Yogurt is a coagulated milk product obtained by lactic acid fermentation through the action of Streptococcus thermophilus and Lactobacillus delbrueckii spp. bulgaricus1 from milk and milk products (pasteurized or concentrated milk) with or without optional addition2. Lactic acid bacteria have a number of well-established and potential benefits. These bacteria can improve lactose digestion, play a role in preventing and treating diarrhea and act on the immune system, helping the body to resist and fight with infection. More research must be conducted to authenticate the role that lactic acid bacteria might play in anti-tumor effects, hyper-cholesterol effects and preventing urogenital infections, alleviating constipation and treating food allergy3.
Yogurt may have two primary defects: Variation in viscosity and/or expulsion of serum (syneresis). Processing, incubation and storage conditions have an effect on these changes. Syneresis can be reduced by adding stabilizers that interact with the casein network. Amatayakul et al.4 stated that stabilizers are commonly used in cultured products to control the texture and reduce whey separation, since they impart good resistance to syneresis and provide a smooth sensation in the mouth by binding water to reduce the water flow in the food matrix space. Stabilizers enhance the viscosity and influence of the texture, creaminess and mouth feel, stabilizers also prevent separation of whey from yogurt5.
Sulistyowati et al.6 reported taro starch (Colocasia esculenta) as a local product with the potential to be developed as an alternative source for the stabilizer industry in Indonesia. When starch is used as a stabilizer, it is employed for its protective colloid properties to modify the physical and sensory properties of foods. Thus, this research was aimed at the development and optimization of yogurt and to evaluate the effect of taro starch (Colocasia esculenta) addition as a stabilizer on the syneresis, pH, total acidity, viscosity and sensory evaluation of yogurt.
MATERIALS AND METHODS
Taro starch: Taro of good quality was obtained. Taro was peeled, washed, cut and soaked in saline solution in a ratio of 4:1 (saline:taro) for 1 h to eliminate the oxalate compound. Taro chunks were crushed and extracted in a ratio of 4:1 (water:taro) and the material was squeezed using a filter cloth. The dregs of taro were added to water at a ratio of 4:1 (water:pulp taro) and re-extracted. Milk starch was deposited for 6-8 h. Starch that formed was dried at a temperature of ±60°C for ±6 h, ground and sifted with a 100 mesh sieve7.
Preparation of yogurt: The process of making yogurt includes heating of pasteurized cow's milk to 90°C for 10 min with the addition of starch taro. The temperature is quickly brought to 43°C by immersing the glass beaker containing milk in cold water. The next stage is addition of inoculation of bacteria starter (3%). Inoculation is followed by yogurt fermentation and anaerobic incubation for 24 h at room temperature after completion of the incubation period followed by analysis the quality of yogurt.
Quality of yogurt: Yogurt was analyzed for its physico-chemical properties. The susceptibility of yogurt to syneresis was determined by centrifuging 20 g of sample at 500 rpm for 5 min and weighing the supernatant7. The amount of supernatant recovered (%, v/w) was measured. Analysis of pH was achieved using a pH meter/ion 510 (Eutech) calibrated pH meter by inserting electrodes into the buffer solution at pH 7 and 4. The total acidity was measured using a 0.1% NaOH solution until the color changed to pink. The viscosity was measured using a Brooke Field viscometer (Model RVA 3D, Scientific Australia)7. The sensory evaluation used hedonic scale scoring.
Methods: A completely randomized design was applied and each treatment had 4 replications, (P0 = Without the addition of taro starch, P1 = Addition of taro starch 1%, P2 = Addition of taro starch 2% and P3 = Addition of taro starch 3%). The variables measured and analyzed were syneresis, pH value, total acidity, viscosity and sensory evaluation.
Statistical analysis: Data were analyzed using a one-way analysis of variance (ANOVA) and means were compared using Duncans multiple range test, significance was accepted at p<0.05.
RESULTS AND DISCUSSION
Laboratory test results related to the nutritional content of the taro starch are presented in Table 1.
Effect of taro starch addition on syneresis of yogurt: Based on the analysis, the addition of starch taro stabilizer provides a highly significant difference (p<0.01) in the syneresis of yogurt as shown in Table 2. The treatment without the addition of stabilizer starch taro (0%) produced a higher syneresis in yogurt than treatment with taro starch stabilizers (1-3%). The syneresis of yogurt in this study ranged from 0.98-11.42%. The higher concentration of stabilizer taro starch produces lower syneresis of yogurt. Hematyar et al.8 used xanthan and carrageenan gums stabilizer to decrease the syneresis of samples. This method can cause anionic hydrocolloids to interact with positive charges on the surface of casein micelles to strengthen the casein network and to reduce syneresis and are classified as adsorbing.
Effect of taro starch addition on pH of yogurt: Based on the analysis of variance, taro starch had a significant influence on the pH of yogurt (p<0.01), which ranged from 3.74-3.98. The pH of the lowest-level yogurt without stabilizer (0%) was 3.74. The use of taro starch stabilizer gave an ideal pH near the pH of yogurt after fermentation ready for consumption, namely, pH 4 as investigated by Temesgen and Yetneberk9. A decrease in pH is affected by the activities and growth of lactic acid bacteria (LAB), which can hydrolyze starch and convert lactose to lactic acid10. Lactic acid production lowers pH.
Effect of taro starch addition on total acidity yogurt: The use of taro starch as stabilizer had a significant influence on the total acidity of yogurt (p<0.01), which ranged from 1.35-1.73% as shown in Table 2.
|Table 1:||Nutritional value of taro starch
|*Based on a 100% dry matter|
The highest value of total acidity in the treatment without taro starch stabilizer as 1.73%. Treatment with taro starch 2 and 3% did not alter total acidity, while stabilizer at 1% increased the total acidity, which was in agreement with the results of Alakali et al.5, using carboxyl methyl cellulose (CMC) 0.50-1%. This acidity of yogurt produces a value of 0.90-0.91%, lower than without stabilizer, exhibiting a total acidity of 0.92%. Therefore, low acid production could be attributed to the formation of highly viscous systems that caused diffusion resistance reduced mobility of reactants. As a result, the fermentation rate of the reacting species (yogurt culture organisms and lactose) decreased.
Effect of taro starch addition on viscosity of yogurt: Table 3 showed that the viscosity of yogurt samples with taro starch addition is higher than the control. Addition of taro starch can increase the viscosity of yogurt. The highest values for consistency were obtained from 3% addition taro starch. The higher concentration of taro starch addition produced the thickest consistency because it had the highest viscosity. Sahan et al.11 reported that viscosity can increase due to the rearrangement of protein and protein contacts. The viscosity is affected by the number of bands between casein micelles in the yogurt and their structure12.
Effect of taro starch addition on sensory evaluation of yogurt: Panelists assessment results for color, taste, aroma and texture of yogurt had high scores, indicating good acceptance. For sensory scores for color, taste and aroma, there were no significant differences (p>0.05) but significant differences (p<0.05) in texture were observed. Scores for color ranged between 4.00 and 4.40 for a maximum of 5 points. The color of yogurt with 3% addition of taro starch was lighter and led to lower panelists scores.
|Table 2:||Average of syneresis, pH, total acidity and viscosity of yogurt
a-dSuperscripts in the same columns indicated highly significant difference (p<0.01), P0: Concentration of taro starch 0%, P1: Concentration of taro starch 1%, P2: Concentration of taro starch 2%, P3: Concentration of taro starch 3%
|Table 3:||Score average sensory evaluation of yogurt
a-bSuperscripts in the same columns indicated significantly different effect (p<0.05), P0: Concentration of taro starch 0%, P1: Concentration of taro starch 1%, P2: Concentration of taro starch 2%, P3: Concentration of taro starch 3%
The taste scores for the yogurt were 3.07-4.53. A strong aroma and sour taste were detected and accepted by the panelists. The best yogurt taste was for the sample with 1% addition of taro starch, while the lowest score was for yogurt without taro starch addition. The addition of taro starch did not influence the aroma of yogurt but did influence the texture due to viscosity. According to the sensory results for texture, the addition of 1% taro starch led to higher scores and was accepted by the panel.
It can be concluded that the addition of 1% taro starch provided the best physico-chemical and sensory evaluation of yogurt. This study showed that yogurt of acceptable quality and sensory evaluation can be produced using locally available taro starch. The incorporation of taro starch can improve the viscosity of yogurt.
The authors gratefully acknowledge the Directorate General of Higher Education, Ministry of Research, Technology and Higher Education, Republic of Indonesia for financially supporting the present project.