Foaming activity of lupin protein isolates in the absence of insoluble protein aggregates

This study investigates the foaming properties of two lupin protein isolates (LPIs) extracted by ultrafiltration (albumin-rich fraction; LPTF) and isoelectric precipitation (globulin-rich fraction; LPTE), respectively. An earlier work on LPIs extracted by similar methods, Alamanou and Doxastakis [1], blamed the insoluble protein aggregates for the worse foamability and foam stability of the LPTE isolate compared to the LPTF isolate. Herein, the foaming properties of LPI are examined after removal of the insoluble fraction in order to appraise solely the effect of dissolved proteins. Foams are produced by whipping 1% w/v of LPIs aqueous solutions at pH 5.5 and 7, alone but also with addition of xanthan gum (0.05% and 0.1% w/v) and NaCl (0.1 M). Foaming ability and stability are assessed globally by volumetric measurements and locally by electrical conductance measurements taken non-intrusively at different heights along the foam. Both LPIs showed satisfactory foaming activity with electrical measurements depicting local drainage features that global volumetric measurements could not capture. It is found that LPTF yields better foamability than LPTE, exactly as it was shown earlier in the presence of aggregates. However, there is a discrepancy with that earlier work regarding foam stability since the two isolates perform comparably in the absence of aggregates. The discrepancy may be explained by considering that the larger size LPTE aggregates could have a stronger destabilizing effect than the smaller size LPTF aggregates. The role of pH, xanthan gum and NaCl in affecting the performance of the two LPIs is also discussed. (C) 2010 Elsevier B.V. All rights reserved.