Textural and organoleptic properties of fat-free buffalo yogurt as affected by polydextrose

The demand for functional, nutraceutical and low calories dairy products has grown in recent decades. The effects of using different concentrations of polydextrose (1.5%, 3%, and 5%) on the textural and organoleptic properties of fat-free buffalo set yogurt (FFBS) were investigated. Addition of polydextrose significantly (P < .05) improved the water-holding capacity (WHC), sensory attributes and texture properties compared to the FFBS control yogurt. The sensory attributes, pH and WHC values were gradually decreased during cold storage of 21 days. Viability of yogurt culture was enhanced in the presence of 3% polydextrose compared with the FFBS control yogurt.

Introduction

The demand of functional, nutraceutical and low calories dairy products has grown in the last decades. Yogurt is the most popular dairy product and has been considered as the first functional foods.[1] Buffalo is the second global milk-producing animal all over the world and widely distributed throughout Asia. It is worth noting that more than 95.8% of the world populations of water buffalos are kept in Asia.[2] Buffalo milk has a high content of fat, protein, lactose, vitamins, and minerals, and consequently high nutrient contents, which has attracted enormous attention by both academic researchers and dairy industry for various dairy products manufacturing.[3] Furthermore, it has been reported that subjects with cow milk allergies are capable of tolerating buffalo milk.[2] Milk fat plays a crucial role in yogurt quality attributes, and therefore fat reduction causes some undesirable qualities in yogurt such as lack of flavor, weak body, and poor texture.[4] Different attempts have been performed to improve the properties of low-fat yogurt i.e. using transglutaminase (TG), increasing solid nonfat, addition of natural or synthetic stabilizers, and polysaccharides like starch and polydextrose.[5]

Polydextrose, is a complex carbohydrate made from glucose, citric acid, and sorbitol, is a water-soluble dietary fiber with calorie content of 1 kcal/g.[6] Polydextrose is an indigestible carbohydrate and has some health effects due to its laxative action and control of glucose and cholesterol levels in blood.[7] Many countries approved using polydextrose as dietary fiber.[8] In addition to its health benefits, polydextrose has some technological properties including forms a highly viscous gel-like matrix contributing to creaminess and mouthfeel.[6] Therefore, it was used as fat replacer in low-fat dairy products. Srisuvor et al.[9] added inulin and polydextrose in concentrations of 1% to 3% to improve physicochemical and sensory characteristics of low-fat yogurt. Furthermore, 6 and 10% polydextrose concentrations were utilized as bodying agents, and as fat replacers in ice cream mixtures.[10,11] In addition, polydextrose was added as prebiotic to produce nonfat symbiotic fermented milk.[8] Addition of inulin and polydextrose mixtures showed promised replacement of sucrose in sugar-free chocolate.[12]

Accordingly, the objective of this study was to evaluate the effect of adding polydextrose with different concentrations (1.5%, 3%, and 5%) as fat replacer on the physicochemical, sensory and rheological properties of fat-free buffalo set yogurt (FFBS). Yogurt quality indices as a function of polydextrose addition were investigated throughout the storage of 21 days at 4°C.

Materials and methods

Materials

Fresh buffalo milk was obtained from buffalo research institute farm, Nanning, China. Yogurt culture YF-1922 (Direct Vat Set), consisting of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, was purchased from Chr. Hansen A/S (Denmark). Polydextrose was obtained from Jiangsu Ruiduo Biological Engineering Co., Ltd (Jiangsu, China).

Yogurt preparation

Whole buffalo milk was separated, giving skim milk of 0.2% fat. Polydextrose was added to the buffalo skim milk at concentrations of 1.5%, 3%, and 5% (w/w). The mixtures were heat-treated at 90°C for 10 min followed by cooling to 45 °C and inoculated with yogurt culture (0.015%, w/w). The inoculated treatments were then filed into polystyrene cup (125 ml) and incubated at 42 °C until coagulation. All yogurts were stored in the refrigerator at 4 ± 1 °C for 21 days. Fat-free and full-fat buffalo set yogurts, without the addition of polydextrose, were prepared as control treatments. Physicochemical, sensory and microbiological analyses were carried out at 1, 7, 14, and 21 days of storage.

pH value and water-holding capacity (WHC)

The pH values were measured by pH-meter (PB-10, Sartorius, Germany). Samples were analyzed in triplicate. WHC was determined as described by Romeih and Hamad.[12] In brief, 50-g yogurt was filled in centrifugation tubes and centrifuged for 15 min at 3000 × g at 20°C (Biofuge, Thermo Fisher Scientific, Germany). The amount of supernatant yogurt serum was removed and weighted.WHC(%,w/w)=Yoghurt(g)−Supernatent(g)Yoghurt(g)×100WHC%,w/w=Yoghurtg−SupernatentgYoghurtg×100

Sensory evaluation

Organoleptic properties of the FFBSY were assessed by 10 trained panelists from the Buffalo Research Institute using evaluation criteria as described by Romeih et al.[4] The panel was asked to evaluate the yogurt treatments using a graduated scale of (30) for appearance, (30) for flavor, (40) for texture and/or (100) total acceptability. All samples were tested within random order in polystyrene plastic cups.

Instrumental textural properties

Texture characteristics, i.e. firmness, adhesiveness, and cohesiveness of the FFBS yogurts after 1d of storage at 4 ± 1°C were measured by a texture analyzer (TMS-PRO, Food Technology Corporation FTC, Virginia, USA) using a 5 kg load cell as described by Moschopoulou et al.[13] Yogurt gels in the cups were penetrated with 1 cm2 cylinder probe (P/1KP) to 10% of their original height. A force–time curve was obtained at a crosshead speed of 1.0 mm/s. The pretest and posttest speeds were 120 mm/min. Measurements were carried out in six replicates.

Yogurt culture viability

Ten grams of each yogurt treatments were diluted with 90 ml of sterile physiological saline (PS). Serial dilutions were prepared in 9 ml PS. One ml of each dilution was poured on MRS agar plate. MRS agar plates were incubated under anaerobic conditions for 48 h at 37 °C. The numbers of colony-forming units (CFU) were counted and presented as CFU/g.

Statistical analysis

The results are presented by the mean values ± standard deviations (SD). Two-way analysis of variance (ANOVA) was performed by IBM-SPSS (ver. 20.0, IBM, USA), followed by assessment of differences by the least significance difference method (LSD) at P < .05.

Results and discussion

pH values and WHC characteristics of FFBS yogurts after 1d storage

Polydextrose was approved as a fat replacer and prebiotic materials in many countries to improve the physiochemical and texture properties of low-fat dairy products. The effects of different polydextrose concentrations on water-holding capacity and pH values of FFBS yogurts after 1d storage at 4 ± 1 °C are presented in Table 1. The results showed that pH values were significantly (P < .01) decreased by increasing the polydextrose concentration and ranged from 4.16 to 4.56 (Table 1). Addition of polydextrose most probably enhanced the growth of yogurt starter culture that consequently resulted in high acid production and decrease in pH values. This result demonstrates the prebiotic effect of polydextrose on the yogurt culture and is in agreement with that of Oliveira et al.[14] and Srisuvor et al.[9] who reported that addition of polydextrose decreased the pH value compared with the control fermented milk. In contrast, Guven et al.[15] reported no significant effect on pH values of yogurt by the addition of inulin as a fat replacer.