Effect of Tragacanth Gum and Wheat Starch as a Substitute for Gelatin on Rheological, Physical and Sensory Properties of Dairy Dessert

 The consumption of milk and its products is considered as a main indicators of development of human societies. In order to promote the health of children and provide the sufficient amount of calcium and nutrients and make the consumption of these products attractive. Necessary measures should be taken to increase milk consumption in society and increase the diversity of these products (Keshtkaran et al., 2013). Gelatin as one of the gelling agents used in dairy desserts is a hydrocolloid with unique functional properties and has been used in various industries for a long time (Karim & Bhat, 2008). Gelatin is extracted from the partial hydrolysis of skin collagen, white connective tissue, and animal bones. One of the problems with using gelatin in dairy desserts is the instability at ambient temperature. Therefore, it is necessary to evaluate the characteristics of other native hydrocolloids as an alternative. Tragacanth is a dried gum secreted from several plant species and Iran is the largest exporter in the world. Structurally, the material is regular, odorless and hard. It can be used in various food products. The simultaneous utilization of hydrocolloid mixtures is used to create a synergistic property and to improve these products. Starch originated from a variety of sources , including corn, rice, wheat, potatoes, cassava, bananas, and sorghum (Murphy, 2000) is used in the food industry. Among the sources of starch production, wheat is cultivated in large quantities in Iran.

 In this study, pasteurized milk was purchased from Salamat Dairy Products Company, gelatin made in Pakistan, tragacanth gum and Gonabad wheat starch were purchased from Kian Shimi Company and sugar and vanilla were purchased from confectionery stores located in Mashhad. Ingredients for dessert production include 80% milk, 10% sugar, 0.1% vanilla, tragacanth gum at concentrations of 1.3%, 1%, and 0.7% and starch at concentrations of 3% , 2%, and 1% by weight (powder selection) according to the method of Tarrega et al. 2004 with a slight modification. A sample containing 1.5% gelatin was considered as a control sample. The mixture was heated to 90 ° C for 20 minutes, stirring at medium speed. The samples were then placed in special containers and cooled to ambient temperature. Prior to testing, the samples were refrigerated (4-5 ° C) for 24 hours. The samples were coded based on the concentration of gum and starch (: T% of tragacanth gum and: S% of wheat starch).

 The flow behavior in all dairy dessert samples was similar to the control  and was pseudoplastic type. The Power model was selected as the best model (R <98, RMSE> 1.6). As the concentrations of tragacanth gum and wheat starch increased, the parameters of hardness, consistency, adhesion and adhesion strength increased significantly, and the samples 0.7T2S, 0.7T3S and 1T, 1S had the same value in terms of all textural parameters. At constant concentration of starch, with increasing tragacanth gum from 0.7% to 1.3%, the rate of syneresis of samples decreased from 0.34% to 0.06% and at constant concentration of gum, with increasing concentration of wheat starch, Syneresis decreased significantly from 0.34 to 0.24%. Increasing the shelf life of the samples naturally led to an increase in syneresis. In the samples 0.7T3S, 1T3S, 1T2S, and 1.3T3S, the increase of syneresis occurred to a lesser extent.. According to the results of analysis of variance, the effect of gum on zeta potential was very significant (p <0.05). Increasing the concentration of hydrocolloids in most samples led to an increase in the amount of negative charge and zeta potential. In most samples, at a constant concentration of tragacanth gum, with increasing the concentration of wheat starch from 1% to 3%, the span decreased, while at a constant concentration of starch, the increase in tragacanth gum from 0.7% 0 to 1.3% led to increase the span. Among the dairy dessert samples, 5 dairy dessert samples include: 0.7T2S, 0.7T3S, 0.7T1S, 1T2S and 1.3T1S, were selected the best in terms of physical and rheological characteristics to the control sample. They were more similar, selected and used for sensory evaluation. The results of comparing the mean of the data showed that different concentrations of tragacanth gum and wheat starch had a significant effect on the sensory characteristics of the samples including: color, flavor, sweetness, adhesion, gum state and overall acceptance (P> 0.05) and increasing the concentration of tragacanth gum and wheat starch had no significant effect on other sensory properties (p> 0.05).

 In general, due to the similarities of the physical, rheological and sensory properties of the treatment samples with the control, 1S1T sample  was selected as the most suitable sample to replace the sample containing gelatin.