Determining Viscoelastic Behaviour in Sweet Potato (Ipomoea batatas L.) Tissue

Abstract

This study determined the stress relaxation behaviour of sweet potato (Ipomoea batatas L.) tissue samples (n=9) having 1 cm³ volume each, taken at an approximate mid-length of a transverse section of three randomly selected root tubers locally grown in Trinidad and Tobago.  A Baoshishan Digital Force Gauge HP-500N with manual force test stand and digital displacement scale was utilised for testing.  A flat disc indenter, having a 15 mm diameter was used to apply a constant deformation of approximately 3.14 mm (s=0.12 mm) at a rate of loading of approximately 0.5 mm/sec.  Force decay was digitally recorded over an approximate period of two minutes.  The method of successive residuals was used to determine stress relaxation times (T_rel) for tissue samples.  The stress relaxation rheological model for sweet potato tissue was determined to be a two-term exponential equation, where σ(t) = 94.12 e^-t/1877.25 + 24.78 e^-t/4.07 (N/cm²).  Theoretical apparent Modulus of Elasticity and Shear Modulus (Poisson’s Ratio, ν=0.43) was calculated to be 373.9 N/cm² and 91.42 N/cm² respectively.  Thin slices of approximately 1mm of each root tuber were prepared for dehydration at 150°F for 12 hours in a convection dryer.  Moisture content was determined to be 69.23% by weight.