ABSTRACT
Sweetened yoghurt or misti dahi is a sweet fermented dairy product popular in eastern part of India. The shelf life of sweetened yoghurt is very less. So, an attempt was made to prepare a shelf-stable yoghurt powder using spray drying. This study aimed at optimizing the spray-drying process conditions in developing a sweetened yoghurt powder (SYP) using response-surface methodology. Process variables were 140-180°C inlet air temperature, 0.3-0.6 L/h feed rate, and 500-1000 kPa atomization pressure; and responses studied were moisture content (% db), bacteria survival ratio (N/N 0), acetaldehyde retention (%), and overall sensory score. Under optimal conditions (inlet air temperature 148°C, feed rate 0.54 L/h, and atomization pressure 898 kPa), the response variables including the moisture content, bacteria survival ratio, acetaldehyde retention, and overall sensory score were predicted and validated as 3.69% (db), 2.37 χ 10-3, 76.39% and 8.0, respectively. Furthermore, the influence of spray-drying conditions on functional and reconstitution properties of SYP was studied. Analysis of experimental data of solubility and dispersibility of SYP revealed negative correlation with inlet air temperature and atomization pressure. Solubility of powder was significantly (p < 0.05) affected by feed rate. Wetting time of powder increased significantly (p < 0.001) with the increase of spray-drying temperature. The bulk, tapped, and particle densities were in the range 160of 344.8-475.7, 551.7-782.5, and 1187.5-1666.7 kg/m3, respectively. A significant quadratic effect (p < 0.001) of inlet air temperature and atomization pressure was observed on bulk density. Particle density increased with the increase in inlet air temperature and atomization pressure (p < 0.001). The results of the present investigation have the credible evidence to support that the processing conditions, namely, inlet air temperature and atomization pressure decreased the water activity and flow property of the SYP, whereas feed rate showed positive effect. Morphology of the optimally developed SYP showed a smooth and uniform texture with round particles. It was concluded that a reasonably good-quality shelf-stable powder can be developed from sweetened yoghurt having adequate bacterial count and acetaldehyde content by spray drying.
