The experiment was conducted in the Laboratory of Food Technology, Division of Dairy Technology, National Dairy Research Institute (NDRI), Karnal, Haryana, India. Chemicals Cow milk was collected from NDRI, Karnal, Haryana, India; analytical grade sulfuric acid and amyl alcohol were purchased from Merck Specialties Pvt. Ltd., Mumbai, India. Potato and corn starch were procured from Aryan International, Jonapur, New Delhi, India. The DELVO DSL Direct set TM Lyophilised starter cultures were procured from DSM Food Specialities, P.O. Box 1, 2600 MA Delft, The Netherlands. Other solvents and reagents used were of analytical grade and from laboratories stock. Determination of fat in milk (Gerber method) Mixture of 90 ml of sulphuric acid with 10 ml of distilled water is called Gerber acid. Gerber acid ten milliliters was transferred into milk butyrometer (range 0 to 10%) using an automatic measure. The milk sample (10.75 ml) was slowly transferred from the side of the butyrometer closed with a lock stopper and shaken well. The mixer contents were centrifuged for 5 min at 1100 to 1200 rpm and the fat that appeared as a colourless column was read directly on the butyrometer stem. Estimation of Solids-Not-Fat (SNF) The SNF is the collective term given to the various components of milk other than fat. The SNF content of milk was calculated by determining the specific gravity of milk. The sample was mixed well avoiding incorporation of air or foam formation. The temperature of milk sample was adjusted to measuring temperature prescribed for the BIS lactometer (27ºC). Sufficient milk was poured into the glass or steel cylinder to allow free floating of lactometer. It was then placed in the milk and allowed to float till it stopped and assumed a constant level. The lactometer reading and temperature of milk was recorded at the same time. This was the lactometer reading (LR). The corrected lactometer reading (CLR) was obtained from the standard table for corresponding temperature. The SNF and/or TS content were calculated using the following formulas:
Percent SNF = CLT/4 + (0.25*fat%) + 0.44 Percent Total Solids = CLT/4 + (0.25*fat%) + 0.44. Preparation of yoghurt Fresh raw cow whole milk was taken and standardized to 3.0% fat and 8.5 SNF. It was heated to 40ºC and added with starch at different levels (0.5, 1.0, 1.5, 2.0 and 2.5%) and power sugar (13%). It was blended and kept for 30 min. After that, it was heated up to 65ºC and homogenized (2000 psi). Another similar batch was prepared without homogenization. Again, the sample was heated up to 95ºC, cooled down to 40ºC and starter culture was added. The sample was kept into incubator at 40ºC for 7 hrs. After that, it was taken out and kept in refrigerator for further analysis. Determination of pH of different starch-based cow milk yoghurt The pH of the sample was measured by using EUTECH Instruments, pH700, pH/mV/ºC/ºF meter at ambient temperature. Colour values measurement The colour of different starch-based sweetened yoghurt was measured using colourflex instrument (Hunterlab, Hunter Associates Laboratory, Teston, Virginia, USA) provided with the universal software (Ver.$ 1.72). Before the test, the instrument was calibrated with standard black glass and white tile as specified by the manufacturer. The light source was dual beam xenon flash lamp. Colour values were expressed in terms of CIELAB software and expressed in terms L*, a*, b* uniform colour specs, where L* indicates lightness, ranges zero (black) to 100 (white), a* indicates redness (0 to +60) and greenness (0 to -60) and b* is the amount of yellowness (0 to +60) and blueness (0 to -60). Hue angle and chroma were determined. The different starch-based sweetened yoghurt samples were put densely up to 1 cm height in glass sample containers and placed on sample port facing upward. Command was given to the instrument to analyze the colour intensity of the sample by taking flash photograph of the product and results were displaced. Measurements were done in triplicates. Texture profile analysis Texture profile analysis plays an important role in understanding various textural attributes. The texture of yoghurt closely related to body and appearance and, hence affects the consumer acceptability. The texture profiles of different starch-based sweetened yoghurt were determined by using Texture Analyzer, TA-XT2i (M/s Stable Micro Systems, UK) fitted with 25 kg load cell. TA-XT2i was calibrated with 5 kg standard dead weight prior to use. The probe P-25 was used for texture profile analysis of different starch-based sweetened yoghurts. Viscosity analysis using modular compact rheometer The viscosity of the different starch-based sweetened yoghurt was measured by using modular compact rheometer instrument (Anton Paar, MCR52, Ostfildern, Germany) controlled by the computer aided software (Rheoplus/32, service ver.3.61). Cone and plate geometry (CP75-1, 1.002º inclination) attachment was used with a gap of 0.149 mm. For steady flow measurements, the rheometer was programmed for the set temperature (20ºC) and equilibrated for 2 min following a programmed shear rate changing from 0.1 to 100 s-1 in 5 min. The temperature controlled by Peltier control system. For each test, the volume of sample placed on the plate was covered completely and excess sample was trimmed off. Each time new sample was used and all rheological measurements were carried out in triplicates. Rheological parameters: constant shear rate (γ = 100) and viscosity (µ) were recorded in software. Sensory analysis Sensory evaluation was done by a panel of seven trained judges to evaluate the samples for various sensory attributes. The 9-point hedonic scale was used to express the liking or disliking of products, and the panel members requested to judge the sensory quality based on colour and appearance, flavour, acidity, body and texture, and overall acceptability. Statistical analysis The data obtained during this study were analyzed using statistical package for social sciences (SPSS version 20).