Effect of combination of inulin-gelatin and polydextrose-gelatin on rheological and sensory properties of low-fat yogurt

Introduction: Yogurt is one of the most popular dairy products that is widely consumed around the world, which has received a lot of attention due to its high nutritional value and the presence of beneficial bacteria (Physman et al.  1999). But this dairy product sometimes contains high amounts of fat, which has caused concern among consumers, so the demand of consumers to consume low-fat yogurt has increased. Reduction of fat and consequent reduction of total solids in non-fat and low-fat yogurts, weakens the body and texture of the product and increases hydration, which as a result is not accept by consumers (Aziznia et al 2008). Over the years, low-fat yogurt manufacturers have resorted to using fat substitutes to overcome these problems. The use of fat substitutes reduces the calories in food intake and can improve the physical and organoleptic properties of low-fat products (Gon et al 2005). Hydrocolloids are one of the fat substitutes that have been widely used in recent years. Some of these compounds, in addition to acting as fat substitutes, are known as a prebiotic compound. Prebiotics are indigestible carbohydrate dietary fibers that stimulate the growth and proliferation of bacteria such as Bifidobacterium and Lactobacillus in the large intestine and thus improve host health (Raber Freud2005, Werbeck 2005). These prebiotic carbohydrates include, inulin and polydextrose. Inulin is a polymer of fructose units linked by β (1-2) bonds, usually with a glucose residue at the end of the chain. Inulin is a fermentable dietary fiber and helps improve intestinal function, has a bifidogenic and prebiotic role, increases calcium absorption, reduces triglycerides in people with high fat, and for each gram produces 1.5 kcal of energy (Abbasi and Farzanmehr 2009). This polysaccharide is widely used as a fat substitute and prebiotic compound (Roberfroid 2005). Polydextrose is a low molecular weight polysaccharide (Frank 2002) consisting mainly of 1-6 alpha-linked glucose units, some of which are linked to sorbitol and citric acid (Oliveira and Et al 2009). This material has fibrous and prebiotic properties, sweet taste and does not have any undesirable color in the product, increases the consistency and viscosity without creating turbidity (Codex 1996). This compound is mainly used as a substitute for sugar, starch and fat in food and also has prebiotic properties and beneficial effects on mucous membrane activity (Jay et al 2000). Gelatin is a protein hydrocolloid that has many properties including the ability to bind to water, gel formation, increase viscosity, film formation and emulsifying role. The amphoteric state of gelatin as well as its hydrophobic regions on the peptide chain make it a strong emulsifying and foaming agent (Cheng et al 2008). Various studies have proven the effectiveness of different hydrocolloids on low-fat yogurt. In one study, Gojisberg et al 2009 stated that the addition of inulin significantly improved the textural and sensory properties of low-fat yogurt. In another study, Pimentel et al 2012 reported that the addition of long-chain inulin to milk-produced yogurt produces a tissue similar to whole-milk yogurt, so it can act as a fat substitute. Serisor et al 2013 also reported that the probiotics inulin and polydextrose improve low-fat yogurt tissue and reduce hydration. In another study, Fiszman et al 1999 showed that adding gelatin to yogurt increases the strength of the product up to 9 times and reduces its hydration. The results of most of the mentioned studies show the beneficial effect of these additives in improving the properties of low-fat yogurt and no adverse effect has been observed. However, previous studies have not reported the combined effect of inulin-gelatin and polydextrose-gelatin on the rheological and sensory properties of yogurt.Scientific evidence has demonstrated that consumption of high-fat foods has direct relation with increasing incidences of various diseases such as obesity, diabetes, hardening of the artery walls and blood pressure. Thus, demand for low-fat foods has increasingly been promoted by health-conscious consumers. However, development of low-fat foods is challenging as fat makes a major contribution to sensory attributes of many foods. The objective of this research was therefore to evaluate the effects of inulin (0, 2 and 3 %),  polydextrose (0, 2 and 3 %) and gelatin (0.5%)as fat replacer on the rheological and sensory characteristics of low-fat yogurt.  
Material and methods: The materials used in this study included milk (1.5% fat and 9.36% no fat dry matter) from East Azarbaijan Selected Pasteurized Milk Company, Yogurt Starter from Christian Hansen Company of Denmark, Inulin from Sansus Company of the Netherlands, Polydextrose from Denisco Company and Gelatin from Merck Company. In order to produce low-fat yogurt, 500 ml of low-fat milk (1.5% fat) was placed on the steam bath and after reaching 45°C, inulin (2 and 3%), polydextrose (0.5%) and gelatin (0.5%) were added to milk separately. Each treatment was pasteurized at 90°C for 10 minutes and after cooling up to 42°C, commercial starters were added to the sample and then the samples were transferred to incubator 42°C after reaching pH=4.6, the samples were placed in the refrigerator at 5°C until the experiments were performed. 
Results and discussion: The results showed that inulin and polydextrose had not significantly (p>0.05) effect on the pH but increasing the inulin and polydextrose caused significant (p < /em>≤0.05) decrease in the whey separation. The addition of constant concentration of gelatin to different samples containing inulin or polydextrose, caused to further lowering the whey separation. Regarding the rheological parameters, it was observed that inulin and polydextrose caused a significant (p < /em>≤0.05) increase in the storage modulus, loss modulus and viscosity. Addition of constant concentration of gelatin to the samples also improves the rheological properties.
Adding different percentages of polydextrose and inulin to low-fat yogurt causes a significant increase (p≤0.05) in color dye compared to the low-fat control yogurt sample, but increases the concentration of inulin and polyene. Dextrose has the opposite effect and reduces the color score, which was statistically significant in relation to samples containing 3% polydextrose and 3% inulin (p≤0.05). Sensory evaluation showed that the overall acceptability of the low-fat yogurt was improved by addition of inulin or polydextrose with constant concentration of gelatin.
Conclusion: It was generally concluded that the addition of inulin and polydextrose improve various characteristics of low-fat yogurt. The results suggest that the addition of constant concentration of gelatin to the low-fat yogurt containing inulin or polydextrose had more improving effect on rheological and sensory properties.