Effect of sucrose concentration on rheological properties of pomegranate juice gels based on gelatin/Ƙ-carrageenan

Background and objectives: In this paper local pomegranate juice (50% v/v final solution) based gels made from gelatin or kappa-carrageenan with sucrose (2.6%, 5.8% w/v final solution) have been examined to determine the textural characteristics and viscoelastic properties. Pomegranate is one of the most important crops cultivated in Iran which extensively used as a source of variety of products as gels, juices, desserts and etc. However, this fruit has especial properties such as low pH, different tastes, red color, phenolic and anthocyanin compounds which may lead to variety effects on producing different products. In this regard, it is required to study the influence of applying natural pomegranate juice in final gel quality. Since gelatin and kappa-carrageenan are two popular examples of protein and polysaccharide hydrocolloids which are mostly available with low cost, in this work pomegranate gels are formed based on these hydrocolloids. Generally, gel formulations include sugar as a taste and texture improver so its interaction with gel components and its effect on textural properties of pomegranate gel is investigated. Materials and methods: In order to determine textural properties like hardness, cohesiveness and springiness, texture profile analyses have been used. Also, to investigate viscoelastic properties, dynamic oscillation measurements were used and their rheological behavior is interpreted by loss tangent, elastic and viscos modulus. Results: The result of texture profile analysis show that incorporation of sucrose in gelatin based gels effectively increases hardness, cohesiveness and springiness. Furthermore, the results of oscillatory dynamic rheological measurements show that Gelatin gels are more elastic and resist higher frequencies when sucrose concentration increased. Since gelatin has the protein nature, in different pHs has different electrostatic charge so lower/higher pHs than pI lead to positive/negative charge on it. In this condition, acidic pH of pomegranate juice which is lower than gelatins pI results in positive charge on it which can interact with anionic agents in the media. In this case, presence of phenolic compounds, flavonoids and tannins in pomegranate juice with anionic nature can enhance these interactions. The results of investigations have shown that applying pomegranate juice and sucrose for preparation gelatin gels leads to formation a high quality gel because of proper pH of the juice and adequate hydrogen and electrostatic interactions.
In the case of Ƙ-carrageenan gels, increasing the sucrose level enhances viscose modulus and leads to a week viscoelastic texture, which causes hardness and cohesiveness to decrease extremely. Although low concentration of sucrose can improve gel textural properties, high concentration of it inversely affects on carrageenan gel elasticity. Also, different compounds of pomegranate juice may have synergistic effect on decreasing textural quality of carrageenan gels by sucrose. The major reason of this effect may be due to sucrose prohibition from cross-linking interactions. Conclusion: The results of textural analysis indicate that high quality pomegranate gels based on gelatin and Ƙ-carrageenan with optimum concentration of sucrose can be produced whereas natural conditions of pomegranate juice like pH and containing ions do not reduce its quality.