The effect of fat content changes on chemical and rheological properties of yogurt contains Jerusalem artichoke powder during storage

Introduction: Consumers across the world are becoming more interested in foods with health promoting features as they gain more awareness of the links between food and health (Tamime et al., 2005). Yogurt is the most consuming fermented milk products, which has a positive effect on human health due to the high nutritional value and particular importance in the diet of individuals. Yogurt in Health and Disease Prevention examines the mechanisms by which yogurt, an important source of micro- and macronutrients, impacts human nutrition, overall health, and disease. Topics covered include yogurt consumption’s impact on overall diet quality, allergic disorders, gastrointestinal tract health, bone health, metabolic syndrome, diabetes, obesity, weight control, metabolism, age-related disorders, and cardiovascular health (Shah, 2017). Helianthus tuberosus L., commonly known as Jerusalem artichoke, produces fibrous roots with short rhizomes that end in underground caulinar tubers, which accumulate fructans, mainly inulin. The plant has four main uses: horticultural, fodder, bioethanol production, and inulin extraction (Rébora, 2008). This crop is highly tolerant to adverse weather conditions and various plant diseases and also grows well in poor land (Saengthongpinit & Sajjaanantakul, 2005). Among other plants rich in inulins, Jerusalem artichoke stands out as an interesting candidate for the industrial production as its tubers accumulate similar levels of inulin (16–20% of fresh tuber) as chicory roots (Franck, 2000) and could be cultivated at a low cost with low input of fertilisers on any type of soil and cool climatic conditions. However, the use of Jerusalem artichoke (JA) tubers for inulin extraction is less well known as they are commonly eaten as vegetable (Paseephol & Sherkat, 2009). Understanding of rheological properties of fruit juices are essential for quality control, process engineering application (designing and selection of proper equipment including heat exchangers, transport systems, evaporators and pumps), calculating energy usage and power requirement for mixing. Rheological information is valuable in product development (Salehi, 2019).
Recently, viscometers have become a valuable and extensively used tool in the study of milk gel structure. Such measurements are sensitive to the initial stage of casein micelles aggregation and demonstrate that gelation begins well before any visual observation of coagulation. A point is reached during aggregation in which a three-dimensional cross-linked network of casein is formed and following this, coagulation can be observed rheologically (Shaker et al., 2000). The goal of this study is investigate on the effect of fat content changes on chemical and rheological properties of yogurt contains Jerusalem artichoke powder during storage.
Material and methods: In this study, Jerusalem artichoke powder to the amount of 0.5, 1 and 1.5% in yogurt formulation with 0.5, 1 and 1.5% fat was used, and its chemical and rheological characteristics was investigated during the 21 days. The cow milk (total solid nonfat 9%) was heated to 92 °C for 5 min then cooled to inoculation temperature (42 °C). Jerusalem artichoke tubers were washed with tap water and brushed to partially remove tuber skins. Tubers were then cut into ∼2 mm slices and dried at 45 °C in an electric oven with convection, until constant weight was achieved. Dried slices were then ground and passed through a 63-mesh sieve to obtain the Jerusalem artichoke flour. The pH was measured directly using a calibrated digital pH-meter (Lutron YK-2001 pH meter, Taiwan). The titratable acidity (TA) with respect to the percentage of lactic acid equivalent was determined by titration of the yogurt samples with 0.11 N NaOH in the presence of phenolphthalein. The viscosity of yogurt was measured using a rotational viscosimeter (Brookfield, USA). The rheological parameters of yogurt at shear rate of 40 s^-1 were studied using spindle LV64 at 8°C. Each measurement was conducted in three repetitions. The experimental data were subjected to an analysis of variance (ANOVA) for a completely random design using a statistical analysis system (SPSS 21). Duncan’s multiple range tests were used to determine the difference among means at the level of 0.05 (Salehi, 2017).
Results and discussion: Addition of Jerusalem artichoke powder was not significantly effect on chemical and rheological characteristics of yogurt. The results showed that the pH of the higher in fat at the end of the maintenance period was significantly higher than the pH of yogurt with 0.5% fat. Also acidity of the samples at 14 days was significantly increased although for an amount of increase in higher fat yogurt in comparison with 0.5 and 1% fat yogurt is lower. At the end of the maintenance period, acidity of yogurt with 0.5% fat was significantly more than yogurt with 1 and 1.5 percent fat. According to consistency and elasticity, the samples with 1.5 % Jerusalem artichoke powder and 1.5 % fat was the best treatment. Rheological properties of plain yogurt during coagulation process, impact of fat content and preheat treatment of milk were studied by Shaker et al. (2000). Their results showed that the increasing in fat content leads to an increases in viscosity. The highest viscosity was manifested by milk heated at 137°C while the lowest value was exhibited by milk heated at 65°C. The process viscosity curves for three different stages are described by mathematical relationships. Finally, a two-parameter power law model was used to describe the flow behavior of the yogurt during coagulation.
Conclusion: Jerusalem artichoke flour resulted an interesting food ingredient due to its high content of prebiotics (inulin) and phenolics, which may be used as a powder substitute to increase the nutritional quality of dairy products. Rheological properties for foods, such as fermented dairy products, are important in the design of flow processes, quality control, storage and processing and in predicting the texture of foods. With increasing Jerusalem artichoke powder and fat content, loss modulus versus shear stress was increased. In general, with increasing shear rate the viscosity of samples were decreased. Also with increasing the percentage of samples fat, the viscosity in the constant shear rate was increased. The increase in viscosity at the highest fat content may be due to increase of total solids of the milk which has a significant effect on the firmness of yogurt gel. Addition of Jerusalem artichoke powder increases the viscosity but the effect of fat on the viscosity was more.