [MFR]), and the gelation regime by determining the fractal dimension (df). Increasing BM protein concentration enhanced rennet gel G' and MFR. Increasing the ratio of BM to SM significantly decreased the G' and MFR values. The df values increased from 2.69 for non-concentrated BM, to 2.76 for RO, and to 2.88 for UF, indicating that the gel density increased, whereas the casein (CN) aggregates decreased in size. CLSM and SEM observations also showed a higher CN particle's packing density in UF gels compared to non-concentrated and RO gels. Application of the gel regime model revealed that all gels were in the transition regime. This result contrasted with the microscopy and df values. We conclude that while the df values are a good representation of the gel structure, the gel regime characterization is not, and the model cannot be applied to complex milk rennet gels. This study provides a new rheological characterization of BM rennet gels. UF of BM enhanced rennet gel formation and might facilitate the use of BM in cheese making.