Abstract:
Feasibility of ohmic heating to overcome gel-weakening in Pacific whiting surimi
was investigated. An ohmic heating apparatus was developed using two rhodium-coated
stainless steel electrodes inside a polyvinyl chloride (PVC) tube, a variable transformer,
and voltage and current transducers. Rapid heating associated with the ohmic process
quickly inactivated endogenous proteinase(s), resulting in significantly high shear stress
and shear strain of surimi gels (78% moisture content, 2% NaCl). Degradation of myosin
heavy chain (MHC) and actin examined by SDS-PAGE were significantly reduced and
continuous gel structure were shown by scanning electron microscopy. Whiting surimi
heated in a 90°C water bath for 15 min exhibited poor gel quality and disordered
microstructure due to proteolysis of MHC.
Electrical conductivity, a critical parameter influencing rate of heat generation
during ohmic heating, was elucidated. Electrical conductivities of whiting surimi pastes
with four moisture contents (75, 78, 81, and 84% wet basis) and added NaCl (1, 2, 3, and 4%) were measured from 10 to 90°C using ohmic heating at voltage gradient of 3.3, 6.7,
and 13.3 V/cm. Electrical conductivity significantly increased with temperature and salt
content and slightly increased with moisture content. The effect of the voltage gradient
was evident at combinations of high moisture (81, 84%) and NaCl content (3, 4%), due
to electrochemical reactions at the electrodes. The empirical model of electrical
conductivity as a function of temperature and compositional characteristics predicted
values with an error range of 0-15.6%.
Finally, kinetic models of textural degradation of whiting surimi were developed
using two different approaches: isothermal and nonisothermal procedure over a range of
temperature (40-85°C) and time (0.5-35 min). The effect of thermal lag was accounted
for using the models derived from the Arrhenius equation. Textural degradation obtained
from both methods followed first order kinetic. Degradation of MHC derived from
nonisothermal procedure was best described by apparent reaction order of 1.4.
Degradation rate of gel texture and MHC increased with temperature and reached the
highest rate at 55 and 57°C, respectively. Then they decreased to minimum rate at 70
and 75°C, respectively. The kinetic model for the loss of MHC satisfactorily estimated
MHC content of the controls with an averaged error of 10.8%. Relationship between
degradation of MHC and gel texture are discussed.
