Ascorbic acid degradation and color changes in acerola pulp during ohmic heating: Effect of electric field frequency
Introduction
Acerola (Malpighia emarginata D.C.) is a tropical fruit of great nutritional value because of its high content of vitamin C (1000–4000 mg/100 g) and the presence of other nutrients such as anthocyanins, carotenoids, iron and calcium. Loss of its vitamins and pigments due to thermal treatment during food processing is a major concern, which may be potentially mitigated by alternative pasteurization processes such as ohmic heating.
Ohmic heating is based on the passage of an alternating current through a sample which responds by generating heat internally due to its inherent resistance (Fryer et al., 1993, Palaniappan and Sastry, 1991a). The energy generation is proportional to the square of the local electric field strength and the electrical conductivity of the product (Goullieux and Pain, 2005, Ruan et al., 2001). This technology is one alternative to thermal processing that allows high-temperature/short-time processing (de Alwis and Fryer, 1992, Stirling, 1987). Ohmic heating can provide a fast and uniform heating with the ability to start/stop thermal processing instantaneously (Bansal and Chen, 2006). These advantages allow reaching the desire holding temperature of the process in a short time, usually few seconds, reducing the treatment time that is critical to avoid excessive thermal damage to labile substances.
The ohmic heating consists of the application of AC voltage to electrodes in contact with the product. When an electric field is applied to a sample, each molecule in the field suffers a slight distortion of the electron distribution, called electronic polarization (North, 1972). The electric field distorts the negative cloud of electrons around the positive atomic nuclei in a direction opposite to the field. This slightly separation of charges make one side of the atom somewhat positive and the opposite side negative. The consequence of each molecular dipole along the direction of the field is a polarization of the whole sample. The rate of the polarization process is determined by the charge mobility. This is irrelevant if the electric field changes slowly, but if the electric field is altered rapidly (as a high frequency alternating voltage) there is not time for an equilibrium polarization to be established (North, 1972).
Studies have been performed to evaluate the effect of the electric field frequency during moderate electric field treatments on food properties and process parameters (Bansal and Chen, 2006, Imai et al., 1995, Kulshrestha and Sastry, 2006, Lima et al., 1999b, Shynkaryk et al., 2010), diffusion and extraction of certain constituents of foodstuffs (Kulshrestha and Sastry, 2003, Lakkakula et al., 2004, Lima and Sastry, 1999, Wang and Sastry, 2002), stimulation of microbial growth (Loghavi et al., 2008, Loghavi et al., 2009) and inactivation kinetics of spores (Somavat et al., 2012). However, little is known about the effect of the electric field frequency on nutrient stability during ohmic heating.
The nutritional components of the acerola pulp, such as ascorbic acid and pigment compounds, are subjected to an electric field distortion when the ohmic heating technology is applied. High electric field frequencies do not allow the polarization to reach the equilibrium and this phenomenon may influence on their degradation kinetics. For this reason, the objective of this work was to evaluate the effect of the electric field frequency (from 10 to 105 Hz) on ascorbic acid degradation and color changes in acerola pulp during ohmic heating and also to compare this technology with the conventional heating process.
Section snippets
Acerola pulp
The acerola pulp was supplied by EcoFruits Company (South Jordan, USA). The nutritional properties of the product were evaluated. The total dry matter and moisture content were determined by the gravimetric method (AOAC, 2005). The soluble solid content and pH values were determined using a refractometer (Sper Scientific, model 300017, Scottsdale, AZ, USA) and a pH meter (Mettler Toledo, pH-/Cond-/DO-/ISE-Meter model, USA), respectively. Monomeric anthocyanins were analyzed using the
Sample characterization
The nutritional properties of acerola pulp are presented in Table 1. The product showed a good content of vitamin C, 1657 ± 45 mg/100 g. This result is in agreement with those found by Vendramini and Trugo (2000). These authors investigated the ascorbic acid content of acerola in three different stages of maturity and found values between 1074 and 2164 mg/100 g. The soluble solids content of the acerola pulp was approximately 11 °Brix. Monomeric anthocyanins and phenolic contents were 2.16 ± 0.17 mg/100 g
Conclusions
This work studied the effect of electric field frequency on degradation kinetics of ascorbic acid and color changes in acerola pulp during thermal treatment by ohmic heating. Ascorbic acid degradation fitted a first-order reaction model and the rate constants ranged from 1.1 to 1.6 × 10−3 min−1. Color parameters L*, a* and b* decreased over time for all treatments, which indicates color changes during the heating treatment.
Low electric field frequency (10 Hz) showed higher ascorbic acid
Acknowledgement
The authors acknowledge the financial support received from CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brasil), as a scholarship to the first author.
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