Elucidation of factors responsible for formation of white flecks in reconstituted fat filled milk powders

Fat filled milk powders (FFMP) are formulated by blending skim milk and vegetable oil, to which lactose, permeate, sugar, maltodextrin, vitamins and minerals may also be added. The liquid mix is usually homogenised and spray dried, with agglomeration and lecithination, to produce powders for a range of end-user applications including drinking milk, yoghurt base and coffee whitening. Thus, it is important to avoid common powder defects such as inconsistent whitening, feathering or white flecking on reconstitution. The phenomenon of white flecking, and in particular, the underlying causes of fleck formation, are poorly understood. To better understand white fleck formation, six size fraction samples from two different FFMPs, a good (low level flecking) and a poor powder (high level flecking), were profiled. Most extensive flecking was observed in the coarse fraction of the poor powder, which also had an 8-fold higher free fat content than all other fractions and displayed poor emulsion stability (D_3,2 = 15 ± 3.9 μm compared to 0.45-0.75 μm for all other fractions). Treatment of the reconstituted emulsions with an anionic surfactant or a reducing agent suggested that integrity of white flecks was based mainly on electrostatic interactions between proteins, with little contribution from covalent bonds. Although the extent of whey protein denaturation ranged between 22.6–47.5%, whey protein insolubility appeared to play a sub-ordinate role in white fleck formation. These results suggest that presence of flecks in FFMP was mainly associated with poor thermal stability of the emulsions.