Abstract:
Set yoghurt, one of the most widely consumed cultured dairy products in Sri Lanka is commonly
stabilized with gelatin derived from animal body tissues. There is an increased consumer
preference for plant-based food additives such as starch over gelatin for yoghurt gel stabilization.
The properties of starch which depends on the isolation method affects the quality of the final
product. Therefore, a study was designed to investigate the physicochemical and physical
properties of cow milk set yoghurt stabilized with sweet potato (Ipomoea batatas L.) starch (SPS)
isolated using 3 different methods. Sweet potato tubers were washed with potable water, peeled,
and divided into 3 equal lots. Starch was isolated by blending each lot separately with distilled
water (1:1) (SPS1), 1M NaCl (1:1) followed by centrifugation at 2200 rpm/15 min (SPS2), and
distilled water (1:1) followed by centrifugation at 2200 rpm/15 min (SPS3). Experimental
yoghurts (3% fat, 7% sugar) were prepared using gelatin (Control), SPS1 (T1), SPS2 (T2) and
SPS3 (T3) as the stabilizer at the rate of 0.5% (w/w). Physicochemical and physical properties of
SPS and yoghurt were examined using standard methods. Completely randomized design was
employed with 3 replicates and the data collected were subjected to the analysis of variance
(ANOVA) procedure followed by Tukey’s test for mean separation using SPSS software (ver. 25).
Moisture content of SPS was significantly (p<0.05) affected by the starch isolation method but
remains in the range recommended for commercial starches. Isolated SPS was acidic and SPS2
had significantly higher pH compared to the other two. Water absorption capacity varied from
0.598±0.004 mg/g in SPS2 to 0.780±0.002 mg/g in SPS1 and was significantly different. At 80 °C,
SPS1 showed the highest swelling power while solubility was significantly lower compared to
SPS2. Titratable acidity and pH of yoghurt after gel stabilization indicated an uninterrupted
starter culture activity and did not depend on the starch isolation method. Control yoghurt
stabilized with gelatin had significantly higher pH compared to the yoghurt stabilized with SPS
isolated using different methods whereas the titratable acidity did not show a special trend and
was above the minimum level (0.6% lactic acid) prescribed for yoghurt. Water holding capacity
of the yoghurt stabilized with gelatin was the highest confirming the effectiveness of gelatin in
binding water in the yoghurt matrix while spontaneous whey syneresis was not detected in any
of the treatments. The above results indicate the possibility of utilizing SPS isolated using the
methods specified, to stabilize cow milk set yoghurt without affecting the quality.
