Carrots (Daucus carota), one of the important root vegetables, are known
for their nutrient contents viz. b-carotene besides appreciable amount of
vitamins and minerals (Walde et al. 1992). Carrots are well known for
their sweetening, antianemic, healing, diuretic and sedative properties. The
enzymes commonly found to have deteriorative effects in carrots are peroxidases
(PODs) and catalase. In order to minimize deteriorative reactions, fruits
and vegetables are heat treated or blanched to inactivate the enzymes. Blanching
of fruits and vegetables are done either in hot water, steam or selected
chemical solutions (Luna-Guzmán and Barret 2000; Severini et al. 2004a,b).
Blanching in calcium chloride solution is used to increase the firmness of
fruits and vegetables, because of the activation of pectinmethylesterase when
immersed in hot calcium chloride solution (Quintero-Ramos et al. 2002).
The inactivation of POD is usually used to indicate blanching sufficiency
as POD is ubiquitous and considered to be among the most heat-resistant plant
constitutive enzymes (Ganthavorn et al. 1991). While blanching leads to some
favorable factors, like the inactivation of enzymes, expelling trapped air in the
intracellular regions and reducing any initial infections, it also causes loss of
nutritional quality (Ramesh et al. 2002). Therefore, the optimization of the
blanching process with respect to nutrient retention (b-carotene, vitamin C
loss) and product yield should be considered along with the inactivation of
enzymes. The variables, such as temperature and time of treatment, and concentration
and nature of the acid or salt in the blanching solution, determine the
effectiveness of the blanching process (Quintero-Ramos et al. 2002; Severini
et al. 2004a).
Over the years, low-temperature longtime blanching, compared to the
conventional high-temperature short-time blanching for better quality retention,
has been emphasized. Low-temperature longtime blanching, in comparison
to high temperature short time, enhances the firmness and reduces the
nutritional and flavor losses of the product (Taylor et al. 1981; Canet and Hill
1987).
Mathematical modeling of enzyme inactivation in heated foods is essential
to estimate the residual activity of enzymes from the temperature–time
profile and enzyme inactivation kinetics. The potential benefits of modeling
588 U.S. SHIVHARE ET AL.
lies in its ability to assess the effect of different heat treatments on residual
enzyme activity without performing numerous trial runs (Adams 1991).
Carrying out inactivation experiments at several temperatures is considered
as a useful tool for testing the validity of mechanisms verified at isothermal
conditions (Polakovicˇ and Vrábel 1996).
In experimental studies, thermal inactivation of enzymes has not been
often found to follow first-order kinetics (Adams 1997; Soysal and Söylemez
2005). Deviation from the ideal first-order enzyme inactivation model occurs
because of the differences in the heat resistivity of the enzyme molecules. For
POD, this behavior has been explained in terms of the existence of two
isoenzyme populations (heat stable and heat labile), present in the native POD
enzyme (Ling and Lund 1978).
The objective of this work was to determine the optimum blanching
conditions for carrots in terms of nutrient (vitamin C and b-carotene) retention
and to study the kinetics of the inactivation of POD in carrot juice. Various
enzyme inactivation models were tested on the basis of statistical and physical
parameters to ascertain a suitable model capable of explaining the POD inactivation
kinetics.