عنوان مقاله [English]
Background and objectives: In high voltage electrostatic field technique, by applying a high voltage electric field (HVEF) in a needle- plate electrode system, air at the surface of food product is ionized, and a volumetric flow known as corona wind is formed, that increases the heat transfer coefficient in food product by volumetric heat generation. In this thechnology, ozone generation followed by the corona discharge and air ionization, are capable to reducing microbial contamination; on the other hand, since ozone is a strong oxidizer, there is a risk of lipid oxidation. The aim of this paper is using high voltage electrostatic field for thawing of chicken breast, and investigating its effects on microbial count and lipid oxidation.
Materials and methods: First, fresh chicken breast were cut into equal cubes (2×2×2 cm3) by a designed cast and sharp razor, then, were frozen by a freezing tunnel with forced air circulation at -30 0C and 1 m/s air velocity, after vacuum packaging in polyethylene bags. The frozen samples were kept at -180C until use. Thawing under HVEF at three different voltages (from the corona start voltage to the breakdown voltage), and specific electrode gaps were done. The control sample was subjected to no HVEF but thawed conventionally by still air method. Fat oxidation of thawed samples were investigated at first, third, fifth, and seventh days intervals during storage at refrigerator. The thawed samples were kept at refrigerator (4±10C) after packaging, and evaluated for microbiological tests (total microbial count and psychrotrophic), and the oxidation kinetics were investigated. Total microbial count during first, third and sixth days of storage at refrigerator, by pour plate at 300C for 72 hours, and psychrotrophic bacteria count during ten days storage at 6.50C by surface plate were investigated.
Results: The equation resulted from TBARS- time changes curve, is linear. By evaluation of zero-order kinetic model, the kinetic rate constant was determined at different treatments. The kinetic rate constant had no significant difference with control at low electric field strength (1.5 kV/cm); while, this factor was more intense with increasing electric field strength (2.25 and 3 kV/cm). Generally, malonaldahyde formation rate of thawed chicken was increased by increasing voltage and decreasing electrode gap during storage. However, at corona start voltages, malonaldahyde formation rate constant was less than control. Results from microbial growth showed total microbial count decreased at HVEF thawed samples compared with control and fresh chicken during six days storage. Total microbial count in the case of control was 1.07×105 cfu/g at first day, that was increased to 7.82 ×106 cfu/g at sixth day, and in the case of HVEF thawed samples at 8 and 10 kV, were 5× 104 cfu/g and 3.36×104 cfu/g at first day, that were increased to 1.36×106 cfu/g and 1×106 cfu/g at sixth day, respectively. However, total microbial count of fresh chicken was increased from 9.73×104 cfu/g at first day to 6×106 cfu/g at sixth day. On the other hand, storage at 6.5 0C during 10 days, decreased the psychrotrophic microorganisms significantly at HVEF thawed samples) P<0.05).
Conclusion: Due to the importance of thawing for products susceptible to spoilage, such as chicken, it is possible to benefit from HVEF thawing method to decreasing spoilage and increasing shelf life, by applying 2.25 kV/cm electric field strength.