Rheological properties of lactose-free yoghurt in relation to enzyme concentrations

Lactose-free yoghurt is one of the main sources of useful nutrients for people who have got lactose intolerance.Therefore preparing lactose-free yoghurt is considered as an actual issue in the Republic of Uzbekistan.Many factors can affect the rheological and textural parameters of lactose-free yoghurt, including enzyme concentration, which is used to convert lactose in glucose and galactose.The aim of the study was to evaluate enzyme concentration influence on the rheological properties of lactose-free yoghurt.Yoghurt samples were made from pasteurized milk (fat content 2.5%, Tukuma piens Ltd., Latvia) using commercial frozen yoghurt starter culture FD-DVS YC-X11 (Danisco Ltd., Denmark) containing Lactobacillus delbrueckii ssp.bulgaricus and Streptococcus thermophilus strains, and different concentrations of enzyme NOLA™ Fit 5500 (Chr Hansen JSC., Denmark) -0; 500; 1000; 1500 and 2000 BLU L -1 .Milk was pasteurized (95±1 °C, 5 min), cooled down (40±1 °C), inoculated with starter and enzyme, mixed and fermented till pH 4.8±0.1 then samples were mixed and cooled down till 5±1 °C.Textural properties and viscosity of yoghurt samples were analysed using texture analyser TA.HD Plus and DV-III Ultra Programmable Rheometer.Lactose, galactose, glucose contents were measured by HPLC (SHIMADZU, Prominence, USA).The textural properties (such as firmness, consistency, cohesiveness) and viscosity of analysed yoghurt samples were significantly (p<0.05)different for samples with higher enzyme concentration, which showed the lowest viscosity.Rheological properties denote a weaker network of yoghurt gel in samples with full lactose hydrolysis.Results from chromatography provided no lactose presence in any experimental sample with enzyme, the galactose was dominant monosaccharide in each sample treated with enzyme.