Low- calorie pistachio butter is a novel food product which in its original form contains pistachio, sweetener and flavor improver. The viscous flow properties of low- calorie pistachio butter, as a novel food product, was studied by response surface methodology (RSM). The flow behavior of 42 formulas was assessed using Power law, Casson, Bingham and Herschel- Bulkley models. The Herschel-Bulkley model gave negative values for yield stress which has no physical meaning. The effects of three fat replacers (Balangu seed gum, Reihan seed gum, xanthan) and two sweeteners (isomalt and sucrose) on the rheological parameters of these models were investigated. All the samples showed a shear thinning behavior. The magnitudes of consistency coefficients and flow behavior indices in Power law model, the Casson yield stress and Casson viscosity and the Bingham yield stress and Bingham viscosity were 32.743- 388.645 Pa.s and 0.106- 0.515, 42.306- 282.319 Pa n and 0.477- 1.440 Pa.s and 94.599- 357.058 Pa and 1.042- 3.736 Pa.s, respectively.

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