Fermented milk products: effects of lactose hydrolysis and fermentation conditions on the rheological properties

Lactose-free dairy products become increasingly important for lactose-intolerant consumers, but there are only few studies concerning the rheological properties of fermented dairy products from lactose-hydrolysed milk. Hydrolysation was performed with commercial beta-galactosidase either before or during fermentation (co-hydrolysis). In each trial, fermentation of the base milk was carried out simultaneously using the same starter cultures for (a) untreated milk (reference) (b) hydrolysed milk as substrate and (c) by performing lactose hydrolysis and fermentation simultaneously (co-hydrolysis). In total, five thermophilic starter cultures and two products (yoghurt and Greek-style yoghurt) were investigated. Results show that the influence of hydrolysis of lactose on the properties of the fermented dairy products strongly depends on starter culture and substrate. For starters C and D, apparent viscosity (extracted from flow curves at a shear rate of 75 s(-1)) of fermented milks was only marginally affected by lactose hydrolysis, ranging between approx. 0.34-0.31 and 0.37-0.31 Pa.s, respectively. Hydrolysed products from starters A and E exhibited significant lower apparent viscosity (0.16 and 0.24 Pa.s) compared with their respective references (0.29 and 0.35 Pa.s). Fermentation of both substrates (regular yoghurt, Greek-style yoghurt) with starter B resulted in a decrease of yield stress and apparent viscosity because of lactose hydrolysis only for Greek-style yoghurt. Furthermore, a trend towards higher EPS synthesis was found when using hydrolysed milk. The results clearly show that products made from lactose-hydrolysed milk with similar rheological properties as the reference product can be obtained but that there is a lack of information concerning the complex interactions between starter culture and milk substrate.