Document Type : Research Article
Authors
Department of Food Science and Technology, Ferdowsi University of Mashhad (FUM), Mashhad, Iran.
Abstract
Introduction: Freezing is one of the ways to extend the shelf life and improve the chemical and microbiological stability of food products. Food products are exposed to different processes and the functional properties of the products change during these processing. Hydrocolloids are used to stabilize the products undergoing different food processing. However the addition of hydrocolloids to food products shifts the foods characters. On the other hand, hydrocolloids decrease the growing rate of ice crystals in dispersions. Lipidium perfoliatum locally called Qodume shahri. The study on the functional properties of Lipidium perfoliatum seed gum (LPSG) proved that this gum was able to bind with a large amount of water and increase the product consistency. The main purpose of this study was to understand the effect of freezing condition on functional properties including the rheological properties (steady shear rate), emulsion particle size distribution and foaming stability at different gum concentration of LPSG.
Materials and methods: The materials were purchased from a local detailer. LPSG was extracted in optimum condition (T: 48±1 ºC, pH=8, proportion water to seed 30 to 1, t: 1.5 h) according to method previously described by koocheki et al. (2009). After the preparation of the freeze-dried gum powder, dispersions of LPSG were prepared in distilled water at different concentrations (0.5, 0.75 and 1% w/v). In order to study the effects of freezing condition on the functional properties of LPSG, the samples were freezed at slow and fast conditions. The flow behavior was described by fitting the shear stress (τ) to shear rate (γ) data with the models to determine the best model to describe the flow behavior of LPSG. For the Emulsion preparation, the aqueous phase was prepared by mixing 2 g WPC into 30 g distilled water and an appropriate amount of LPSG (0.1 and 0.2 g) into 50 g distilled water on a magnetic stirrer for 10 min at room temperature. The dispersions were then left overnight at 4 ºC prior to emulsion preparation. The emulsion was prepared by mixing 20 g sunflower oil with WPC using a magnetic stirrer for 10 min. The mixture was subsequently pre-homogenized with a laboratory homogenizer at a rate of 20,000 rpm for 2 min at room temperature. After that, the gum solution was added to the emulsion and homogenized for 4 min at the same rate. The particle size analyzer was used to measure the mean diameter. For the foaming stability, the gum dispersions were prepared at 0.5% gum concentration and left overnight in a refrigerator to ensure a complete hydration. Afterwards, they were treated with different temperatures. 2% of egg white powder was added to the solutions and mixed by a homogenizer for 2 min at 20000 rpm. The foam stability was calculated as the foam volume after 30 minutes. A completely randomized design with the factorial arrangement was used for statistical analysis. All experiments were statistically analysed by Analysis of Variance (ANOVA) in Minitab R14. The p-values of < 0.05 were considered significant. All measurements were triplicated and the average values were reported.
Results and discussion: Results showed that the LPSG apparent viscosity increased insignificantly after the fast freezing condition. All samples illustrated non-Newtonian shear thinning behavior. Herschel- bulkley model was the best model to describe the flow behavior of the LPSG solution with the high determination coefficients. Among the selected rheological models, the flow behavior indices and consistency coefficients were unchanged. However freezing condition had no significant effect on the emulsion particle size. After 30 min foam stabilized by LPSG was constant. Therefore, LPSG can be considered as an appropriate stabilizer and thickening agent during freezing condition.
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